the hydronium ions. Le Chatelier's principle then drives the equation to the right, creating more hydronium ions. The hydroxide ions have reacted, lowering the concentration of OH-, but the hydronium ion concentration is approximately the same. Therefore, the pH remains unchanged. Thus a small amount of an acidic solution or of a basic solution can be added to the buffer solution with very little change in the pH.
The pH of a buffer solution changes according to the Henderson-Hasselbach equation:
Since the concentrations of the conjugate base and weak acid are only present in the logarithm, a small change in their concentrations has minimal effect on the overall equation. The minimal effect of the changing concentrations is reflected in the behavior of the buffer. For a buffer solution consisting of a weak base, the Henderson-Hasselbach equation takes the following form:
