Notes On Redox Reactions As A Basis For Titration - CBSE Class 11 Chemistry
Titrations that involve oxidation- reduction reactions are known as redox titrations. Redox titrations are used to determine the strength of a reductant (or) oxidant by using a redox sensitive indicator. Redox titrations can be performed using oxidising agents such as potassium permanganate, potassium dichromate and iodine. Potassium permanganate is a dark purple coloured liquid and a versatile and strong oxidising agent used in the estimation of many reducing agents such as Fe+2 ions, oxalate ions etc. Indicators are not used in these titrations as potassium permanganate acts as a self indicator. During the titration between potassium permanganate and a reducing agent, the pink colour of potassium permanganate is discharged till the reductant is completely oxidised. A further addition of a single drop of potassium permanganate gives a tinge of pink colour to the solution. This indicates the end point of the titration. In the redox titration reactions where the oxidising agent doesn't act as self indicator, indicators such as diphenylamine or N-phenylanthranilic acid are used. Ex: In the estimation of Fe using Potassium dichromate, diphenylamine is used as an indicator to detect the end point. The titrations in which iodine is directly titrated against a reducing agent are called iodimetric titrations. Ex: In the estimation of hypo (or) sodium thiosulphate, iodine directly reacts with sodium thiosulphate in the presence of starch as an indicator and produces an intense blue colour. Iodimetric titration involves addition of an oxidising agent to excess iodide ions to produce iodine which is then titrated with standard thiosulphate solution. Ex: In the estimation of Cu +2 ions iodine is liberated by oxidising iodide ions from potassium iodide. An intense blue colour develops when starch solution is added to the liberated iodine. This solution is titrated against sodium thiosulphate solution. The colour disappears as soon as the iodine is consumed by the thiosulphate ions. Thus, the end point is detected by the colour change. It must be noted that iodine being non-polar, is very slightly soluble in water. However, it dissolves in water in the presence of potassium iodide due to the formation of I3- ions.

#### Summary

Titrations that involve oxidation- reduction reactions are known as redox titrations. Redox titrations are used to determine the strength of a reductant (or) oxidant by using a redox sensitive indicator. Redox titrations can be performed using oxidising agents such as potassium permanganate, potassium dichromate and iodine. Potassium permanganate is a dark purple coloured liquid and a versatile and strong oxidising agent used in the estimation of many reducing agents such as Fe+2 ions, oxalate ions etc. Indicators are not used in these titrations as potassium permanganate acts as a self indicator. During the titration between potassium permanganate and a reducing agent, the pink colour of potassium permanganate is discharged till the reductant is completely oxidised. A further addition of a single drop of potassium permanganate gives a tinge of pink colour to the solution. This indicates the end point of the titration. In the redox titration reactions where the oxidising agent doesn't act as self indicator, indicators such as diphenylamine or N-phenylanthranilic acid are used. Ex: In the estimation of Fe using Potassium dichromate, diphenylamine is used as an indicator to detect the end point. The titrations in which iodine is directly titrated against a reducing agent are called iodimetric titrations. Ex: In the estimation of hypo (or) sodium thiosulphate, iodine directly reacts with sodium thiosulphate in the presence of starch as an indicator and produces an intense blue colour. Iodimetric titration involves addition of an oxidising agent to excess iodide ions to produce iodine which is then titrated with standard thiosulphate solution. Ex: In the estimation of Cu +2 ions iodine is liberated by oxidising iodide ions from potassium iodide. An intense blue colour develops when starch solution is added to the liberated iodine. This solution is titrated against sodium thiosulphate solution. The colour disappears as soon as the iodine is consumed by the thiosulphate ions. Thus, the end point is detected by the colour change. It must be noted that iodine being non-polar, is very slightly soluble in water. However, it dissolves in water in the presence of potassium iodide due to the formation of I3- ions.

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