Estimation of Copper as Copper (1) Thiocyanate Gravimetry
Determination of the amount of copper in a solution of Copper(II)sulphate.
This can be achieved by precipitating copper as copper(I)thiocyanate and massing the precipitated copper(I)thiocyanate, hence this is a gravimetric estimation.
2CuSO4 + 2NH4CNS + 2H2O + SO2 => 2CuCNS + (NH4) 2SO4 + H2SO4
Transfer the given copper(II)sulphate solution to a 100 mL volumetric flask, 20 mL of this solution is pipetted into a 250 mL Erlenmeyer flask. Add about 5 mL of 1 M H2SO4, followed by about 25 to 30 mL of H2SO3 which has been freshly prepared, by passing SO2 though distilled water. This solution is diluted to about 150 mL and heated to boiling. Add about 10 mL of 10 % NH4CNS in small quantities to the warm solution with constant stirring. The NH4CNS should be in excess. As the limiting reagent is the Copper(II) ions.
When the copper(I)thiocyanate is precipitated the solution should be colourless, if not use more ammonium thiocyanate, at this time the solution still should have the odour of sulfur dioxide.
After 24 hours the solution is filtered through a previously washed cleaned, dried and weighed sintered glass crucible (IG-4). The precipitate is washed 10 to 15 times. Add 1 mL of 10 % ammoniumsulphocyanide (NH4CNS) and 5 drops of saturated sulphurous acid to 100 mL of distilled water, cool it and use this liquid for washing the precipitate. Once this is done the final washing is done several times using 20% ethanol to remove the ammonium thiocyanate. The precipitate is dried at 110 to 120 oC for an hour, the precipitate is cooled and massed as CuCNS. The process of heating cooling and weighing until the mass remains constant.
Calculation
Mass of CuCNS = x grams
Mass 1 mole of CuCNS = 121.54g/mol
1 Mole of CuCNS contains 63.54 grams of copper
‘y’ = Mass of copper = 63.54/121.54 * (x mass of CuCNS)
Since we had taken 20 mL of the copper ion solution the total mass of copper(II) present in the solution provided is y*5
Things to note
H2SO3 is used to reduce Cu2+ to Cu+
Keep the pH low to reduce the solubility of the precipitate
Excess ammonium thiocyanate would help in complexing copper ions so should be avoided