(a) One of the reagents is itself intensely coloured and may be used as an indicator. For example, \( KMnO_4 \) or \( MnO_4 \) acts as the self indicator. In \( KMnO_4 \) titrations, when the whole of the reductant (\( Fe^ \ or \ C_2O_4^ \)) is oxidised, the endpoint is reached. At the endpoint, the violet colour of the \( MnO_4 \) solution disappears, and a lasting tinge of pink colour appears. This colour change is so sensitive that it can be easily detected even at very low concentrations of \( MnO_4 \).
(b) If there is no dramatic auto colour change, there are indicators which are immediately oxidised after the last drop of the reactant has reacted producing a dramatic colour change. Unlike \( MnO_4 \), \( C_2O_7 \) is not a self-indicator. Therefore in \( K_2Cr_2O_7 \) titrations, some indicator is used to detect the endpoint.
The following two types of indicators are usually used:
Internal indicators:
Before the start of the titration, a few drops of these indicators are added to the reactant taken in the titration flask. Examples are diphenylamine and N- phenylanthranilic acid. With diphenylamine at the endpoint, colour changes from bluish green or purple to blue-violet. However, with N-phenylanthranalic acid, at the endpoint, the colour changes from green to violet red.
External Indicators:
Potassium ferricyanide is used as an external indicator during the titration of ferrous salts against \( K_2Cr_2O_7 \) solutions. Several drops of the indicator are placed side by side on a glazed white tile. A drop of the oxidised solution is taken out of the titration flask with a glass rod and touched with the drop of the indicator from time to time during titration. In the beginning, the colour is blue, which changes to green and finally to brownish yellow. Thus at the endpoint, the colour is brownish yellow, or there is no change in the indicator colour.
(c) Another indicator which is used in redox reactions is starch. It produces intense blue colour with molecular iodine. However, the use of this indicator is limited to only those reagents which can either oxidise \( I^- \) ions or reduce \( I_2 \). Thus, when \( I^- \) ions are oxidised to molecular \( I_2 \) intense blue colour appears at the endpoint. However, when \( I_2 \) is reduced to \( I^- \) ions, that blue colour disappears at the endpoint.
The titration of \( KMnO_4 \) against oxalic acid is an example of a redox reaction. Potassium permanganate acts as a powerful oxidising agent and is used for quantitative analysis in an acidic medium. Dilute sulphuric acid is used in this titration. HCl and nitric acid is not used in this titration. Oxalic acid reacts with potassium permanganate according to the following equation.
Reduction half-reaction : \( 2KMnO_4+ 3H_2SO_4 \rightarrow K_2SO_4+ 2MnSO_4+ 3H_2O + 5[O] \)
Oxidation half-reaction : \( H_2C_2O_4 + [O] \xrightarrow C> 2CO_2 + H_2O] \times 5 \)
Complete reaction : \( 2KMnO_4 + 3H_2SO_4 + 5H_2C_2O_4 \rightarrow K_2SO_4 + 2MnSO_4 + 8H_2O + 10CO_2 \)
Mn is reduced, and C is oxidised from \( KMnO_4 \) and \( (COOH)_2 \), respectively. Initially, the solution becomes colourless, but after the complete combustion of oxalate ions at the endpoint light pink colour is produced by adding an extra drop of \( KMnO_4 \). In the beginning, the rate of reaction is slow but increases as the chemical reaction proceeds.
Some main applications of this titration include
Some main advantages and disadvantages of this method are
Advantages
Disadvantages
The table below lists the differences between these two processes.
| Redox Titration | Acid-Base Titration |
| Redox titration can be used to determine the exact amount of 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. | Acid-base titration can be used to find out the exact amount of an acid or the base present in a given solution by reacting it against the solution of a standard base or the acid. |
| Some redox species act as self indicators, and in most cases, redox indicators are used. | Indicators that are usually used are weak acids and weak bases. |
| These titrations are mostly associated with d block elements. | These titrations are more common. |
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More Articles for ChemistrySome indicators used in redox titration are starch, potassium ferricyanide, diphenylamine and N- phenylanthranilic acid.
Hydrochloric acid is usually avoided because it reacts with \( KMnO_4 \) forming chlorine which is itself an oxidising agent.
Phosphoric acid is added to reduce the electrode potential for the \( Fe^ \rightarrow Fe^ \) reaction by stabilising the ferric ion.
Redox titration can be used to determine the exact amount of 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.
Following precautions must be taken care of while dealing with redox titrations:Burette must be well cleaned.The titration mixture should be swirled gently to avoid splashing the solution.Spillage of solution from the pipette when transferring the base should be avoided.Burette must not be leaking during the process.Reading must be noted accurately.