The Major Difficulty Encountered During Complexometric Titrations

1 The Major Difficulty Encountered During Complexometric Titrations 

The major difficulty encountered during complexometric titrations is the simultaneous formation of several complexes according to different stoichiometries with the same metallic ion and the same ligand. To be convinced of it, it is sufficient to examine Table 24.1 of Chap. 24, which mentions some cases of such complexations. The result of these cases is the existence of uncertain equivalence points that do not correspond to a well-defined stoichiometry. 

Exercise 1 A titration method of iodide ions with mercuric nitrate has been proposed. According to it, the equivalence point might be indicated by the appearance of the red precipitate of mercuric iodide. Let s consider the titration of 100 ml of a 0.1 mol/L sodium iodide solution with a 0.1 mol/L mercuric nitrate solution. What is the volume added at the equivalence point in these conditions What conclusion can we make  

Recall that mercuric nitrate is totally ionized in water. Let s also recall (see Chap. 2 and Part V) that when the precipitation of an ionizing product occurs, its solubility product is satisfied. In the present case, A s(Hgl2) = (Hg +)(I ) = 10 -. Finally, 

This exercise is solved by writing the mass balance relations on the Hg + and 1 ions (among others that are also satisfied—see below)  

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