Correct Chemistry FINAL

7/28/2019 Correct Chemistry FINAL

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AAS : ATOMIC Absorption Spectrometry

bd Butane-1,4-diamine

bdH+ : Butane-1, 4-diamine monoprotonatedBPR : Bromo pyrogallol Red

BPT : 4,7 -biphenyl-1, 10-phenanthroline

bpy Bipyridine

BunNH2 n-butyl amine

Bupy Butyl pyridine

CMP Cytidine-5-monophosphate

CN- Cyanide

Cydta 1,2 -Diaminocyclohexane N,N, N'N'-tetra acetic acid

Dacco Diazo cyclo octane

D-Mechanism : Dissociative Mechanism

DM NA : N,N'-Dimethyl (p-nitroso) aniline

DPA : Diethylene diamine Penta acitic acid

dto dithio ozalate

DTPA : Diethylene tetraamine penta acetic acid

1,4-DT : 1,4-dithiane

1,3 -DT : 1,3-dithiane

EBDP : Electrons Backscatter Diffraction Pattern

Edda : Ethylene diamine-N, N, N'N,-diacetic acid

EDTA : Ethylene diamine-N,N,N',N'-tetracetic acid

EGTA : Ethylene glycol bis(2-amino ethyl ether)- N,N, N',N'- tetraacetic acid.

en : Ethylene diamine

enH+ : Ethylene diamine monoprotonatedETAAS : Electro thermal atomic absorption spectrometry

EtNH2 : Ethyl amine

FIA : Flow Injection Analysis


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Fuchsin : Fuchsin is a magneta dye having chemical formula

C19S17N3.HCl, when dissolved in water and named Fuchsed after German translation

of French Ronald Companie's in 19th Century.Hedta : B-(2-hydroxy) ethylene diamine tetraacitic acid

HEEDTA : (Hydroxy ethyl) ethylene diamine triacetic acid

HMBPTS 2-Hydroxy-4-methoxy benzo phenone thiosemicarbazide

hxd : Hexane-1,6-diamine monoprotonated

I2 Iondine

ICP-MS Inductively coupled Plasma-Mass spectrometry

IDA : Imino diacetic acid

KCM : Kinetic Catalytic Method

MIDA : Methyle Imino diaceti acid

MNDT : Maleonitrilo dithiolate

Mpz+ : Methyl Pyrazinium ion

NAA : Neutron Activation analysis

NDA : Nitroso diphenyl amine

N-R-salt : Nitroso-R-Salt

NN : -Nitroso -Naphthol

PAN : Pyridyl azo resorcinol

pd : Propane-1, 3-diamine

pdH+ : Propane-1, 3-diamine monoprotonated

Pdta : 1,2-Diamino propane tetro acetic acid

Ph : Phenyl

1,10-Phen : 1,10-Phenanthroline

Phy : Phenylhydrazineptd : Pentane-1, 5-diamine

ptdH+ : Pentane-1, 5-diamine monoprotonated

Py : Pyridine

Pz : Pyrazine


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R2dtc : Di-alkyl dithio carbonate

SMs : synthetic Mixture

TCC : Temperature cell compartmentterpy : Terpyridine

Tet/Tetren : Tetra ethylene pentamine

TMDTA : etra methylene diamine tetra acetic acid

,,,-TPPS : Tetra phenyl porphine sulphonate

Trien : Triethylene tetra amine

20-TSPP 20-Tetrakis (4-Suphonato phenyl) porphine

ttha : Triethylene tetramine hexa acetic acid

1,4-Tx : 1,4-Thioxane

Zincon : 2-carboxy-2-hydroxy-5'-Sulphoformazyl benzene


4/54

CHAPTER I

INTRODUCTION AND LITERATURE SURVEY

1.1 Scope and Purpose of Work :

Environmental contamination and exposure to heavy metals such asmercury cadmium and lead and many others is serious growing problem through out the

world. Human exposure to heavy metals has risen dramatically in the last 50 years as

result of an exponential increase in the use of heavy metals in industrial processes andproducts.

In today's industrial society, there is no escaping exposure to toxic

chemicals and metals. In the United states tons of toxic industrial waste are mixed with

liquid agricultural fertilizers and dispersed across America's farmlands. This

controversial practice", which is presently legal in the US, has been reported in ninestates. While the spreading of arsenic, lead, cadmium, nickel, mercury and uranium on

soil that is utilised to produce food for human consumption is a political and economicissue. The potential for adverse health effects is well documented. In general, heavymetals (HM) are systematic toxins with specific neurotoxic, nephrotoxic, fetotoxic and

teratogenic effects. Heavy metals can directly influence behaviour by impairing mental

and neurological function, influencing neurotransmitter production and utilization andaltering numerous metabolic body processes. System in which toxic metal elements can

induce impairment and dysfunction include the blood and cardiovascular, eliminative

pathways (colon, liver, kidneys, skin), endocrine (hormonal), energy production

pathways, enzymatic, gastrointestinal, immune , nervous, (central and peripheral) ,

reproductive and urinary.

Many occupation involve daily heavy metal exposure, over 50 professionsentail exposure to mercury alone. The greatest source of mercury in the biosphere is

currently of human origin Mercury is considered to be global pollutant capable of

spreading far beyond its source area. Methyl mercury is extremely toxic form ofmercury that biomagnifies in aquatic food chains. It is a potent neurotoxin and the

easiest form for animals to store in their tissues. It binds to proteins and easily crosses


5/54

cell membranes, including the blood-brain barrier and the placenta. Solutions to thecomplex problem of mercury pollution have been impeded by conflicting informaiton

on the sources, transport and accumulation of mercury in the environment.

Due to the above post problems there is a need for the development of

methods for detection, estimation and removal of pollutants, which has recently becomean active field of analytical chemistry. This search has resulted in the emergence of an

entirely new area of research called the "Kinetic methods of Analysis". Many

possibilities of analytical interest are provided by the study of ligand substitution

reactions. But before any indicator reaction can be chosen for an analytical application adetailed kinetic picture is quite often a necessary pre-requisite for the same.

Other important considerations such as scope, sampling and standardrequirements, cost of equipment and time of analysis, are also of great practical

significance. A number of methods such as AAS, ETAAS, ICPMS, NAA, FIA Ion

chromatography and anodic stripping analysis can be used for determination of tracemetal ions. The advantages of instrumental methods are low detection limits, high

sensitivity and selectivity, possibility for multi component analysis, non destructive

nature, distance analysis and analysis "invivo". Along with these advantages there arecertain limitations to the above stated methods. Many of these methods require

complicated and expensive instruments and these techniques are usually not available in

most routine laboratories. A recent addition to the above list is the "Kinetic Method

analysis", which ranks high among the analytical procedures Fig. 1.1 and offers some

distinct advantages over the conventional methods such as simplicity, specificity,accuracy and economy. By kinetic method which is sometimes also reaction rate

method it is often possible to measure immediately after mixing the reactants, the rateof change of some parameter 'P' of the particular reactant (s) whose concentration is to

be determined or product of the reaction and not wait for the reaction to go to

compelition or attain equilibrium. This saving in time may or may not be significant,

depending on the specific reaction, but there are good examples [2-7] of obtainingquantitative rate results in seconds for some selective reactions that would have required

many minutes or hours to go to completion. Another important aspect of these kinetic

methods is that they can determine the concentration of two or three chemically closely

related constituents in a mixture, without separating them physically by usingdifferential rate methods.


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Fig. 1.1 Limits of applicability of the most important trace analysis methods + in

flame, * without flame

In the past few years several excellent monographs on the kinetic methods

of analysis have appeared [8-16] and a significant drive has been made towards thedevelopment of analytical methods for compounds, both inorganic and organic and

compounds of biological interests, in a variety of complex samples.

Before any reaction can be used for its analytical purpose, it becomes

absolutely necessary, to study in detail the kinetics and mechanism of that reactions.Once this is done it is a relatively easy matter to choose experimental conditions like

concentration, pH, temperature and ionic strength etc. that would provide maximum

sensitivity, selectivity and precision for estimation of the desired chemical species.

Catalytic determinations are the most widely used of the kinetic methods.

The field of catalytic methods includes the methods of determination of traceconcentration of metal ions, anions and many organic substances. Low detectable

quantities and high sensitivities are recognized as major advantages of catalytic


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determinations. Selectivity, on the other hand, can be considered to limit the practicalapplication of these determinations.

In recent years many catalytic methods have been studied. The evolution

and the innovations introduced, over the last two decades have been reviewed

comprehensively (17-40). The importance of kinetic studies goes beyond their directapplication in determinations, since most physical or chemical processes used in

contemporary analytical chemistry have their kinetic aspects. Mottola (3) has for

example, discussed systematically the aspects of kinetics that have become part of

modern analytical chemistry. Every process, whatever its nature, takes place at a finiterate, tending to an equilibrium, state. The two states, the kinetic (dynamic) state, and the

equilibrium (static) state are both of high informing power (4). Reaction -rate methods

are becoming increasingly important practically in analytical chemistry; progress

however, relies heavily on better elucidation of the mechanisms of chemical reactions.Recent developments in instrumental design and, especially, in the incorporation of

microcomputers for the control of experiments and data evaluation allow for improved

precision, limits of detection, rapidity and automation of such methods.

On the other hand majority of easy chemical methods suffer in sufficientsensitivity. That is why sometimes it is considered that possibilities of chemical

reactions in trace analysis are exhausted. Exceptions to this are enzymatic catalytic

methods. The high turnover numbers of enzymes allow one particle of catalyst to take

part in a great number of elementary reactions. Moreover, the high selectivity of

enzyme action ensures good selectivity of reaction.

In most cases the reaction rate is monitored photometrically. The enzymatic

catalytic methods combine low limits of detection, high selectivity, simple and available

technique. That is why these methods compete successfully with the instrumental

methods. Moreover, they are irreplaceable for determination of enzyme activity inanalytical practice [41].

Keeping this in view further discussion has been divided into two parts. Inthe first part, a brief survey of the significant developmens in the ligand substitution

reactions is reported while the second half deals with the principles and applications of

ligand substitution reactions for trace determination by kinetic catalytic methods (KCM)of analysis, characterization, classification and methodology.


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REVIEW PART- I

1.2 In coordination chemistry, a ligand is an ion or molecule (functionalgroup) that binds to a central metal atom to form a coordination complex. The bonding

between metal and ligand generally involves formal donation of one or more of the

ligands electron pairs. The nature of metal ligand bonding can range from covalent toionic. Furthermore, the metal-ligand bond order can range from one to three, Ligands

are viewed as Lewis bases, although rare cases are known involving Lewis acidic

ligand.

Metals and metalloids are bound to ligands in virtually all circumstances,

although gaseous naked metal ions can be generated in high vacuum. Ligands in a

compelx dictate the reactivity of the central atom, including ligand substitution rates,the reactivity of the ligands themselves, and redox reactions. Ligand selection is a

critical consideration in many practical areas, including bioinorganic and medicinalchemistry, homogeneous catalysis and environmental chemistry.

Ligands are classified in may ways : their charge, their size (bulk), theidentity of the coordinating atom(s), and the number of electrons donated to the metal

(identicity or hapticity). The size of a ligand is indicated by its cone angle.

Co-ordination compounds are classified in terms of the central metal Mn+

,about which a variety of ligands L, L', L" and so on, may be placed in an unlimited

number of combinations. The overall charge on the resulting complex [MLx LyLz] isdetermined by the charge on M and sum of the charges on the Ligands.

The ability of a complex to engage in reactions that result in replacing one

or more ligands in its coordination sphere in called its lability. Those complexes for

which such substitution reaction are rapid, are called labile whereas those for whichsuch substitution reactions proceed slowly (or not at all) are called inert. It is to be noted

that these terms should not be confused with thermodynamic stability and unstability

[42].

For example :

[Cu(OH2) 6]2+

(aq) + NH3 (aq) instantaneous

[Cr(OH2) 6]3+

(aq) + NH3 (aq) several hours


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As for metal hexa-aquo ions in the first transition series, most are labile.

The only two tha are not are Cr(III) and Co(III). And that is why we often start off witha solution of Co(II) when we want to make complexes of Co(III) : Co (II) is labile so we

can do rapid substitution reactions on it and then oxidize it pretty easily to Co(III),

whereas if we started off with Co(III), which is inert, our substitution reaction would bemuch slower.

We can measure the rate of these reactions by dissolving a hexa-aquocomplex in water labelled with

18O and then monitoring how long it takes for the

labelled water to exchange with the unlabelled ones in the inner coordination sphere.

You ought to be aware that for some hexa-aquo ions in the 2nd

, and particularly the 3rd

transition series, substitution reactions can take a long time thousands of years insome cases !

While were on the subject of hexa-aquo ions, we ought to be aware that

they are to some extent acidic. Attaching a water molecule to a metal centre takes some

of the electron density from the oxygen and consequently makes the hydrogens easier topull off as H

+. Hence, if you take something like hydrated iron (III) chloride, dissolve it

in water, and measure the pH, you will find that it is somewhat acidic.

A practical definition of the terms labile and inert can be given here. Inertcomplexes are those whose substitution reaction have half life longer than a minute.

Such reactions are slow enough to be studied by the classical techniques where thereagents are mixed and changes in absorbances, pH, gas evolution is observed. Labile

complexes are those that have half lives for a reaction under a minute. Special

techniques are required for collecting data during such reactions, as they may appear to

be finished within the time of mixing.

1.3 Kinetics and Mechanism of Ligand substitution reaction.

Two extreme mechanistic possibilities may be considered for any ligand

substitution process or for any single step in a series of substitution reaction [43]. First,there is the dissociative (D) mechanism in which the ligand to be replaced dissociates

from the metal centre and the vacancy in the co-ordination sphere is taken by the new

ligand. (Eqn. 1.1)


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Thus, we see that the most important feature of such a mechanism is that thefirst step (dissociation of the leaving group) is rate determining step, once formed by the

cleavage of the bond to the leaving group X, five coordinate intermediate will react withthe new ligand Y, almost immediately. The kind of mechanism is synonymous to SN

1

mechanism in organic systems, since the formation of the intermediate with reduced co-

ordination number is unimolecular as well as rate determining.

The other extreme possibility for ligand substitution is the addition

elimination mechanism, or the associative (A) mechanism. In this case the new ligand

Y, directly attacks the original complex to form a seven membered intermediate in therate determining step Eqn. 1.2. After this step, the leaving group X is lost in a fast step.

This mechanism can be compared to the SN2

mechanism of organic system, since the

rate determining step is bimolecular.

(1.1)


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In many substitution reactions, well defined five or seven membered

intermediates are not observed, instead the transition state found, has some degree of

bond breaking and some degree of bond making, abbreviated as I mechanism i.e.

Interchange mechanism [44]. They may be of Ia [45] or Id [46] type as explained below.

In the case of the Ia mechanism we could say that the bond from the incomingligand starts to form before the leaving one starts to break. In the Id mechanism, we

could say that an existing metal-ligand bond starts to lengthen or weaken before the

(1.2)


12/54

incoming ligand arrives. A pure interchange would have the leaving ligand-metalbond weaken at the same time that the incoming ligand-metal bond forms.

There are three most important type of cases which show some sort ofcomplication present in reaction mechanism.

(i) Solvent Intervention(ii) Ion Pair Formation(iii) Conjugate base Formation

(i) Solvent InterventionMany reaction of complexes have been studied in solvents that are

themselves ligands. Water, for example is a very good ligand of this category and is

present in aqueous solution in high and effectively constant concentration. The

substitution of X by Y might take place as under Eqn. 1.3 and 1.4.

MXn + H2O MXn-1 H2O + X (1.3)

MX n-1 H2O + Y MXn-1 Y+ H2O (1.4)

Intervention of solvent like water obscures the molecularity of the ratedetermining step, the reaction will necessarily be observed to be of first order because

of high and constant concentration of the entering ligand, H2O.

(ii) Ion Pair FormationIt occurs when the reacting complex and the entering ligand are both ions,

especially when both have high charges Eqn. 1.5.

[M X5]n+

+ Ym-

{(M X5)Y}n-m

(1.5)

Polycationic complexes tend to form ion pairs with anions and these ionpairs often undergo reactions via the Ia pathway. The electrostatically held nucleophile

can exchange positions with a ligand in the first coordination sphere, resulting in net

substitution. An illustrative process comes from the anation (reaction with an anion)

of chromium (III) hexaaquo complex :

[Cr(H2O)6]3+

+ SCN-

{[Cr(H2O)6], NCS}2+

(1.6)

{[Cr(H2O)6], NCS}2+

[Cr(H2O)5NCS]2+

+ H2O (1.7)


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When ion pairs are featured as intermediates in the reaction path that leadsto ligand substitution, then the observed rate laws will be of second order, whether or

not mechanism at the rate determining step involves, Associative and dissociative

Pathways [42, 43].

(iii) Conjugate base FormationWhen experimental rate laws contain [OH

-] then there is a question whether

OH-

actually attacks the metal in a true associative fashion or whether it appears in the

rate law through operation of mechanism Eqn. 1.8 and 1.9.

[Co (NH3)5Cl]2+

+ OH- [Co(NH3)4 NH2Cl]

++ H2O fast (1.8)

[Co (NH3)Cl]2+

+ Y-

OH-

[Co(NH3)5 Y]2+

+ Cl-

slow (1.9)

In this conjugate base (CB) mechanism, the rate for the hydrolysis of cobalt

(III) ammine halide complexes are deceptive appearing to be associative but proceedingby an alternative pathway. The hydrolysis of [Co(NH3)5 Cl]

2+follows second order

kinetics; the rate increases linearly with concentration of hydroxide as well as the

starting complex. Based on the information, the reactions would appear to proceed vianucleophilic attack of hydroxide at cobalt. Studies show, however, that in the hydroxide

deprotonates one NH3 ligand to give the conjugate base of the starting complex, i.e.

[Co(NH3)4 (NH2)Cl]+

. In this monocation, the chloride spontaneously dissociates. This

mechanism is compable to the SN

1

CB mechanism of organic system.

Water Exchange in Aqua Ions

Of the many reactions in which complexes are formed occur in aqueous

solution, one of the most fundamental reactions in which the water ligands in the aqua

ion [M(H2O)n]m+

are displaced from the first co-ordination shell by other ligands comeunder this category. In this, a new ligand is another water molecule, i.e. the water

exchange reaction [42].

It is convenient to classify metal ions in four categories based on the measuredexchange rates :

Class I : Very fast (diffusion controlled); k > 108

s-1

Alkali, larger earth alkaline metals like Cd2+

, Hg2+

, Cr2+

, Cu+

come under this category.


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Class II : Rate between 10410

8s

-1

Divalent first-row transition metals (except V2+

, Cr2+

, Cu2+

) Ti3+

, Mg2+

, trivalent

lanthanides, depict this category.

Class III : Rate between 1104

s-1

Be2+

, V2+

, Al3+

, Ga3+

, show this rate.

Class IV : Kinetically inert; rate between 10-6

-10-2

s-1

Ions like, Cr3+

, Co3+

, Rh3+

, Ru2+

, Ir3+

, Pt2+

, belong to this category.

Many other reviews on the substitution reactions involving specific metal

ions viz. Ni (II) [47], Fe III [48], Cd [49], Pd(II) [50] have been attempted by previousinvestigators. Therefore, only a brief description based on the following broad heading

will be presented.

1.4 Formation Reaction

The formation of metal complexes take place in media where usually

water acts as a solvent. The rate of there reactions vary from very slowly to very fast.

The generally accepted mechanism for complex formation was originally proposed by

Eigen and Tamm [51-53]. For complexes of unidendate ligands it involves the

formation of an outer sphere complex between solvated metal ion and the incomingligand followed by loss of a solvent molecule from this outer sphere compelx to give

the desired species. The mechanism of formation of complexes of multidentate ligandcan be extended to the formation of complexes of multidentate ligands with the minor

modification that the ring closure may constitute the rate determining step. The Eigen

and Tamm mechanism on the formation of labile complexes involving divalent cation

is, by now, fairly well enstablished [55]. The formation of M(III) complexes in generaland Fe(III) in particular have already been described in detail by a review on the subject

[56].

1.5 Dissociation Reaction

The dissociation of metal complexes can be considered as the reverse of thecomplex formation. These reactions are, generally much slower than the formation

reactions. The mechanism proposed for the complex formation also accounts for the

dissociation rates of complexes bearing unidentate, bidentate or ambidentate ligands. Incase of complexes of bidentate or ambidentate ligands the rate constant depends upon


15/54

opening of the chelate ring or sometimes rupture of penultimate metal-ligand bonds.Both these situations have been encountered frequently with outgoing bidentate or

multidentate ligand groups. The presence of an acid generally enhances the dissociation

rate because of protonation of the released ligand stablises the intermediate relative tothe fully coordinated form [57, 58].

1.6 Ligand Exchange Reactions

Ligand substitution reactions of coordination compounds have been

studied as intensively as any class of inorganic reactions. The kinetics of theseprocesses have been investigated extensively for octahedral and to a lesser extent for

square planar complexes. A very wide span of rates is found ranging from the extremely

slow exchange of CN-

with NiCYDTA2-

( no evidence for the formation of [Ni(CN)4]2-

in 60 days) [59] to the almost diffusion controlled exchange of H2O between[Cu(H2O)6]2+

and water (t = 108

sec.) [60]. There are the four types of substitution

reaction met with coordination chemistry.

a. Monodentate ligand displacement reaction (excess ligand)b. Multidentate ligand displacement reaction (excess ligand)c. Metal exchange reactions between ligands (excess metal)d. Double exchange reactions.

The above exchange reactions are often sluggish in nature because thereaction involves the breaking of a series of coordinate bonds in succession. In case of

displacement of the multidentate ligand EDTA, for example six bonds must be brokenduring exchange process.

1.6.1 a. Monodentate Ligand Displacement Reactions

This section is concerned with the kinetic and mechanism of substitution in

complexes where monodentate ligands exchange with monodentate or multidentate

ligands.

1.6.1a1. Unidentate by Unidentate Ligands

The substitution of one unidentate ligand by another is the simplest

situation to consider and has been extensivey used for investigating the mechanism ofsubstitution in some octahedral complexes as well as some square-planar complexes.


16/54

The general reactions involve the replacement of a monodentate ligand present in theinner coordination sphere in the solvent media.

There are two basic mechanisms for monodentate ligand exchange in aqueous

solution. Considering the nickel-hexaamine system the first mechanism would be a

dissociate type :

r.d.s

[Ni (NH3)6]2+

[Ni (NH3)6]2+

+ NH3 (1.10)

+NH3

[Ni (NH3)5]2+

[Ni (NH3)6]2+

(1.11)

The second would be bimolecular mechanism involving water molecules :

r.d.s

[Ni (NH3)6]2+

+ H2O [Ni (NH3)5 H2O]2+

+ NH3 (1.12)

[Ni (NH3)5H2O]2+

+NH3 [Ni (NH3)6]2+

+ H2O (1.13)

It is difficult to distinguish between such mechanism in aqueous solution.

However, the lack of NH3 attack on [Ni (NH6)5]

2+

and the fact that a change of 30% inH2O concentration produced no observable effect support a dissociative mechanism

[61].

The kinetics of substitution reaction of a series of complexes of the type

[Ni Fe(CN5) L](3n)

have been studies by Toma and Melin [62, 63] where L was anaromatic nitrogen heterocycle and the substitution ligand was Nitroso-R-salt ( N-R-

salt). The rate of substitution varied with the nature of L and a saturation kinetic, typical

of rate determining loss of L from the complex, followed by rapid addition of the

incoming ligand was reported. This is the first study [65] about monodentate ligandsubstitution reactions of simple low-spin Fe(II) complexes which generally proceed by

D or SN1

(lim) mechanism according to following scheme (Eqn. 1.14).


17/54

k

Ld

Fe(CN5) L](3-m)-

[Fe(CN5)]3

+L

m

kLkL1 + L

n1 (1.14)

Fe(CN5) L1](3n)

Where m and n are charges on L and L1 respectively.

A similar study [66] of the reaction of [Fe (CN)5SO3]5

with CN-

has also

been reported. Other examples include aquation of [Fe (CN)5NO]2

[67] and its reaction

with ammonia hydroxylamine or hydrazine [63]; the reaction of [Fe (CN)5(3,5 Me2-Py)]3

with CN-Pyrazine ( Pz) and imidazole ( Imid H) [69], the reaction of [Fe (CN)5

PhNO]3

[70], of [Fe (CN)5SO3]5

[71], of [Fe (CN)5Py]3

with cyanide ion. The

photochemical reaction of [Fe(CN)5NO]2-

with thiourea or diethylethiourea in methanolhave also been study and shown to follow second order rate law [74]. Several reports

have appeared on the complex formation reactions involving [Fe(CN)5H2O]3-

. The

tendency of this ion to dimerise at higher concentration and the nature of the dimericspecies [75, 76] involved have also been discussed. The rate data of some work on the

formation and dissociation reactions of [Fe(CN)5L](3-m)-

complexes have been compiled

from literature and are given in Table 1.1. Kinetic and thermodynamic data from

mechanistic studies are consistent with a dissociative 'D' mechanism involving the five

coodinated intermedia [Fe(CN)5]

3-

is likely for most of these reactions although someresearchers still favour on Id mechanism [77, 78].

Recently Abu Gharib et al. [64] have reported the kinetic data for the

reaction of [Fe (CN)5(4-CN-Py)]3

with a variety of incoming ligands over a range of

concentration in order to provide a good illustration of the saturation kinetics. Togetherwith respective double reciprocal plots. These authors claim that hese results are

compatible with a limiting dissociative mechanism.

Malin and co-workers [79] have observed that the intermediate [Fe

[Fe(CN)5]3

is quite insentive to the nature and charge of the attacking reagent. In order

to examine the influence of charges on the attacking reagents. Bradic et al. [80] studiesthe kinetics of replacement with L1

n. A being PhNO, 3-CN-Py, DMNA, SCN

-, CN

-and

SO32-

, and the leaving ligand Lm being DMNA, PhNO, SO32-

and H2O respectively.

Finally, they concluded that the magnitudes of second order rate constant kL1 are similarand vary with Ln a in the range of approximate 200-300 M

-1s

-1, 42 to 60 M

-1s

-1and 33


18/54

M-1

s-1

for uncharged molecule, uninegative ion and SO32-

respectively. The effect ofionic strength fits fairly well with the Bronsted -Bjerrum equation [80]. Limiting

reaction rates, at sufficiently large concentrations of entering ligand L1n

have been

observed with all leaving ligands, except water where the replacement obey a secondorder rate law [63, 79, 80] as given in Eqn. 1.15.

(1.15)

Table 1.1

Kinetic and activation parameters at 298o

K for the formation and dissociation of

various pentacyano (ligand) ferrate (II) compelexes

Ligand KfM

-s

-

1

Kd

x

104,s

-1

H

(kJ mol-1

)

S

(JK-1

mol-1

)

Ref.

1 2 3 4 5 6

Pyridine 365 11.0 103.7 (67.3)* 46.0 (29.3)* 62, 79

4-Methylpyridine - 11.5 100.3 37.6 62

Isonicotinamide 296 7.3 108.7 (66.0) 58.5 (25.1) 62, 79

4-Picoline 354 - - (63.1) - (16.7) 79

Pyrazine 380 4.2 110.3 (64.4) 58.5 (20.9) 62

n-Metylpyrazinium 550 2.8 114.9 (70.2) 74.6 (41.8) 62

Dimethylsulphoxide 240 0.75 110.8 (64.4) 46.0 (16.7) 79

4,4bipyridine - 6.2 110.8 66.9 62

Thiourea 286 390.0 69.4 (65.6) -37.6 (20.9) 76Allylthiourea 196 451.0 68.1 (69.8) -41.8 (33.4) 76

Dimethylthiourea 238 813.0 75.2 (64.8) -12.5 (16.7) 76

Glycinate 28.0 26.7 97.0 (61.5) 29.2 (-12.5) 77

Imidazole 240 13.3 101.6 (63.5) 41.8 (12.5) 77

N'-Histidine 320 5.3 105.3 (64.4) 46.0 (20.9) 77

N -Histidine 320 1090 91.1 (644) 41.8 (20.9) 77

Cyanide 38 - - - 80

Thiocyanate 64 - - - 80

3-Cyanopyridine 370 - - - 80

Nitrile 42 - - - 80

DMN - 12.0 - - 80Aniline - Ca0.20 - - 78

Cyclohexylamine - 13.56 96.5 46.0 78

Ethanolamine - 7.72 97.8 38.0 78

Morpholine - 7.16 103.2 53.9 78

Continued on page no.


19/54

Table 1.1 (continued)

1 2 3 4 5 6NH3 190 12.0 102.4 63.1 78

MeNH2 130 4.46 103.2 53.9 78

MeNH 80 7.79 100.3 49.7 78

Me3N 60 12.2 91.4 28.8 78

BunNH2 250 7.47 105.7 66.9 78

EtNH2 180 7.54 104.1 38.0 78

PinNH2 200 7.38 107.0 71.0 78

Bipyridine - 8.43 94.9 29.0 78

En 330 51.5 97.0 37.7 81

enH 620 104.0 99.90 50.1 81

Sulphite - 0.57 Ca 119.5 Ca 75.2 81Nitrosobenzene - 0.016 Ca 117.0 Ca 41.8 81

N-Methylimidazole 418 32.4 81.5 - 82

Isonicotinohydrazide 325 7.3 107.8 59.8 83

Pd - 54.0 100.5 58.7 84

pdH+

- 83.0 100.5 53.7 84

bd - 46.0 102.4 58.7 84

bdH+

- 69.0 104.5 62.7 84

Ptd - 45.0 103.7 58.7 84

ptdH+

- 64.0 101.6 53.7 84

hxd - 41.0 100.5 45.8 84

hxdH - 53.0 96.5 37.8 841,4-Tx - 5.71 112 71 85

1,4-DT - 5.58 105 44 85

1,3-DT - 3.39 108 50 85

* Numbers in parenthesis give the values of H

and S

for the formation reactions.In recent years one of the diagnostic tests that is being widely applied for

elucidating the substitution mechanisms, is the volume of activation (V#) [86-88].

Thus, the most convincing evidence in favour of D mechanism comes from the

determination of V#. If the leaving group of D mechanism comes from the

determination of V#. If the leaving group L

mis to dissociate completely in the

transition state then a positive value ofV#

is interpreted as representating a stretchingof a metal-ligand bond in the transition state and this favours a D mechanism.

Activation volumes [69] for ligand substitution reaction on several pentacyano (ligand)ferrate (II) complexes by few incoming ligands are isted in Table 1.2. The values listed


20/54

in Table 1.2 are all the positive values.This suggests that either a dissociativemechanism or a mechanism involving charge dispersal in the transition state is obeyed

[97].

In the D mechanism of the scheme given in Eqn. 1.5 there is a competition

between incoming ligand Ln

1 and the outgoing Lm

for the transient intermediate[Fe(CN)5]

3-produced from [Fe(CN)5 L]

(3- m)-which is characterized by the ratio of the

second order rate constant kL1 / KL. This gives an idea of the reactivities of a variety of

ligands for the intermediate [Fe(CN)5]3-

. Reactivity ratio for various nucleophiles in

aqueous solution are listed in Table 1.3. Solvent effects on relative reactivities of[Fe(CN)5]

3-in various binary aqueous mixtures have also been discussed [98].

Table 1.2

Activation volumes ( V#) for some ligand substitution reactions on pentacyano

(ligand) ferrate (II) complexes by various incoming ligands in aqueous and non-

aqueous medium.

Reaction Solvent V(cm mol

-) Ref.

1 2 3 4

Fe(CN)5L3-

+CN-Fe(CN)64- +L

L = 4-(1-butylpentyl) pyridine

L=4-phenylpyridineL=N-)n-pentyl)pyrazinium (Na2salt)

L= Pyrazine

H2O-

MeOH

H2OH2O

H2O

+16

+10+10

+13

89

Fe(CN)5L3-

+CN-Fe(CN)64- +L

L=4-CNpyL=4,4' - bpy

L=4-t

Bupy

H2OH2O

H2O

+19.00.5

+13.50.7

+11.41.0

90

Fe(CN)5L3-

+CN-Fe(CN)64- +L

L = p- (CH3CH2CH2)CHC5H4N

L=p-(C5H4N)2

L=p-(C6H5)(C5H4N)L=p-CH3(CH2)5NC4H4N

+

L=p-C6H4N2C2H2L=p-(CH3)3CC5H4N

L=p-NCC5H4N

L=p-CH3CC4H4N2+

20%

MeOH

H2O

H2OH2O

H2OH2O

H2O

H2O

+16.31.4

+13.60.5

+10.40.5

+9.60.8

+17.90.4

+11.41.0

+19.01.0

+0.90.5

91


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L=p-C4H4N2

L=p-CH3C4H4N2+

H2O

H2O+12.51.2

+20.90.5

Fe(CN)5(4-CNpy)3-

+CN-Fe(CN)64- +4CNpy H2O +19.01.0 92

Fe(CN)5(NH2R)3-+pyFe(CN)5(py)3- + NH2RR=H

R=CH3R=C2H5R=PhCH2R=

1Pr

H2O

H2OH2O

H2O

H2O

+16.40.6

+24.01.0

+16.31.5

+17.41.4

+18.50.6

93

Fe(CN)5H2O3-

+Ln-

Fe(CN)5L(3+n)- +H2OL

n-= imidazole

Ln-

= histidine

Ln-

= methionineL

n-= glutathione

Ln-

= glycine

Ln-

= -alanine

H2O

H2O

H2OH

2O

H2O

H2O

+15.50.7

+17.00.4

+17.90.6

+14.10.4+16.40.6

+16.80.2

94

Fe(CN)5H2O2-

+L Fe(CN)5L2- +H2OL = cytosine

L = cytidineL= CMP

H2O

H2OH2O

+2.50.5

+9.51.2

+12.81.1

95

Fe(CN)5(NO2)24-

+H2OFe(CN)5(H2O)3-+NO2- H2O +20.11.0 96

Table 1.3

Reactivity ratios KL1/KL for the replacement of a lignd Lm

by an incoming ligand

Ln

1 in the omplexes of the type [Fe(CN)5-L](3-m)-

in aqueous solution at 250C

Outgoing ligand Lm

Incoming ligand Ln

1 Ratio of rates

KL1/KL

Ref.

4-CN-Py CN-

0.03 98

3-CN-Py CN-

0.05 98

p-Me2NC6H4NO CN-

0.10 80

p-Me2NC6H4NO PhNO 0.80 80SO3

-CN

-9.0 66

H2O CN-

12000 70


22/54

1.6.1 a2 . Multideatable By Unidentate

The exchange of multidendate ligands by unidendate ligands is another example of

unidendate ligands exchange reactions.Recently Nigam and coworkers have investigated the kinetics and mechanims for

replacement of aminocarboxylates from mono (aminocarboxylato) hydroxoferrate (III)

complexes by cyanide ions [99-101]. The general mechanistic scheme for [NiL](2-n)

-CN- replacement reactions requires that three cyanides are bonded to Nickel (II) ion

while four cyanides are required in [Fe L(OH)](2-n)

-CN-systems (n= charge on ligand L)

in their respective rate determining step. The last cyanide adds rapidly forming

[Ni(CN)4]2-

or [Fe(CN)5OH]3-

as the case may be.

Some bis [99-101] and binuclear [104-106] nickel (II) complexes of multidentate

ligands react with cyanide ion by a cyanide independent dissociative path (Eqn. 1.16,1.19) and cyanide dependent associative path (1.17, 1.18, 1.20, 1.21)

Bis NiL2kd NiL + L (1.16)

NiL2 + CN- NiL (CN) + L (1.17)

NiL(CN)+3CN- NiL (CN)4

2-+ L ( in steps) (1.18)

Binuclear Ni2Lkd NiL + Ni

2+(aq) (1.19)

Ni2L + CN- NiL (CN) + Ni

2+(aq) (1.20)

NiL(CN)+3CN- NiL (CN)4

2-+ L ( in steps) (1.21)

The [FeL(OH)](2-n)

-CN- reaction presents some complications and takes place in three

distinct stages [99-101]. The mechanism for the first stage i.e. the formation of

[Fe(CN)5(OH)]3-

from [FeL(OH)](2-n)

complexes as already been reported for manyaminocarboxylates as hown in eqn. 122-1.26

K1

[FeL(OH)](2-n)

+ CN-

[FeL (OH)(CN)](1-n)

fast (1.22)

K2

[FeL(OH)(CN)](1-n)

+ CN-

[FeL (OH)(CN)2]n-

fast (1.23)K

3

[FeL(OH)(CN)2]n- + CN- [FeL (OH)(CN)3]

(n+1)- fast (1.24)

k4[FeL(OH)(CN)3]

(n+1)-+CN

-[FeL (OH)(CN)4]

(n+2)-r.d.s. (1.25)

k-4

k5

[FeL(OH)(CN)4](n+2)-

+CN-

[Fe(CN)5(OH)]3

+ Ln-

fast (1.26)


23/54

Where L = HPDTA

4-, HIDA

2-, EDTA

4-, HEDTA

3-, DTPA

5-, PDTA

4-, and

TTH A6-

.The second stage of reaction is the conversion of [Fe(CN)5 OH]3-

to [Fe(CN)6

]

3-

due to reaction with cyanide ions, present in excess according to Eqn. 1.27

[Fe(CN)5 OH]3-

+CN

- [Fe(CN)6 ]

3-+ OH

-(1.27)

The third stage involves the oxidation of aminopolycarboxylates released in

the first stage Eqn. 1.26 by [Fe(CN)6 ]3-

formed in the second stage as shown in Eqn.

1.28

[Fe(CN)6 ]3-

+

Ln-

[Fe(CN)6 ]3-

+ oxidation product of Ln-

( 1.28)

The cyanide is a potential unidentate ligand having the capacity of

displacing multidentate liands viz. microcyclic ligand, polyamines,polyaminocarboxylates and thioligands from their metal complexes. The reactions

involving [NiL](2-n)

compelexes ( L = polyaminoarboxylates [59, 107-108] and

polyamines [109, 110] microcyclic ligands [111, 112]) with cyanide ions have beenstudied extensively by many workers.

1.6.1 a3. Unidentate by Multidentate

This type of reactions have been studied extensively in connection with the

replacement of coordinated water during the formation reactions of multidentate ligandcomplexes [113]. The reactions of open chain and macrocyclic polyamines with Cu(II)

in strongly basic solutions have been studied [114] to examine the kinetic behavior of

the unprotonated ligands. Some kinetic information is also available on the reactions of

[Ni(CN)4]2-

with aminopolycarboxylate ligands. A mixed [Ni(CN)3 L](n+1)-

complex isformed before the rate determining step in which an additional CN

-is lost. The behavior

with trien is different, however, becaue the rate-determining step is the reaction of the

ligand directly with [Ni(CN)4]2-

by an associative mechanism [115]. Also the

disappearance of [Ni(CN)4]

2-

is greatly accelerated by the polyamine concentration incomparison to the much slower reaction of [Ni(CN)4]2-

with aminocaboxylates [59,

107]. Crouse and Margerum [79] have reported the reaction of [Ni(CN)4]2-

with a few

polyamines in presence and absence of I2 as a scavenger for cyanide ion. In presence ofI2 the released cyanide has no effect on the forward reaction rate but in absence of I2

there is an inverse effect.


24/54

1.6.2 Multidentate ligand displacement reactions

The mechanisms by which one multidentate ligand displaces another from ametal ion depends upon the ability of both ligands to coordinate with the metal ion

simultaneously. There are four accepted mechanisms for this type of reactions.

(i) The outgoing ligand (L-L) is completely dissociated from the metal cation (M)before association between M and the incoming ligand (L-L)* starts.

(1.29)

(1.30)

(ii) The association of the metal cation with the incoming ligand may begin beforedissociation of the outgoing ligand is complete. In this mechanism there will bean intermediate, in which both ligands are bonded to the same cation.

(1.31)

(iii) A variation on the above mechanism in the case of pre equilibrium partialdissociation of he starting complex.

(1.32)


25/54

(iv) In the last scheme, there is a modification of the previous scheme, one whichcan apply when two multidentate e.g., tetradentate ligands are replaced by

another multidentate e.g. quinqui of hexadentate ligand.

(1.33)

The rate determining step of overall reaction is the cleavage of any one ofthe several bonds between metal and the leaving group which must be broken in the

course of the reaction. As example, Ni Tet2+

reacts with EDTA [116], TMDTA [117],

PDTA or DTPA [118] and Nitrien2+

reacts rapidly with EDTA [119], HEDTA[120] or

DTP [121] forming mixed ligand intermediates and give respective products byunwrapping of Tet or Trien.

A review of on multidentate ligand exchange reaction and their application

to analytical chemistry has appeared for system involving EGTA and 4(2-Pyridylazo)

resorcinol [122]. Other multidentate ligand exchange reactions of metal ions such as

Cu(II) [123-125], Zn (II) [126], Hg (II) [127], Cd (II) [128-130] and Ni (II) [131-135]have been investigated by many workers.

The elucidation of substitution mechanism of a coordinated ligand to Fe(III)

centre by another incoming ligand has been the subject of considerable interest for

many workers [136-140], including us [101]. Most of these studies are largely centred

around exchange of a polydentate by a monodentate or monodentate by polydentateligand. On the contrary, there have been limited reports on the kinetic and mechanistic

studies involving the exchange ofa polydentate ligand coordinated to Fe(III) by another

polydentate ligand [141-143].

We have recently investigated the kinetics and mechanism of the exchangeof HIDA coordinated to Fe(III) viz. from [Fe HIDA (OH2)]

-by Par (2Pyraylazo

resorcinol). This reaction seen to proceed through the following mechansitic scheme

Eqn. 1.34-1.36.

k1[FeL(OH)

-+ HR

-[RFeL]2 +H2O fast (1.34)

k-1

k2

[RFeL]2

[FeR]* +L3-

r.d.s. (1.35)k-2


26/54

k3

[FeR]+

[FeR2] + H+

fast (1.36)

k-3

This mechanism is in confirmation with the mechanism proposed earlier by

Nigam et al. [142] and us [143] for FeL-Par (L = HEDTA and NTA). Mentasti et.al.[141] has investigated the kinetics of displacement of metallochromic indicators (In)

from their complexes of Fe(III), represented by the Eqn. 1.37.

Fe In + EDTA Fe EDTA + In (1.37)

In = Salicyclic acid H2S4I or Tiron = 1, 2-di hydroxy benezene disulphonicacid (H2T).

In many multidentate ligand replacement reactions the rate expressioncontains terms in various powers of the hydrogen concentration, which arise from the

possible ways of protonating the ligands both free and in complexed form. It has been

possible to resolve the rate constants due to various protonated reactant by algebraicmanipulation and mathematical analysis of reaction rates measured over a wide pH

range and resorting to some simplifying assumptions about species distribution.

1.6.3 Metal Exchange and Displacement Reactions

The displacement of one metal cation from its complex, with a multidentateligand by another metal ion (or the exchange with an identical labelled metal ion) is

represented by Eqn. 1.38.

M + M'L M' + ML (1.38)

Mechanism of metal displacement can be either dissociative

(1.39)


27/54

or associative in nature.

(1.40)

The mechanism of some metal cation displacement reactions are indicated

in Table 1. 4

Table 1.4

Mechanism of some cation displacement reactions from

Multidentate Ligand Complex

Complex Replacing cation Type of

mechanism

Ref.

Ca (II)-TTHA M(II) (M-Co, Ni,

Cu, Zn)

DA 144

Cd (III)-CYDTA Cu(II) D 145

Cd (III)-EDTA Cu(II) DA 146

Cd (II)-(EDTA)H-

Cu(II) DA 147

Cu (II)-EDTA Cu(II) DA 148

Fe (III)-HEDTA, -DTPA Cd(II) D 149

Fe (III)-HEDTA, -EGTA, -

DTPA

Ga(III) D 150

[Fe(III) (EDTA) H2O]-

In (III) - 151

[MEDTA]- (m=Gaor In) Fe (III) D 152

Bi (III) EDTA Fe (II) D 153


28/54

Reactions following Eqn. 1.39 are represented by D and Eqn. 1.40 by DA.

1.6.4 Double Exchange Reactions

An interesting situation exists when two multidentate complexes are mixedand thermodynamics dictates a double exchange reactions [154]. The exchange of

ligands of the type given in Eqn. 1.41 between two metal ion chelates has been studied

[155-157] and was found to be a slow process if the reaction goes via a dissociation

sequence.

ML + M'L' M'L + ML' (1.41)

No evidence for the formation of a dicomplex intermediate has been given

so far, but the rates of such coordination change reaction were found to increase

enormously if small amounts of either ligand or metal ion is added [158-160] to thereaction mixture. This increase is the consequence of a coordination change mechanism.


29/54

REVIEW PART-II

1.7 Kinetic Catalytic Methods (KCM) of Analysis : Characterization,

Classification and Methodology

The bulk of analytical chemistry is based on chemical reactions at metal ioncentres in liquid media, particularly in aqueous solutions. Their study, understanding

and applications constitute a large portion of todays tasks of analytical chemists. The

use of Kinetics by analytical chemists has increased tremendously during the past few

years. Reaction rate techniques have been used in the development of analyticalmethods for estimation of inorganic and organic compounds present in industrial

enviromental and biological sample. A brief survey of the significant developments

made in this field will be presented here though the discussion will be mainly centred

around the recent advances in the applications of ligand substitution kinetics to thedetermination/analysis of component, in mixtures of catalytic species present in or

added to suitable reaction system.

Mottola [19] has for example, discussed systematically the aspects of

kinetics that have become part of modern analytical chemistry. Every process, whateverits nature, takes place at a finite rate, tending to an equilibrium state. The two states, the

kinetic (dynamic) state, and the equilibrium (static) state are both of high informing

power[20]. Reaction-rate methods are becoming increasingly important practically inanalytical chemistry; progress however relies heavily on better elucidation of the

mechanisms of chemical reactions. Recent developments in instrumental design andespecially in the incorporation of microcompouters for the control of experiments and

data evaluation allow for improved precision, limits of detection, rapidity andautomation of such methods.

1.8 Classification of Kinetic Methods based on the Chemistry of the Reactions

EmployedGenerally, kinetic method are classified into two broad categories :

1. Methods based on catalysed reaction.2. Methods base on uncatalysed reaction.Here, methods only based on catalyzed reactions will be discussed.


30/54

1.8.1a. Methods based on catalysed (Non-enzymatic) Reactions

These continue to be the most popular methods in the literature of kineticmethods of determination. Their popularity is reflected in Fig. 1.1 which illustrates the

growth of the catalytic determination through the years, with information derived from

the reviews in the journal, analytical chemistry. Commonly, catalysed reaction are usedfor the determination of the catalyst(s) and seldom for the reactant(s). The recent

development of catalytic methods of analysis/determination is a result of their high

sensitivity combined with relatively simple procedures. A variety of catalytic effects on

reactions have been employed in analytical determinations. Catalytic determinations canbe broadly viewed as:

A. Use of primary catalytic rates (determination of catalyst) andB. Use of modified catalytic rates (determination of modifiers)

1.8.1 a1 . Kinetic Methods Based on Primary Catalytic Effects

Two facts must be accounted for catalytic determination :

(a) The uncatalysed reaction proceeds simultaneously with the catalysed

reaction and (b) The rate of catalysed reaction is proportional to the concentration of the

catalyst. The latter is a consequence of cyclic regeneration of the catalyst so that its

concentration remains constant. Another practical requirement for successfulapplications is that the concentration of reactants other than the catalyst or the species,

whose changes in concentration is monitored, must be such as to make the reaction ratepseudo-zero order. The species whose change in concentration is being monitored is

adjusted to give first order dependence. Thus, for the generalized reaction :

(1.41)

Where S (monitored species) and R are reactants. P and Y are products and

C is the catalyst, the general rate expression can be written as :

(1.42)


31/54

Here kc and ku are the rate coefficients containing some concentration terms

for catalysed and uncatalysed reactions respectively and is the initial concentrationof the catalyst in the reaction system.

Focussing our attention on the term for the catalysed path in equation (1.27)

and accounting for the presence of the catalytic cycle, one can use a simplified two-stepreaction scheme as represented by Eqn. 1.43 and 1.44

Depending upon the relative magnitudes of rate coefficients for reactions

shown in Eqn. 1.43 and 1.44 two situations may arise :

(a) Pre-equilibrium case and (b) a steady state situation. After resorting to

some valid approximations both these conditions demonstrate the proportionality

between catalyst concentration and the rate of reaction as given by Eqn. 1.45 [19]

(1.45)

Where k'2 is a composite rate constant made up of some rate constants and

concentration terms.

Thus, under stipulated conditions and also recognizing that the uncatalysed

reaction is invariably taking place, Eq. 1.42 can be written in a general form for boththe above conditions as :

(1.46)

(1.43)

(1.44)


32/54

Where F[C]0is a linear function of the catalyst concentration and F is therate due to uncatalysed path. Eq. 1.45 can be used to determine the concentration of

catalyst C using either a differential or an integral approach.

In Differential methods :

Direct evaluation of d (Signal)/dt, is calculated,

- Initial rate measurements, in which the initial reaction rate is determined andutilized for the evaluation of concentration.

- Slope measurements, in which the slope of the response curve at a selected pointis measured and related to the concentration.

In Integral methods (or integration methods):

Evaluation of the corresponding rate expressions over a finite, constant and normally

small time interval t is done,- Fixed time measurements, in which the change of a parameter, related to the

concentration of a reaction or product, is measured over a predetermined time

interval.

- Fixed concentration or variable- time method, in which the period of time,required to bring about the same predetermined change in the concentration of areactant or product is measured.

Other methods include :

- Methods based on the measurements of the length of induction periods.- Special methods such as oscillating chemical reaction.

Differential and integral methods are most frequently used for catalyzedreaction. Detailed descriptions of methods for the determination of catalytically active

substances and a discussion of accuracy and precision are given by Mottola [19] and

Perez-Bendito and Silva [20].


33/54

It is very important to note that the precision of kinetic methods depends onthe reliability of the analytical technical technique used to measure the changes in the

concentration of a given component (indicator substance) as a function of time.

The overwhelming majority of indicator reactions chosen for catalytic

determinations involve redox system. However, sometimes a reaction involving theexchange of ligands in a complex can be catalysed homogeneously by a metal ion

provided that the metal ion has an affinity for the leaving ligand and the experimental

conditions are so selected that the catalyst can be regenerated. As an illustrated

example, the metal ion catalysed replacement of CN- from [Fe(CN)6]4-

has been usedfor the determination of small amount of catalyst metal ions. Pavlovic and Asperger

[161] found a method for determining as low as 2.7 10-7

M of Hg2+

in biological

materials with relative standard error of about 20%. Nigam et al., have determined

down to 1 10

-7

M of Hg

2+

using p- NDA [162] and Mpz+ [163] as the entering ligand.Table 1.5 gives a summary of analytical applications of catalysed ligand substitution

reaction of [Fe(CN)6]4-

and [Fe(CN)5NH3]3-

.

Table 1.5

Reactions involving exchange on hexacyanoferrate (II) And pentacyano (ligand)

ferrate (II) in presence of catalysts

Indicator Reaction Analyte Remarks* Ref.

1 2 3 4

[Fe(CN)6]

-

+ PhNO Hg

+

528nm (2.7 10

-

M)pH = 4.1, 20% 161

[Fe(CN)6]-+ Bipy Ag

+, Au

+520nm(1.6710

-M

Ag+, 1.6510

-5M

Au3+

)

164

[Fe(CN)6]-+ Bipy Hg

+485 nm (1.0 10

-M) 165

[Fe(CN)6]-+ PhNO Hg

+ 525 nm (0.5 g/2ml)

pH=4.1, 7%

166

[Fe(CN)6]-+ PhNO2 Hg

+- 167

[Fe(CN)6]-+ Phen Au

+(10

-M), pH =3.5 168

[Fe(CN)6]-+ L

(in presence of catalyst)

Phen, Bipy (510-

M) , pH =2.4 169

[Fe(CN)6]-

+ Nitroso-R-

Salt

Pd+

, 720 nm (0.04 ppm)

pH = 5.05

170

[Fe(CN)6]-

+ Nitroso-R-

Salt

Ag+

, 720 nm (0.05 ppm)

pH = 4.0, 5%

171

Continued page no.


34/54

Table 1.5 (Continued)

1 2 3 4

[Fe(CN)6]-+ Ferrozine Hg

+, Ag

+, Au

+562 nm (0.02 ppm

Ag+, 0.1 ppm Au3+,0.01 ppm Hg

2+),

pH=4-6, 4.7%, 4.3%,

2.7% respectively

172

[Fe(CN)6]-

+ Nitroso-R-

Salt

Hg+

, Ag+

, Au+

625 nm (0.01 ppm

Hg2+

, 0.005 ppm Ag+,

0.05 ppm Au3+

),

pH=4.5-6.0, 3.75%,2.15%, 5.44%

respectively.

173

[Fe(CN)6]4-

+ NN Hg 630 nm (1 10-

M)

pH=3.5, 6%

174

[Fe(CN)6]-+ p-NDA Hg

+ 640 nm (3.6 10

-8M)

pH=5.0, 5%

162

[Fe(CN)6]-+ Mpz

+Hg

+ 655 nm (3.6 10

-8M)

pH=5.0, 1.5%

163

* Detection limit is given in parenthesis and error in %.

Catalysed substitution reaction involving displacement of one-ligand byanother in a metal complex have been widely used to determine microgram quantities of

catalysts [175]. Tabata and Tanaka have reported kinetic methods for determination of

nanogram amounts of Hg2+

[176] and Cd2+

[177] by their catalytic effects on the metal

ion incorporation of Mn2+

with , , , - TPPS. Cu2+

catalysed metal-metal exchangereaction of Zn

2+with NiEDTA

2-has been exploited by Bydalek and Margerum [178] for

determination of Cu2+

at concentration 10-5

M. Double exchange reactions are very

sensitive to trace catalysis and inhibition. These properties have led to the development

of sensitive analytical methods for concentration down to 10-8

M of metal ion orligands. [179-181]. Table 1.6 gives an over view of catalytic methods developed for

some chemical species using ligand substitution or complex formation reactions.


35/54

Table 1.6

Determinations based on catalytic effect of various ions on ligand substitution or

complex formation reaction

Indicator Reaction Analyte Remarks* Ref.

1 2 3 4

Zn EGTA +Zincon Ca+ Polarographic (10

-M) 182

Mn EGTA +PAN Ca+ Polarographic 182

Zn EDTA +NH3 Ca Polarographic 183

Cd EDTA +NH3 Ca Polarographic 184

Cu EGTA +PAR Ca Spectrophotometric

(10-5

M), pH =9

185

Hg CPC + CYDTA NH3 Spectrophotometric

(510-5

M), pH =9.5, 5%

186

Cu H2O+o-Cresolphthalein

CN- 568 nm (0.25 g/25 ml)pH =11.5

187

Hg PAR +CYDTA I-

500 nm (10-

M) pH = 8.5 175

Cr (H2O)6 +Xylenolorange

CO3- 188

Co (CN)-5+O2

(coupled to chain

coordination)

O2 (10-

M), 8% 180

Co (en)2 salicylate +I2 CH3COO-

HCO3

In accetate buffer 189

Ni EDTA + Zn+

Cu+

380 nm (10-

M),

pH = 5

178

Ni Trien+ Cu EDTA Ni 550 or 590 nm

(10-7

M), pH =8, 5%

179

Ni Tetren + Cu EDTA EDTA Stopped flow mixing (10-

M),

pH =7-12, 5-10%

190

Ni Tetren + Cu TTHA Tetren Polarographic (10-

M) pH

=8.0

191

Ni Trien + Cu EDTA Tetren 550 nm (10-

M) pH =7.4 181

MnII

+ ,, , -TPPS Hg+

413 nm (10-

M) pH=6-7 176

MnII

+ ,, , -TPPS Cd+

413 nm (10-

M) pH=8.0 177

Mn +5, 10, 15, 20-TSPP Pb 413 nm (10

-

M) 192

* Detection limit is given in parenthesis and error in %.


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1.8.2 Mechanisms of analytically important catalytic reactions

Indicator reactions for catalytic (non enzymatic) methods may be classifiedon the basis of reaction mechanisms as follows [193]

a. Redox reactions

1. With valency change of the catalyst

2. Without valency change of the catalyst

b. Reactions with carbonyl compounds

1. Hydrolysis reactions

2. Decarboxylation reactions

c. Exchange reaction with coordination compounds

1. Nucleophilic substitution

2. Electrophilic substitution

3. Coordination chain reactions

Knowledge about the mechanisms of the reactions is far more important forcatalytic methods than for methods based on chemical equilibrium. In the latter

pocedures, in most cases, proportionality between the concentration of the substance to

be determined and on parameter of the reaction at equilibrium is sufficient for analytical

purposes, regardless of the stage of reaction at which the equilibrium is attained.

The more knowledge is obtained on the mechanisms of a catalytic reaction,

the more easily one will find the most favourable conditions for analytical applicationsof the reaction with regard to senstivity, accuracy and selectivity.

1.8.2a. Redox reactions

The most common indicator reactions are redox in nature and involve oxidantssuch as H2O2, O2, BrO3

-, ClO3

-, IO3

-, S2O8

2-, Fe

III, Ce

IV, etc., and inorganic reductants


37/54

such as SnII, Fe

II, As

III, S2O3

2-and organic reductants such as amines, phenols, azo dyes

and leuco bases of dyes, etc.[194]. Redox reagents can be classified into two main

groups, namely, reactants acting by electron transfer through their d-orbitals and

reactants acting by electron transfer through their s- and p- orbitals (Table 1.7).Reactions with oxidants of the second group are as a rule slower than those of the first

group and thus are convenient as indicator reactions for determining catalytically active

substances.

Table 1.7

Typical Oxidising and Reducing Agents Used for the Determination of Catalysts

Oxidising Agents Reducing Agents

d-orbitals Ce

4+

Cu

2+

Ag

2+

Mn

3+

Cr

2+

Fe

2+

Ti

3+

Fe3+

VO2+

MnO4-

V3+

VO2+

CrO22+

MoO22+

OsO4

s, p - orbitals Sn4+

Sb5+

Pb4+

Bi5+

I-

Sn2+

Br-

Cl-

Tl3+

N3-

Polyatomic species H2O2 NO3-

ClO3-

ClO4-

ArOH, ArNH2, azo dyes,

BrO3-

S2O82-

IO4-

leuco bases of dyesAsO

2-, S2O3

2-, C2O4

2-

ascorbic acid

The reactions can be divided into two groups, depending on whether the ion

acting as the catalyst changes its oxidation state during the reaction or not.

Reactions in which the oxidation state of the ion-catalyst changes are

marked by high sensitivity. Catalyst concentrations from 10-9

to 10-7

g cm-3

may be

commonly determined and in some cases (Co, Os, Ru) down to 10-11

g cm-3

. The

mechanism of reactions of this type involves the steps

Red + C(n+1)+

P +Cn+

(1.47)

Cn+

+ Ox C(n+1)+

+ Z (1.48)

where and C(n+1)+

and Cn+

are the two diffferent valency forms of the

catalyst in the two oxidation states, Red and Ox are the reactants in the redox reaction

and P and Z are the reaction products. In these reactions, ions of the transition elementsand halides usually function as catalysts.


38/54

The other large group of catalytic redox reactions comprises the reactions inwhich the oxidation state of the ion-catalyst remains unchanged. These include catalytic

oxidations by hydrogen peroxide in acidic medium. Catalytic activity is exhibited by

cations with high positive charges that readily form peroxocomplexes (e.g. Fe

III

, Ti

IV

,Hf

IV, Th

IV, Nb

V, Ta

V, Cr

VI, Mo

VIand W

VI)

General overviews of the mechanism of catalytic reactions of analyticalinterest have been given by Bontchev [195] and Mottola [20].

1.8.2b. Reactions with carbonyl compounds and catalytic ligand exchange

reactions

These catalytic reactions include catalytic decompositions, hydrolysis and

catalytic substitution reactions of both organic and inorganic substrates [195].

The reactions are analytically important, because they are often catalyzed

by elements from the main groups of the periodic table whose ions have no vacant d-orbitals and thus cannot catalyze redox processes.

The mechanism of catalysis by catalytic decomposition and hydrolysis

inolves the effect of the catalyst on the polarization of the bonds in the reactants or on

the orientation of the reactants, thus making the reaction possible. An important step in

the catalysis is then often chelation : the ions of the catalyst form stable chelates with

the substrates. An example is the hydrolysis of monoalkyl phosphates ROPO32-

. Thesesubstances alone do not hydrolyze in alkaline media. In the presence of Cu2+

or Mg2+

ions, neutral chelates are formed, the negative charge on the substrate is neutralized andhydrolytic decomposition can begin. From the analytical point of view, catalytic

substitution reactions of complexes are also important [193].

Typical reactions of this type are the substitution reactions of

hexacyanoferrate (II) with various substituting ligands is represented in a simplified

form as :

[Fe(CN)6]

4-

+ L [Fe(CN)6L]

3-

+ CN

-

(1.49)

where, L = substituting ligand.

Reaction 1.49 is catalyzed by all metal ions that form stable complexes with

cyanide (e.g. mercury, silver, palladium, gold) and can be used to determine these


39/54

metals (198). The Table 1.5 provides a summary of analytical applications of catalysedligand substitution reactions of [Fe(CN)6]

4-and [Fe(CN)6NH3]

3-.

Other methods based on ligand - exchange reactions are quite recent and

have thus been studied less. They have a promising future as they allow the

determination of non-transition metals such as alkaline-earth metals as well as oflanthanides ammonia and some other species. Three general types of ligand - exchange

reaction can be considered in this context, as follows [18, 20].

1.8.2c. Exchange Reaction with Coordination Compounds

This category consists of exchange of same ligand between two metal ions,

exchange of same metal between two ligands and mutual exchange between metal ionsand ligands.

1.8.2c1. Exchange of a ligand between two metal ions

The system

NiY2-

+ Zn2+

ZnY2-

+ Ni2+

(1.50)

Has been described by Margerum et al [196]. This reaction moves slowly

and is monitored photometrically at 380 nm by measuring the disappearance of the

NiY

2-

complex. The addition of a small amount of Cu

II

to the system results in twofurther reactions :

NiY2-

+ Cu2+

CuY2-

+ Ni2+

(1.51)

CuY2-

+ Zn2+

ZnY2-

+ Cu2+

(1.52)

The CuY2-

complex is rapidly formed, immediately reacts with zinc ions to

form the ZnY2-

complex and the CuII

ions are liberated. CuII

ions are thus not consumed

and can be considered as catalyst. Copper ions can be determined on this basis.

1.8.2c2. Exchange of a metal between two ligands

Consider the reaction between CuII

and the ligands oxybis (ethylenenitrilo)

tetra-acetic acid (EGTA) and 4(-2-pyridylazo) resorcinol (PAR)


40/54

Cu-EGTA + PAR Cu - PAR (1.53)

This reaction is catalyzed by traces of CaII. Mg

IIcauses no interference, as

its EGTA complex is much less stable than that formed with CaII

. Calcium ions can thusbe determined at concentrations between 0.4 and 40 g cm

-3by spectrophotometrically

monitoring (at 515 nm) the rate of appearance of the Cu-PAR complex [197].

1.8.2c3. Dual exchange reactions (coordination chain reactions)

Olsen and Margerum have found, that complex ligand exchange occurs in

the reaction of Cu-EDTA with Ni-tren.

NiT2+

+ CuY2-

CuT2-

+ NiY2-

(1.54)

where T = trien = triethylenttramine

This reaction develops very slowly, though it can be accelerated by simplyadding an excess of one of the ligands. Thus, a small excess of EDTA (Y

4-) attacks the

Ni-trien complex to yield Ni-EDTA and free trien (T), which in turn reacts with the Cu-

EDTA complex, from which it displaces the ligand :

Y4-

+ NiT2+

NiY2-

+ T (1.55)

T + Cu Y2- CuT

2++ Y

4-(1.56)

The chain is propagated cyclically and a steady state is rapidly reached. The

reaction rate is monitored photometrically by means of the absorbance at 550 nm,

which corresponds exclusively to the Cu-trien complex and is proportional to the EDTA(or trien) concentration added. This allows the determination of small concentrations of

ligand. Metal ions can act as an inhibitor for the ligand-catalyzed reaction. In this way,

trace concentrations (10-7

to 10-10

g cm-3

) of metals forming stable complexes with eg.EDTA canbe determined. Other examples for the analytical use of substitution reactions

are described by Kopanica et al., [31 ,32].

1.8.3. Application of oscillating reactions

This type of reaction has limited but increasing analytical interest. [33].Reactions catalyzed by metal ions capable of exchanging a single electron at normal

potential values between 0.9 and 1.6 V (e.g. the CeIV

/CeIII

system) are typical


41/54

representatives of oscillating reactions, as are the CeIV

-catalyzed oxidation of malonicacid (Belousov-Zhabotinski reaction system, B-Z system) succinic acid or citric acid by

bromate.

Mechanistically these processes are very complex. For the bromatemalonic

acid reaction catalyzed by CeIV

-CeIII

, a 10-step process for the mechanism has been

proposed [33-35]. The process (e.g. B.Z. system) is monitored either by photometricmeasurement of the changes in the Ce

IVconcentration or by non-equilibrium

measurements of the redox potential of the CeIV

/CeIII

pair, which shows the periodic

variation of the CeIV

concentration, or by using a bromide-selective electrode.

It is analytically important that the number of cycles per unit of time is

proportional to initial reactant concentrations. However, this reactions is only obeyed

closely for a short time after the initiation of the reactions; when reagent concentrations,decrease, so does the cycle frequency. The Kinetic determination of hexacyanoferrate

(710-8

to710-6

mol dm-3

) by its inhibition of an oscillating chemical reaction (B-Zsystem) was described by Jiang et. al[36].

1.9. Determination Based on Inhibition

The general practice followed for such determination has been to monitor

the decrease in reaction rate in a system containing a constant amount of catalyst by

adding increasing amount of inhibitor to obtain calibration curves, either by integral or

differential rate measurements. This approach offers relatively good limits of detectionbut has a limit concentration range amenable to determinations. Table I.8 lists some

typical redox indicator reaction systems and inhibitors determined by them.

Several other references on the determination of metal ions as well as

organic compounds based on their inhibitory action are cited in the literature [199-205].Most of these kinetic methods involve redox systems as indicator reactions. From the

Table I.8 below it is evident that surprisingly very little attention has been paid to the

determination of chemical species based on their inhibitory effect involving ligand

substitution processes.


42/54

Table I.8

Typical redox systems used for determination of inhibitors by kinetic methods

Indicator Reaction Catalyst Analyte(Inhibitors)

DeterminationRange

Ref.

1 2 3 4 5

Malachite

green+Periodate

Mn+ EDTA 10

-206

Malachite

green+Periodate

Mn+ Glycine, DL-

Phenylalanine, DL-

Glutamic acid, DL-

Sarine and L-Arginine

10-

M 207

Pyrocatechol violet +

H2O2

Co+

Histamine.

Neoantergan andSynopen

0.1-2.910-9

M 208

Pyrocatechol violet +

H2O2

Co+

Nicotinic acid,

Nicotinamide,Vitamine B1 and

B6

1-510-9

M 209

Luminol + H2O2 Cu+

Amino acids in

natural water

- 210

Alizarin S + H2O2 Mn+

8-

Hydroxyquinoline,

Bacteriostatics and

Fungicides of 8-hydroxyquinoline

type

0.3-1.110-7

M 211

Sb (III)+ Peridoate Mn+

EDTA and DTPA 1-1010-7

M 212

Phosphinate+ Peridoate Mn CYDTA - 213

Azorubine + H2O2 Mo+

Antibiotics(Tetracycline,

Methacycline and

Demecycline)

18-160g/mL 214

Azorubine + H2O2 Mo+

Gentisic and

chromotropic acid

6.2-49.3 and

13.2-105.3

g/mLrepectively.

215

[Fe(Phen)3]++Cr(IV) Oxalic

acidUric acid 0.85-

5.95g/mL

216

Pyrocatechol violet +H2O2

Cu+

Thyroxine and 5-Hydroxytryptophan

1-1010-6

and

7.7-3210-7

M

217


43/54

Pyridoxal-2

pyridylhydrazone +

H2O2

Pb+

Phosphate and

Plyphosphates110

-7M 218

4,4'-Dihydroxybenzophenone

thiosemicarbazone+

H2O2

Cu EDTA and EGTA 10-

M 219

2,4-Diaminophenol+

H2O2

Fe Oxalate.citrate and

fluoride

ng level 220

Autodecomposition of

H2O2

Cu+

1,10-

phenanthroline2-20 g/mL 221

p-Phenetidine+Periodate Fe Thiocarbamide,

somedithiocarbamtes,

pesticides andMercaptans

110-2g/mL 222

Perborate + I-

Zr+

Fluoride 1-1010-9

M 223

Perbromate + I-

Fe+

EGTA, DTPA and

EDTA1.5-1510

-7M 224

O-Dianisidine+ H2O2 Peroxidase Hg+

- 225

Hydrolysis of sucrose t-Fructofuranosidase

Hg+in water and

carbonated soft

drink

100-270

g/mL

226

[Co(III)-EDTA]+

Hypophosphite

Pd Hg in sphalerites

and

pharmaceuticals

13-120 ng/mL 227

Sulfanilic

acid+Peroxodisulphate

Ag+

Cysteine 0.5-410-6

M 228

Phloxin + Persulphate Ag+

Cysteine 1-1210-6

M 229

K4[Fe(CN)6]+pNDA Hg+

Cysteine,

thioglycolic acid,

thiosulphate

- 230

K4[Fe(CN)6]+Mpz+ Hg+

Cysteine 2.0-2010-7

M 231

K4[Fe(CN)6]+Mpz+ Hg+

MNDT as low as 510-

8M

232

1.9. Determination Based on Activator

In many metal catalysed reactions a substance called activator some times

combines with the catalyst to promote the catalytic activity of the metal ion. If the initial

rates of such reaction are proportional to the concentration of the activator (until all the


44/54

catalyst combines with activator), rate measurement can be employed to determine theconcentration of activator species the same manner as for direct analysis of catalyst i.e.

metal ion (vide supra). Such metal chelate activation may arise due to the fact that a

metal chelate can offer labile and free aquo positions on the metal ion through whichthe reaction with a substrate may take place. In such cases the chelating ligand acts as a

carrier for the metal ion and catalysis occurs through the formation of a mixed ligand-

chelate compound. Some workers have made use of activators to lower the limits ofdetection for metal ion catalysts (233). Ascorbic acid, for example, has been used to

lower the detection limit of Cu(II) , from 0.05 g/mL to 0.2g/mL using Cu(II)catalysed oxidation of 2-thiosemicarbazone by hydrogen peroxide [233]. The sensitivity

and selectivity of the catalytic determination of vanadium catalyst in the oxidation of o-

phenylmediamine by bromate ion is said to be improved by the presence of Tiron actingas an activator [234]. NTA acts as an activator for Mn catalysed periodate - Sb(III)

reaction and thus lowers the detection limit of Mn (II) [235].

1.10 Selectivity of catalytic methods of analysis

According to IUPAC terminology [236], selectivity denotes the extent towhich the method is free of interference by other chemical species. Total or complete

specifity is the case in which no interferent effects are known. In practice total specifity

is rare for catalytic methods apart from enzyme catalyzed reactions. The catalyticproperties of an inorganic ion depends on its size, structure, charge and coordination

sphere, chemically similar species exhibit similar catalytic effects and therefore highly

selective catalytic determination in the presence of chemically related elements is not

common. Consider the indicator reaction

A + B P (1.57)

which takes place in the presence of catalyst C at a rate defined by the Eqn. 1.57

(1.58)

where A, B are the reactants, k', k are the rate constants for the uncatalyzedand the catalysed reaction, respectively and symbols p', q', p, and q are, in a simple case,the stoichiometric coefficients. In the presence of different catalysts C1,C2,....Cn the

expressions for the rate of the catalysed reaction under different reaction conditions can

be described by Eqn. 1.59, 1.60.


45/54

1-10 = k1111 [C1] + k1212 [C2] +...........+ k1n1n [Cn] (1.59)n-n0 = kn1n1 [C1] + kn2n2 [C2] +...........+ knnnn [Cn] (1.60)

where 1.....n characterise the rates of the catalyzed reaction; i0 the rates

of the uncatalyzed reaction and ik = [A]pik

+[B]qik

, where i, k=1,.....n the coefficients

p,q.,,,, are determined by the actual mechanism.

For the activity of a certain catalyst to be highly selective, the activities of

the individual catalysts must differ considerably. Under the given reaction conditions,

kii kik or, the individual catalysts must react according to different reaction

mechanisms i.e. pii, qii pik, qik.

The selectivity of a catalytic determination can often be improved by

optimizing the procedure by selective masking or by using separation techniques.

The following scheme shows ways to improve the selectivity of catalyticmethods.

Optimisation of the procedure Use of separation techniques

Variation of reaction conditions Ion-exchange

- pH- reagent concentration

- temperature- suitable co-ordination sphere

- catalyst-substrate interaction

- photoactivation

mechanism-optimisation

(e.g. by means of the graph method)

Ion-exchangechromatographic methods

microdiffusinco-precipitation

distillation

electrophoresis

liquid-liquid-extraction- back-extraction or composition of the

organic solvent.

- extraction-catalytic determinations


46/54

Table 1.9

Survey of the elements which can be determined using the catalytic activity of their

ions. The numbers beside the symbols of the elements denote the detection limit

expressed as the negative logarithm of the corresponding concentration in g cm-3

[20, 193].

Ia IIa IIIb

IVb

Vb

VIb VIIb

VIII

ib IIb

IIIa

IVa

Va

VIa

VIIa

H

L

i

Be

9

B C N O F1

0

N

a

M

g6

Al

6

Si

6

P8 S1

0

C1

7

K Ca6

Sc Ti8

V10

Cr9 Mn10

Fe9

Co11

Ni9

Cu11

Zn9

Ga

Ge7

As5

Se9

Br8

R

b

Sr Y

7

Zr

8

N

b7

Mo

10

Tc8 Ru

11

Rh

10

P

d9

Ag

10

C

d6

In

8

Sn Sb

8

Te

10

I

10

C

s

Ba L

a

Hr

7

Ta

7

W1

0

Re

0

Os

11

Ir1

1

Pt

7

Au

9

H

g9

Ti Pb

8

Bi

7

Po At

F

r

Ra A

c

T

h7

Pe U9

Liquid-liquid extraction is of greatest importance, followed by ion-exchange andother chromatographic methods. In extraction separation the simplest procedure

preferable; in other words back-extraction or the thermal decomposition of the organicsolvent should be avoided. It is advantageous to carry out the catalytic determinationdirectly in the extract or in the extract dissovled in a mixed solvent (extraction -

catalytic determinations) (237).

The following conditions must be satisfied to enable an extraction catalytic

determination :(i) the indicator reaction must proceed in the organic or mixed solvent and

the catalyst activity must be retained in this medium,

(ii) an extraction system must be available for a highly selective separationof the test metal and the extraction agent, if extracted itself, must not interfere with the

catalytic reaction.

Extraction-catalytic methods are very attractive analytically, because they alsoenable analyses in very complex matrices. Table 1.10 gives an overview of extraction-catalytic methods that have been developed and used until the present time.

The nomenclature of kinetic methods of analysis (terms and definitions) is proposed in

an IUPAC paper by Svelha [238].


47/54

Table 1.10

Extraction-catalytic dtermination [177]

Metal Ion Indicator reaction Extraction System Reaction MediumAg Bromopyrogallol Red

(BPR) + S2O82-

+ 1, 10

Phenanthroline

1, 10-

phenanthroline,

BPR/nitrobenzene

nitrobenzene-dioxan

water

Ca sulphanilic acid + H2O2 +

pyridinep-ethoxyaniline +H2O2

pyridine,

salicylate/CHCl3neocuproine/CHCl3

-CHCl3-ethanol-

water-CHCl3-ethanol-

water

Cr 3, 3' dimethoxybenzidine+ H2O2

HCl/MIBK MIBK-ethanol-water

Fe p-ethoxyaniline + H2O2+1,10-phenanthroline

p-ethoxyaniline + H2O2+1,10-phenanthroline

p-ethoxyaniline + H2O2+

1,10-phenanthroline

LiCl/MIBKseveral systems

1, 10 -phenanthroline

MIBK-ethanol waterCHCl3-ethanol-water

CHCl3-ethanol-water

Mo 1-naphthylamine + BrO3-

oxine/ChCl3 CHCl3-ethanol-water

Nb o-aminophenol + H2O2 benzoin oxime/

CHCl3

CHCl3-ethanol-water

Ti o-phenylenediamine +

H2O2

pyrocatechol,

dioctyl-amine/

butane

CHCl3-ethanol-water

V o-phenylenediamine +BrO3

-

pyrocatechol, octyl-dimethylamine/

butanol

butanol-ethanol-water


48/54

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19. Mottola, H.A., Kinetic Aspects of Analytical Chemistry ,1988(Vol. 96 of the series,Chemical Analysis), New York, John Wiley & Sons.

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21. Muller,H.,and Crit, CRC. 1982,Rev. Anal. Chem. Vol.13,pp. 313.22. Schwing, J.P. ,WolfT, Ch.M.and Albrecht-Gary, A.M.,April 1985, L' actualite

chimique, Vol.53.23. Gaal,F.and Abramovic ,1984,Talanta, Vol.31,pp.987.24. Guyard, M.A.,1876,Bull. SOC. Chim.Paris Vol.25,pp. 58.25. Bontchev, P.R.,1970, Talanta ,Vol.17,pp. 499.26. Yatsimirskii,K.B.,1966, Kinetic Methods of Analysis, Oxford, P

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