Determination of hydrogen ion concentration in softwater lakes using carbon dioxide equilibria

Abstract

A strategy for determining the hydrogen ion content of fresh waters is proposed that involves total dissolved inorganic carbon (DIC or σCO 2) and CO 2 partial pressure ( P CO 2 ) measurements rather than pH electrode measurements. This recommendation derives from discrepancies between pH and carbon dioxide equilibria measurements made on several softwater lakes at the Experimental Lakes Area, northwestern Ontario. The pH calculated from DIC, P CO 2 , and the first dissociation constant of carbonic acid ( K 1) data was consistently higher than that directly measured with a pH electrode. Similarly, calculation of P CO 2 of surface waters from pH, DIC, and K 1 data gave values up to twice that of atmospheric saturation despite repeated equilibrations with atmospheric P CO 2 . Laboratory experiments demonstrated that the high dissolved organic carbon content of these waters appears to alter the electrode response yielding pH values lower than the true values. Furthermore, the uptake of protons by weak organic acid anions appear to be the cause of the measured difference between total (Gran) and carbonate (DIC — dissolved CO 2) alkalinity. Therefore bicarbonate ion concentration must be calculated from the difference between the total dissolved inorganic carbon content and uncharged dissolved CO 2 content. These procedures should provide more accurate and consistent results in the pH trend in surface waters and hence yield a solid baseline against which the effects of acid precipitation can be assessed.