One of the important significance of redox reactions in solutions is their frequent use in volumetric analysis. Volumetric analysis involves estimation of the substance by accurate easurement of the volumes of the solution undergoing a chemical change. Here, a definite mass of the substance to be estimated is dissolved in water to get some exact volume ·of the solution. A known volume of this solution is allowed to react completely with some standard solution of oxidising agent (or reducing agent). The process by which two solutions are allowed to react is called titration. The final result is calculated from the volumes of the two reacting solutions. In this section, our main focus will be on redox titrations and the calculations based on the molar relationship between the oxidising and reducing agents.
In acid-base systems we come across with a method of making one solution react against another, using glassware (pipette and burette) and indicators to establish the exact end point. The objective is to find out something previously unknown about one of the solutions.
In a similar way, redox titrations can be used to determine the exact amount of an oxidising (or a reducing agent) in a given solution by titrating it against the standard solution* of a suitable reducing agent (or the oxidising agent). However, acid-base and redox titrations differ from each other in the choice of indicators.
Choice of indicators. The main difference between the two types of titrations is that the indicators used in acid-base titrations to detect the changes in pH are not useful for redox titrations. This problem can be solved in the following three ways:
1. One solution to the problem of use of indicator is to choose one of the reagents, which is itself intensely coloured, as indicator, e.g. , permanganate ion, MnO4-. Here, KMnO4 or MnO 4- acts as the self indicator.
In potassium tetraoxomanganate(VII) (KMnO4) titrations, when whole of the reductant (Fe2+ or C2O4) is oxidised, end point is reached. At the end point, the violet colour of MnO 4- solution disappears and a lasting (permanent) tinge of pink colour appears. This colour change is so sensitive that it can easily be detected even at very low concentrations (10 -6 mol L -1) of MnO -4
2. If there is no dramatic auto-colour change (as with KMnO4 titrations), there are indicators which are immediately oxidised after the last drop of reactant has reacted producing a dramatic colour change. For example, Cr2Ol- (or K2Cr207) is not a self indicator, therefore, diphenylamine is used as an indicator in K2Cr207 titrations. It produces intense blue colour at the end point.
3. Another indicator which has been used in redox reactions is starch which produces intense blue colour with molecular iodine. The use of this indicator is, however, limited to only those reagents which can either oxidise I- ions such as Cu2+ ions or reduce I2 such as thiosulphate ion (S2O3) ions.
Thus, when I- ions are oxidised to molecular I2. Intense blue colour appears at the end point. However, when I2 is reduced to I- ions, the intense blue colour disappears at the end point.
It may be noted here that although I2 is insoluble in water, it can be easily dissolved in KI solution where it exists as KI3