What response would you expect in the apparatus of Figure...

What response would you expect in the apparatus of Figure $5-4$ if the solution tested were $1.0 \mathrm{M} \mathrm{HCl}$ ? What response would you expect if the solution were both $1.0 \mathrm{M} \mathrm{HCl}$ and $1.0 \mathrm{M} \mathrm{CH}_{3} \mathrm{COOH} ?$

Key Concepts

pH Measurement and the Nernst Equation

Electrochemical sensors, such as pH electrodes, convert the activity of hydrogen ions into an electrical potential. This conversion is governed by the Nernst equation, which shows that the electrode potential is a logarithmic function of the hydrogen ion concentration. Thus, in a solution where HCl is fully dissociated, the high concentration of hydrogen ions will produce a potential corresponding to a very low pH, typically near zero for 1.0 M HCl.

Common Ion Effect in Mixed Acid Solutions

When both a strong acid and a weak acid are present in a solution, the high concentration of hydrogen ions from the strong acid can suppress the ionization of the weak acid (an effect known as the common ion effect). As a result, the contribution of the weak acid to the overall hydrogen ion concentration is minimal, and the measured electrode response is largely governed by the strong acid. This concept is critical in predicting that the response in a mixed solution will be very similar to the response for the strong acid alone.

Acid Dissociation

This concept involves understanding that acids differ in their degree of ionization. A strong acid, like HCl, completely dissociates in water, producing a high concentration of hydrogen ions. In contrast, a weak acid such as acetic acid only partially dissociates, generating fewer free hydrogen ions in solution. This distinction is foundational in explaining why the measured response in an electrochemical system might vary depending on which acids are present.

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