Activity coefficients of molecular species

Activity coefficients for uncharged, molecular species generally obey the empirical Setchenow equation up to high ionic strengths (cf, Lewis and Randall 1961 Miller and Schreiber 1982). Such species include dissolved gases, weak acids, and molecular organic species. The Setchenow equation is 

Values of K, for molecular species in NaCl solutions at 25°C are given in Table 4.5. Pytko-wicz (1983) lists additional AT, values for seawater. MINTEQA2 assumes A = 0.1 for all uncharged species. The largest Kj values in Table 4.5 equal about 0.2 for several species. In fresh, potable waters (TDS 500 ppm, / 0.01 m), the activity coefficients of these species still equal 1.00. Even in brackish waters with TDS values of about 5000 ppm (/ 0.1 mol/kg), for = 0.2, molecular species activity coefficients equal 1.02. Thus, to a good approximation the y, of such species can be takeri equal to unity in fresh and brackish waters. 

TABLE 4.5 Values of the salting out coefficient, K where log y = K l. for some molecular species in NaCl solutions at 25°C based on various sources 

Source Harned and Owen (1958) Millero and Schrieber (1982) Millero (1983) Marshall and Chen (1982). 

The stoichiometric activity coefficient of bicarbonate ion is less in a 0.7 moVkg solution of NaHCO, than it is in a 0.7 mol/kg solution of NaCl. Why might this be the case  

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