The activity coefficients of non-electrolytes in seawater

Abstract

Abstract The solubility of non-electrolytes in electrolyte solutions has frequently been examined using the Setschnow equation ln (S 0 /S)= ln γ N =k S m where S0 and S are the solubilities in water and solution, kS is the salting coefficient of the non-electrolyte (N), γN is the activity coefficient, and m is the molality. The values of the salting coefficients are available for a number of non-electrolytes in some electrolyte solutions. Values are however not readily available for many of the major salts that make up natural waters (MgCl2, CaCl2, MgSO4, and Na2SO4). This requires the development of methods that can be used to estimate the salting coefficients of unknown salts for calculations of the solubility of non-electrolytes in natural waters. The measured values of kS have been shown to be in reasonable agreement with the values calculated using the Scaled Particle Model (kS=kα+kβ, where kα is related to the free energy of making space in the solution for the non-electrolyte and kβ is the free energy related to the interaction of the ions and the non-electrolyte). The values of kS in a given electrolyte have also been determined using the semi-empirical equation k S =a+b(e/k) where e/k is an energy parameter related to the polarizibility of the non-electrolyte, while the values in different electrolytes have been determined using correlations of the measured values of kS of two non-electrolytes k S (N)=a′+b′k S ( O 2 ) (a, a′, b, and b′ are adjustable parameters). These semi-empirical equations yield calculated values of kS that are in better agreement with the measured values than the Scaled Particle Theory. The measured and estimated values of kS have been used to estimate the Pitzer coefficients for 21 non-electrolytes (He, H2, Ne, N2, CO, N2O, O2, Ar, NO, CH4, CF4, Kr, C2H2, SF6, C2H4, Xe, C2H6, Rn, CCl4, C6H6, H2S) in major seasalts. The Pitzer parameters for the ions (λNi which is related to the interaction of ion I with the non-electrolyte N) have been used to estimate the activity coefficients in seawater. The calculated values are in good agreement with the measured values (H2, Ne, N2, CO, N2O, O2, Ar, CH4, Kr) and are in better agreement (σ=0.033) than the estimates made using the Scaled Particle Model (σ=0.077). A simple correlation of lnγN=0.0938+0.3404 kS (NaCl) gives an excellent fit (σ=0.014) of the experimental measurements in seawater (S=35 and t=25°C). This empirical equation can be used to determine solubilities for non-electrolytes in seawater from measurements made in NaCl solutions.