When pH is not directly measured, which method is commonly used to estimate pH in buffer systems?

• A. Henderson-Hasselbalch equation
• B. Nernst equation
• C. Michaelis-Menten kinetics
• D. Arrhenius equation

Answer: (A)

In buffer systems, pH can be estimated when direct measurement isn’t practical by using the Henderson-Hasselbalch relationship. This equation links pH to the pKa of the weak acid in the buffer and the ratio of its conjugate base to the acid: pH = pKa + log([A-]/[HA]). The pH is governed by how much of the weak acid and its conjugate base are present, so knowing their concentrations lets you calculate the pH without a meter.

For example, if the buffer is acetate with a pKa around 4.76, equal amounts of acetate and acetic acid give pH ≈ 4.76. If you increase the base form relative to the acid, the pH rises; if you increase the acid relative to the base, the pH falls. Temperature changes can shift pKa, so the calculation assumes a constant temperature and that activity effects are negligible.

Other equations listed aren’t used for estimating pH in buffers: the Nernst equation deals with electrochemical potentials, not acid-base ratios; Michaelis-Menten kinetics describe enzyme reaction rates; Arrhenius temperature dependence describes how reaction rates change with temperature.