Chemistry of natural waters—III carbonic acid

Abstract

The total carbonic acid content of a water can be represented as a function of the hydrogen in concentration and the total alkalinity. A very convenient approximation equation to enable the carbonic acid to be calculated from the pH and total alkalinity is derived and presented in tabular form, and the direct determination of the carbonic acid is briefly discussed. A similar equation and table for the calculation of the free carbonic acid is also given. The solubility of carbon dioxide in natural waters is discussed and it is shown that, at 25°C, this depends on the total alkalinity of the water. Equations and a numerical table are given to enable the percentage saturation of a water with carbon dioxide to be calculated, and it is concluded that this percentage saturation is a more reliable parameter for the detection of pollution than is the concentration of free carbonic acid. Data for unpolluted rivers of Natal shows that they are virtually in equilibrium with atmospheric carbon dioxide. If this is a general feature of surface waters, it explains immediately why the pH values of such waters, even those with very poor buffering powers, remain so constant. It also explains why waters in treatment works often do not show the pH response to chemical dosing that they theoretically should.