2.8:

Acid–Base Equilibria: Activity-Based Definition of pH

JoVE Core
Analytical Chemistry
Un abonnement à JoVE est nécessaire pour voir ce contenu.  Connectez-vous ou commencez votre essai gratuit.
JoVE Core Analytical Chemistry
Acid–Base Equilibria: Activity-Based Definition of pH

393 Views

01:10 min

April 04, 2024

For an ideal solution, the pH is defined as the negative logarithm of the hydrogen ion concentration. For a non-ideal solution, an accurate measurement of the pH must consider the negative logarithm of the hydrogen ion activity rather than concentration. In such a solution, the pH can be more accurately defined as the negative logarithm of a product of the hydrogen ion concentration and its activity coefficient.

In solutions of very low ionic strength—for example, pure water—the activity coefficient of the hydrogen ion is close to one when the ionic strength of the solution increases due to the addition of an electrolyte that does not donate or accept a proton. This results in a slight decrease in the pH of the solution. In other words, the addition of an electrolyte increases the hydrogen ion activity, or the effective hydrogen ion concentration in the solution, which decreases the pH of the solution.