SEMINAR ON COMPLEXOMETRY TITRATIONSEMINAR ON COMPLEXOMETRY TITRATION

Under the Guidance of Dr. Satish Kumar Shetty

Presented by, Sudhindra.g

CONTENTCONTENT DefinitionDefinition ChelatesChelates LigandLigand CLassification of ligandCLassification of ligand Titration curveTitration curve Complexometric titration involving EDTAComplexometric titration involving EDTA Methods of End Point DetectionMethods of End Point Detection Types of Complexometric TitrationsTypes of Complexometric Titrations

1. Direct Titration1. Direct Titration

2. Back titration2. Back titration

3. Replacement of one complex by another3. Replacement of one complex by another

4. Alkalimetry titration of metals 4. Alkalimetry titration of metals

Definition:Definition:Complexometric titration is the Complexometric titration is the

volumetric method of analysis in which volumetric method of analysis in which the simple metal ion is converted into the simple metal ion is converted into complex by addition of reagent known complex by addition of reagent known as ligand or complexing agent and is as ligand or complexing agent and is used to estimate polyvalent ions i.e., used to estimate polyvalent ions i.e., divalent, trivalent.divalent, trivalent.

CHELATECHELATE:: Chelate is a complex that is formed by the Chelate is a complex that is formed by the

combination of a polyvalent metal ion with a combination of a polyvalent metal ion with a molecule which contain two or more groups molecule which contain two or more groups that can donate electrons.that can donate electrons.

ExampleExample: copper with a co-ordination no 4 : copper with a co-ordination no 4

formsforms

Cationic amine complex Cationic amine complex

Neutral complex with glycineNeutral complex with glycine

Copper bonds with the both oxygen of Copper bonds with the both oxygen of carboxyl group and nitrogen of amine groupcarboxyl group and nitrogen of amine group

LIGANDLIGAND : : A A complexing agentcomplexing agent is an electron donating ion or molecule is an electron donating ion or molecule

capable of forming one or more covalent or coordinate bonds capable of forming one or more covalent or coordinate bonds (dative bond) with metal ions, producing a complex which has (dative bond) with metal ions, producing a complex which has

different properties from that of metal ions.different properties from that of metal ions.

A ligand uses a lone pair of electrons (Lewis base) to bond to the metal ion (Lewis acid)

All an ions, and molecules such as NHAll an ions, and molecules such as NH33, H, H22O, NO, CNO, NO, CN--, and , and

halides are called as Ligands, because they can donate pair of halides are called as Ligands, because they can donate pair of electrons to metal ion.electrons to metal ion.

AgAg++ + 2 CN + 2 CN- - ⇌ [Ag (CN) 2]⇌ [Ag (CN) 2]- -

HgHg22+ + + 2 Cl+ 2 Cl- - ⇌ HgCl⇌ HgCl22

Classification of LigandsClassification of Ligands Unidentate ligandUnidentate ligand Ligands that are bound to metal ion only at one place are called Ligands that are bound to metal ion only at one place are called

unidentate ligands.unidentate ligands.

Bidentate ligandBidentate ligand If the ligand has two co-ordinating site then it is called as If the ligand has two co-ordinating site then it is called as

Bidentate Ligand.Bidentate Ligand. E.gE.g.: .: ↓ ↓ ↓ ↓

HH22N −CHN −CH22−CH−CH22−NH−NH22

Ethylene diamine [bidentate]Ethylene diamine [bidentate]

Tridentate ligandTridentate ligand If the ligand has three coordinating site then it is called as If the ligand has three coordinating site then it is called as

Tridentate LigandTridentate Ligand..

E.gE.g.: -↓ ↓ ↓.: -↓ ↓ ↓

HH22N −CHN −CH22−CH−CH22−NH− CH−NH− CH22−CH−CH22−NH−NH22

Multidentate ligandMultidentate ligand Ligand having many sites is called Ligand having many sites is called Multidentate Ligand.Multidentate Ligand.

E.g.:-E.g.:-EDTA- disodium saltEDTA- disodium salt..

TITRATION CURVESTITRATION CURVES So in the EDTA titrations if pM (negative logarithm of the So in the EDTA titrations if pM (negative logarithm of the

free metal ion concentration pM = -log[Mfree metal ion concentration pM = -log[Mn+ n+ ]) is plotted ]) is plotted against the volume of EDTA solution added .against the volume of EDTA solution added .

And a point of inflexion occurs at equivalence point in some And a point of inflexion occurs at equivalence point in some

instance it may exceed 10pM units.instance it may exceed 10pM units.

There are two factors that are important in determining the magnitude of break in titration curve at end point.

1. The stability of complex formed: The greater the stability constant for complex formed, larger the charge in free metal concentration (pM) at equivalent point and more clear would be the end point.

2. Effect of pH:. Control of pH is important since the H+ ion plays an important role in chelation. Most ligands are basic and bind to H+

ions throughout a wide range of pH. Some of these H+ ions are frequently displaced from the ligands (chelating agents) by the metal during chelate formation.

Equation below shows complexation between metal ion and H+

ion for ligand: Thus, stability of metal complex is pH dependent. Lower the pH of the solution, lesser would be the stability of

complex. Only metals that form very stable complexes can be titrated in

acidic solution, and metals forming weak complexes can only be effectively titrated in alkaline solution.

COMPLEXOMETRIC COMPLEXOMETRIC TITRATION INVOLVING EDTATITRATION INVOLVING EDTA

The spatial structure of the EDTA which has 6 donor atoms The spatial structure of the EDTA which has 6 donor atoms enables it to satisfy the coordination number of 6 frequently enables it to satisfy the coordination number of 6 frequently encountered with the metal ion and forms strain less five encountered with the metal ion and forms strain less five membered rings on chelation .EDTA can form a hexadentate membered rings on chelation .EDTA can form a hexadentate ligand.ligand.

Effect of pH on complex formation

Ethylenediamine tetra-acetic acid ionizes in four stages (pK1=2.0, pK2=2.67, pK3=6.16 and pK4=10.26).

since the actual complexing species is Y4-, complexes will form more efficiently and be more stable in alkaline solution.

On the other hand, at lower pH values when the concentration of Y4- is lower, the stability constant of the complexes will not be so high.

Complexes of most divalent metals are stable in ammonia solution.

Trivalent metal complexes are usually still more firmly bound and stable in strongly acid solutions.

Example:- cobalt(III) edetate complex is stable in concentrated hydrochloric acid.

STABILITY OF COMPLEXESSTABILITY OF COMPLEXES

The general eqn: for the formation of a 1:1 chelate The general eqn: for the formation of a 1:1 chelate

complex , MX iscomplex , MX is

Where M is the metal ion and X is the chelating ion Where M is the metal ion and X is the chelating ion Therefore stability constant K = [MX] / [M] [X]Therefore stability constant K = [MX] / [M] [X]

Methods of End Point DetectionMethods of End Point Detection

End point in complexometric titration can be detected by the End point in complexometric titration can be detected by the

following methodsfollowing methods::

1.1. IndicatorsIndicators

2. 2. Spectrophotometric detectionSpectrophotometric detection

3. 3. Amperometric titrationAmperometric titration

4. 4. Potentiometric titrationPotentiometric titration

METAL ION INDICATORSMETAL ION INDICATORS

The end point in complexometric titrations is shown by means of The end point in complexometric titrations is shown by means of

pM indicators.pM indicators.

If K is the stability constant, If K is the stability constant,

K = [MX]/[M][X] K = [MX]/[M][X]

then, [M] = [MX]/[X]K then, [M] = [MX]/[X]K

or log [M] = log [MX]/[X] – log K or log [M] = log [MX]/[X] – log K

and pM = log [X]/[MX] – pKand pM = log [X]/[MX] – pK

The pM indicator is a dye which is capable of acting as a The pM indicator is a dye which is capable of acting as a chelating agent to give dye-metal complex .chelating agent to give dye-metal complex .

The complex is different in colour from the dye itself and also The complex is different in colour from the dye itself and also has a lower stability constant than the chelate metal complex..has a lower stability constant than the chelate metal complex..

As soon as there is a slight excess of edetate , the metal dye As soon as there is a slight excess of edetate , the metal dye complex decomposes to produce free dye , this is complex decomposes to produce free dye , this is accomplished by a colour change.accomplished by a colour change.

The colour of dye and of the metal complex vary with the pH The colour of dye and of the metal complex vary with the pH

therefore it is also essential to buffer the solution to maintain therefore it is also essential to buffer the solution to maintain the required pH during titration.the required pH during titration.

Mechanism of action of Mechanism of action of

indicatorindicator Let the metal be denoted by M, indicator by I and chelate by Let the metal be denoted by M, indicator by I and chelate by

EDTA. EDTA.

At the onset of the titration, the reaction medium contains the At the onset of the titration, the reaction medium contains the metal-indicator complex (MI) and excess of metal ion.metal-indicator complex (MI) and excess of metal ion.

When EDTA titrant is added to the system, a competitive When EDTA titrant is added to the system, a competitive reaction takes place between the free metal ions and EDTA. reaction takes place between the free metal ions and EDTA. Since the metal-indicator complex (MI) is weaker than the Since the metal-indicator complex (MI) is weaker than the metal-EDTA chelate, the EDTA which is being added during metal-EDTA chelate, the EDTA which is being added during the course of the titration is chelating the free metal ions in the course of the titration is chelating the free metal ions in solution at the expense of the MI complex. solution at the expense of the MI complex.

Finally, at the end point, EDTA removes the last traces of the Finally, at the end point, EDTA removes the last traces of the metal from the indicator and the indicator changes from its metal from the indicator and the indicator changes from its complexed colour to its metal free colour. complexed colour to its metal free colour.

EXAMPLES FOR pM EXAMPLES FOR pM INDICATORSINDICATORS

Some examples of metal ion Some examples of metal ion indicatorsindicators

MUREXIDEMUREXIDE

Murexide is a ammonium salt of purpuric acid Murexide is a ammonium salt of purpuric acid solution. solution.

pH =9(HpH =9(H44D) reddish- violet D) reddish- violet

ppH H 9 to pH 11 (H9 to pH 11 (H33DD2 -2 -) violet , and above ) violet , and above

pH 11(HpH 11(H22DD3 -3 -) blue-violet . ) blue-violet . Murexide from complexes with many metal ions .Murexide from complexes with many metal ions .

Only these with Cu, Ni,Co,Ca,and the lanthanides Only these with Cu, Ni,Co,Ca,and the lanthanides are sufficiently stable to find the application in are sufficiently stable to find the application in analysis .analysis .

There colours in alkaline solutions are orange There colours in alkaline solutions are orange (copper), yellow (nickel and cobalt) & red (calcium).(copper), yellow (nickel and cobalt) & red (calcium).

Murexide may be employed for direct EDTA titration Murexide may be employed for direct EDTA titration of calcium at pof calcium at pHH=1 the colour change at the end =1 the colour change at the end points is from red to blue–violet .points is from red to blue–violet .

In strongly acidic solutions the dye tends to polymerise to a red-brown In strongly acidic solutions the dye tends to polymerise to a red-brown product, and therefore the indicator is rarely applied in the EDT A titration of product, and therefore the indicator is rarely applied in the EDT A titration of solutions more acidic than pH = 6.5.solutions more acidic than pH = 6.5.

The sulphonic acid group gives up its proton long before the pH range of 7-12, The sulphonic acid group gives up its proton long before the pH range of 7-12, which is of more interest for metal-ion indicator use.which is of more interest for metal-ion indicator use.

Only the dissociation of the two hydrogen atoms of the phenolic groups need, Only the dissociation of the two hydrogen atoms of the phenolic groups need, and so the dyestuff may be represented by the formula and so the dyestuff may be represented by the formula HH22DD--. The two p. The two pKaKa

values for these hydrogen atoms are 6.3 and 11.5 respectively.values for these hydrogen atoms are 6.3 and 11.5 respectively.

Solochrome Black (Eriochrome black T )

Below pH = 5.5, the solution of Solochrome Black is red (due to

H2D-).

Between pH 7 and 11 it is blue (due to HD2-).

And above pH = 11.5 it is yellowish-orange (due to D3-).

In the pH range 7-11 the addition of metallic salts produces

colour change from blue to red.

M2+ + HD2- (blue) →MD- (red) + H+

Xylenol OrangeXylenol Orange

This indicator is prepared by the condensation of O-This indicator is prepared by the condensation of O-

cresolsulphonephthalein (Cresol Red) with formaldehyde and cresolsulphonephthalein (Cresol Red) with formaldehyde and

iminodiacetic acid.iminodiacetic acid.

Special feature is dyestuff retains the acid-base properties of Special feature is dyestuff retains the acid-base properties of

Cresol Red and displays metal indicator properties even in Cresol Red and displays metal indicator properties even in

acid solution (pH = 3-5).acid solution (pH = 3-5).

Acidic solutions of the indicator are coloured lemon-yellow Acidic solutions of the indicator are coloured lemon-yellow and those of the metal complexes coloured red.and those of the metal complexes coloured red.

Direct EDTA titrations of Bi, Th, Zn, Cd, Pb, Co, etc., are Direct EDTA titrations of Bi, Th, Zn, Cd, Pb, Co, etc., are readily carried out and the colour change is sharp. readily carried out and the colour change is sharp.

Mordant black IIMordant black II

Indicator is blue in colour at pH 10. Indicator is blue in colour at pH 10. On complexation with metal ions pink colour is formed.On complexation with metal ions pink colour is formed.

Below pH 6.3 and above 11.5 it is reddish in colour hence it is Below pH 6.3 and above 11.5 it is reddish in colour hence it is used at pH 10.used at pH 10.

It is used in the estimation of metal ions like Calcium, It is used in the estimation of metal ions like Calcium, Magnesium, Zinc, Cadmium, Lead and Mercury.Magnesium, Zinc, Cadmium, Lead and Mercury.

It can’t be used with oxidizing ions like ferric, cerric or with It can’t be used with oxidizing ions like ferric, cerric or with reducing ions like stannous and titannous.reducing ions like stannous and titannous.

It can’t be used with ions like copper, aluminum, cobalt, silver It can’t be used with ions like copper, aluminum, cobalt, silver as these ions form more stable complexes with indicator than as these ions form more stable complexes with indicator than with chelating agent.with chelating agent.

TYPES OF COMPLEXOMETRIC TYPES OF COMPLEXOMETRIC TITRATIONSTITRATIONS

1. Direct titration1. Direct titration

2. Back titration2. Back titration

3. Replacement of one complex by 3. Replacement of one complex by

anotheranother

4. Alkalimetric titration of metals4. Alkalimetric titration of metals

The solution containing the metal ion to be determined is The solution containing the metal ion to be determined is buffered to the desired pH and few drops of indicator is added.buffered to the desired pH and few drops of indicator is added.

The contents are titrated against standardized disodium edetate The contents are titrated against standardized disodium edetate solution till the end point shown by the colour change.solution till the end point shown by the colour change.

A blank titration is carried out omitting the substance to be A blank titration is carried out omitting the substance to be determined, but contains all the other solutions like buffer & determined, but contains all the other solutions like buffer & indicator. indicator.

The volume of edetate consumed in blank titration is subtracted The volume of edetate consumed in blank titration is subtracted from that obtained in the original estimation.from that obtained in the original estimation.

1. Direct Titration

Examples of such estimationExamples of such estimation:: Bismuth - Nitrate, carbonate, oxynitrate, sub nitrate.Bismuth - Nitrate, carbonate, oxynitrate, sub nitrate.

Calcium - Chloride, gluconate, lactate, carbonate.Calcium - Chloride, gluconate, lactate, carbonate.

Magnesium - Carbonate, oxide, stearate, sulphate, trisilicate.Magnesium - Carbonate, oxide, stearate, sulphate, trisilicate.

Zinc - Sulphate, oxide.Zinc - Sulphate, oxide.

Assay of magnesium Assay of magnesium sulphatesulphate

PrinciplePrinciple:: Magnesium sulphate forms stable complex with disodium Magnesium sulphate forms stable complex with disodium

edetate in presence of strong ammonia-ammonium chloride edetate in presence of strong ammonia-ammonium chloride buffer. buffer.

It is assayed by direct titration method.It is assayed by direct titration method. Mordant black mixture is used as indicator for detection of end Mordant black mixture is used as indicator for detection of end

point {red to blue} and standard 0.05 M disodium edetate is point {red to blue} and standard 0.05 M disodium edetate is employed as titrant.employed as titrant.

Materials required:Materials required:

Standard disodium edetate 0.05 M, Magnesium sulphate Standard disodium edetate 0.05 M, Magnesium sulphate sample (0.4 g), sample (0.4 g),

Ammonia-ammonium chloride buffer (10ml), Mordant black Ammonia-ammonium chloride buffer (10ml), Mordant black mixture as indicator.mixture as indicator.

► Weigh accurately about 0.3 g and dissolve in 50 ml of water. Weigh accurately about 0.3 g and dissolve in 50 ml of water.

► Add 10 ml of strong ammonia-ammonium chloride solution, and Add 10 ml of strong ammonia-ammonium chloride solution, and titrate with O.05M EDTA using 0.1 g of mordant black 11 mixture titrate with O.05M EDTA using 0.1 g of mordant black 11 mixture as indicator, until the pink colour is discharged from the blue. as indicator, until the pink colour is discharged from the blue.

Each ml of O.05M disodium ethylenediaminetetraacetate is equivalent Each ml of O.05M disodium ethylenediaminetetraacetate is equivalent to 0.00602 g of MgS04to 0.00602 g of MgS04

Assay:

Mg2+ + EDTA → Mg EDTAMg2+ + EDTA → Mg EDTA Mg2+ + [H2X]2- → [Mg X ]2- + 2 H + Mg2+ + [H2X]2- → [Mg X ]2- + 2 H + . . . . . . Mg SO4 ≡Mg2+≡Na2H2X.2 H2O Mg SO4 ≡Mg2+≡Na2H2X.2 H2O . . . . . . 120.38 gm Mg SO4 ≡ 1000 ml M 120.38 gm Mg SO4 ≡ 1000 ml M

Back titration is necessary in the following cases:Back titration is necessary in the following cases: Metals which are precipitated as hydroxides in the pH usedMetals which are precipitated as hydroxides in the pH used For insoluble substances like Lead sulphate, Calcium oxalateFor insoluble substances like Lead sulphate, Calcium oxalate Those react slowly with disodium acetateThose react slowly with disodium acetate Those form stable metal chelate complex than with indicatorThose form stable metal chelate complex than with indicator MethodMethod:: In this method, a known excess of disodium edetate, buffer In this method, a known excess of disodium edetate, buffer

solution and few drops of indicator is added. solution and few drops of indicator is added. The complexation is favored by heating, then cooled and the The complexation is favored by heating, then cooled and the

excess edetate is back titrated against magnesium sulphate or excess edetate is back titrated against magnesium sulphate or zinc sulphate.zinc sulphate.

ExampleExample

Aluminum Hydroxide gel, dried- aluminum hydroxide gelAluminum Hydroxide gel, dried- aluminum hydroxide gel Calcium phosphateCalcium phosphate

BACK TITRATION

Assay of ALUMINIUM HYDROXIDE Assay of ALUMINIUM HYDROXIDE GELGEL

Weigh accurately about 5 g and dissolve in 3 ml of HCl by Weigh accurately about 5 g and dissolve in 3 ml of HCl by warming on a water-bath; cool to below 20° and dilute to warming on a water-bath; cool to below 20° and dilute to 100.0 ml with water. 100.0 ml with water.

Pippet 20.0 ml of above solution, add 40 ml of O.05M EDTA Pippet 20.0 ml of above solution, add 40 ml of O.05M EDTA + 80 ml of water + 0.15 ml of methyl red solution, and + 80 ml of water + 0.15 ml of methyl red solution, and neutralize by the drop wise addition of NaOH. neutralize by the drop wise addition of NaOH.

Warm on a water-bath for thirty minutes, add 3 g of hexamine, Warm on a water-bath for thirty minutes, add 3 g of hexamine, and titrate with O.05M lead nitrate, using 0.5 ml of Xylenol and titrate with O.05M lead nitrate, using 0.5 ml of Xylenol orange solution as indicator. orange solution as indicator.

Each ml of 0.05 M EDTA is equivalent to 0.002549 g of AlEach ml of 0.05 M EDTA is equivalent to 0.002549 g of Al220033

REPLACEMENT OF ONE REPLACEMENT OF ONE COMPLEX BY ANOTHERCOMPLEX BY ANOTHER

In this method the metal, which is to be analyzed, displaces In this method the metal, which is to be analyzed, displaces quantitatively the metal from the complex. quantitatively the metal from the complex.

When direct or back titrations do not give sharp end points, the When direct or back titrations do not give sharp end points, the metal may be determined by the displacement of an equivalent metal may be determined by the displacement of an equivalent amount of Mg or Zn from a less stable EDTA complex. amount of Mg or Zn from a less stable EDTA complex.

Mn displaces Mg from Mn EDTA solution. Mn displaces Mg from Mn EDTA solution. The freed Mg metal is then directly titrated with a standard EDTA The freed Mg metal is then directly titrated with a standard EDTA

solution. solution.

In this method, excess quantity of Mg EDTA chelate is added to In this method, excess quantity of Mg EDTA chelate is added to Mn solution. Mn solution.

Mn quantitatively displaces Mg from Mg EDTA chelate.Mn quantitatively displaces Mg from Mg EDTA chelate.

This displacement takes place because Mn forms a more This displacement takes place because Mn forms a more stable complex with EDTA. stable complex with EDTA.

By this method Ca, Pb, Hg may be determined using By this method Ca, Pb, Hg may be determined using Eriochrome blackT indicator Eriochrome blackT indicator

ExampleExample: Calcium, Cadmium, lead and mercury can be : Calcium, Cadmium, lead and mercury can be determined by this methoddetermined by this method

Assay of CALCIUM Assay of CALCIUM GLUCONATE TABLETSGLUCONATE TABLETS

CaCa++ ++ + Mg-EDTA → Ca-EDTA + Mg+ Mg-EDTA → Ca-EDTA + Mg++++

MgMg++ ++ + EDTA → Mg-EDTA+ EDTA → Mg-EDTA

Transfer an accurately weighed quantity of about 0.5 g of calcium Transfer an accurately weighed quantity of about 0.5 g of calcium gluconate to a 250 ml conical flask.gluconate to a 250 ml conical flask.

Add 50 ml water to dissolve it. Add 50 ml water to dissolve it.

Add 5 ml of magnesium sulphate (0.05 M) and 10 ml of strong Add 5 ml of magnesium sulphate (0.05 M) and 10 ml of strong ammonia-ammonium chloride solution.ammonia-ammonium chloride solution.

Titrate the mixture with disodium edetate solution (0.05M) using Titrate the mixture with disodium edetate solution (0.05M) using mordant black mixture as indicator to blue colour. mordant black mixture as indicator to blue colour.

Each ml of 0.05 M disodium edetate ≡ 0.02242 g of calcium Each ml of 0.05 M disodium edetate ≡ 0.02242 g of calcium gluconate. gluconate.

4. Alkalimetry titration of metals4. Alkalimetry titration of metals

It is clear from the metal-EDTA complex formation It is clear from the metal-EDTA complex formation reaction that protons from disodium edetate are reaction that protons from disodium edetate are liberated resulting in formation of acid.liberated resulting in formation of acid.

The liberated acid is titrated with standard alkali. The liberated acid is titrated with standard alkali. Either visual pH indicator or Potentiometric method Either visual pH indicator or Potentiometric method is adopted for detecting the endpoint. is adopted for detecting the endpoint.

That is when a solution of disodium EDTA, Na2H2Y That is when a solution of disodium EDTA, Na2H2Y is added to a solution containing metallic ions; is added to a solution containing metallic ions; complexes are formed with the liberation of two complexes are formed with the liberation of two equivalent hydrogen ions.equivalent hydrogen ions.

MnMn++ ++ HH22YY2-2-⇌ (MY⇌ (MY)(n-4))(n-4) + + 2H2H+ +

REFERENCESREFERENCES

The text book of Pharmaceutical The text book of Pharmaceutical Analysis by Kenneth A. Connors.Analysis by Kenneth A. Connors.

The text book of Pharmaceutical The text book of Pharmaceutical Analysis by P.Parimo.Analysis by P.Parimo.

The text book of quantitative The text book of quantitative chemical analysis by Vogel’s 5chemical analysis by Vogel’s 5 thth edition .edition .

Fundamentals of analytical chemistry Fundamentals of analytical chemistry 7th edition by Skoog.7th edition by Skoog.

Pharmaceutical drug analysis by Pharmaceutical drug analysis by Ashutosh kar. Ashutosh kar.