What do we call the logarithm of the reciprocal of hydrogen-ion concentration?

The term that describes the logarithm of the reciprocal of hydrogen-ion concentration is pH. pH is a scale used to specify the acidity or basicity of an aqueous solution. By definition, it is calculated as the negative logarithm (base 10) of the concentration of hydrogen ions (H⁺) in a solution. This logarithmic relationship means that as the hydrogen-ion concentration increases, the pH value decreases, indicating a more acidic solution. Conversely, as the hydrogen-ion concentration decreases, the pH value increases, indicating a more basic solution.

In contrast, options that refer to "alkaline," "concentration gradient," or "enzyme" do not accurately describe this relationship. Alkaline specifically refers to a basic solution with a pH above 7, and concentration gradient describes the difference in concentration of a substance across a space. An enzyme is a biological catalyst, unrelated to the measurement of acidity or basicity. Thus, pH is the precise term that represents the logarithmic relationship with hydrogen-ion concentration, confirming that it is the correct answer.