Determination of Stannous Tin

The oxidation of tin + II by the air dioxygen may lead to an inaccurate value. The reaction of titration of tin + II with iodine provides an example of determining a metallic salt at its minimum (of oxidation). 

Determination of Mercuric Salts and to That of Reducing Organic Substances 

Mercury(I) salts (mercurous salts) such as the dichlorodimercury(II) HgaCla (improperly called calomel or mercurous chloride) and other metallic salts at their minimum can be titrated with a iodine solution in the presence of iodide ions according to the reaction 

The complex tetraiodomercurate(II) ion [Hgl4 ] is formed. It is a back titration. The excess iodine is titrated with a thiosulfate solution. We notice that mercury(I) is oxidized into mercury(II), an oxidation state that it exhibits in the ion complex tetraiodomercurate. Simultaneously, one molecule of iodine is reduced to two iodide ions. We notice also that the iodide ions formed are not free after the reaction, as in a classical direct iodometry, but are complexed. Now, we must wonder why this reaction is possible from a thermodynamic standpoint. There is indeed, at first sight, a sort of contradiction between the occurrence of this reaction and the values of the standard potentials of systems under study  

Hg(II) should not be formed. But the studied reaction is more complicated than the simple redox reaction between the couples Hg +/Hg2 + and I2/I that can be predicted at first sight. The above redox reactions are coupled to the reactions 

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