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. PROCEDURE WITH PHENOLPHTHALEIN. Add 4 drops of phenolphthalein indicator to 50 or 100 cc. of the sample in a white porcelain casserole or an Erlenmeyer flask over a white surface. If the solution becomes colored, hydroxide or normal carbonate is present. Add N/50 sulfuric acid from a burette until the coloration disappears.