Qualitative analysis of Group II(B) cations

  

In the last post we separated group IIB cations from IIA cations by using yellow ammonium polysulphide (NH₄)₂S_x solution, which dissolves the IIB cations. By doing this we obtained them in the form of filtrate. In the filtrate, the cations are in the form of thiosalts (NH₄)₃AsS₄, (NH₄)₂SbS₄, and (NH₄)₂SnS₃ and for further analysis we have to dissociate the salts, remove excess of sulphur and re-precipitate the cations.

How to re-precipitate IIB cations: Add hydrochloric acid HCl to the filtrate until it becomes slightly acidic (test with litmus paper, acid turns blue litmus into red) then warm it and stir for 1-2minutes. It dissociates thiosalts and precipitates excess of sulphur in the form of fine white or yellow precipitate. If a yellow or orange flocculant (colloidal) precipitate is obtained, it may contain As₂S₅, As₂S₃, Sb₂S₅ and sulphur S. Wash the precipitate with a little hydrogen sulphide H₂S water and reject washing.

2AsS₄^3- + 6H⁺ ⟶ As₂S₅ ↓ + 3H₂S ↑

2SbS₄^3- + 6H⁺ ⟶ Sb₂S₅ ↓ + 3H₂S ↑

SnS₃^2- +2H⁺ ⟶ SnS₂ ↓ + H₂S ↑

Separation of Arsenic: Sulphides of arsenic are insoluble in concentrated hydrochloric acid HCl while sulphides of antimony and tin are soluble in it by forming chlorides. Transfer the precipitate in a conical flask and add 5-10ml concentrated hydrochloric acid HCl then place a funnel in the mouth of flask. Boil gently for 5 minutes. Sulphides of antimony and tin will dissolve and you will get arsenic sulphide as residue. Some of the arsenic may have dissolved in it, to re-precipitate it add 2-3ml water and pass hydrogen sulphide H₂S gas for 1 minute. Keep the filtrate to test for antimony and tin.

Tests for arsenic: Arsenic is found in two states As(III) and As(V). In strongly acidic medium As(III) is stable and in strongly alkaline medium As(V) predominates. As(V) exists in solutions as AsO₄^3- ion.

Dissolve the precipitate in 3-4ml warm ammonia NH₃ solution. Add 3-4ml 3% hydrogen peroxide H₂O₂ solution and warm for a few minutes, it oxidizes arsenite As(III) to arsenate As(V). Add 1-2ml of magnesium nitrate reagent (a solution of Mg(NO₃)₂, NH₄Cl and a little NH₃). A whilte precipitate of magnesium ammonium asenate Mg(NH₄)AsO₄.6H₂O is obtained.

            AsO₄^3- + Mg²⁺  + NH₄⁺ ⟶ Mg(NH₄)AsO₄↓

Filter off this white precipitate and add 1ml silver nitrate AgNO₃ solution containing 6-7 drops of 2M acetic acid CH₃COOH. Red precipitate of silver arsenate Ag₃AsO₄ is formed.

            Mg(NH₄)AsO₄ ↓ + 3Ag⁺ ⟶ Ag₃AsO₄ ↓ + Mg²⁺  + NH₄⁺

Scheme for the separation of II^nd group cations

Test for Sb and Sn: Take the filtrate to test for Sb and Sn. Boil to expel H₂S gas then cool it. Divide the solution into three parts. In two parts we will test for Sb and in third part for Sn.

Part 1: add 2M ammonia NH₃ solution to make it just alkaline. Ignore any slight precipitation and add 1-2g solid oxalic acid (COOH)₂, boil and pass H₂S gas for 1 minute into hot solution. Orange precipitate of Sb₂S₃ is obtained.

Part 2: place 1ml solution in a spot plate and add a minute crystal of sodium nitrite NaNO₂ to oxidise Sb(III) to Sb(V) state. Add 1ml of Rhodamine-B reagent (tetraethylrhodamine). A bright red colour of reagent is changed to violet.

Part 3: Partly neutralise the solution with 2M ammonia NH₃ solution. Take 1m solution add 1cm clean iron wire, warm gently to reduce Sn(IV) to Sn(II).

Sn⁴⁺ + Fe ⟶ Fe²⁺  + Sn²⁺

Filter it and pour into a solution of mercury(II) chloride. Tin reduces Hg(II) to Hg(I) state and you will obtain a white precipitate of mercury(I) chloride Hg₂Cl₂.

Sn²⁺  + 2HgCl₂  ⟶ Hg₂Cl₂ ↓ + Sn⁴⁺ + 2Cl^-

If Sn²⁺ ions are in excess, the precipitate turns grey on warming, because of further reduction of Hg(I) ion to Hg metal.

Sn²⁺  + Hg₂Cl₂ ↓  ⟶ Hg↓ + Sn⁴⁺ + 2Cl^-

In the next post we will discuss identifying tests for 3^rd group cations in the filtrate you got after the removal of precipitate of 2^nd group cations. 

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