The ability of electrolytes to conduct electric current is known as conductivity or conductance. Similar to metallic conductors, electrolytes follow Ohm’s law, which states that the electric current  flowing through a conductor is directly proportional to the voltage applied across it.

Here,  represents the potential difference between two ends (measured in volts), and  is the resistance, expressed in ohms (Ω). The resistance  of a conductor is directly proportional to its length  and inversely proportional to its cross-sectional area . Expressed as,

Where ρ (rho)   is the proportionality constant known as resistivity or specific resistance. The value of ρ depends on the material of the conductor. From equation (1), we can derive..

The ability of a substance to conduct electricity, known as conductivity, is the inverse of resistance. The reciprocal of specific resistance (resistivity) is called specific conductance or specific conductivity.

STRONG AND WEAK ELECTROLYTES

Electrolytes are substances that conduct electricity when dissolved in water. They are classified into two main types:

(a) Strong Electrolytes

• Definition: Substances that ionize completely in solution, producing a large number of ions.
• Characteristics:
  • Almost all molecules are ionized.
  • Solutions are excellent conductors of electricity.
  • Have high equivalent conductance, even at low concentrations.
• Examples:
  • Strong Acids:
    • Hydrochloric acid (HCl)
    • Sulfuric acid (H₂SO₄)
    • Nitric acid (HNO₃)
    • Perchloric acid (HClO₄)
    • Hydrobromic acid (HBr)
    • Hydroiodic acid (HI)
  • Strong Bases:
    • Sodium hydroxide (NaOH)
    • Potassium hydroxide (KOH)
    • Calcium hydroxide [Ca(OH)₂]
    • Magnesium hydroxide [Mg(OH)₂]
  • Salts:
    • Most salts such as sodium chloride (NaCl), potassium chloride (KCl), etc.

(b) Weak Electrolytes

• Definition: Substances that ionize only partially in solution, producing relatively few ions.
• Characteristics:
  • Only a small fraction of molecules are ionized.
  • Solutions are poor conductors of electricity.
  • Have low equivalent conductance.
• Examples:
  • Weak Acids:
    • Acetic acid (CH₃COOH)
    • Oxalic acid (H₂C₂O₄)
    • Sulphurous acid (H₂SO₃)
    • Other organic acids
  • Weak Bases:
    • Most organic bases
    • Example: Ethylamine (C₂H₅NH₂) and other alkyl amines
  • Weak Salts:
    • Mercury(II) chloride (HgCl₂)
    • Lead(II) acetate [Pb(CH₃COO)₂]

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Measurement of Electrolytic conductance

Conductance is the reciprocal of resistance, meaning it can be calculated by measuring the resistance of an electrolytic solution. This measurement is typically carried out in the laboratory using a Wheatstone bridge. The solution whose conductance is to be measured is placed in a specialized container called a conductance cell. A basic type of conductance cell commonly used in laboratories is illustrated in Fig. 24.6. The cell contains platinum electrodes coated with platinum black, which are attached to platinum wires sealed within two narrow glass tubes. Electrical contact with the circuit’s copper wires is established by immersing them in mercury contained within these tubes. Electrical contact with the copper wires of the circuit is established by immersing them in mercury contained within the glass tubes.

The setup commonly used to measure the resistance of a conductance cell is illustrated in Fig. 24.7. In this arrangement, a headphone is used instead of a galvanometer to detect the balance point. AB represents a manganin wire that is tightly stretched along a meter scale marked in millimeters. A sliding contact, denoted by H (indicated with an arrow), can move along the wire. The component labeled R is a resistance box, while C is the conductance cell containing the electrolytic solution. An induction coil, labeled I, supplies alternating current to the circuit as shown in the diagram.

When current flows, all resistances in the resistance box  are disconnected. The sliding contact  is then adjusted until the sound in the headphones is at a minimum. At this point, the system is in balance, and we have:

The resistance of a solution measured using the conductance cell can be converted to specific conductance using the equation:

The ratio l/A has been put equal to x. That is,

The value of  is constant for a given cell and is referred to as the cell constant. Once the specific conductance ( ) is determined, the equivalent conductance (Λ) and the molar conductance of the solution can be calculated using the relevant formulas.  

Where N is the gram-equivalent and M is the gram-mole of the electrolyte.

Determination of the Cell constant

The exact value of the cell constant ( ) can be determined by measuring the distance between the electrodes ( ) and their cross-sectional area ( ). However, accurately measuring these dimensions is quite difficult. Therefore, an indirect method is commonly used to determine the cell constant.

We know that:

To determine the cell constant, a standard solution of potassium chloride (KCl) with a known specific conductance at a given temperature is used. A KCl solution of the same concentration is then prepared, and its conductance is measured experimentally at the same temperature.

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