What is the pH endpoint for alkalinity titrations using phenolphthalein?

The correct pH endpoint for alkalinity titrations using phenolphthalein is 8.3 pH. At this pH level, phenolphthalein changes color, indicating that the solution transitions from a basic to a more neutral environment. This is relevant for measuring the total alkalinity of water, which typically consists of bicarbonates, carbonates, and sometimes hydroxides.

When performing an alkalinity titration, phenolphthalein is specifically used to detect the presence of carbonate ions. The endpoint at 8.3 pH corresponds to the point at which the carbonates and bicarbonates in the water are effectively neutralized, signifying the amount of alkalinity contributed by these species. This is particularly important in wastewater treatment, as it helps in assessing the buffering capacity of the water and its ability to maintain stable pH levels.

Other pH levels like 7.0 or 4.5 do not correspond with the endpoint for alkalinity titrations. A pH of 7.0 signifies neutrality, whereas 4.5 is below the range we observe for carbonates and bicarbonates. Similarly, a pH of 10.0 indicates a more basic solution where a different indicator would