If phenolphthalein alkalinity is equal to total alkalinity, what is the source of all alkalinity?

When phenolphthalein alkalinity is equal to total alkalinity, it indicates that the water sample contains only hydroxide ions contributing to its alkalinity. This situation arises because phenolphthalein alkalinity primarily measures the concentration of hydroxide (OH-) and carbonate (CO3^2-) ions, while total alkalinity measures contributions from hydroxide, bicarbonate (HCO3-), and carbonate.

If the phenolphthalein alkalinity matches the total alkalinity, it suggests that there are no bicarbonate or carbonate ions present in the sample, as these typically contribute additional alkalinity that would cause total alkalinity to exceed phenolphthalein alkalinity. Instead, the entire alkalinity is derived from the presence of hydroxide ions, which can contribute to the overall alkalinity significantly, particularly in a more basic environment. Hence, the source of all alkalinity in this case is hydroxide alkalinity.