The alkalinity of a natural water represents its content of carbonate,
bicarbonate, borate, silicate, phosphate, and hydroxide. Alkalinity is
determined by neutralization with standard sulfuric acid or potassium
bisulfate in the presence of phenolphthalein and either methyl orange,
erythrosine, or lacmoid as indicators. Methyl orange may be used except
in waters containing aluminium sulfate or iron sulfate. The relations
between estimates in presence of these indicators and the carbonate,
bicarbonate, and hydroxide radicles are indicated in Table 8. The
alkalinity of carbonates in the presence of phenolphthalein is different
from that in the presence of methyl orange, partly because of loss of
carbon dioxide and partly because of defects in phenolphthalein as an
indicator in such conditions.
Table 8.—RELATIONS BETWEEN ALKALINITY TO PHENOLPHTHALEIN AND THAT TO
METHYL ORANGE, ERYTHROSINE, OR LACMOID, IN PRESENCE OF BICARBONATE,
CARBONATE, AND HYDROXIDE.
─────────────────┬─────────────────────────────────────────────────────
Result of │ Value of radicle expressed in terms of calcium
titration.[C] │ carbonate.
─────────────────┼─────────────────┬─────────────────┬─────────────────
│ Bicarbonate. │ Carbonate. │ Hydroxide.
─────────────────┼─────────────────┼─────────────────┼─────────────────
P = 0 │ T │ 0 │ 0
P < 1/2T │ T − 2P │ 2P │ 0
P = 1/2T │ 0 │ 2P │ 0
P > 1/2T │ 0 │ 2(T − P) │ 2P − T
P = T │ 0 │ 0 │ T
─────────────────┴─────────────────┴─────────────────┴─────────────────
Footnote C:
T = Total alkalinity in presence of methyl orange, erythrosine, or
lacmoid. P = Alkalinity in presence of phenolphthalein.
_Reagents._—1. Sulfuric acid or potassium bisulfate. A N/50 solution.
2. Phenolphthalein indicator. Dissolve 5 grams of a good quality of
phenolphthalein in 1 liter of 50 per cent alcohol. Neutralize with N/10
potassium hydroxide. The alcohol should be diluted with boiled distilled
water.
3. Methyl orange indicator. Dissolve 0.5 gram of a good grade of methyl
orange in 1 liter of distilled water. Keep the solution in the dark.
4. Lacmoid indicator. Dissolve 2.0 grams of lacmoid in 1 liter of 50 per
cent alcohol. Dilute the alcohol with freshly boiled distilled water.
5. Erythrosine indicator. Dissolve 0.5 gram of erythrosine (the sodium
salt) in 1 liter of freshly boiled distilled water.