Pharmaceutics Figure 4

Semi-log Plot of k' versus pH

Investigate the effect of changing k₀, k_H⁺, k_OH^-, k_a, k_b and C_a values on the semi-log plot of observed k' versus pH. Explore the equation by changing the parameter values.

The observed first order rate constant k' may be calculated a function of the specific reaction rate, k₀, the specific acid-base catalytic coefficients, k_H⁺ and k_OH^-, and the general acid-base catalytic coefficients, k_a and k_b, according to Equation 1.

Equation 1. Observed rate constant versus pH

k₀, the specific reaction rate is independent of pH

With specific acid catalysis of the reaction k_H⁺ is multiplied by the concentration of hydrogen ion, H⁺.

Specific base catalysis is included by multiplying k_OH^- by the concentration of hydroxyl ion concentration, OH^-.

General acid catalysis is included in equation 1 by multiplying k_a by the concentration of general acid, HA, given by equation 2.

Equation 2. Concentration of the acid form of a monoprotic acid

General base catalysis is included in equation 1 by multiplying k_b by the concentration of general base, A^-, given by equation 3.

Equation 3. Concentration of the base form of a monoprotic acid