The amount of copper present in the supplied sample can be determined by the following way:
Prepare a standard solution of potassium dichromate and standardize the supplied thiosulphate solution in the usual manner.
Dilute the supplied sample to 250 mL in a volumetric flask and shake well (B). Take 25 mL of the previously diluted solution (B) with a pipette and deliver into a 250 mL conical flask. Neutralize the free mineral acid with dilute Na2CO3 solution by adding drop-wise until a faint permanent precipitate remains. Dissolve the precipitate with dilute acetic acid, and then add 1 mL in excess. Add 1-2 g of solid KI (or 15 mL of 10% KI solution), shake and keep in dark place for about 5 minutes. Titrate with the standardized sodium thiosulphate solution until the brown color of the iodine fades. Then add 1 mL of starch solution and continue the addition of the thiosulphate solution until the blue color commences to fade. Finally add about 1 gram of KCNS or NH4CNS and complete the titration as quickly as possible.
1 mL 1M Na2S2O3 ≡ 0.06357 g Cu