SEPARATION AND ESTIMATION OF TWO METAL IONS
COPPER-NICKEL
Estimation of both by gravimetric method:
Principle: In a mixed solution of both, copper is estimated first as cuprous thiocyanate
(CuSCN) and then in the filtrate, nickel is estimated as nickel dimethyl glyoximate
[Ni(HDMG)2]. Copper is precipitated as cuprous thiocyanate from a mixed solution of copper
and nickel by adding ammonium thiocyanate solution in slightly acidic medium in the presence
of reducing agent such as sulphurous acid to reduce Cu (II) to Cu (1)
After removing copper, dimethyl glyoxime reagent (1% solution of dimethylglyoxime in ethanol) is added to hot slightly acidic filtrate followed by slight excess of ammonia solution. Scarlet red coloured precipitate of nickel dimethylglyoximate is formed.
Chemical required: 50 mL mixed solution of copper sulphate and Nickel ammonium sulphate. (25 mL copper sulphate (16 g/L) and 25 mL nickel ammonium sulphate (11 g/L) 10% Ammonium thiocyanate solution.
•Saturated sulphurous acid solution. .1% solution of dimethylglyoxime in ethanol
1:1 ammonia solution. dil. HCl, aquaregia.
Procedure for copper estimation: Take 50 ml mixture solution in 400 mL beaker. Add 1-3 mL of dil. HCl and then a slight excess of (-20 mL) freshly prepared saturated sulphurous acid solution. Dilute this solution to about 150 mL. Heat it to boiling and add freshly prepared 10% ammonium thiocyanate solution slowly with constant stirring till complete precipitation occurs. Allow the white precipitate of cuprous thiocyanate to settle. Test for complete precipitation by adding ammonium thiocyanate solution
to the clear supernatant liquid by means of glass rod. Allow the precipitate to settle for 2-3 hrs. Filter the precipitate through a weighed G4 sintered glass crucible, wash the ppt 2-3 times with cold and dilute ammonium thiocyanate solution (1 mL of 10% ammonium thiocyanate solution diluted to 100 ml and added with 5-10 drops of sulphurous acid solution). Finally wash the precipitate with cold
Keep reserve the filtrate and washings for the gravimetric estimation of nickel.
Calculations:
(a) Weight of sintered glass crucible = W18-
(b) Weight of sintered glass crucible + precipitate=W28-
Precautions: A large excess of the precipitant (ammonium thiocyanate) should be avoided, since CuSCN is soluble in its excess forming a complex thiocyanate ion. The solution also must be free form any oxidising agent like HNO, etc.
Estimation of Nickel
glyoxime in
ia solution.
2H
Transfer the filtrate and washings to a beaker and evaporate the solution to about 100 mL. Heat this solution with about 20-25 mL aquaregia to almost dryness on a sand bath. This will destroy the excess of thiocyanate (added in excess during copper estimation). To this add 100 mL of distilled water and 1 mL of dil. HCl. Heat the solution to about 70°C, add slight excess of 1% dimethylglyoxime reagent followed by addition of dilute ammonia solution dropwise with constant stirring till the solution becomes ammoniacal. Digest the precipitate on water bath for kel ammonium about 30 minutes. Test the supernatant liquid for complete precipitation. Filter the precipitate through a previously weighed sintered crucible (G4). Wash the precipitate with hot water until the washings sare free from chloride ions. Dry it in oven at 110-120°C, cool in a disiccator and weigh. Repeat the heating, cooling and weighing till a constant weight is obtained
Calculations:
(a) Weight of sintered glass crucible = W₁ g
(b) Weight of crucible + precipitate = W₂ 8-
