Depending upon the nature of the oxidising agent, these are divided into the following categories:
1. Potassium Tetraxomanganate (VII) titrations
In these titrations, reducing agents like iron (II) ions (Fe )2+ from FeSO4, Mohr’s salt [(NH4) 2S04. FeSO4.6H20], oxalic acid (COOH)2, or ethanedioate (C2O4) ions from ethanedioic etc., are directly titrated against KMnO 4 as the oxidising agent in acidic medium. For example, (i) Oxidation of iron (II) salts:
It may be noted that in titration involving ethanedioate (oxalate) ions the flask contain C20 ions in acidic medium is heated to about 60°C. This is to provide activation energy to the reactants. However, as the titration progresses, the manganese (II) ions (Mn2+) formed during the reaction catalyse the reaction.
It may be noted that in neutral or basic condition a brown precipitate is formed which interfere with end point of the equation involved is
2. Potasssium Dichromate Titrations
In these titrations, the above listed reducing agents are directly titrated against k2Cr207 as the oxidising agent in acidic medium. For example,
(i) Oxidation of iron (II) salts:
3. Iodimetric Titrations
These titrations involve the direct use of iodine as the oxidising agent (in neutral or slightly acidic medium) using starch as an indicator. The various reducing agents used in these titrations are thiosulphates, sulphites, arsenites and antimonites.
4. Iodometric titrations
These titrations are carried out in two steps. In the first step, oxidising agents such as KMnO4, K2Cr2O7, CuSO4, etc. are treated with an excess of KI when I2 is liberated quickly and quantitatively. For example,
In the second step, the liberated iodine is titrated against a standard solution of sodium thiosulphate using starch as an indicator. All such titrations in which iodine liberated from potassium iodide with the help of an oxidising agent is titrated against a standard solution of sodium thiosulphate are called iodometric titrations