The formation of complex ions is very useful in the systematic analysis of inorganic anions.
1. Separation of silver and mercury from each other: Silver ions are precipitated as white silver chloride precipitate which is soluble in ammonia due to the formation of silver ammonia complex.
\(Ag^+ + Cl^- \longrightarrow Ag Cl \downarrow\) ; \(Ag Cl + 2NH_3 \longrightarrow [Ag(NH_3)_2]Cl\)
The silver ammonia complex is soluble in water, owing to which it is separated from Hg2Cl2. It decomposes easily on treatment with dilute nitric acid in excess into silver chloride back.
\([Ag (NH_3)_2]Cl + 2HNO_3 \longrightarrow AgCl \downarrow +2NH_4 NO _3\)
On the other hand, the white precipitate of Hg2Cl2 which is formed by the addition of HCl to mercury salt turns into black complex compound treatment with ammonia.
\(Hg_2 Cl_2 + 2 NH_3 \longrightarrow \underset {Black}{[NH_2 - Hg - Cl + Hg]}\)
2. Separation of IIA group elements from II B : Since the sulphides of IIA group elements (e.g. \(\mathrm{Hg}, \mathrm{Pb}, \mathrm{Cu}\) and Cd) and IIB group elements (viz \(\mathrm{As}, \mathrm{Sb}\) and Sn) are precipitated together as sulphides, they are separated from each other by means of yellow ammonium sulphide. The separation is based on the fact that the sulphides of IIB group elements form soluble complexes with yellow ammonium sulphide or even with normal ammonium sulphide (except SnS) whereas the IIA group elements remain unaffected.
\(\mathrm{As}_{2} \mathrm{~S}_{3}+3\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}
\longrightarrow 2\left(\mathrm{NH}_{4}\right)_{3} \mathrm{AsS}_{3} \) , \( \mathrm{As}_{2} \mathrm{~S}_{5}+3\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S} \longrightarrow 2\left(\mathrm{NH}_{4}\right)_{3} \mathrm{AsS}_{4} \)
\(\mathrm{Sb}_{2} \mathrm{~S}_{3}+3\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S} \longrightarrow 2\left(\mathrm{NH}_{4}\right)_{3} \mathrm{SbS}_{3}\) , \(\mathrm{Sb}_{2} \mathrm{~S}_{5}+3\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S} \longrightarrow 2\left(\mathrm{NH}_{4}\right)_{3} \mathrm{SbS}_{4}\)
\( \mathrm{SnS}_{2}+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S} \longrightarrow \left(\mathrm{NH}_{4}\right)_{2} \mathrm{SnS}_{3} \) , \( \mathrm{SnS}+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}_{x} \longrightarrow\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SnS}_{3}+(x-2) \mathrm{S}\)
3. Separation of copper and cadmium: The separation of copper and cadmium which are precipitated as their corresponding sulphides is based on the fact that on treatment with potassium cyanide the two ions form the complexes in the following manner.
\( \mathrm{CuS}+2 \mathrm{KCN} \longrightarrow \mathrm{Cu}(\mathrm{CN})_{2}+\mathrm{K}_{2} \mathrm{~S}\) , \(2Cu (CN)_2 \longrightarrow Cu _2 (CN)_2 + (CN_2),\)
\(Cu_2 (CN)_2 + 6KCN \longrightarrow 2K_3 [Cu (CN)_4],\)
\(CdS \,\text {or}\,CdCl_2 + 2 KCN \longrightarrow cd (CN)_2 + K_2S \,\text {or}\, 2KCl\)
\(Cd (CN)_2 + 2KCN \longrightarrow K_2[Cd (CN)_4]\)
4. Separation of cobalt and nickel. The mixture of cobalt and nickel sulphide is treated with a mixture of hydrochloric acid and potassium chlorate when the insoluble sulphides are converted into soluble chlorides.
\(\mathrm{CoS}+2 \mathrm{HCl}+\mathrm{O} \longrightarrow \mathrm{CoCl}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{S}\) ,
\(Nis + 2 HCl +O \longrightarrow NiCl_2 +H_2O + s\)
5. Separation of zinc and manganese. It is based on the fact that on adding excess of NaOH to their salts, only the former forms soluble complex sodium zincate. But \(\mathrm{Mn}(\mathrm{OH})_{2}\) (white precipitate) is oxidised by the atmospheric oxygen to manganic oxide (brown precipitate).
\(ZnCl_2 + 2NaOH \longrightarrow Zn (OH) _2 + 2NaCl\) ,
\(Zn (OH)_2 + 2 Na OH \longrightarrow \underset {Sodium\,zincate}{Na_2 Zn O_2 + 2H_2O}\)
\(MnCl_2 + 2 Na OH \longrightarrow Mn (OH)_2 \downarrow +2 NaCl\) ,
\(\underset {Brown} {2Mn(OH)_2}+O \longrightarrow Mn_2O_3 \downarrow +2H_2O\)
6. Separation of calcium and strontium : It is based on the fact that when excess of ammonium sulphate solution is added to \(\mathrm{Ca}^{2+}\) and \(\mathrm{Sr}^{2+}\) salts ; the latter is precipitated as its sulphate, whereas the former remains in the solution due to the formation of a complex.
\(Sr Cr O_4 + (NH_4)_2 SO_4 \longrightarrow \underset {(white)}{SrSO_4}\downarrow + (NH_4)_2 CrO_4\)
\(CaCr O_4 + (NH_4)_2 SO_4 \longrightarrow CaSO_4 + (NH_4)_2 CrO_4,\)
\(CaSO_4 + (NH_4 )_2 SO_4 \longrightarrow (NH_4)_2[Ca (SO_4)_2]\)
7. Complexes is quantitative analysis : There are numerous reactions in quantitative analysis where complexes are formed. To mention a few are
(a) Estimation of nickel and other metals as dimethyl glyoxime complex.
(b) The most common application is the estimation of several cations \((Mg^{2+},Ca^{2+},Zn^{2+},Ni^{2+} \text {and}\,Al^{3+})\) using EDTA as titrant in presence of suitable indicator, e.g. murexide solution.
(c) EDTA is used in the estimation of Ca2+ and Mg2+ ions in water (total hardness of water).
(d) EDTA is used in softening of hard water.
