In iodimetry, quantitative oxidation of reducing agents, such as arsenious acid (H2AsO3) may be carried out by employing standard solutions of iodine as shown under :
This type of assay is known as ‘direct method of iodimetry’.
In another situation, a known excess quantity of standard iodine solution is added in the substance (a reducing agent) to be assayed and then the excess iodine may be titrated with the help of standard sodium thiosulphate solution, such as : the estimation of sodium bisulphite :
This category of assay is termed as ‘residual method of iodimetry’.
In iodometry, an equivalent amount of iodine is liberated when the given sample of an oxidizing agent oxidizes potassium iodide in an acidic medium, for example : the determination of cupric sulphate (CuSO4) :
2CuSO4 + 4 KI → 2CuI ↓ + I2 + 2K2SO4
Consequently, the equivalent amount of iodine generated by the above reaction may be conveniently assayed by titration against a standard sodium thiosulphate solution. In this context a point of caution must be observed while KI is being oxidized under a strongly acidic medium so as to avoid simultaneous oxidation of the iodide by atmospheric oxygen that may result high erroneous titer values leading to false estimations.
It is, however, pertinent to mention here that iodometric assays are never performed in a strongly basic medium, because of the fact that the reaction between I2 and OH– produces hypoiodide and iodate ions respectively as shown below :
I2 + OH– → HI + IO–
3IO– → IO3– + 2I
The said two ions partially oxidize thiosulphate to a higher oxidation form, such as sulphate (SO42–) thereby the stoichiometry achieved is always false.