Sulphate titration

Procedure. Weigh out accurately about 10 g of the salt and dissolve it in 250 mL of water in a graduated flask. Pipette 25 mL of this solution into a 250 mL conical flask, add about 20 mL of 10 per cent potassium iodide solution, 2 mL of 1M sulphuric acid, and 15 mL of a solution containing 2.0 g crystallised zinc sulphate. Titrate the liberated iodine immediately with standard 0.1M sodium thiosulphate and starch add the starch solution (2 mL) after the colour has faded to a pale yellow. The titration is complete when the blue colour has just... 

PERMANGANATE, DICHROMATE AND CERIC SULPHATE TITRATION METHODS... 

A number of pharmaceutical substances and dosage forms may be determined by the help of ceric sulphate titration methods as given in Table 6.1. 

Table 6.1 Redox Titrations Ceric Sulphate Titration Method...

Discuss the various theoretical aspects involved in the assay of permanganate, dichromate and ceric sulphate titration methods. Give equations to explain your logical stand. 

When the alkali concentration is 0.5-1.0M, dithionite can be titrated with standard hexacyanoferrate(IIl) solution. Because of the possibility of oxidation in air, the titration vessel should be provided with an inlet and an outlet for purging and maintaining an oxygen-free nitrogen atmosphere. Under these conditions, the dithionite is oxidised to sulphite. When the alkali concentration is raised to 4-4.5 M and the temperature is raised to 50°-60°C, the sulphite thus produced or already present together with the thiosulphate, the anions are oxidised by the iron(IIl) complex to sulphate. Titration is carried out in presence of osmium tetroxide as a catalyst. A second titration with iodine in acid solution oxidises the thiosulphate to tetrathionate. This can be titrated with the iron(III) complex at 50°C when the alkali concentration is 5 M. The end-point could be detected potentiometrically or by the dead-stop set up. 

Ammonia may be estimated by dissolving the gas in a known volume of standard acid and then back-titrating the excess acid. In a method widely used for the determination of basic nitrogen in organic substances (the Kjeldahl method), the nitrogenous material is converted into ammonium sulphate by heating with concentrated sulphuric acid. The ammonia is then driven off by the action of alkali and absorbed in standard acid. 

Other examples are the use of osmium(VIII) oxide (osmium tetroxide) as catalyst in the titration of solutions of arsenic(III) oxide with cerium(IV) sulphate solution, and the use of molybdate(VI) ions to catalyse the formation of iodine by the reaction of iodide ions with hydrogen peroxide. Certain reactions of various organic compounds are catalysed by several naturally occurring proteins known as enzymes. 

Fill a 250 mL separatory funnel with ca 0.25M sodium sulphate solution. Allow this solution to drip into the column at a rate of about 2 mL per minute, and collect the effluent in a 500 mL conical flask. When all the solution has passed through the column, titrate the effluent with standard 0.1 M sodium hydroxide using phenolphthalein as indicator. 

B. Back-titration. Many metals cannot, for various reasons, be titrated directly thus they may precipitate from the solution in the pH range necessary for the titration, or they may form inert complexes, or a suitable metal indicator is not available. In such cases an excess of standard EDTA solution is added, the resulting solution is buffered to the desired pH, and the excess of the EDTA is back-titrated with a standard metal ion solution a solution of zinc chloride or sulphate or of magnesium chloride or sulphate is often used for this purpose. The end point is detected with the aid of the metal indicator which responds to the zinc or magnesium ions introduced in the back-tit ration. 

Sulphate may be determined by precipitation as barium sulphate or as lead sulphate. The precipitate is dissolved in an excess of standard EDTA solution, and the excess of EDTA is back-titrated with a standard magnesium or zinc solution using solochrome black as indicator. 

The dyestuff is thoroughly mixed with 100 times its weight of sodium sulphate, and 1 g of the mixture is used in each titration. The indicator is not very stable in alkaline solution. 

Solutions of EDTA of the following concentrations are suitable for most experimental work 0.1M, 0.05M, and 0.01 M. These contain respectively 37.224 g, 18.612g, and 3.7224 g of the dihydrate per litre of solution. As already indicated, the dry analytical grade salt cannot be regarded as a primary standard and the solution must be standardised this can be done by titration of nearly neutralised zinc chloride or zinc sulphate solution prepared from a known weight of zinc pellets, or by titration with a solution made from specially dried lead nitrate. 

Pipette 25 mL of an aluminium ion solution (approximately 0.01 M) into a conical flask and from a burette add a slight excess of 0.01 M EDTA solution adjust the pH to between 7 and 8 by the addition of ammonia solution (test drops on phenol red paper or use a pH meter). Boil the solution for a few minutes to ensure complete complexation of the aluminium cool to room temperature and adjust the pH to 7-8. Add 50 mg of solochrome black/potassium nitrate mixture [see Section 10.50(C)] and titrate rapidly with standard 0.01 M zinc sulphate solution until the colour changes from blue to wine red. 

Every millilitre difference between the volume of 0.01 M EDTA added and the 0.01 M zinc sulphate solution used in the back-titration corresponds to 0.2698 mg of aluminium. 

Palladium(II) compounds can be determined by a similar procedure, but in this case, after addition of the cyanonickelate, excess of standard (0.01 M) EDTA solution is added, and the excess is back-titrated with standard (0.01 M) manganese(II) sulphate solution using solochrome black indicator. 

Procedure. Prepare a manganese(II) sulphate solution (approx. 0.05M) by dissolving 11.15 g of the analytical-grade solid in 1 L of de-ionised water standardise the solution by titration with 0.05 M EDTA solution using solochrome black indicator after the addition of 0.25 g of hydroxylammonium chloride — see below. 

After the end point, add 2.5 g of sodium fluoride, stir (or agitate) for 1 minute. Now introduce the standard manganese (II) sulphate solution from a burette in 1 mL portions until a permanent red colour is obtained note the exact volume added. Stir for 1 minute. Titrate the excess of manganese ion with EDTA until the colour changes to pure blue. 

The sulphate is precipitated as barium sulphate from acid solution, the precipitate is filtered off and dissolved in a measured excess of standard EDTA solution in the presence of aqueous ammonia. The excess of EDTA is then titrated with standard magnesium chloride solution using solochrome black as indicator. 

The iron(III) indicator solution consists of a cold, saturated solution of ammonium iron(III) sulphate in water (about 40 per cent) to which a few drops of 6M nitric acid have been added. One millilitre of this solution is employed for each titration. 

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