Phenolphthalein Alkalinity Test
Phenolphthalein alkalinity is present only when free carbon dioxide (CO2) is absent and therefore exists only when the pH exceeds 8.3. Phenolphthalein alkalinity should never be over half the total alkalinity; otherwise, a caustic alkalinity is produced.
The apparatus and reagents needed for making this test are the same as described for the Total Alkalinity Test, with the exception that the indicator, phenolphthalein, is employed in the place of methyl orange or methyl purple.
- Measure out 100 mL of the water to be tested and pour into a clean while porcelain evaporating dish.
- With a dropping bottle, add 5 drops of phenolphthalein indicator to the sample. If the solution becomes pink, phenolphthalein alkalinity is present. No color indicates the phenolphthalein alkalinity is zero and this test is complete. No color also indicates free carbon dioxide is present, and the same sample may be used to carbon dioxide test.
- If pink color results after phenolphthalein indicator is added, add N/50 sulfuric acid slowly and carefully from the burette to the contents of the dish until the coloration disappears. While adding the acid, the contents of the dish should be gently stirred with the glass stirring rod. (A pH meter can be used to determine this end point also since the color disappears at a pH of 8.3).
- The number of milliliters of the acid required to remove the pink color multiplied by 10 equals the phenolphthalein alkalinity in ppm: that is, each 0.1 mL of the acid used is equal to 1 ppm of phenolphthalein alkalinity. Example: If 2.4 mL of acid were used, the phenolphthalein alkalinity would be 2.4 × 10 = 24 ppm.
Remarks: The phenolphthalein alkalinity is also recorded as calcium carbonate (CaCO3) since the strength of the sulfuric acid (H2SO4) used in the test is such that each milliliter used represents 10 ppm of CaCO3. If the presence of phenolphthalein alkalinity is over half of the total alkalinity in the water, then the water is caustic. Caustic water should be avoided in a domestic supply. Since it causes burning sensation when tasted.
It is recommended that drinking water shall not have a pH greater than 10.6 nor a normal carbonate alkalinity greater than 120 ppm. Although not stated in the standards, an appreciable alkalinity should always be present.
If the phenolphthalein alkalinity (P) and the total alkalinity (T) is known, the proportion of bicarbonate, carbonate, and caustic alkalinities present as CaCO3 can be computed from the table below.
Relationship between hydroxide, carbonate and bicarbonate, and
phenolphthalein and total alkalinity
Amount of Alkalinity Present as
Results of P and T
tests in ppm as
Carbonate (not to
exceed 120 ppm)
(1) P equals 0 0 0 T (2) 2P is less than T 0 2P T minus 2P (3) 2P is equal to T 0 2P 0 (4) 2P is more than T 2P minus T 2(T minus P) 0 (5) P equals T T 0 0
Assume that the phenolphthalein, or P, alkalinity is 0 and the total, or T, alkalinity is 80.
Note from the table that when P equals 0, all alkalinity is bicarbonate; therefore, there are 80 ppm alkalinity as bicarbonate.
Assume that P equals 50 ppm and T equals 80 ppm. In this instance, the situation is found in Line 4 of the table. It is noted that when 2P (2 × 50 = 100) is more than T (80 ppm), 2P minus T equals the hydroxide or caustic alkalinity.
Therefore, (2 × 50) - 80 = 100 - 80 = 20 ppm caustic alkalinity; and 2 (80 - 50) = 2 × 30 = 60 ppm carbonate alkalinity.