DISSOCIATION CONSTANTS FOR CARBONIC ACID DETERMINED FROM FIELD MEASUREMENTS

Abstract

A number of workers have recently shown that the thermodynamic constants for the dissociation of carbonic acid in seawater of Mehrbach et al. are more reliable than measurements made on artificial seawater. These studies have largely been confined to looking at the internal consistency of measurements of total alkalinity (TA), total inorganic carbon dioxide (TCO2) and the fugacity of carbon dioxide (fCO(2)). In this paper, we have examined the field measurements of pH, JCO(2), TCO2 and TA on surface and deep waters from the Atlantic, Indian, Southern and Pacific oceans to determine the pK(1), pK(2) and pK(2) - pK(1). These calculations are possible due to the high precision and accuracy of the field measurements. The values of pK(2) and pK(2) - pK(1) over a wide range of temperatures (-1.6-38degreesC) are in good agreement (within +/-0.005) with the results of Mehrbach et al. The measured values of pK(1) at 4degreesC and 20degreesC are in reasonable agreement (within +/-0.01) with all the constants determined in laboratory studies. These results indicate, as suggested by internal consistency tests, that the directly measured values of pK(1)+pK(2) of Mehrbach et al. on real seawater are more reliable than the values determined for artificial seawater. It also indicates that the large differences of pK(2) - pK(1) (0.05 at 20degreesC in real and artificial seawater determined by different investigators are mainly due to differences in pK2. These differences may be related to the interactions of boric acid with the carbonate ion. The values of pK(2) - pK(1) determined from the laboratory measurements of Lee et al. and Lueker et al at low fCO(2) agree with the field-derived data to +/-0.016 from 5degreesC to 25degreesC. The values of pK(2) - pK(1) decrease as the fCO(2) or TCO2 increases. This effect is largely related to changes in the pK(2) as a function of fCO(2) or TCO2. The values of fCO(2) calculated from an input of TA and TCO2, which require reliable values of pK(2) - pK(1), also vary with fCO(2). The field data at 20degreesC has been used to determine the effect of changes of TCO2 on pK2 giving an empirical relationship: pK(2)(2)(TCO) = pK(2) - 1.6 x 10(-4) (TCO2 - 2050) which is valid at TCO2 > 2050 mumol kg(-1). This assumes that the other dissociation constants such as K-B for boric acid are not affected by changes in TCO2. The slope is in reasonable agreement with the laboratory studies of Lee et al. and Lucker et al. (-1.2 x 10(-4) to -1.9 x 10(-4)). This equation eliminates the dependence of the calculated CO2 on the level of fCO(2) or TCO2 in ocean waters (sigma = 29.7 muatm in fCO(2)). An input of pH and TCO2 yields values fCO(2) and TA that are in good agreement with the measured values ( +/-22.3 muatm in fCO(2) and +/-4.3 mumol kg(-1) in TA). The cause of the decrease in pK(2) at high fCO(2), is presently unknown. The observed inconsistencies between the measured and computed fCO(2) values may be accounted for by adding the effect of organic acid (similar to8 mumol kg(-1)) to the interpretation of the TA. Further studies are needed to elucidate the chemical reactions responsible for this effect. (C) 2002 Elsevier Science Ltd. All rights reserved.