What indicates that the phenolphthalein alkalinity of a water sample is zero?

The phenolphthalein alkalinity of a water sample is determined primarily by the presence of hydroxide (OH-) and carbonate (CO3=) ions. A phenolphthalein indicator turns pink in alkaline conditions, specifically when the pH is greater than 8.3, which typically indicates the presence of sufficient hydroxides contributing to phenolphthalein alkalinity.

When a water sample has a pH continually measured above 8.3, this indicates that there is enough of these hydroxide or carbonate constituents to suggest that alkalinity exists. Therefore, a test result showing phenolphthalein alkalinity as zero implies that the pH is not high enough to support the presence of these ions, leading us to conclude that if the indicator color shows no change with a pH greater than 8.3, phenolphthalein alkalinity is effectively zero.

Considering the role of pH values, if the pH is less than 7, it indicates an acidic condition with no alkalinity present at all. A pH of exactly 8.0 represents a low alkaline condition but may still reflect some presence of phenolphthalein alkalinity which typically requires a pH higher than 8.