When no phenolphthalein (P) alkalinity is present, what does this indicate about the forms of alkalinity?

When no phenolphthalein alkalinity is present, it indicates that there is no carbonate alkalinity in the water and that only bicarbonate is present. In water chemistry, phenolphthalein alkalinity is measured at a pH range of about 8.3 to 10.0 and is primarily associated with the presence of carbonate ions (CO3^2-). If phenolphthalein alkalinity is absent, this signifies that the pH is below 8.3 and that any carbonate ions would have converted to bicarbonate ions (HCO3^-).

In this situation, the water will primarily exhibit bicarbonate alkalinity because bicarbonate is stable at lower pH levels and can effectively neutralize acids, which is a key characteristic of the alkalinity in such circumstances. Therefore, the absence of phenolphthalein alkalinity clearly indicates that the carbonate form has been depleted and only bicarbonate remains, which aligns with the properties of alkalinity.