Dissociation Constant.pptx

Dissociation constant is defined as the amount of reactant that dissociates reversibly to form component products.

1. Dissociation Constant

2. The strength of an acid depends on its ability to transfer its proton (H+) to A Base to form a conjugated base. When an acid (HA) dissolves in water It transfers its proton to water (as bronsted base) to form hydronium ion (H 3 O + ) and conjugated base. HA + H 2 O ⥨ H 3 O + + A - Conjugate base

3. HA + H 2 O ⥨ H + + A - This equation represent the dissociation of HA into H + and A - Applying law of mass action Where Ka is called the acid dissociation constant.

4. Similarly for bases According to a Arrhenius concept, base is a substance produce OH - ions. Let us consider base BOH BOH ⥨ B + +OH - Applying law of mass action Where Kb is called base dissociation constant.

5. So, strength of an acid and base is defined as the concentration of the H + ions and OH - ions, respectively in its aqueous solution at room temperature. So, value of dissociation constant (for both acid and base) measures its acid strength and base strength or acidity and basicity respectively. The most common way to represent the dissociation constant by taking - Log 10 of dissociation constant. Represented as pKa pKa = -log 10 (Ka).

6. Applications The acid base dissociation constant ( pka ) of a drug is a physiochemical parameter which influences biopharmaceutical properties. The knowledge of the pka value is important for quantitative analysis of system involving acid base equilibrium. It is also essential for working with buffers. It is used with pH indicators.

7. Measurement of dissociation constant Ka and kb or pka can be determined by: Potentiometric titration. Spectrophotometric methods. Capillary electrophoresis. Liquid chromatography. Conductivity method. Solubility method.

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