Complexometric Analysis: Assay Methods

ASSAY METHODS

The complexometric titrations involving various inorganic pharmaceutical substances may be catego-rized into three broad heads, namely :

(i) Direct titration methods,

(ii) Masking and demasking agents, and

(iii) Residual titration methods.

1. DIRECT TITRATION METHODS

In direct titration, usually an appropriate buffer solution and a suitable indicator are added to the M²⁺ (metal-ion) solution and subsequently the resulting solution is titrated with previously standardized disodium-EDTA until the indicator just changes colour. Sometimes, a simultaneous blank determination is also recommended to have a check for the presence of traces of metallic impurities in the reagents.

1.1. Preparation of 0.05 M Disodium Ethylenediamine Tetracetate Solution (Disodium Edetate 0.05 M)

Materials Required : Disodium ethylenediaminetetracetate : 18.6 g.

Procedure : Weigh accurately 18.6 g of disodium ethylenediaminetetracetae, dissolve in sufficient DW in a 1 litre volumetric flask and make up the volume upto the mark.

Calculations :

1.2. Standardization of 0.05 M Disodium Edetate Solution

Materials Required : Granulated zinc : 0.8 g ; dilute HCl (10% w/v of HCl) : 12.0 ml ; bromine water (3 ml Br₂ in 100 ml H₂O) : 5 ml ; sodium hydroxide (2 N) : 20.0 ml ; ammonia buffer (pH 10.0) (dissolve 5.4 g of NH₄Cl in 70 ml of 5 N ammonia and dilute with water to 100 ml) : 100 ml ; Mordant Black II mixture (mixture of 0.2 part Mordant Black II with 100 parts of NaCl) : 50 mg ; disodium edetate : 0.05 M.

Procedure : Weigh accurately about 0.8 g of granulated zinc, dissolve by gentle warming in 12 ml of dilute hydrochloric acid and 5 drops of bromine water. Boil to remove excess bromine, cool and add sufficient DW to produce 200 ml in a volumetric flask. Pipette 20 ml of the resulting solution into a flask and neutralize carefully with 2 N sodium hydroxide. Dilute to about 150 ml with DW, add to it sufficient ammonia buffer (pH 10.0) to dissolve the precipitate and add a further 5 ml quantity in excess. Finally add 50 mg of Mordant Black II mixture and titrate with the disodium edetate solution until the solution turns green. Each 0.003269 g of granulated zinc is equivalent to 1 ml of 0.05 M disodium ethylenediaminetetracetate.

Calculations :

1.3. Calcium Chloride

Materials Required : Calcium chloride dihydrate : 0.15 g ; dilute hydrochloric acid (10% w/w of HCl) : 3.0 ml ; 0.05 M disodium edetate ; sodium hydroxide solution (20% w/v in water) ; calcon mixture (a mixture of 1 part of calcon with 99 parts of freshly ignited anhydrous Na₂SO₄) : 0.1 g.

Equations :

Procedure : Weigh accurately about 0.15 g of calcium chloride dihydrate and dissolve it in 50 ml of DW. Titrate with 0.05 M disodium ethylenediamine tetracetate to within a few ml of the expected end point, add 8.0 ml of sodium hydroxide solution and 0.1 g of calcon mixture and continue the titration until the colour of the solution changes from pink to a full blue colour. Each ml of 0.05 M disodium ethylene disodium tetracetate is equivalent to 0.007351 g of CaCl₂ . 2H₂O.

1.4. Magnesium Sulphate

Materials Required : Magnesium sulphate heptahydrate : 0.3 g ; strong ammonia-ammonium chloride solution (6.75 g NH₄Cl in 74.0 ml strong ammonia solution add water q.s. to produce to 100 ml) ; 0.05 M disodium edetate ; Mordant Black II mixture (mixture of 0.2 part mordant black II with 100 parts of NaCl) : 0.1 g.

Equations :

The assay of MgSO₄.7H₂O is based upon the reactions designated by the following equations :

Procedure : Weigh accurately about 0.3 g of magnesium sulphate heptahydrate and dissolve in 50 ml of DW. Add to it 10 ml of strong ammonia-ammonium chloride solution, and titrate with 0.05 M disodium ethylenediaminetetracetate employing 0.1 g of mordant black II mixture as indicator, until the pink colour is discharged from the blue. Each ml of 0.05 M disodium ethylenediaminetetracetate is equivalent to 0.00602 g of MgSO₄.

1.5. Cognate Assays

A number of pharmaceutical inorganic substances may be assayed by the direct titration method using disodium ethylenediaminetetracetate. A few typical examples are cited in the following Table 9.2.

2. MASKING AND DEMASKING AGENTS

The disodium ethylenediaminetetracetate usually complexes with a wide spectrum of cations, which ultimately renders the selectivity of the titration procedure adversely, thereby providing enough scope for the accompanying metal impurities to be titrated along with the ion it is aimed at for actual estimation. Therefore, in a situation where one or two ions present in a mixture of cations is specifically required to be determined with a view to eliminate completely the possible effects of unwanted impurities that may enhance the titre value, a third substance is added, which is known as the Masking Agent. These agents must fulfil the follow-ing three requirements, namely :

(a) should act by precipitation,

(b) should form complexes that are definitely more stable than the interfering ion-edetate complex, and

(c) colour developed by either precipitates or auxiliary complexes should not obscure the end-point.

A few typical examples are cited below in Table 9.3 where masking has been accomplished by precipitation.

3. RESIDUAL TITRATION METHODS

Direct titration method offers a serious limitation for the assay of aluminium and bismuth containing pharmaceutical inorganic substances because of the precipitation of the metal as their corresponding hydroxides in alkaline media thereby introducing undesirable errors.

In actual practice, an excess of the standard solution of disodium edetate is added to the sample, pH is adequately adjusted for the residual titration with a metal-ion solution e.g., ZnSO₄ and employing an appropriate indicator which is sensitive enough to the respective titrant. However, the metal ion under estimation remains firmly complexed with the EDTA and offers little interference with the Zn-EDTA complex formed. It has been established experimentally that bismuth readily yields a highly stable complex which may be titrated conveniently between pH 1 and 2. Bismuth forms a stable complex by reacting with EDTA quantitatively at pH 4.0 and, therefore, dithizone is employed as an indicator to detect the end-point for it has a transition state of colour at pH 4.6.

3.1. Potassium Alum, KAl(SO₄)₂, 12H₂O

Materials Required : Potassium alum : 1.7 g ; 0.05 M disodium edetate : 30.0 ml ; hexamine : 1.0 g ; 0.05 M lead nitrate ; xylenol orange solution (0.1% w/v in water) : 0.4 ml.

Theory : The solution of potassium alum is heated with an excess of disodium edetate to ensure complete formation of aluminium-edetate complex. Hexamine serves as a buffer thereby stabilizing the pH between 5 and 6, the ideal pH for the titration of the disodium edetate not required by the Al with 0.05 M lead nitrate employing xylenol orange as indicator. The various reactions involved may be represented by the following equations :

Procedure : Weigh accurately 1.7 g of potassium alum and dissolve it in suffcient DW in a flask. Heat the contents of flask over a water-bath for 10 minutes to allow completion of complexation and cool to ambient temperature. Now, add 1 g hexamine to act as buffer and titrate with 0.05 M lead nitrate employing 0.4 ml of xylenol orange solution as an indicator. The colour shall change from that of the indicator (yellow at the pH of the titration) to the corresponding reddish purple, the colour of the lead complex of the indicator. Each ml of 0.05 M disodium edetate is equivalent to 0.02372 g of KAl(SO₄)₂, 12H₂O.

3.2. Glycobiarsol [Bismethyl-N-glycolyl-arsanilate]

Materials Required : Glycobiarsol : 0.2 g ; 0.05 M disodium edetate : 10.0 ml ; acetic acid-ammo-nium acetate buffer (mix 13.6 g of sodium acetate and 7.7 g of ammonium acetate in water to make 100 ml. Add 25.0 ml of glacial acetic acid and mix) : 10.0 ml ; alcohol : 25.0 ml : dithizone solution (0.05% w/v in chloroform) : 2.0 ml ; 0.025 M ZnSO₄ solution.

Procedure : Weigh accurately 0.20 g of glycobiarsol into a 250-ml conical flask and add 10.0 ml of 0.05 M disodium edetate. Warm the contents of the flask over a water-bath until glycobiarsol gets dissolved completely and then cool the contents to the room temperature (25°C). Add to it 10.0 ml of acetic acid-ammonium acetate buffer, 25.00 ml of alcohol and 2 ml of dithizone solution as an indicator. Titrate the excess of disodium edetate with 0.025 M zinc sulphate until the resulting solution turns rose pink in colour. Each millilitre of 0.05 M disodium edetate consumed is equivalent to 10.45 mg of Bi.

Note : The content of Bi, calculated on dried basis, lies between 38 to 42.5%.

3.3. Cognate Assays

A number of inorganic pharmaceutical substances may be assayed by adopting the residual titration method as depicted in Table 9.4.