Assay of Copper sulphate by Iodometry
Pharmaceutical Analysis I Practical: Limit test for (1) Chloride (2) Sulphate (3) Iron (4) Arsenic Preparation and standardisation of (1) Sodium hydroxide (2) Sulphuric acid (3) Sodium thiosulfate (4) Potassium permanganate (5) Ceric ammonium sulphate Assay (1) Ammonium chloride (2) Ferrous sulphate(3) Copper sulphate (4) Calcium gluconate (5) Hydrogen peroxide (6) Sodium benzoate (7) Sodium Chloride Determination of Normality (1) Conductometric titration of strong acid against a strong base (2) Conductometric titration of strong acid and weak acid against a strong base (3) Potentiometric titration
Aim
To determine the percent purity of copper sulfate in a given sample
Principle
This is example of Iodometry. The determination of copper compounds (copper sulphate) depends on the instability of cupric iodide formed by the reaction between the copper sulphate and potassium iodide in the presence of Acetic acid. The unstable cupric iodide is decomposed to cuprous iodide (a nearly white precipitate) and free Iodine. This reaction is reversible, since cuprous iodide is slightly soluble, the backward reaction may be promoted. The liberated iodine can be titrated with standard sodium thiosulphate solution, using starch mucilage as indicator. Towards almost the end of the titration when the blue colour is still there, a small quantity of potassium thiocyanate is added. This converts the small quantity of cuprous iodide into cuprous thiocyanate which is sparingly soluble to prevent the backward reaction. The titration is continued till the blue colour is discharged. Potassium thiocyanate is added only at the end of the titration, as otherwise Iodine will be adsorbed by the cuprous thiocyanate
Standard
Copper sulfate contains not less than 98% and not more than 105 % of CuSO4.5H2O
Procedure
Accurately weigh the sample about 1 gm into an Iodine flask.
Add 50 ml of water to dissolve and add a little sodium carbonate until turbid to remove any free mineral acid.
Add acetic acid to clear the solution and then 5 ml in excess.
Add 3 gm of potassium iodide, dilute to about 50 ml with water and titrate the liberated Iodine with 0.1 M sodium thiosulfate solution. During the titration, the brown color of the Iodine becomes less intense. When a pale yellowish color has become obtained add 1 ml of starch mucilage. The solution should now be blue.
Continue the titration drop by drop and when near the endpoint add 2 gm of potassium thiocyanate and shake well.
Complete the titration, shaking well and allowing 10 seconds between each drop, because the endpoint is rather slow.
The endpoint is white or flesh color and 1 drop of 0.1 M sodium thiosulphate should result in a change from the blue color to this endpoint
Reference
Indian Pharmacopoeia 2014; Vol. 1; Pg. No.313
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