The solubility of oxygen in the major sea salts and their mixtures at 25°C
Abstract
The solubility of gases and other nonelectrolytes in electrolyte solutions are normally examined using the Setschenow (1899) equation, which for oxygen is given by: ln{[O_{2}] ^{0}/[ O_{2}]}= lnγ(O_{2})= k_{S}m The ratio of the solubility of oxygen (μmol kg ^{1}) in water [O _{2}] ^{0} and solution [O _{2}] is equal to the activity coefficient (γ) at a given molality (m), and k _{S} is the salting coefficient. In this paper, measurements of the solubility of oxygen in the major sea salts (NaCl, MgCl _{2}, Na _{2}SO _{4}, and MgSO _{4}) from dilute solutions to saturation at 25°C are examined. Measurements were also made on mixtures of the major sea salts (NaCl + Na _{2}SO _{4}, NaCl + MgCl _{2}, Na _{2}SO _{4} + MgSO _{4}, MgCl _{2} + MgSO _{4}, NaCl + MgSO _{4} and Na _{2}SO _{4} + MgCl _{2}). The solubilities [O _{2}] have been fitted to equations of the form: ln[O_{2}]= A+ Bm+ Cm^{2} where A, B, and C are empirical constants. The salting coefficient k _{S} is equal to (B + C m), and the value of (∂lnγ/∂m) _{m = 0} = B. The mixtures were also fitted to similar equations, where m is replaced by the total molality (m _{T}). The values of k _{S} for the mixtures can be estimated from the endmembers by: k_{S}( NaCl+ MgSO^{4})= N_{NaCl}k_{S}( NaCl)+ N_{MgSO4 }k_{S}( MgSO_{4}) where N _{i} is the mole fraction of the salt (i). This equation predicts solubilities for the mixtures that are in reasonable agreement with the measured values (±20 μmol kg ^{1}). The division of the values of k _{S} of the salts into ionic components [k _{S}(NaCl) = k _{S}(Na ^{+}) + k _{S}(Cl ^{})] yields different values for k _{S}(SO4 ^{2}) from Na _{2}SO _{4} and MgSO _{4} solutions. This is related to different interactions of SO4 ^{2} with Na ^{+} and Mg ^{2+} ions. These interactions are accounted for using the Pitzer equations: lnγ _{O2 }=2Σ _{c}λ _{O2c}m_{c}+2Σ _{a}λ _{O2a}m_{a}+2Σ _{N}λ _{O2N}m_{N}+Σ _{c}Σ _{a}m_{c}m_{a}ζ _{O2ca} where λ _{O2i} is a parameter that accounts for the interaction of O _{2} with cations (c) and anions (a) and ζ _{O2ca} accounts for interactions for O _{2} with the cation and anion (ca). The component N is used to denote the interactions between O _{2} with neutral species (e.g., the MgSO _{4} ion pair). The λ _{O2i} and ζ _{O2ca} coefficients determined for the major sea salt ions are in reasonable agreement with the tabulations of Clegg and Brimblecombe (1990). The calculated activity coefficients and solubilities in the mixtures were found to be in good agreement with the measured values. The solubilities of oxygen in the mixtures agree to within ±5 μmol kg ^{1} except for the Na _{2}SO _{4} + MgCl _{2} solutions (10 μmol kg ^{1}). This can be related to the formation of the MgSO _{4} ion pair in the mixture. Calculations of the solubility of O _{2} in seawater and artificial Dead Sea water with the model were found to be in good agreement with the measured values (±4 mol kg ^{1}).
 Publication:

Geochimica et Cosmochimica Acta
 Pub Date:
 July 2002
 DOI:
 10.1016/S00167037(02)008384
 Bibcode:
 2002GeCoA..66.2349M