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The solubilities of apatitcs of varying compositions were measured as a function of pH in seawater at 10" and 25°C. Wide ranges of solubilities were observed; at pH ,= 7.4 they ranged from 0.4 to 11 PM Poe-P liter-l, depending on the nature of the samples. Results are interpreted in terms of complex coatings formed on the surface of the apatitcs.The goals of our work were to define the solubility of apatites in seawater better and to examine the differences in solubility of apatites of different composition, We found that the solubility behavior is best described in terms of complex reactions on the apatite surface rather than by simple solubility theory.Studies of the solubility oE apatites in seawater have heretofore been based on the assumption of simple phases equilibrating with the seawater. Sill&r (1961) suggested that hydroxyapatite controls the equilibrium concentration of phosphate in seawater. Kramer (1964) and Roberson (1966) postulated that francolite, a carbonate fluorapatite, rather than hydroxyapatite is the solid phase which occurs in seawater and that its solubility is the one that should be studied. Differences in their results and those of Smirnov et al. (1962), and the relatively poor precision obtained by Roberson, made it impossible to determine accurately the saturation state of seawater with respect to apatite.The apatite surface is very susceptible to rearrangement or to complex formation in distilled water solutions (Dictz et al. 1964; Smith ct al. 1974). Greenwald ( 1942) reported surface area effects on the solubility oE calcium phosphates, although he used a poorly defined solid phase in his work. Levinskas and Neumann (1955) found a decrease in the solubility of hy-

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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.