When measuring for phenolphthalein alkalinity, the sample is titrated to an end-point pH of what?

The correct approach for measuring phenolphthalein alkalinity involves titrating the water sample to an endpoint pH of 8.3. At this pH, phenolphthalein changes color, which indicates the transition point between acid and base conditions in relation to the presence of bicarbonates and carbonates in the sample.

Phenolphthalein alkalinity is primarily associated with the presence of carbonate ions, which contributes to the buffering capacity of water. A pH of 8.3 aligns with the typical transition point where carbonate ions begin to deprotonate to form bicarbonate ions. Thus, reaching this specific pH during titration signifies that carbonate is buffering the pH up to this point, effectively measuring the strength of the alkalinity attributable to this component.

Understanding the specific pH endpoint is essential for accurately assessing water quality, particularly in evaluating its capability to neutralize acids and maintain pH stability. This knowledge is crucial for laboratory analysts working with drinking water to ensure it meets health and safety standards.