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BACKGROUND
An assay is an investigation of an analyte with a proper procedure in medicines, pharmacology, environmental and molecular biology for qualitative and quantitative measurement of the presence of the amount, or functional activity of the targeted entity.
The assay is done on the basis of the oxidation-reduction reaction of iodine/thiosulphate. A solution of copper sulphate is first treated with potassium iodide and acetic acid. Cuprous iodide (CuI) is formed with iodine and the liberated iodine is titrated with 0.1 N sodium thiosulphate.
Aim: To perform the assay of copper sulphate.
Reaction:
Eq. wt. of sodium thio-sulphate (Na2S2O3. 5H2O): 248.18
Eq. wt. of copper sulphate (CuSO4. 5H2O): 249.68
i.e. Each ml of 0.1 N sodium thiosulfate is equivalent to 24.968 mg of CuSO4 · 5H2O.
Preparations:
1. Standard 0.1 N sodium thiosulphate solution-
24.8 g sodium thiosulfate pentahydrate (Na2S2O3.5H2O) or 15.8 g anhydrous sodium thiosulfate (Na2S2O3) is dissolved in 1 litre water. 0.01 g anhydrous sodium carbonate and 0.4 ml chloroform are added to stabilize the solution. It is mixed thoroughly by shaking or by aerating with nitrogen for about 15 min, and stored in a glass-stoppered, reagent bottle.
2. Starch Indicator Solution-
1 g of corn or potato starch is added into 10 ml of distilled water, well-shaken and poured into 100 ml of boiling distilled water. The solution is stirred thoroughly and boiled for 1 minute. Then it is left in air to cool down. If the precipitate forms, the supernatant is decanted and used as the indicator solution.
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REQUIREMENTS
Chemicals: Copper sulphate
sulfuric acid,
Potassium iodide,
Acetic acid,
Sodium thiosulphate,
Starch solution,
Sodium thiocyanate.
Apparatus: Volumetric flask,
Iodine flask
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PROCEDURE
An accurately weighed sample is dissolved in water with the aid of a small amount of dilute sulfuric acid and the volume is made up to 100 ml mark in volumetric flask. A solution equivalent to 1g is added into the iodine flask and treated with an excess of potassium iodide (3 g), 5ml of acetic acid is added and shaken vigorously. The liberated iodine is titrated with 0.1 N sodium thiosulphate solution till the dark iodine colour in the iodine flask changes to yellow. At this stage 1ml of freshly prepared starch solution (indicator) is added. A blue colour forms, and the titration is continued to faint blue colour. Towards the end of titration 2g of potassium thiocyanate is added, the contents are stirred, and the titration is continued until the blue colour gets disappeared. The procedure is repeated for a concordant reading. A blank determination is noted following the similar procedure and taking distilled water in place of test solution, to minimise the error.
Observation table
| No. of Observation | Initial Burette Reading (IBR) (ml) |
Final Burette Reading (IBR) (ml) |
Difference (ml) |
Mean (ml) |
A-B (ml) |
|
| For Test |
1 | A= | ||||
| 2 | ||||||
| 3 | ||||||
| For Blank |
1 | B= | ||||
| 2 | ||||||
| 3 |
Calculation
Wt. of pure copper sulphate = Volume of titrant × Equivalent Factor
= (A-B) × 24.968 mg = _______ (X)mg
% purity = (Wt. of pure copper sulphate × 100)/ Wt. of copper sulphate sample taken
= (X × 100)/ 1000 = ________
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CONCLUSION
From the above experimental procedure it is concluded that the percent of purity of the supplied sample is ______.
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REFERENCES
-
Indian Pharmacopoeia. 1996, 2016
-
Pharmaceutical Chemistry Inorganic, G.R Chatwal
- Available at: http://en.m.wikipedia.org/wiki/Assay&grqid=_OydgnEb&hl=en=IN.
