Liebig-Deniges’ method

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni(II) salt in ammoniacal solution is heated with excess of cyanide ion, the [Ni(CN)4]2 ion is formed quantitatively since it is more stable than the [Ag(CN)2] ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA see the following sections. 

Liebig -Deniges method determines cyanide ions with silver ions. 

Figure 37.4 shows titrations curve obtained according to the principle of Liebig-Deniges method, by using several ammonia concentrations. 

The excess of cyanide ions is titrated with a standard silver nitrate solution according to a principle close to that prevailing in Liebig-Deniges method. If a small quantity of silver iodide is added to the initial ammonia solution containing Ni + ions, the turbidity it induces because of its insolubility disappears after an addition of cyanide ions because of the occurrence of the reaction... 

Deniges developed a very nice amelioration to Liebig s method. He proposed using an ammonia solution instead of pure water as solvent and, moreover, adding iodide ions as potassium iodide to the solution. The equivalence point is detected by the precipitation of silver iodide, which is very insoluble [X s(AgI) = 10 - ]. 

The essential difference between Liebig s and Deniges methods lies in the fact that the constants that govern the phenomena occurring in Deniges modification exhibit values different from those found in Liebig s titration since silver ions stand as ammine complexes, above all as the diamminesilver(I) complex. 

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