In the total alkalinity test, the sample is titrated to an end-point pH of what?

In the total alkalinity test, the sample is titrated to an end-point pH of 4.5. This is the correct endpoint because total alkalinity primarily measures the buffering capacity of water, which is largely due to carbonate and bicarbonate ions that can neutralize acids. Achieving an endpoint pH of 4.5 effectively indicates that all bicarbonate ions have reacted and that the pH is low enough to ensure the presence of acid and the completion of the reaction.

At this pH level, it can be reliably determined that residual buffering capacity is predominantly due to the hydroxide and carbonate forms that are stable in higher pH conditions. Therefore, the choice of pH 4.5 is crucial for accurately assessing the sample's total alkalinity in order to understand its ability to resist changes in pH.