Aim: To perform the assay of copper sulphate by iodometry.
Requirements:
- Apparatus: Beaker, Funnel, Pipette, Burrette.
- Chemicals: Copper sulphate.
Theory: Iodometric determination of copper has based on the oxidation of iodides to iodine by copper (II) ions, which get reduced to Cu+.
A comparison of standard potentials for both half-reactions (Cu2+ /Cu+ Eo = 0.17 V, I2/I− Eo = 0.54 V) suggests that it is iodine that should be acting as an oxidizer. However, that is not the case, as copper (I) iodide CuI is very weakly soluble (Ksp = 10−12). That means the concentration of Cu+ in the solution is very low and the standard potential of the half-reaction Cu2+ /Cu+ in the presence of iodide is much higher (around 0.88 V).
In effect reaction taking place in the solution is
2Cu2+ + 4I− → 2CuI(s) + I2
and produced an equivalent amount of iodine can be titrated with thiosulfate solution. For the best results reaction should take place in the slightly acidic solution (pH around 4-5), correct pH is obtained by the addition of ammonia and acetic acid, effectively creating an acetic buffer. Lab practice shows that the endpoint is sharper when we add some thiocyanate to the solution. Copper (I) thiocyanate is slightly less soluble than iodide, which makes the concentration of Cu+ even lower, increasing the oxidation potential of the Cu2+ /Cu+ system.
The solution should be free of other substances that can oxidize iodides to iodine (for example Fe3+ or nitrates).
Procedure:
(A) Standardisation: 0.1 M sodium thiosulphate: Preparation of 0.1 M sodium thiosulphate: Dissolve 25 g of sodium thiosulphate and 0.2 g of sodium carbonate in carbon dioxide-free water and dilute to 1000 ml with the same solvent.
Procedure: Dissolve 0.200 g of potassium bromate, weighed accurately, in sufficient water to produce 250.0 ml. To 50.0 ml of this solution add 2 g of potassium iodide and 3 ml of 2 M hydrochloric acid and titrate with the sodium thiosulphate solution using the starch solution, added towards the end of the titration, as an indicator until the blue color is discharged.
1 ml of 0.1 M sodium thiosulphate is equivalent to 0.002784 g of KBrO3·
(B) Assay: Pipette aliquot containing copper (II) into 250 ml conical flask with a glass stopper. Add concentrated ammonia till the solution turns dark blue. Add concentrated acetic acid till the solution loses dark blue color, and then about 3 ml. Add 2 g of solid potassium iodide, swirl well. Put the stopper on the flask and put the solution in a dark place for 5 minutes.
Titrate swirling the flask, until a pale-yellow color. Add 5 ml of the starch solution. Add 1 g of potassium thiocyanate. Titrate swirling the flask, until the blue color disappears.
Observation table: (Standardization)
| Sr. No. | Parameters | Reading |
| 1. | Burette solution | Sodium thiosulphate solution |
| 2. | Conical flask solution | Potassium iodide, potassium bromate, HCl |
| 3. | Indicator | Starch solution |
| 4. | Endpoint | Blue to colorless |
| Burette reading B.R (ml) | Trial 1 | Trial 2 | Trial 3 | Average (BR) |
Observation table: (Assay)
| Sr. No. | Parameters | Reading |
| 1. | Burette solution | Sodium thiosulphate |
| 2. | Conical flask solution | Copper sulfate, potassium iodide |
| 3. | Indicator | Starch solution |
| 4. | Endpoint | Blue to colorless |
| Burette reading B.R (ml) | Trial 1 | Trial 2 | Trial 3 | Average (BR) |
Result: The % purity of copper sulphate is …….
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