Preparation and Analysis of Sodium Copper I Thiosulphate

When copper sulphate solution reacts with sodium thiosulphate, copper(II) is reduced to 

Cu(I) and its thiosulphate crystallises out with the sodium thiosulphate. The crystals can 

Dissolve 10 g of copper(II) sulphate pentahydrate in SO cm warm water and dissolve 18 g of sodium thiosulphate pentahydrate in 30 cm of warm water. Heat each solution to 40°C. Stir the copper solution well while adding the solution of thiosulphate. Allow the mixture to settle so that complete precipitation of the yellow crystals takes place in about an hour. Filter the crystals under suction and wash them with distilled water and 

Weigh out accurately about 0.5 g of your preparation and dissolve in 50 cm water in a glass stoppered bottle. Add a few cm of acetic acid and 2 cm of freshly prepared starch solution. Run from a burette 0.05 M iodine solution until a deep blue colour persists. Note the titre and then titrate the excess iodine with 0.02 M standardised sodium thiosulphate until the blue colour has been just discharged. In this case all the thiosulphate of the compound had reacted witfi iodine whereas copper remains as Cul precipitate. 

For the determination of copper, weigh out accurately about 0.5 g of your preparation into a conical flask. Add 30 cm dilute sulphuric acid and 2.5 g ammonium persulphate and shake well to dissolve the solids. Heat the solution gradually to the boiling point. Continue until all the copper appears as the blue copper(II) sulphate disregarding a small residue of sulphur which floats on the surface. Continue boiling for 15 minutes to decompose any remaining persulphate. Filter into a clean conical flask and wash on the filter with water. Add to the filtrate concentrated ammonia until the deep blue copper(lI) complex is formed. Ttien add dilute acetic acid to restore the pale blue colour of aqueous Cu(II) ions and then add an equal volume of the acid already added. Dissolve 2 g of KI and titrate the liberated iodine with standardised 0.05 M sodium diiosulphate until the solution is straw yellow. Dilute with 100 cm water, add 2 cm ficshly prepared starch solution and continue titration. When the blue colour fades add 1 g of purest ammonium thiocyanate and continue titration rapidly until the blue colour disappears for 1 minutes. The pale flesh-coloured Cul remains as a precipitate. Calculate the molar ratio Cu thiosulphate. 

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