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Dissociation Constant, Determinations and Applications

Working in the laboratory requires a lot of caution, as strong acids could burn the skin if handled carelessly.

Dissociation constants

Working in the laboratory requires a lot of caution, as strong acids could burn the skin if handled carelessly. When determining which acids are weak or strong, how do we do that? Comparison of the dissociation constants between the acids is one method to accomplish this. Essentially, the dissociation constant is the ratio of original acid (reactants) to the dissociated ions (products). Ka is the abbreviation for it.

Acids (HA) dissociate into bases (A-) and the ion hydronium (H3O+) when they are in contact with water

Our typical method for determining the dissociation constant is to measure how much is dissociated in water. The conjugate base of an acid is formed whenever water is added to an acid, with some of the hydrogen atoms from the acid being transferred to the water. During this process, hydrogen atoms travel from the acid to the water, and then some of these hydrogen atoms transfer back to the acid (reforming it). And these hydrogen atoms continue moving between the acid and water. Until the reaction reaches equilibrium, the products and reactants are exchanged. The equilibrium of a reaction is when, over time, no concentration changes of the reactant or product occur. To calculate the dissociation constant, we need to reach equilibrium in the reaction. A quotient can be used to calculate it since it is the ratio of products to reactants:

In brackets, these molecules are grouped by their concentration. In brackets, these molecules are grouped by their concentration.

A dissociation constant is used in chemistry, biochemistry, and pharmacology to measure the tendency of a larger object to divide into smaller components, such as when molecules separate into their component molecules, or when ions separate into their components. Inversely related to the association constant is the dissociation constant. The ionization constant or dissociation constant can alternatively be referred to as an ionization constant in the case of salts

Determinations of dissociation constant

Different methods, such as conductivity, visible or UV absorption spectrometers, potentiometers, etc. are available. The most commonly used method is potentiometric pH.

A measure of acid dissociation

Ka= [H3O+ ] [A--]/ [HA]

Salt A- and an acid [HA] are present in equimolar concentrations, so the dissociation constant Ka--- is the same as the hydronium ion concentration.

Ka= [H3O+ ] when [A- ] = [HA]

Method

A solution containing an equimolar concentration of an acid and a strong base salt of an acid may be sampled and the pH determined. pKa is the concentration at which measurement can be made. By calculating ka from pKa. In the case of dissociation constants, pKa is the negative logarithm.

Applications

A drug's dissociation constants are determined by its physiological and pharmaceutical activities, solubility, solution rate, side of binding (protein binding), and rate of absorption.

Example - Because Unionized drugs are more readily available from the stomach, there is a higher rate and extent of absorption from the stomach area for weakly acidic drugs. Similarly, the intestinal tract is better at absorbing weak bases
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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