Dissociation constants of calcite and CaHC0 3 + from 0 to 50°C

Details Dissociation constants of calcite and CaHC0 3 + from 0 to 50°C Dissociation constants of calcite and CaHC0 3 + from 0 to 50°C

Feb 1974

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1974gecoa..38..301j&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 38, Issue 2, pp.301-318

Computer Science

4

Scientific paper

From conductance measurements, the negative logarithm of the dissociation constant of the CaHCO 3+ ion pair, p K ( CaHCO 3 + ), is 0.7, 1.0 and 1.35 within ±0.05 units at 0, 25 and 60°C, respectively. A revaluation of published and unpublished data yields p K ( CaCO 3 0 ) 3.2 at 25°C. Use of these p K 's to compute the dissociation constant of calcite ( Kc ) from published calcite solubility measurements in pure water gives p Kc values which increase markedly with ionic strength. However, if the ion pairs are ignored, computed p Kc values are nearly constant with ionic strength. All reasonable attempts to eliminate the trend in p Kc by adjusting ion activity coefficients, and/or values of K ( CaCO 3 0 ) failed, so the dilemma remains. Kc values computed from the most reliable published calcite solubility data are in good agreement with such values based on solubility data measured in this study at 5, 15, 35 and 50°C. Study results ignoring ion pairs are accurately represented by the equation log Kc = 13.870 -- (3059/ T ) -0.04035 T , and correspond to -8.35, -8.42, and -8.635 at 0, 25 and 50°C, respectively. The logarithmic expression leads to H r o = -2420 ± 300 cal/mol, Cp = -110 ± 2 cal/deg mol, and S r o = -46.6 ± 1.0 cal/deg mol for the calcite dissociation reaction at 25°C. The dependence of Kc on temperature when CaCO 3 0 and CaHCO 3 + are assumed, is described by log Kc = 13.543 - (3000/ T ) - 0.0401 T which yields -8.39, -8.47, and -8.70 at 0, 25 and 50°C. This gives H r o = -2585 ± 300 cal/mol, Cp = -109 ± 2 cal/deg mol, and S r 0 = -47.4 ± 1.0 cal/deg mol at 25°C.

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