Answer to Question #274618 in Inorganic Chemistry for Sohan

Question

Estimation of copper in the supplied solution using standardized sodium thiosulphate solution. Discussion about this.

Answer 1

Sodium thiosulphate makes for an inexpensive, safe and effective reducing agent to react with I2.

Since starch turns to a dark blue in the presence of I2, then back to white when the I2 is depleted, starch makes a good indicator for the endpoint of this reaction.

The oxidation reaction of iodides to iodine by copper(II) ions is most effective under slightly acidic conditions, with a pH of around 4-5. This is the reason for the addition of acetic acid and sodium carbonate. Whenever a procedure calls for the addition of a weak acid and a weak base, there is a pretty good chance that the purpose is to form a buffer solution at some desired pH. At least in this case, that is exactly the purpose. You will notice in the reference above that they use ammonia and acetic acid instead. Sodium carbonate is preferred in some situations for safety reasons.