Application of Oxidation-Reduction Titrations

Application of Application of

Oxidation-Reduction TitrationsOxidation-Reduction Titrations

Hassan.F.Askal, Ph.D.

1

Importance of Redox Reactions in Biological systems and

Pharmaceutical Applications Oxidizing agents act on human tissues by one of the following mechanisms:

1. Germicides through liberation of oxygen in the tissues such as hydrogen peroxide

2. Bactericidal against anaerobes as KMnO4 (for wounds) or Cl2 (for dental therapy) through denaturing the proteins by direct oxidation.

Oxidation can have deleterious effect on the cells and tissues of human body because it produces reactive oxygen and nitrogen species, such as OH, NO, NO2

and alkoxyl radicals RO which damage cells and other body components.

Antioxidants (reducing agents) can counteract the effect of the reactive oxygen and nitrogen species and protect the other compounds in the body from oxidation.

2

Importance of Redox Reactions in Biological systems and

Pharmaceutical Applications Typical antioxidants include vitamin A, C and E; minerals such as selenium; herbs such as rosemary.

Antioxidants act by one or more of the following mechanisms:

1.Reaction with the generated free radicales can counteract the effect of the reactive oxygen and nitrogen species

2.Binding the metal ions needed for catalyzing the formation of reactive radicales

3.Repairing the oxidative damage to the biomolecules or induction of the enzymes that catalyze the repair mechanisms.

3

Redox reactions are included not only in inorganic or organic drug analysis but also in the metabolic pathways of

drugs. Catabolic pathways: oxidation of C-atoms of carbohydrate

or lipid or proteinAnabolic pathways: reduction of C-atoms of carbohydrate

or lipid or proteinPhase-I Metabolic Pathways (Functionalization reactions)I- Oxidative reactionsA.Oxidation of aromatic compounds → hydroxy aromatic compoundsB.Oxidation of olefins → EpioxidesC.Oxidative dealkylation

Redox Reactions and Biological Action4

Examples for the oxidative dealkylation pathways:

1. N-dealkylation, R-NH-CH3 → R-NH2 + CH2O

2. Deamination, R-CH2-CH2-NH2 → NH3 + R-CO-CH3

3. N-oxide formation, R3 - N → R3 N→O

4. N-hydroxylation. R2NH → R2N-OH

5. O-dealkylation. R-O-CH3 → R-OH + CH2O

6. S-dealkylation, R-S-CH3 → R-SH + CH2O

7. S-oxidation, R2S → R2S=O + CH2O

8. Desulphuration R2C=S → R2C=O

Redox Reactions and Biological Action5

RH(Parent drug)

P-450-(Fe3+)

RH─P-450-(Fe3+) RH─P-450-(Fe2+)

NADPH + flavoprotein reductase

H2O

ROH (oxidized product)

RH─P-450-(Fe2+)

O2-

Redox Reactions in Biological System 6

Example of redox reaction in biological systems ethyl alcohol can be oxidized in liver into acetaldehyde

II- Reductive reactions1. Reduction of aldehydes and ketones

2. Reduction of nitro and azo compounds3. Miscellaneous reductive reactions

N

H

R

C NH2

O

NAD+

+ + CH3CH2OH

Ethyl alcohol

Alcoholdehydrogenase

N

R

CONH2

HH

+ CH3CHO + H+

NADH)(

( )Nicotinamide adenine dinucleotide

7 Redox Reactions in Biological System

Properties of Oxidizing Properties of Oxidizing AgentsAgents

1. Potassium permanganate (KMnO1. Potassium permanganate (KMnO44))

2. 2. Potassium dichromate Potassium dichromate (K(K22CrCr22OO77)) 3. Ceric sulphate3. Ceric sulphate Ce(SO Ce(SO44))22

4. Iodine (I4. Iodine (I22))5. Potassium iodate (KIO5. Potassium iodate (KIO33))

7. 7. Potassium bromatePotassium bromate (KBrO(KBrO33)) 8. Bromate-bromide mixture 8. Bromate-bromide mixture (KBrO(KBrO33/KBr)/KBr)

8

6. Sodium thiosulphate (Na6. Sodium thiosulphate (Na22SS22OO33))

1. Potassium permanganate (KMnO1. Potassium permanganate (KMnO44))

Powerful oxidant E° = 1.52 V.In acid medium:

can oxidize: oxalate, Fe2+, Ferrocyanide, As3+, H2O2, and NO2.

In alkaline medium: H2SO4 is the suitable acid since HCl reacts with it:

2KMnO4 + 16HCl → 2 KCl + 2MnCl2 + 5Cl2 + 8 H2O

Zimmermann’s reagent prevents reaction of KMnO4 with Cl. Not primary standard, may contain MnO2 which catalyses

auto-oxidation that is accelerated by light and acids. 4MnO4

+ 2H2O → 4 MnO2 + 3O2 + 4OH

It is prepared after removal of MnO2 through glass wall (filter paper contains organic matter can decompose permanganate).

Self indicator. Standardized by: 1- Oxalate. 2- Fe2+. 3- Arsenious oxide.

MnO4 + 8H+ + 5e → Mn2+ + 4H2O

MnO4 + e → MnO4

2

9

2. 2. Potassium dichromate Potassium dichromate (K(K22CrCr22OO77)) Less strong than KMnO4, E°= +1.33 V.

Primary standard (highly pure and stable).

Not easily reduced by organic impurities in water.

Not reduced by HCl in dil. solution.

Not self indicator; since its orange colour is reduced to the green (Cr3+).

Used for determination of Fe2+ (Cl does not interfere); ferroin indicator.

10

More powerful oxidant than KMnO4 and KK22CrCr22OO77.

Reacts in acid medium, it has 2 E° according the type of acidIn HNO3 medium E°=1.61V. and in H2SO4 E° = 1.45V

In neutral or alkaline medium its hydroxide or basic salt is ppt.

Not a primary standard (difficult to obtain pure)

Not affected by HCl or Cl on cold. It can be used as self indicator, because its

reduction product is colorlessbut it is better to use an internal redox indicator such as ferroin.

Standardized as KMnO4.

3. Ceric sulphate Ce(SO4)211

Slightly soluble in water and volatile. KI added to increase solubility and decrease

volatility (triiodide).

Titrations must be applied in stoppered flasks Standard I2 prepared by:

a) Dissolving it in water after addition of KI (iodide is easily oxidized to iodine, specially in acid medium 4I + O2 + 4H+ 2I2 + 2H2O

b) Mixture of iodate and iodide, solution is very stable in neutral on acidification I2 liberated. IO3

+ 5I + 6H+ 3I2 + H2O Standardized by titration with Na2S2O3 using

starch as indicator. I2 + 2Na2S2O3 2NaI + Na2S4O6 (sod. tetrathionate)

4. Iodine (I4. Iodine (I22))

I2 + I → I3 (triiodide

ion)

12

Stronger oxidant than iodine.

It is obtained very pure.

It is more practical to prepare molar solution; due to variations in Eq. wt. according to the medium:

IO3 + 5I + 5H+ = 3I2 + 3H2O (0.1 N

HCl) Eq. wt. = MW/5

IO3 + 2I2 + 6H+ = 5H+ + 3H2O (4-6N

HCl) Eq.wt. = MW/4

5. Potassium iodate (KIO5. Potassium iodate (KIO33))13

Not used as primary standard, its solution is unstable; decomposed by CO2 and certain type of bacteria.

S2O32 SO3

2 + S

Decomposition avoided by: using recently boiled and cold distilled water and addition of Na2CO3 or borax.

Eq. wt. = M.w./2

6. Sodium thiosulphate (Na2S2O3)14

Primary standard purity. Powerful oxidizing agent in acid medium. Its solution is

stable. BrO3 + 6H+ + 6e ↔ Br + 3H2O E° = 1.44 V

7. 7. PotassiumPotassium bromate bromate (KBrO(KBrO33))

Mainly used :(a) For determination of strong reductants as: As+3, Sn+2, Sb+3 &

[Fe(CN)6]4

BrO3 + 3H3AsO3 Br + 3H3AsO4

At end point, first excess BrO3 reacts with Br Br2 which

decomposes the indicator.(b) As a source of standard bromine solution: Bromine solution is a mixture of BrO3

and Br when it is

acidified gives Br2.

BrO3 + 5Br + 6H+ 3Br2 + 3H2O

15

1. Determination of primary aromatic amines, phenols, quinolines, acetylsalicylic acid, sulphanilic acid, etc.

BrO3 + 5 Br + 6 H+ 3 Br2 + 3 H2O

dark

OH

+ 3Br2

OH

BrBr

Br

+ 3HBr

2,4,6-TribromophenolPhenol

The excess Br2 is determined by the reaction with iodide:

Br2 + 2I I2 + 2 Br & I2 + 2 Na2S2O3 Na2S4O6 + 2 I

Chloroform is added to dissolve TBP & as indicator.

16 Uses of bromine solution

Known ex.standard

2- Indirect determination of some cations such as Mg+2, Al+3

Mg+2 + C9H6NOH (oxine) → Mg(C9H6OH)2 + 2H+

The ppt is separated, dissolved in acid Mg(C9H6NO)2 + 2H+ → 2C9H6NOH + Mg+2

2C9H6NOH + 4Br2 → 2C9H5ONBr2 + 4 HBr

17

≠ Standard Na2S2O3 Using starch as ind.

Known excess standard + 2KI → 2KBr + I2

Uses of bromine solution

3- Determination of unsaturation:

Determination of unsaturation in oils and fats using bromine-dioxane.

O

O

O

O

BrBr

BrBr

+ Br2

18

─CH=CH─ + Br2 → Br ─CH─CH─Br + 2 HBr

Dioxane Dioxane tetrabromide

≠ Standard Na2S2O3 using starch as ind.

Known excess standard + 2KI → 2KBr + I2

Uses of bromine solution

19 Examples of Applications of Examples of Applications of Redox TitrationsRedox Titrations

1. Free elements a. Metallic and reduced iron b. Zinc powder

c. Sulphur d. Free halogens2. Determination of peroxides:

a. H2O2 b. ZnO2 c. Higher oxides of heavy metals.3. Determination of cations A. Determination of iron: 1. Ferrous 2. Ferric

3. Substances that reduce Fe+3 to Fe+2

4. Substances that oxidize Fe+2 to Fe+3

5. Ferro and ferricyanides B. Determination of copper salts C. Determination of HgCl2 D. Determination of cations that form insoluble

oxalates.

20 Examples of Applications of Examples of Applications of Redox TitrationsRedox Titrations

4. Determination of Anions A. Determination of soluble oxalates B. Determination of sulphide, sulphite, thiosulphate, sulphate & persulphates. C. Determination of halides, chlorate and hypochlorite. D. Determination of nitrite and nitrate5. Determination of aldehydes.6. Determination of moisture content (Karl-Fischer reagent)7. Bromometric determination of phenolic compounds.8. Determination of organic pharmaceutical compounds9. 9- Analysis of mixtures.

a. Metallic iron (Fe) It can be determined by dissolving it in ferric chloride solution and the produced ferrous chloride formed is titrated with standard permanganate in the presence of Zimmermann reagent.

Iron oxides do not interfere.

Fe + 2FeCl3 3FeCl2

1. Free elements 21

≠St. KMnO4

(self indicator) in the presence of

Zimmermann reagent

b. Zinc powder (Zn)is determined by reaction with ferric sulphate and the produced ferrous iron, is acidified with dilute H2SO4 and titrated with standard KMnO4.

This method determines the free zinc and not zinc oxide, because the oxide will not reduce ferric salt

Zn + Fe2(SO4)3 ZnSO4 + 2FeSO4

1. Free elements (cont.)

22

≠St. KMnO4

(self indicator) in the presence of

Zimmermann reagent

+ H+

c. Sulphur (S) Sulphur is converted by refluxing with Na2SO3 to

Na2S2O3 which is then titrated with standard iodine solution.

I2 + 2Na2S2O3 2NaI + Na2S4O6

The excess Na2SO3, which also reacts with I2, is

converted by addition of formaldehyde into formaldehyde bisulphite.

Na2SO3 + S Na2S2O3

23

≠Stand. I2

(Starch indicator)


24

d. Free halogens Iodine can be determined by direct titration with

sodium thiosulphate solution. Bromine or chlorine displaces iodine from potassium iodide.

I2 + 2Na2S2O3 2NaI + Na2S4O6 (sod. tetrathionate)

Br2 + 2KI I2 + 2KBr Cl2 + 2KI I2 + 2KCl


≠Stand. Na2S2O3 (Using starch as

indicator)

1. Hydrogen peroxide A. as reducing agent By direct KMnO4. (self indicator)

2MnO4 + 5H2O2 + 6H+ → 2Mn+2 + 5O2 + 3H2O

By direct Ce+4 (using ferroin indicator) 2Ce+4 + H2O2 → 2Ce+3 + O2 + 2H+

B. as oxidizing agent (iodometrically) H2O2 + 2H+ + 2I → I2 + 2H2O

2. Zinc peroxide ZnO2 + 2H+ → H2O2 + Zn+2

KMnO4 or + KI I2 S2O3

-2.

2. Determination of peroxides25

≠Stand. Na2S2O3 (Using starch as

indicator)

Organic peroxides1. Carbamide peroxideTopical antiseptic and disinfectant solution

H2N-CO-NH2..H2O2 → H2N-CO-NH2 + H2O2

is assayed for H2O2 content iodometrically

2. Hydrous benzoyl peroxideKeratolytic and keratogenic agent for acne.It can be determined iodometrically.

(C6H5COO)2 + 2I + 2H+ 2C6H5COOH + I2

26 2. Determination of peroxides (cont.)

1. Higher oxides of manganese and heavy metals as MnO2, PbO2, Pb3O4

a. Iodometrically MnO2 + 4HCl MnCl2 + Cl2 + 2H2O

Cl2 + 2KI I2 S2O32

b. Indirect titration with reducing agents MnO2 + 2Fe 2+ + 4H+ Mn2+ + 2Fe 3+ + 2H2O

MnO2 + C2O42- + 4H+ Mn2+ + 2CO2 + 2H2O

2MnO2 + 2AsO33- + 4H+ 2Mn2+ + 2AsO4

3- + 2H2O

3. Determination of oxides27

Known excess standard

≠ Stand. KMnO4 (self indicator)

As2O5 and Sb2O5 can oxidize I I2 or I2 oxidizes As2O3 and Sb2O3 As2O5 and Sb2O5

depending on the pH of the medium: In presence of much acid as HCl or HI, The Eº of As5+/As3+ or Sb5+/Sb3+ (it oxidizes I I2)

As2O3 + 2I2 + 2H2O ← As2O5 + 4I + 4H+

Sb2O3 + 2I2 + 2H2O ← Sb2O5 + 4I + 4H+

In slightly acid or neutral medium, The Eº of As5+/As3+ or Sb5+/Sb3+ (I2 oxidizes As2O3 and

Sb2O3 As2O5 and Sb2O5)

Oxides of As or Sb (As2O3, As2O5, Sb2O3 and Sb2O5)

≠Stand. Na2S2O3

(starch as indicator)

28 3. Determination of oxides (cont.)

(iodometric)

(Iodimetric)

Addition of mild alkali (NaHCO3) keep the oxidation going on.

As2O3 ≠ 2I2 + 2H2O → As2O5 + 4I + 4H+

Sb2O3 ≠ 2I2 + 2H2O → Sb2O5 + 4I + 4H+

using starch as indicator

Na2CO3 or NaOH (pH>8) can not be used as they react with iodine forming hypoiodite and iodate which react with both oxides to form iodinum ions

I2 + OH IO + I

3IO IO3 + 2I self oxidation reduction


NaHCO3

NaHCO3--------→

--------→

As2O3 and Sb2O3 can be determined by direct reaction with potassium bromate in acid medium As2O5 and Sb2O5 using methyl orange as irreversible redox indicator

3As2O3 ≠ 2BrO3 → As2O5 + 2Br

3Sb2O3 ≠ 2BrO3 → Sb2O5 + 2Br

BrO3 + 5Br + 6H+→ 3Br2 + 3H2O

Br2 + methyl orange → irreversible oxidation

(bleaching color)

H+

H+


PbO can be determined by dissolving the sample in glacial acetic acid lead acetate and precipitating it as lead oxalate

PbO + CH3COOH → Pb (CH3COO)2 + 2H+

Pb (CH3COO)2 + H2C2O4 → Pb C2O4 + 2

CH3COOH

Filter and wash the ppt


≠ Stand. KMnO4 (self indicator)


I. Ferrous salts: ferrous sulphate, ammonium sulphate (Mohr’s salt), carbonate, ferrous sulphide,

1. Fe+2 KMnO4 after addition of Zimmermann’s reagent, self indicator.

2. Fe+2 K2Cr2O7 after addition of H3PO4 and internal redox (diphenylamine) or external indicator.

3. Fe+2 Ce+4 , irreversible methyl red indicator.4. Fe+2 I2 in presence of F or PO43 using starch

4. Determination of CationsA. Iron

32

II. Ferric salts: by three methodsFerric (Fe3+) reduced to (Fe2+) by pre-reductants1- SnCl2 + Fe+3 → SnCl4 + Fe+2 KMnO4 after addition of Zimmermann’s reagent, self

indicator SnCl2 + 2HgCl2 → Hg2Cl2 + SnCl4

2- Zn and H2SO4:

2Fe+3 + Znº → Zn+2 + 2Fe+2 KMnO4 H2SO4 accelerates the reduction by Znº

and the unreacted Znº is removed by filtration

3- Amalgamated Znº (Znº + HgCl2). Excellent reducing agent; (Jones reductor).

334. Determination of Cations

A. Iron (cont.)

Iodometrically: Fe+3/Fe+2 system E°= 0.77 I2/2I system E°=

0.54 You must difference between the 2 systems either by I conc. by addition of xss I or I2 conc. by

extraction with immiscible solvent as CHCl3 or CCl4.

Fe+3 + I → Fe+2 + I2 S2O3-2 using starch

Direct titration with titanous chloride using methylene blue (irreversible) or thiocyanate as

indicators (the end point is colorless due to disappearance of either the blue color of MB or the red color of Fe(SCN)3 which is formed at the beginning)

FeCl3 ≠ TiCl3 FeCl2 + TiCl4

standard

34 4. Determination of CationsA. Iron (cont.)

III. Reductants that reduce Fe+3 → Fe+2 : 1- SnCl2 + Fe3+ → SnCl4 + Fe2+ KMnO4

2- Znº + 2Fe3+ → Zn+2 + 2Fe2+ KMnO4

3- Feº + 2Fe3+ → 3Fe2+ KMnO4

IV. Oxidants that oxidize Fe+2 → Fe+3 : 1- K2S2O8 + 2Fe2+ + 2H+ → 2Fe3+ + 2KHSO4

2- KClO3 + 6Fe2+ + 6H+ → 6Fe3+ + KCl + 3H2O

3- MnO2 + 2Fe2+ + 4H+ → 2Fe3+ + Mn2+ + 2H2O Known excess standard

≠ Stand. KMnO4 or K2Cr2O7 (self indicator) (Ferroin

ind.)


Persulphate

Chlorate

V. Ferrocyanide By direct titration KMnO4 or Ce4+

1- 5[Fe(CN)6]4- MnO4- +8H+→ 5[Fe(CN)6]3- + Mn2++3H2O

(self indicator) 2- [Fe(CN)6]4- Ce4+ → [Fe(CN)6]3- + Ce3+

(ferroin indicator) Ferricyanide 1- By iodometrical titration 2[Fe(CN)6]3- + 2I- → 2[Fe(CN)6]4- + I2 S2O3

-2 (starch)

we add H2SO4 and ZnSO4 (to precipitate Zn2[Fe(CN)6]) to ↑↑ the E° of [ferri]/[ferro] to oxidize iodide to iodine.

2- [Fe(CN)6]3- + pre-reductants → [Fe(CN)6]4- MnO4

-

Remove xss

e.g. Sulphite or sulphide or sod. peroxide


4. Determination of CationsB. Determination of copper salts

In presence of tartarate or citrate I2 oxidizes Cu+ to cu+2 as these anions form stable complexes with Cu+2

Iodometrically: Cu+2/Cu+ system E°= 0.15 I2/2I system E°=

0.54ExpectedActually

2Cu+2 + 4I → 2CuI2

The instability of CuI2 results in formation of Cu2I2 ppt which Cu+ → ↑ E° of Cu+2/Cu+ system to become able to oxidize I → I2

using starchunstable

37

I2 oxidize Cu+ → Cu+2 The reverse occurs due to instability of CuI2

→ Cu2I2 + I2 S2O3-2

We add KSCN… Why?

4. Determination of CationsC. Determination of HgCl2

38

HgCl2 is reduced firstly to Hg° by HCHO in Ca(OH)2 medium.

Hg2+ + HCHO + 3OH- Hg° + HCOO- + H2O

Hg° + I2 HgI2 red ppt colorless

complexknown

Excess stand≠

Stand. Na2S2O3 (starch as indicator)

+ 2I- [HgI4]2-

4. Determination of CationsD. Cations form insoluble oxalates.

(Ca2+, Ba2+ Sr2+, Mg2+, Cd2+, Bi3+, Zn2+, Ni2+, Co2+ & Pb2+)

Ca2+ + H2C2O4 → Ca C2O4 + xss. H2C2O4

(i) The washed precipitate is dissolved in dil. H2SO4 and ≠ KMnO4 at 60°C. or

(ii) The excess oxalic acid in the filtrate and washing is back titrated with standard KMnO4 at 60°C.Oxidizing substance such as K2S2O8, KClO3, NO3

-, MnO2

can be determined by treatment with a known excess oxalic acid and the residual oxalic acid is then back titrated with standard KMnO4.

39

known Excess stand

then follow one of the 2 ways

Filter and wash the ppt

40

Soluble oxalates KMnO4 or Ce+4 in presence of sulphuric acid and heating to 60°C.

The reaction is slow at start becomes

rapid after formation of reduction product (Mn+2 or Ce+3).

5. Determination of AnionsA. Soluble oxalates.

(A) Errors due to iodine (I2)

I2, being volatile, is subjected to loss. So, iodimetric and

iodometric titrations should be carried out in a glass-stoppered flask.

Iodine may undergo changes to hypoiodite and iodideI2 + H2O HIO + HI

(B) Errors due to iodide ionIodides are easily oxidized by atmospheric oxygen to

iodine, specially in acid medium. 4I- + O2 + 4H+ 2I2 + 2H2O

(C) Errors due to starchStarch may be decomposed by micro-organisms, these

decomposition products give non-reversible reddish colour with iodine that masks the true end point.

Sources of errors in iodine titrations41

Also glucose which may result from hydrolysis of starch can give error as it is reducing agent, Boric

acid may be added as a preservative or better starch solution is freshly prepared.

(D) Errors due to thiosulphateThiosulphate is unstable being slowly decomposed

with the precipitation of sulphur:S2O3

2- + H+ HSO3- + S

Decomposition may be caused by CO2 (in distilled water) or by bacterial action. Therefore, the

solution should be prepared with recently boild and cooled dist. water and Na2CO3 is added to prevent

bacterial activity (pH = 9-10).

42 Sources of errors in iodine titrations

5. Determination of Anions b. S2-, SO3

2-, S2O32-,SO4

2- & S2O82-

. Direct titration1. Sulphide S2 ≠ I2 → 2I + Sº

Use dilute soln. to decrease the inclusion of I2 by Sº

2. Thiosulphate 2S2O32 ≠ I2 → S4O6

2 + 2I

Back titration.3. Sulphite SO3

2 + I2 + H2O → SO42 + 2I + 2H+

S2 + I2 → 2I + Sº

4. Sulphate SO42 + BaCrO4 → BaSO4 + CrO4

2 (filtrate)

2CrO42 (filtrate) + 2H+ Cr2O7

2 + H2O

Known xss. standard

≠ St. S2O32

(starch indicator)

+2KI I2 S2O32 (starch)Iodometrically

43

ClO3- + 6I- + 6H+ Cl- + 3I2 + 3H2O

ClO- + 2I- + 2H+ Cl- + I2 + H2O

Chlorates (ClO3) & Hypochlorites (ClO)

Iodometrically

Chlorate

Hypochlorite≠

Stand. Na2S2O3 (starch as indicator)

5. Determination of AnionsC. Determination of halides, ClO3

and ClO44

e.g.1: Chlorine in bleaching powder

+ 4H+ Cl2 + Ca2+ + 2H2O

Acetic acid used for acidification not HCl, since calcium chlorate present as a result of hypochlorite decomposition will react slowly with I and liberate iodine.

e.g.2: N-chloro-organic compounds as water disinfectant, these in water release hypochlorous acid (HOCl) which is the active germicidal species. ClO + 2I + 2H+ Cl + I2 + H2O (iodometrically)

5. Determination of AnionsC. Determination of halides (cont.)

45

(calcium hypochloriteCa(OCl)2

+ basic chloride). CaCl2.Ca(OH)2.H2O

Organically combined iodine compound Thyroxine, diiodohydroxyquinoline & chiniofon

These are converted into iodide by:A. Iodoxyl

B. Thyroxine

I + 3Br2 + 3H2O → IO3 + 6Br + 6H+

5. Determination of AnionsC. Determination of halides (cont.)

46

xss Br2 removed by phenol.

ignition at 700°C K2CO3

I

+ xss I I2 S2O32

Znº + gl. HOAc Reflux

ZnI2 filter, cool ≠ IO3-

C. Oxygen flask combustion method Iodide determined iodometrically.

1. The compound is wrapped in a filter paper attached to pt. wire and sealed in the stopper of a special oxygen filled flask containing dilute Na2S2O5 (sodium metabisulphite)

solution.

2. Combustion is complete within 30 sec at 1200°C.

3. The resulting iodine is absorbed (reduced) by Na2S2O5 forming iodide

47 Oxygen flask combustion method

nitrite sample is added to an excess of acidified KMnO4 soln. (the

tip of the pipette containing the nitrite solution should be below the surface of the liquid during addition in order to stop volatility of the unstable nitrous acid, liberated from acidified nitrite solution).

Nitrates

2MnO4- + 5NO2

- + 6H+ 2Mn2+ + 5NO3- + 3H2O

2NO3- + 3C2O4

2- + 8H+ 2NO + 6CO2 + 4H2O

5. Determination of Anionsd. Determination of nitrites and nitrates


≠ Stand. Fe2+ (self

indicator)


≠ Stand. KMnO4 or K2Cr2O7 (self indicator) (Ferroin

ind.)

48

Nitrites

Determination of Aldehydes

• e.g. formalin (formaldehyde), glucose, and lactose.

• 2R-CHO + I2 + 2NaOH → 2R-COOH + 2 NaI

• Sucrose after acid hydrolysis (inversion) of sugar

• C12O22O11 + H2O → H+→ C5H11O5-CHO + C5H11O5CO

(glucose) (fructose)

I2 + 2NaOH → NaOI + NaI + H2O

C5H11O5-CHO + IO → C5H11O5-COOH + I

Change in oxidation number = 2 (Glucose Sucrose)

Kn. Xss. St.

└→ acidification → I2 ≠ St. S2O32

(starch indicator)

Gluconic acidGluconic acid

49

• Reagent: I2 + SO2 + anhyd. CH3OH and anhyd. pyridine.

• Sample containing moisture + reagent until appearance of yellow color of the xss iodine.

Determination of moisture content (Karl-Fischer reagent)

SO2 + I2 + H2O → SO3 + 2HI

N N-HI N-SO3

NSO4CH3

H

3 + SO3 + 2HI 2 +

+ CH3OH

50

Compound substitution productsReaction time

(min)

OH

COOH

OCOCH3

COOH

Bromometric determinations(Bromine substitution method)

OH

BrBr

Br

+ 3HBr

OH

BrBr

Br

+ 3HBr

+ CO2

OH

BrBr

Br

+ 3HBr

+ CO2

NaOH

OH

COONa

+ CH3COONa + H2O

OH

51

Salicylic acid

Phenol

Aspirin

45

45

45


(min)

OH

CH

CH2

NH.HCl

CH3OH

N

OH

OH

Cl

Bromine substitution method (cont.)52

p-Chlorophenol

Oxine

Phenylephrine HCl

45 (4ºC)

30

15

OH

BrBr

Cl

+ 2HBr

OH

BrBr

Br

CH

CH2

NH.HCl

CH3OH

+ 3HBr

N

OH

Br

Br

+ 2HBr


(min)OH

OH

OH

OHC6H13

Bromine substitution method (cont.)53

Resorcinol

Hexylresorcinol

3

10

OH

BrBr

C6H13

OH

+ 2HBr

OH

BrBr

Br

OH

+ 3HBr

Vitamin C (ascrobic acid) Iodimetrically

Miscellaneous Applications

Polyhydroxy Alcohols e.g. glycerol can be determined iodometrically:3C3H8O3 + 7K2Cr2O7 + 28H2SO4 →

9CO2 + 40H2O + 7Cr2(SO4) + 7K2SO4

54

OO

OHHO

CH CH2

OH OH

+ I2H+

OO

OO

CH CH2

OH OH

+ 2HI

≠ Standard Na2S2O3 (chloroform)

Known excess standard + 2KI → I2

Mercaptans (Thiol) compounds

Generally 2RSH ≠ I2 → RS-SR + 2HI e.g., Dimercaprol, Thioglycollic acid, D-penicillamine;

can be determined iodimetrically.

55

Dimercaprol (BAL)

dimercaprol

Thioglycollic acid

2

CH2

CH

CH2

SH

OH

SH + 2I2

CH2

CH

CH2

S

OH

S

S

S

CH2

CH

CH2OH

+ 4HI≠

disulphide

+ 2HICH2

COOH

S S CH2

COOH

CH2

COOH

SH+ I2

NaHCO32 ≠

Analysis of Mixtures

1. Acetic & formic acids (Total ≠ OH ─ [ph.ph.] & formic ≠ MnO4

─)

2. Oxalic acid & sulphuric acid (Total ≠ OH ─ [ph.ph.] & oxalic ≠ MnO4

─)

3. Phenol & salicylic acid (Total ≠ bromometrically & salicylic acid ≠ OH ─ [ph.ph.])

4. I2 & KI (Total I2/I by Andrew's & I2 ≠ S2O3

2─ )5. Ammonium oxalate & ammonium chloride

(Total by formol ≠ OH ─ [ph.ph.] & oxalate ≠ MnO4

─)6. Ferrous oxalate & oxalic acid (protoxalate)

(Total ≠ MnO4 ─ & the produced Fe3+ pass

through Jones red. ≠ MnO4 ─ or iodometrically)

56

7. Persulphate & potassium acid sulphate (persulphate + xss ferrous ≠ permanganateand acid sulphate by ≠ st. NaOH using ph.ph.

8. Glucose & sucrose Home work9. Lead oxide & lead acetate mix.(lead subacetate)10. Cl─, Br ─ & I ─ mixture (Fractional oxidation)11. Sulphide & thiosulphate mixture12. Ferrous fumarate or ferrous gluconate13. Iron ammonium citrate14. Tartar emetic (Antimony potassium tartarte)15. Arsenicals16. Chloramine-T (as hypochlorite)

57 Analysis of Mixtures (cont.)

Hassan.F.AskalHassan.F.Askal

Documents

Application of Oxidation-Reduction Titrations