Ricardo M. Pytkowicz

The solubility of calcite in seawater at atmospheric pressure and 35%permil; salinity

The apparent (stoichiometric) solubility product of calcite in artificial seawater of salinity 35‰ was measured by a saturometer technique. The value of the apparent solubility product was found to be (4·59 ± 0·05) × 10−7 moles/(kilogram of seawater)2 at 25°C with a temperature coefficient of −0·0108 × 10−7/°C between 2 and 25°C. These values are significantly smaller than those found by MacIntyre (1965) and other workers. The effect of these results on the saturation of the oceans with respect to calcite is examined.

Ionization of water in seawater

Abstract The ion product of water in seawater and the total activity coefficients of hydroxide and hydrogen ions were determined over the temperature range 2° to 35°C, and the salinity range 20‰ to 44‰. At 25°C and 35‰ salinity, the measured values are p K W SW = 13.20, f OH = 0.22, f H = 0.71 on the molar concentration scale.

Phosphate association with Na+, Ca2+ and Mg2+ in seawater

Abstract Association constants of orthophosphate ion with Na + , Ca 2+ , and Mg 2+ were measured at μ = 0.68 and 20°C. The results were used to calculate phosphate speciation in seawater. Free HPO 4 2− ion and MgHPO 4 o are found to be the predominant species.

Effect of temperature and pressure on sulfate ion association in sea water

Abstract The temperature dependences between 2° and 25°C of the stoichiometric association constants of NaSO4− and MgSO4° ion-pairs were determined by a potentiometric procedure. The constant for NaSO4− ion-pairs was also measured at pressures up to 1000 atm. These data were used with estimates of the effect of pressure on MgSO4° ion-pairs to evaluate the effects of temperature and pressure on the major chemical species of sulfate present in sea water. The results indicate that sulfate ion-pairing in sea water increases as the temperature decreases. The NaSO4− ion-pairs dissociate as pressure increases, and the sulfate released causes an increase in the concentration of MgSO4° ion-pairs. These results indicate that the ultrasonic absorption of sea water due to MgSO4° ion-pairs may increase with depth in the ocean, rather than decrease, as is predicted from single-salt solutions.

The solubility of calcite — probably containing magnesium — in seawater

The apparent solubility product of calcite was measured by saturometry as a function of temperature and salinity. Simplified equations for the carbonic-acid dissociation constants of Mehrbach et al., 1973 (Limnol. Oceanogr., 18: 897–907) have been derived from their experimental data and used to calculate apparent solubility product, K′sp, K′sp at 25°C and 35‰ salinity, was found to be K′sp = 4.70 × 10−7(mol2 kg seawater−2) An equation was fitted to the experimental data, resulting in pK′sp = 6.5795 − 3.7159 × 105(TS) + 0.91056(T/S) − 22.110(1.0/S) The mean activity coefficients, γ±CaCO3, were calculated at various temperatures and salinities, using the thermodynamic solubility product of Jacobson and Langmuir, 1974 (Geochim. Cosmochim. Acta, 38: 301–318) and the apparent solubility products quoted in their paper. The change in K′sp at each salinity, as a function of temperature, was used to calculate the apparent enthalpy of dissociation for calcite, ΔH′, and the extrapolated value of ΔH0 was in good agreement with that of Jacobson and Langmuir. Finally, this work was used to calculate saturation profiles for oceanic stations and as a basis for comment of the accuracy of in-situ saturometry, as well as the applicability of in-situ K′sp pressure corrections.

Interaction of metal complexes with coulombic ion-pairs in aqueous media of high salinity

The competition for anions between the cations of the alkali and alkaline earth metals (to form ion pairs) and the cations of heavy metals (to form complexes) is investigated. The interaction is shown to affect the stability constants of the heavy metal complexes and the nature of the ionic species present in aqueous media of high salinity. The theory is discussed with special reference to NaCl-NaClO4 solutions, seawater and the labile complexes of lead and cadmium.

Theoretical model for the formation of ion-pairs in seawater

Abstract Predictions of association constants for ion-pair formation in seawater were compared to measured constants as a test for the existence of ion-pairs. A fair agreement was obtained in most cases, although the theoretical electrostatic calculations indicate that significant chloride ion-pairing should occur which has not been observed in most experimental investigations.

High Pressure Solubility of Calcium Carbonate in Seawater

The saturation concentration of calcium carbonate, equilibrated with aragonitic oolites, is about 2.7 times larger after exposure to 1000 atmospheres than it is when saturation is achieved at 1 atmosphere. This sugests that deep ocean waters may be undersaturated with calcium carbonate, in which case the alkalinity of the oceans would increase as sinking calcareous organisms dissolve.

Ion association of chloride and sulphate with sodium, potassium, magnesium and calcium in seawater at 25°C

Abstract Stoichiometric association constants, which have been measured for the ion pairs of Cl − and SO 4 −2 with Na + , K + , Mg 2+ , and Ca 2+ , were used to determine the speciation in an artificial sea water containing only these ions. The resulting distribution is quite different to that found in earlier models in which chloride ion association was ignored. The concentrations of chloride ion pairs with the cations are 4 to 5 times larger than the concentrations of the sulphate ion pairs with the same cations. The total activity coefficients of the neutral salts in sea water calculated from the model are in good agreement with the experimentally measured values. The concentration of MgSO 4 0 calculated to be present agrees with the amount determined from ultrasonic absorption data. The calculated solubility of gypsum is also in good agreement with the measured values.

Carbonate cycle and the buffer mechanism of recent oceans

Abstract Existing estimates of the fluxes of carbon dioxide and carbonates through the oceans are extended to include solution of calcium carbonate and oxidative regeneration of carbon dioxide at depth. The relative roles of carbonates and silicates in the buffer mechanism of the oceans are examined and it is shown that, within a short time scale, carbonates are the primary buffering agents.