Redox reactions involve a change in oxidation number or transfer of electrons. The standard solution is either oxidizing or reducing agent. The oxidation-reduction reaction that takes place between the analyte and the titrant is the basis for redox titration. It is also one of the techniques used most frequently to determine the concentration of unidentified analytes. A redox indicator or a potentiometer is frequently required for redox titration. Redox titration is a technique used in laboratories to measure the concentration of an analyte by generating a redox reaction between the analyte and the titrant.
Fe 2+ + Ce 2+ → Fe 3+ + Ce 3+
Primary standard solution
The primary standard solution can be prepared by direct weighting o standard substance followed by dilution to give the definite volume of the solution. E.g., Na2CO3, NaCl, K2Cr2O7
Secondary standard solution
A secondary standard solution should be standardized by titrating against the primary standard solution. Examples: HCl, NaOH, KMnO4
2 Mg + O2 → 2 MgO
- Loss of electron
- Addition of oxygen
- Increase in oxidation state
- Removal of hydrogen
MgO + H2 → Mg + H2O
- Gain of electron
- Loss of oxygen
- Decrease in oxidation state
- Addition of hydrogen
In an oxidation reduction reaction, the reducing agent loses electrons.
Fe2+ → Fe3+ + e-
- Potassium permanganate (KMnO4)
- Potassium dichromate (K2Cr2O7)
- Potassium iodate (KIO3)
- Potassium bromate (KBrO3)
In an oxidation reduction reaction, the oxidizing agent gains electrons.
Ce 4+ + e- → Ce 3+
- Fe (II)
- Tin (II)
- Sodium thiosulphate (Na2S2O3)
- Arsenic (III) oxide (As2O3)
Some redox reactions
i. Dichromate titration
It is redox titration. In this reaction, potassium dichromate is used as the oxidizing agent. Although potassium dichromate is a weaker oxidizing agent than potassium permanganate it has several advantages. It can be used as a primary standard substance. Additionally, the solution of dichromate is quite stable.
For example: Determination of Fe2+by using potassium dichromate (K2Cr2O7)
K2Cr2O7 + 6 Fe (NH4)2 (SO4)2 + 7 H2SO4 → 3 Fe2(SO4)3 + Cr2(SO4)3 + K2SO4 +
6 (NH4)2SO4 + 7 H2O
II. Permanganate titration
Potassium permanganate is a strong oxidizing agent. Its solution is used to estimate reducing agents including hydrogen peroxide, oxalic acid, ferrous salt, oxalates, and many more. It acts as self indicator during the titrimetric analysis. For example, during the titration between potassium permanganate and oxalic acid potassium permanganate becomes colourless at the end point.
5 H2C2O4 + 2 KMnO4 + 6 H2SO4 → 10 CO2 + 2 MnSO4 + K2SO4 + 8 H20
III. Iodometric and iodimetric titration
Iodine is a solid that cannot be dissolved in water. However, in aqueous potassium iodide, it is quite soluble. The use of iodine in volumetric analysis is based on its ability to behave as a mild oxidizing agent. Iodine is used for two different kinds of titrations. They are as follows:
Iodimetry titration is the redox titration that involves the titration between a standard iodine solution with a solution whose concentration needs to be determined. The iodine and analyte are directly titrated in this process. It’s used to determine the reducing agent’s concentration.
For example: the titration between sodium thiosulphate and iodine
Na2S2O3 + I2 → Na2S4O6 + 2 NaI
Iodimetry titration involves the indirect titration between iodine and analyte. In iodometric titration the oxidizing agent allowed to react with excess of potassium iodide in acidic or neutral medium, to liberate free iodine. Free iodine is equivalent to the amount of oxidizing agent which make the basis for the calculation. The amount of liberated iodine is estimated by using the standard solution of sodium thiosulphate.
2 Cu 2 + + 2 KI → 2 Cu+ + 2 K+ + I2
I2 + 2 Na2S2O3 → 2 NaI + Na2S4O6
Redox reaction using potassium bromate are known as the bromatometry titration. Potassium bromate is usually used as the oxidizing agent for the determination of various pharmaceutical compounds.
BrO3– + I– → IO3- + Br–
IO3- + 5 I– + 6 H+ → 3 I2 + 3 H2O
I2 + 2 S2O32- → 2I- + S4O62-
The redox reactions that that uses Ce4+ as oxidizing agent is known as cerimetry. In an acidic solution, ammonium ceric sulphate acts as a strong oxidizing agent and has a bright yellow color. The cerous salt obtained after the reduction of this salt is colorless. This makes it a self-indicator. Typically, sulfuric acid or perchloric acid are used for this titration. Permanganate-based titrations can be performed using ceric (IV). It has a lot of benefits because it is an extremely potent oxidizing agent and the choice of acid used might change its potential. Additionally, salt of cerium (IV) can be obtained as primary standard and it don’t require standardization of salt solution during titration.
Ce 4+ + e- → Ce 3+
Applications of redox titration
- Redox titration is used to analyse the wide range of inorganic analytes.
- Redox reaction is also used for the determination of organic compounds This reaction, for instance, is used to determine the chemical oxygen demand of both natural and waste water.
- Many chemicals like caustic soda and chlorine can be produced by using redox reaction.
- Glavanization process involves redox reaction.
- . This is frequently used in the food industry to determine the concentration of vitamin E and C or the amount of salt and sugar in food goods.
- It is utilized in metallurgical operations such as metal extraction from ores and fuel combustion.
- Evaluating the chlorination of public water sources is one of the most significant industrial applications of redox titrations.
- Redox titration is employed in pharmaceutical analysis, such as the detection of valganciclovir hydrochloride (VLGH) in tablets and pure medications.
Advantages of redox titration
- As little equipment is needed, redox titration is typically inexpensive.
- It does not necessitate a great level of expertise.
- It can be easily performed in laboratory.
- The process is frequently quick.
Disadvantages of redox reaction
- It is destructive method usually require large quantity of analytes.
- The reaction is takes place in solution phase which may not be suitable for analysis of some compounds.
- It has low accuracy.