Iodometric titration of copper with sodium thiosulphate potentiometry
Answers
Iodometric determination of copper is based on the oxidation of iodides to iodine by copper (II) ions, which get reduced to Cu+.
Comparison of standard potentials for both half reactions (Cu2+/Cu+ E0=0.17 V, I2/I- E0=0.54 V) suggests that it is iodine that should be acting as oxidizer. However, that's not the case, as copper (I) iodide CuI is very weakly soluble (Ksp = 10-12). That means concentration of Cu+ in the solution is very low and the standard potential of the half reaction Cu2+/Cu+ in the presence of iodides is much higher (around 0.88 V).
In effect reaction taking place in the solution is
2Cu2+ + 4I- → 2CuI(s) + I2
And produced 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 addition of ammonia and acetic acid, effectively creating acetic buffer. Lab practice shows that the end point is sharper when we add some thiocyanate to the solution. Copper (I) thiocyanate is slightly less soluble than iodide, which makes concentration of Cu+ even lower, increasing the oxidation potential of the Cu2+/Cu+ system.
Solution should be free of other substances that can oxidize iodides to iodine (for example Fe3+ or nitrates).
Reaction
As it was already explained, first reaction taking place is:
2Cu2+ + 4I- → 2CuI(s) + I2
This is followed during titration by the reaction of the iodine with the thiosulfate:
2S2O32- + I2 → S4O62- + 2I-
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