Dichrometry analysis first semester analysis PDF

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DICHROMETRY Titration involving potassium dichromate as the oxidizing agent is called dichrometry. The method is based on the oxidation of dichromate ion. The oxidizing action is due to the conversion of dichromate ion containing sexivalent chromium to Cr3+ Cr2O72- + 14H+ + 6e-

2Cr3+ + 7H2O

Since this conversion requires H+ ions, the reaction take place in acidic medium. Advantages of potassium dichromate over potassium permanganate 1. Easy to prepare the chemically pure substance where as potassium permanganate is not chemically pure due to the presence of MnO2 and other reduce products, so exact weighing is not possible. But dichromate solution can be prepared by exact weighing. 2. Potassium permanganate is not a stable solution. It decomposes in the presence of light, acid etc, whereas potassium dichromate when kept in closed vessel is highly stable. It doesnot decompose even when the acidified solution is boiled 3. Potassium dichromate is used as primary standard because it is available in the pure form, thermally stable, has definite shelf life and is less expensive. A disadvantage of potassium dichromate as an oxidizing agent is that the titration give rise to Cr3+ ions which imparts green colour to the solution, so it is difficult to detect the end point. The usual indicators used are N-phenyl anthranilic acid, diphenylamine sulfonate, sodium or barium salt of diphenyl amine sulphonic acid is more suitable because it is more soluble than diphenyl amine in water and gives a sharp colour change from colourless-green- red violet. Applications 1. Determination of iron in iron ore Ore is dissolved in hydrochloric acid where iron (III) is reduced to iron (II) and the solution is titrated with standard dichromate solution. Cr2O72+ + 6Fe2+ + 14H+

2Cr3+ + 6Fe3+ + 7H2O

2. Determination of chromium in chromium (III) salt 3. Chemical oxygen demand can be determined by dichrometry Preparation of potassium dichromate (0.0167M) Weigh 4.9 g of potassium dichromate, previously powdered and dried in a desiccator for 4 hours, and dissolve in sufficient water to produce 1000 ml.

Standardisation of 0.0167 M potassium dichromate To 20.0 ml of the solution add 1 g of potassium iodide and 7 ml of 2M hydrochloric acid. Add 250 ml of water and titrate with 0.1 M sodium thiosulphate, using 3 ml of starch solution, added towards the end point of the titration, as indicator until the colour changes from blue to light green. 1ml of 0.1 M sodium thiosulphate is equivalent to 0.0049 g of K2Cr2O7 References 1. Pharmaceutical Analysis-I by Dr Uttam Singh Baghel, Dr Pragi Arora, page no: 79 2. IP Volume 1 2010, page no: 633 3. Basics of Drug Analysis by G Vidya Sagar, page no: 213-214...