Question

When the cations are separated into subgroups what is the purpose of potassium hydroxide?

Answer 1

II(A) cations

In the systematic separation of cations we perform successive separation of group cations with the use of group reagent. Group reagents react with corresponding group cations and convert them into insoluble salts like chlorides, sulphides, and carbonates. First, we prepare a solution of the given mixture, then we add group reagent of Ist group which converts the cations of Ist group into insoluble chlorides and separate them as precipitate and then we test for IInd group cations in the filtrate of Ist group.

You have learnt in previous post that group reagent of 2nd group is Hydrogen sulphide H2S (gas or saturated aqueous solution).  Here you will see how cations get precipitated by common ion effect. Hydrogen sulphide is a weak acid, it dissociates partly. Hydrochloric acid, which we added initially, is a strong acid and dissociates completely.

H2S ⇌ 2H+ + S2-

HCl ⟶ H+ + Cl-

Here H+ is a common ion among H2S and HCl. Due to complete dissociation of HCl concentration of H+increases in the solution which shifts equilibrium of reaction 1 backwards. This results in precipitation of cations in the form of sulphides.

H2S ⟶ 2H+ + S2-

Take the filtrate in a boiling test tube and heat till it is nearly boiling and then pass H2S gas under pressure in excess (30 seconds -1 min). If 2nd group cations are present, you will get coloured precipitate of sulphides.

Black precipitate: Mercury(II) sulphide HgS, lead(II) sulphide PbS, copper(II) sulphide CuS.

Brown precipitate: Bismuth(III) sulphide Bi2S3, tin(II) sulphide SnS.

Yellow precipitate: Cadmium(II) sulphide CdS, arsenic(III) sulphide As2S3, arsenic(V) sulphide As2S5, tin(IV) sulphide SnS2.

Orange precipitate: Antimony(III) sulphide Sb2S3, antimony(V) sulphide Sb2S5.

Filter the precipitate and wash with dilute hydrochloric acid HCl. The precipitate may contain IIA or IIB or both cations. To differentiate them add an excess of (5ml) yellow ammonium polysulphide (NH4)2Sx solution and heat to 50-60°C for 3-4 minutes with constant stirring. sulphides of sub group IIA (Cu sub group) are insoluble in (NH4)2Sx while sulphides of sub group IIB (As sub group) dissolve in it by forming thiosalts. First we will test IIA group in the precipitate and preserve the filtrate for the test of IIB group.