Principles of Electrochemistry and Chromatography

Posted on Jul 9, 2026 in Chemistry

Oxidizing and Reducing Agents

Definitions

Oxidizing Agent: An oxidizing agent is a substance that accepts electrons and is reduced during a chemical reaction.

Reducing Agent: A reducing agent is a substance that donates electrons and is oxidized during a chemical reaction.

Example Reaction

Zn + CuSO₄ → ZnSO₄ + Cu

• Zn: Reducing Agent
• Cu²⁺: Oxidizing Agent

Note: There is no difference between Galvanic and Voltaic cells; both terms refer to the same electrochemical system.

Comparison of Electrochemical Cells

The Nernst Equation

Definition

The Nernst equation is used to calculate the electrode potential under non-standard conditions.

Mathematical Formula

E = E° – (0.0591 / n) log Q

Where:

• E: Cell potential
• E°: Standard potential
• n: Number of electrons transferred
• Q: Reaction quotient

Effective Concentration and Activity

Definition

Effective concentration is referred to as activity. It represents the actual concentration of ions participating in a chemical reaction.

Mathematical Formula

a = γC

Where:

• a: Activity
• γ: Activity coefficient
• C: Concentration

Principles of Chromatography

Adsorption Chromatography

Adsorption chromatography is a technique in which the components of a mixture are separated based on their different adsorption affinities on the surface of a solid stationary phase.

• Principle: Separation depends on adsorption.
• Stationary Phase: Solid (e.g., Silica gel or Alumina).
• Mobile Phase: Liquid or Gas.
• Examples: Thin Layer Chromatography (TLC), Column Chromatography.

Partition Chromatography

Partition chromatography is a technique in which the components of a mixture are separated according to their distribution between a liquid stationary phase and a mobile phase.

• Principle: Separation depends on the partition coefficient.
• Stationary Phase: Liquid.
• Mobile Phase: Liquid or Gas.
• Example: Paper Chromatography.

Comparison Table

Specific Chromatography Techniques

Thin Layer Chromatography (TLC)

TLC is an adsorption chromatography technique in which a thin layer of silica gel or alumina is coated on a glass, plastic, or aluminum plate to separate components.

• Principle: Adsorption
• Stationary Phase: Silica gel or Alumina
• Mobile Phase: Suitable solvent
• Uses: Identification of compounds, purity testing, and drug analysis.

Column Chromatography

Column chromatography is an adsorption technique where the stationary phase is packed inside a vertical glass column and a liquid solvent acts as the mobile phase.

• Principle: Adsorption
• Stationary Phase: Silica gel or Alumina
• Mobile Phase: Liquid solvent
• Uses: Purification, separation, and isolation of compounds.

Analytical Metrics and Formulas

R_f Value Definition

The R_f value is the ratio of the distance traveled by the compound to the distance traveled by the solvent front.

Formula: R_f = (Distance traveled by solute) / (Distance traveled by solvent)

Statistical Analysis: Standard Deviation

Standard deviation (SD) is the statistical measure of the spread of data around the mean.

Formula: SD = √[Σ(x – x̄)² / (n – 1)]

• Lower SD: Indicates higher precision.
• Higher SD: Indicates lower precision.

Formal Potential (E°’)

Formal potential is the electrode potential measured under specific experimental conditions such as pH, ionic strength, and complex formation.

HETP and Column Efficiency

HETP (Height Equivalent to a Theoretical Plate) is the height of a chromatographic column equivalent to one theoretical plate.

Formula: H = L / N

Where:

• H: HETP
• L: Column length
• N: Number of theoretical plates

Note: Lower HETP indicates higher column efficiency.

Partition Coefficient

The partition coefficient is the ratio of the concentration of a solute in two immiscible liquids at equilibrium.

Formula: K = C₁ / C₂

Capacity Factor

The capacity factor is a measure of the retention of a solute in chromatography.

Formula: k’ = (t_R – t_M) / t_M

Where:

• t_R: Retention time
• t_M: Dead time (void time)

Classification of Electrodes

1. Electrode of the First Kind

Consists of a pure metal in contact with a solution containing its own metal ions. The potential depends on the ion concentration.

• General Representation: M(s) | Mⁿ⁺
• Electrode Reaction: Mⁿ⁺ + ne^– → M(s)
• Examples: Zn | Zn²⁺, Cu | Cu²⁺

2. Electrode of the Second Kind

Consists of a metal covered with its sparingly soluble salt, immersed in a solution containing the anion of that salt.

• General Representation: M | MX(s) | X^–
• Electrode Reaction: MX(s) + e^– → M(s) + X^–
• Examples: Ag | AgCl | Cl^– (Silver–Silver Chloride), Hg | Hg₂Cl₂ | Cl^– (Calomel)

3. Electrode of the Third Kind

Consists of a metal coated with a sparingly soluble salt, immersed in a solution containing another metal ion that forms an insoluble salt with the same anion.

• General Representation: M | MX₁ | MX₂ | M₂ⁿ⁺
• Example: Silver–Silver Sulfide electrode used for determining heavy metal ions like Pb²⁺, Cd²⁺, and Hg²⁺.
• Electrode Reaction (Example): Ag₂S + 2e^– → 2Ag + S^2-