Waters which contain sodium or potassium carbonates or bicarbonates
contain all of their calcium and magnesium as carbonates or
bicarbonates. That is, they possess no non-carbonate hardness (sulfates,
nitrates or chlorides of calcium and magnesium).
The most accurate method is to determine the total alkalinity by
titration with N/50 sulfuric acid, using methyl orange, erythrosine, or
lacmoid as an indicator; then determine the calcium and magnesium
content; and subtract from the total alkalinity the computed alkalinity
due to the calcium and magnesium expressed in terms of calcium
carbonate. The remainder is the alkalinity due to carbonates and
bicarbonates of sodium and potassium.
This determination may also be made by applying the method, for
non-carbonate hardness with soda reagent (see p. 35), and by noting the
excess of acid required to neutralize the alkaline carbonates originally
present.
With present information as to solubilities of the normal carbonates of
calcium and magnesium, it is difficult in their presence to measure
slight quantities of carbonates of sodium or potassium.