An electrolyte solution is a solution in which the solute, after dissolving in a solvent, completely or partially dissociates into ions. The solute is the electrolyte, and conductivity is a characteristic of electrolyte solutions. Acid, base, and salt solutions are all electrolyte solutions. Electrolyte solutions are formed by the positively charged cations and negatively charged anions dissociated from the electrolyte, which move directionally towards their corresponding electrodes under the influence of an external electric field and discharge there.
The main factors affecting conductivity are degree of ionization, conductivity, ion mobility, ion transport number, ion activity, and ion strength.
1. Degree of ionization
At ionization equilibrium, the degree of ionization is the ratio of the number of ionized electrolyte molecules to the total number of molecules, expressed as a percentage. A high degree of ionization indicates the formation of more ions and stronger conductivity. At a given temperature, the degree of ionization of an electrolyte increases as its concentration decreases. The quantitative relationship between the degree of ionization, concentration, and ionization constant is determined by Osterhua's dilution law. Experiments show that weak electrolytes with very low ionization readily obey the dilution law, while strong electrolytes largely do not, because strong electrolytes are practically almost completely ionized. There is no electrical balance in the solution. Due to the strong ion interactions in strong electrolyte solutions (unless infinitely diluted), the degree of ionization of a strong electrolyte does not reflect its true ionization. Therefore, the degree of ionization of a strong electrolyte is called the apparent degree of ionization.
2. Electrical conductivity
The reciprocal of resistance is consistent with the general meaning of conductance in electrical engineering. The conductance of electrolyte solutions can be expressed in two ways: specific conductance and equivalent conductance. Specific conductance refers to the conductance of an electrolyte solution with an electrode area of 1 square centimeter and an electrode distance of 1 centimeter. Equivalent conductance refers to the conductance of a solution containing 1 gram of electrolyte between two parallel electrodes 1 centimeter apart.
3. Ion mobility
The ion mobility is the velocity of ions when the potential gradient between the two electrodes is 1 V/cm, also known as the absolute ion mobility. Ion mobility decreases with increasing solution concentration and increases with increasing temperature. The greater the ion mobility of an electrolyte, the greater its equivalent conductivity.
4. Ion transport number
The fraction of the total charge carried by a particular ion during migration, relative to the total charge passing through the solution, is called the ion transport fraction. Two ions with significantly different mobility will have vastly different transport numbers. In industrial electrolysis, the amount of charge carried by a particular ion and its concentration changes near the electrode can be determined based on its mobility, serving as a basis for controlling electrolysis conditions.
5. Ion activity
The corrected ion concentration, also called the effective concentration, is equal to the product of the actual ion concentration and the activity coefficient. The activity coefficient is equal to the ratio of activity to concentration. Except for extremely dilute solutions, due to the complex interactions between ions and flux molecules in the solution, the ion concentration is not equal to the activity, i.e., the activity coefficient is not equal to 1. Introducing the concept of ion activity, that is, replacing ion concentration with ion activity, allows some thermodynamic formulas that only apply to ideal solutions to be used in real solutions.
6. Ionic strength
The mean activity coefficient of ions is half the sum of the concentrations of all ions in a solution multiplied by the squares of their valence numbers. The mean activity coefficient of ions decreases with increasing ionic strength, and the higher the valence of the ion, the greater the decrease. Ionic strength reflects, to some extent, the strength of the interactions between ions.
