Bicarbonate and carbonate ion-pairs and a model of seawater at 25°C1

The apparent (stoichiometric) association constants for the formation of ion-pairs between bicarbonate and carbonate and the major cations of seawater at 25°C were dctcrmined experimentally. The results wcrc used, in conjunction with earlier data on sulfate ion-pairs, to calculate the concentrations of the major species present in scawatcr. Here we measured the apparent (stoichiometric) association constants for the formation of bicarbonate and carbonate ion-pairs with sodium, calcium, and magnesium ions at 25°C and 0.72 ionic strength. The results were used, in conjunction with the sulfate association constants determined by Kester and Pytkowicz ( 1969)) to detcrmine the major ionic species present in seawater at 25°C and 34.8z0 salinity. We will show later that the proportions of ion-paired and free species are insensitive to the salinity. Activity coefficients are lower and solubility products higher in some mixed elcc.trolyte solutions such as seawater than in chloride solutions ( Kester and Pytkowicz 1969). This is usually attributed to ionic interactions, which can range from the formation of ion-pairs (outer sphere interactions due to electrostatic attraction) to that of true complexes (inner sphere bonds with loss of water of hydration). Partial molal volume data (Kestcr 1969) and results from Raman spectra (Daly et al. 1972) suggest that the major ions of seawater may form ion-pairs, although the potentiometric method we used does not distinguish between ion-pairs and complexes. Ionic interactions are important because, by affecting the charge distribution of solutes, they influence properties such as electrical conductivity, osmotic pressure, solubility of minerals, and sound attenua‘This work was supported by National Science Foundation Grants GA-17011 and (X-36057X and by Office of Naval Research Contract N00014-07A-0369-0007. tion. The effect of interactions on activity coefficients can be successfully accounted by two approaches; the specific interaction model of Br@nstcd ( 1922) and Guggenheim ( 1935)) in which interaction terms of an unspecified nature arc measured in bi.nary solutions and are applied to seawater ( Leyendclkers 1972; Robinson and Wood 1972; Whitfield 1973) and the ionpair concept of Bjerrum, which was introduced to oceanography by Carrels and Thompson (1962), and in which association constants are determined and applied to seawater. The two methods differ in their algebra but have much in common bccause they both require measurements in simple solutions and their application to seawater by the use of an extended ionic strength principle, namely, that interaction coefficients and association constants are independent of the medium composition at a given ionic strength. The validity of this principle was tested successfully (Pytkowicz and Kcster 1969; Kcster and Pytkowicz 1969; ILeyendekkers 1972; Robinson and Wood 1972; Whitfield 1973) because species distributions and activity coefficients calculated from measurements in binary solutions agreed with results in seawater, and because apparent association constants were found to be independent of composition at a given ionic strength. Both methods are compatible with Harned’s rule (Pytkowicz and Kester 1969) and lead to successful predictions of activity coefficients (Leyendekkers 1972; Whitfield 1973). It should be emphasized that, although apparent association constants can be meaLIMNOLOGY AND OCEANOGRAPHY 223 MARCH 1974, V. 19(2) 224 Pytkowicx and Hae Thompson 1966; Thompson and Ross 1966; Pytkowicz and Gates 1968; Pytkowicz and Kester 1969; Kester and Pytkowicz 1968, 1969) but there are significant differences for several species concentrations, especially anionic ones. This occurs because Garrels and Thompson used estimated singIe ion activity coefficients which are uncertain, especially in the case of sulfate ions, as these are associated in all solutions that could be used for the mean salt method, and because they assumed, as a result of the scarce data available at the time, that the activity coefficients of charged ionpairs were equal to those of bicarbonate while those of ion-pairs with zero net charge were the same as those of nonelectrolytes, even though ion-pairs are dipolar entities. We avoided questionable assumptions about activity coefficients in our choice of experimental methods as far as possible. In the earlier sulfate work we used ratios of activity coefficients of cations, thus canceling errors in part, and at no time did we have to resort to estimating activity coefficients of anions and of ion-pairs. The difficulty of estimating such coefficients accurately makes it difficult to apply the thermodynamic association constants determined by Hostetler (1963), Nakayama ( 1971), and Reardon and Langmuir (in prep.) to seawater. In the sulfate work chloride solutions were used as reference solutions (in which association does not occur) and cation activities were compared in chloride and seawater to determine the extent of ion-pairing. In this work on bicarbonate and carbonate ions, changes in the dissociation equilibria of carbonic acid with composition at constant ionic strength provide the ion.pairing information. We have used reference electrodes with salt bridges and assumed that liquid junc,tion potentials, to a first approximation, are independent of composition at a given ionic strength rather than avoiding salt bridges and having to make assumptions about the activity coefficient of chloride ions. Either approach has to be viewed with caution and the results considered tentative until confirmed by other independent methods. Our assumption about liquid junction potentials appears justified, within the precision of the method, because the association constants that we determined for sulfate ion-pairs (Pytkowicz and Kester 1969) and for bicarbonate ion-pairs ( this work) are independent of composition and lead to a successful prediction of the activity coefficients of cations (Kester and Pytkowicz 1969). In the case of ca:cbonates we are forced to invoke triple ions to explain the measurements although the experimental results may reflect changing liquid junction potentials. Still, the relative strength of the several triple ions will be justified from an energetics viewpoint and the extent of carbonate association shown to be compatible with data on carbonic acid dissociation and on calcium carbonate solubility. Earlier workers did not find triple ions because they did not measure the concentration dependence of the association. The ion-pair concept is used in preference to the specific interaction model because data on sound attenuation (Fisher 1967) and from Raman spectroscopy (Daly et al. 1972) indicate that ion-pairs may indeed exist. Still, the specific interaction model is a valuable tool for the study of multielectrolyte solutions. Details of experimental and calculation procedures not presented here are available in Hawley ( 1973). We thank C. Culberson and S. Ingle for assistance in this work and. S. Williams for Bicarbonate and carbonate ion-pairs 225 typing and proofing the many versions of the manuscript.