Conductance

Conductance and Electrical resistance

Conductance or electrical conductance is the reciprocal of resistance (G = 1/R) describes the property of an electrolyte solution that helps to conduct electricity in the solution. In chemistry or physics, the unit of conductance is equal to ohm⁻¹ or mho and siemens in the SI system. The conducting ability of the electrolyte solution is the direct proof of the existence of ions in the solution and the experimental determination of conductance provides the behavior of ions in the solution.

What is electrical resistance?

From Ohm’s law,

R = V/I

Conductance, G = 1/R = I/V

Where V = potential difference between the two electrodes dipped into the electrolytic solution
I = current strength
R = electrical resistance that obstructs the flow of current.

The electrical resistance of a given solution depends on

1. The nature of the electrolyte solution.
2. The distance of separation of two electrodes.
3. The area of electrodes.

Specific Conductance

The specific conductance (L) of an electrolytic solution is the conductance of the solution enclosed between two electrodes 1 cm apart and 1 cm² area. In other words, the L may be defined as the conductance of 1 cc solution enclosed between the electrodes 1 cm apart. The unit of L is equal to ohm⁻¹ cm⁻¹ or Siemens m⁻¹.

Specific conductance (L) depends on,

1. The number of ions present per cc of the solution.
2. The charge of the ions.
3. The speed of the ions.
4. It also depends on the temperature of the electrolytic solution and increases by a 2 percent per degree rise in temperature.

Therefore, with the dilution of the strong electrolyte solution, the number of ions per cc decreases while the speed of ions increases. Here the first factor predominates over the second and L decreases with the dilution of the solution.

Equivalent Conductance

It is defined as the conductance of a solution containing 1 gram equivalent electrolyte and the solution placed between two parallel electrodes 1 cm apart.

When the concentration of the electrolyte solution = C (N). The volume of the solution containing 1 gram equivalent electrolyte, V = 1000/C (N) cc.

The equivalent conductance
λ = V × L
or λ = 1000L/C (N)

Equivalent conductance unit

The unit of λ is equal to ohm⁻¹ cm²/gram equivalent or Siemens m²/gram equivalent.

It is used to compare the conductance of different electrolyte solutions. Since the amount of electrolyte or 1 gram equivalent is fixed for λ.

Molar conductivity

Molar conductivity (μ) is defined as the conductivity of a solution containing 1 mole electrolyte and the solution placed between two parallel electrodes 1 cm apart.

The unit of μ = ohm⁻¹ cm² mole⁻¹ or Siemens m² mole⁻¹

Kohlrausch Law

The strong electrolyte solution is fully dissociated in any concentration and the total number of ions fixed in 1 gram equivalent electrolyte solution is always fixed. Therefore, with the dilution, the speed of the ions increases due to less interionic attraction among the ions.

At a certain dilution, the interionic attraction forces vanish and λ riches limiting values. It is called equivalent conductance at infinite dilution (λ₀). A strong electrolyte solution always obeys the Kohlrausch law of independent migration.

Kohlrausch Law of infinite dilution

Kohlrausch equation for strong electrolyte,

λ_c = λ₀ − k√C

Where k = constant that depends on the temperature and dielectric constant (D) of the medium.

For electrolysis of weak electrolyte solution, the Kohlrausch formula or plot is not used for the determination of λ₀. The law states that at infinite dilution, each ion moves independent of the influence of its co-ion and contributes a definite share to the λ₀ of the electrolyte.

Application of Conductivity Measurement

• The conducting ability of the electrolyte solution is the direct proof of the existence of ions in the solution. The experimental determination of conductance provides the behavior of ions in the solution.
• The electrical conductance measurement is used for the determination of various physical quantities like the endpoint in an acid base titration, the solubility of sparingly soluble salts, the ionic product of water, and the chemical kinetics of the reactions.
• The electrical conductivity measurement of soil is extremely important for the health and growth of crops in the agricultural industry. It is used to monitor essential nutrients of soil like phosphates, nitrates, calcium, and potassium which are used for plant growth.
• Conductance offers a very simple and convenient way of determining the solubility of a sparingly soluble salt like silver chloride (AgCl).