What is the Difference Between Phenolphthalein Alkalinity and Total Alkalinity?

05/17/2025 • Giovanni Medeiros

The difference between phenolphthalein alkalinity and total alkalinity lies in the pH level at which they are determined and the forms of alkalinity they measure.

  • Phenolphthalein Alkalinity: This is determined by titration to a pH of 8.3 and indicates the total hydroxide and half of the carbonate present in the water sample. Phenolphthalein alkalinity measures the sum of all carbonate (CO3=), bicarbonate (HCO3–), and hydroxide (OH–) alkalinity above pH 8.3.
  • Total Alkalinity: This is determined by titration to a pH of 5.1, 4.8, 4.5, or 3.7, depending on the amount of carbon dioxide present. Total alkalinity measures all carbonate-bicarbonate alkalinity and hydroxide alkalinity at a pH of 4.5. It includes the alkalinity due to bicarbonate, carbonate, and hydroxide ions.

In summary:

  • Phenolphthalein alkalinity measures the alkalinity in a water sample at a pH of 8.3, focusing on the total hydroxide and half of the carbonate present.
  • Total alkalinity measures the alkalinity in a water sample at a pH of 4.5, accounting for all carbonate-bicarbonate alkalinity and hydroxide alkalinity.

Comparative Table: Phenolphthalein Alkalinity vs Total Alkalinity

The difference between phenolphthalein alkalinity and total alkalinity lies in the constituents they measure and the pH range at which they are measured. Here is a table summarizing the differences:

Parameter Phenolphthalein Alkalinity Total Alkalinity
Constituents Measures hydroxides and half of the carbonates Measures all carbonates, bicarbonates, and hydroxides at a pH of 4.5
pH Range Not specific, but usually measures alkalinity at a higher pH Measures alkalinity at a pH of 4.5
Unit of Measurement Not specific, but usually measured in ppm (parts per million) or mg/L CaCO3 (milligrams per liter of calcium carbonate) Measured in ppm (parts per million) or mg/L CaCO3 (milligrams per liter of calcium carbonate)
Purpose Indicates the alkalinity in a water sample Measures the water's ability to resist changes in pH

In summary, phenolphthalein alkalinity measures the alkalinity in a water sample due to hydroxides and half of the carbonates, while total alkalinity measures all the carbonates, bicarbonates, and hydroxide alkalinity at a pH of 4.5. Total alkalinity is closely associated with pH and is a measure of water's resistance to changes in pH.

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