Kohlrausch’s Law and its Applications

Introduction

For the analysis of dilute solutions and the analysis of electrochemical cells, Kohlrausch’s law and its applications are very useful. Kohlrausch first proposed this concept in 1876. He made extraordinary contributions to the development of physical chemistry and was a pioneer in the field of electrochemistry at the time. Kohlrausch law refers to the limited molar conductivity of an electrolyte with respect to the ions it contains. Calculating limiting molar conductivity involves adding up the contributions of individual ions and cations. The Kohlrausch law is commonly referred to as the law of independent migration of ions. One of the most important applications of this law is to determine the limiting conductivity of a weak electrolyte. 

The Kohlrausch law is named after Friedrich Kohlrausch, a German physicist who gave it between 1875-1879. Kohlrausch made significant contributions to the development of physical chemistry. Several renowned and important chemists utilised his experiments. To determine the behaviour of the electrolytes and to determine the anomalies in them, he explored their conductive properties. Friedrich Kohlrausch found that equivalent conductivity at infinite dilution equals cation conductivity plus anion conductivity at infinite dilution.

The volume of conductance solution containing one mole of an electrolyte at the given concentration is the molar conductivity of the solution. This is positioned between two electrodes with cross-sections unit area and unit distances. A solution’s molar conductivity increases with a decrease in concentration, which can be attributed to an increase in the overall volume containing an electrolyte mole. Molar conductivity is called the limiting molar conductivity when the electrolyte concentration goes to zero.

By comparing the limiting molar conductivities of several strong electrolytes, Kohlrausch identified specific patterns. Kohlrausch posited that an electrolyte limiting molar conductivity could be approximated as the sum of individual contributions from its anions and cations based on his observations. The Kohlrausch law is commonly referred to as the law of independent migration of ions. The limiting conductivity of sulphuric acid is an example of this law. Hydrogen cation plus sulphate anion contribute equally to the limiting molar conductivity of sulfuric acid. 

Electrolytes fall into two categories: strong and weak. The conductivity of strong electrolytes is high since they are ionised completely, while weak electrolytes are only partially ionised. Dilution increases the molar conductivity of strong electrolytes because of the decrease in solute-solute interaction. Specific conductivity is characterised by an increase in the concentration of electrolytes when the conductivity increases. Specific conductivity relates to ions per unit volume present in a solution. The dissociation process increases with dilution, increasing the number of ions that contain current. As a result of dilution, a unit volume of a solution has fewer ions, which reduces its conductivity.

A material’s conductivity refers to its ability to allow ions to pass through it and conduct electricity. Generally, it can be defined as the reciprocal of a material’s resistance. The conductance SI unit is Siemens. An electrolytic solution’s conductivity is determined by:

• Electrolyte type and concentration
• Ion size and solvation
• The temperature 
• The type of solvent and its viscosity

Electrolytes dissociate or move easily under a potential gradient because of their charge, concentration, size, and ion concentration. Various electrolytes have different solution conductivities when dissolved in the same solvent at the given temperature. Therefore, we refer to an electrolyte solution as having a molar conductivity. This is calculated by dividing the specific conductivity by the electrolyte concentration. Kohlrausch law formula is given below: 

eq = c+ a

Here, equivalent conductivity at infinite dilution equals cation conductivity plus anion conductivity at infinite dilution.

Applications of Kohlrausch’s Law

In many areas, such as electrolytes and ions, Kohlrausch’s law can be used to determine molar conductivity. The following are some examples of the application of Kohlrausch law:

• Analysing weak electrolytes: By extrapolation, you cannot determine an electrolyte’s molar conductivity at infinite dilution. The molar conductivity of weak electrolytes at infinite dilution is very difficult to calculate. Using Kohlrausch’s equation, one can determine the value of weak electrolytes.
• Analysing weak electrolyte ionisation degree: A weak electrolyte’s degree of ionisation can be determined by Kohlrausch’s law no matter what concentration it is at.
• Finding the weak electrolyte ionisation constant: Aqueous solutions containing weak electrolytes ionise very little. A measure of the degree of ionisation describes the extent of ionisation. Between solution’s ions and unionised molecules, a dynamic equilibrium exists. This equilibrium is described by the ionisation constant. Kohlrausch’s law can be used to find weak electrolyte ionisation constants.
• Solvable salt solubility determination: When calculating the solubility of a moderately soluble salt, Kohlrausch’s law is used. Moderately or sparingly soluble salts dissolve in water in extremely small amounts. For instance, barium sulphate.
• Finding limiting molar conductivities: In any electrolyte, the Kohlrausch law assists us in determining the limiting molar conductivity.  

Conclusion 

In his research on the conductivities of electrolytes, German physicist Friedrich Kohlrausch discovered that migrating ions have unique limiting molar conductivities. Kohlrausch’s law of independent migration of ions was created as a result. The Kohlrausch law states that an electrolyte limiting molar conductivity equals the sum of the limiting molar conductivity of its constituent cations and anions. Any electrolyte, weak or strong, can be subjected to this law. The law can be used to analyse weak electrolytes, find the weak electrolyte ionisation constant, find limiting molar conductivities, find solvable salt solubility, and many more.