Distribution Law

Definition of Distribution Law:

• The distribution law, also known as Nernst’s distribution law, states that when a solute is distributed between two immiscible solvents at a constant temperature, the ratio of its concentrations in the two solvents is constant.

  • • • • • $\frac{[\text{Solute}]_A}{[\text{Solute}]_B} = K_d$

• $[\text {Solute}] _A \quad \text{and} \quad [\text {Solute}] _B$
  are the concentrations of the solute in solvents A and B, respectively.
• K_d​ is the distribution coefficient or partition coefficient

Assumptions:

• The solute does not react chemically with any of the solvents.
• The solute exists in the same molecular form in both solvents.
• The system is at a constant temperature.
• The solvents are immiscible.

Limitations of the Distribution Law

1. Chemical Changes: If the solute reacts chemically or changes state in one of the solvents, the law doesn’t apply.
2. Solvent-Solvent Interaction: Even slight miscibility between solvents can affect the partition coefficient.
3. Temperature Variations: Changes in temperature can alter the partition coefficient, affecting distribution.
4. Concentration Dependence: High concentrations can lead to solute saturation or self-interaction, deviating from expected behaviors.
5. Physical Form Variations: Differences in solute ionization or physical state between solvents can impact distribution.

Applications of the Distribution Law

1. Drug Design and Pharmacokinetics: Essential for understanding drug distribution across body compartments, impacting dosing and delivery.
2. Environmental Science: Helps predict the movement of pollutants between phases like water and oil.
3. Extraction Processes: Guides the selection of solvents for efficient compound extraction in chemical and pharmaceutical industries.
4. Analytical Chemistry: Basis for solvent choice in liquid-liquid extraction, optimizing separation of analytes.
5. Food Industry: Influences the formulation by predicting how additives distribute between different phases.

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