An assessment of excess carbon dioxide partial pressures in natural waters based on pH and alkalinity measurements

ethods of estimating excess partial pressures of carbon dioxide in river waters using pH and Gran alkalinity measurements are considered using data from several UK lowland rivers covering a spectrum of industrial, urban and agricultural catchments. Two simple equations are shown to be eminently suitable except for the most demanding accuracies at pH values greater than 9 when carbonate and hydroxide ions as well as calcium complexes with bicarbonate, carbonate and hydroxide become larger. The most basic of these equations, which simply allows for the averaged effects of temperature and ion activity, is View the MathML source The second equation, which allows for variation in temperature and average ionic strength, is View the MathML source Within this equation, the 0.95 term represents the average factor which converts the chemical concentration of monovalent ions into chemical activities and t°C is temperature in degrees Celsius. For more demanding situations, such as at high pHs, the following equation is suggested View the MathML source As in the previous case, the 0.95 term allows for activity concentration relationships for univalent ions. For this equation, pKwater and pK2 represent minus the logarithm of the equilibrium constants for the respective reactions H2O=H++OH− and HCO3−=H++CO32−, where View the MathML source and View the MathML source The (1+2.38∗10pH−pK2) term allows for the effect of the carbonate ion contribution to the alkalinity and it is the ratio of the carbonate ion to the sum of carbonate and bicarbonate with both the numerator and denominator in units of equivalent concentration. The 2.38 term converts the ratio of carbonate to bicarbonate from an activity to an equivalent concentration ratio. Comparisons of measured EpCO2 for the River Frome agree well with estimates based on the above equations and this adds to the confidence of the methodologies above pH 6. For pH<6, other methods are required owing to interferences from organic acids and aluminium