Question

The pH is directly proportional to pKa?

(a) True

(b) False

The question was asked in a job interview.

The origin of the question is Buffers topic in chapter Introduction to Fermentation Processes & Media for Industrial Fermentation of Bioprocess Engineering

Answer 1

The correct answer is:

(b) False

The pH is not directly proportional to the pKa of a buffer. Instead, pH is related to the pKa through the Henderson-Hasselbalch equation, which expresses the relationship between pH, pKa, and the concentrations of the acid and its conjugate base in a buffer solution.

The equation is:

pH=pKa+log⁡([A−][HA])\text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right)pH=pKa+log([HA][A−]​)

Where:

• pH is the acidity of the solution,
• pKa is the acid dissociation constant,
• [A−][\text{A}^-][A−] is the concentration of the conjugate base,
• $[\text{HA}]$ is the concentration of the weak acid.

So, while pH and pKa are related, pH is not directly proportional to pKa. The pH of a solution depends on the ratio of the concentrations of the acid and conjugate base, not just the pKa value.