What does the absence of phenolphthalein (P) alkalinity in a sample indicate?

The absence of phenolphthalein (P) alkalinity in a water sample indicates that all alkalinity is in the bicarbonate form. Phenolphthalein alkalinity measures the concentration of carbonate and hydroxide ions, which are present at higher pH levels (typically above 8.3). If P alkalinity is absent, it suggests that the pH of the water sample is below this threshold, meaning that the carbonate and hydroxide ions are not available in significant quantities. Thus, the remaining alkalinity must be attributed to bicarbonate ions, which are stable in a pH range of approximately 6.5 to 8.4.

By understanding this relationship between pH and the forms of alkalinity, one can accurately interpret water chemistry and its implications for treatment and distribution processes in drinking water systems.