Non-carbonate hardness may be calculated for waters which are soft or moderately hard in a fairly satisfactory manner by deducting the total alkalinity from the total hardness by the soap method (pp. 31–34). For waters that are very hard, and particularly those that contain much magnesium, this method is not advised.

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.

Standard methods for the examination of water and sewage — Part 30

Table 8.—RELATIONS BETWEEN ALKALINITY TO PHENOLPHTHALEIN AND THAT TO METHYL ORANGE, ERYTHROSINE, OR LACMOID, IN PRESENCE OF BICARBONATE, CARBONATE, AND HYDROXIDE.

Standard methods for the examination of water and sewage — Part 30

─────────────────┬───────────────────────────────────────────────────── 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 ─────────────────┴─────────────────┴─────────────────┴─────────────────

Standard methods for the examination of water and sewage — Part 30

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.