What is the endpoint pH for total alkalinity?

The correct endpoint pH for determining total alkalinity is 4.5 pH. Total alkalinity is a measure of a water's ability to neutralize acids, and it primarily reflects the presence of bicarbonate, carbonate, and hydroxide ions. In a titration procedure used to determine alkalinity, the endpoint at a pH of 4.5 indicates the point at which bicarbonate ions are converted to carbonic acid, effectively representing the complete neutralization of alkalinity.

At this specific pH, the system transitions from the presence of alkalinity that can buffer against acid inputs to a state where the buffering capacity is diminished, allowing for further acidification. Titration beyond this endpoint would primarily affect the free carbonic acid present rather than the alkalinity itself.

Other pH values such as 7.0, 8.3, and 10.0 do not represent the accurate point for total alkalinity titration, as they do not effectively indicate the transition state of bicarbonates in the presence of acids, which is critical for accurate alkalinity assessments.