If your phenolphthalein alkalinity is 2.5 mg/l and total alkalinity is 10 mg/l, what is your bicarbonate alkalinity?

To determine bicarbonate alkalinity from the given values, it's important to understand the relationship between phenolphthalein alkalinity, total alkalinity, and bicarbonate alkalinity.

Total alkalinity is the sum of all forms of alkalinity in the water, predominantly bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions, as well as hydroxide ions (OH⁻) in some cases. Phenolphthalein alkalinity specifically indicates the presence of carbonate and hydroxide ions, but does not account for bicarbonate.

In this case, phenolphthalein alkalinity is reported as 2.5 mg/L, which represents the carbonate (and any hydroxide) present. The total alkalinity is the overall measure of alkalinity, reported as 10 mg/L. To find bicarbonate alkalinity, we subtract the phenolphthalein alkalinity from the total alkalinity:

Bicarbonate alkalinity = Total alkalinity - Phenolphthalein alkalinity
Bicarbonate alkalinity = 10 mg/L - 2.5 mg/L
Bicarbonate alkalinity = 7.5 mg/L

Thus, the bicarbonate alkalinity is correctly