Some assumptions were made for the modified equation of the internal pressure theory originally suggested by McDevit and Long~[2].
κ_s={V~0_n(V_s-V~0_s)}/(2.303RTβ_0)·a/(a+b) (1)
When the distance between ion and nonelectrolyte molecule in aqueous salt solution is calculated, the ionic hydration should be considered for the larger nonelectrolyte molecules; and the nonelectrolyte molecule must be excluded from the primary solvation shell of the ion. For 1-1 electrolytes, we assume that: (a) Latimer's radii are used instead of the radii of ions, i.e., r_L~+=r_P~+_P+0.085 (nm), r~-_L=r~-_P+0.010 (nm), where r_P is the Pauling′s radius; (b) a in equation (1) is the average of the radii of cation and anion, i.e., a=1/2(R~-_L+r~-_L); (c) b is the radius of the nonelectroiyte molecule, i.e., b=1/2(V_n/N)~1/3, in which V_n is the molar volume of the nonelectrolyte, N is the Avogadro′s number. In order to test these assumptions, it has been calculated for the salting-out constant of five benzene derivatives and four fatty acids in the solution of alkaline halides by the theories of (I) Debye-McAulay, (II) Conway-Desnoyers-Smith, (III) McDevit-Long and (IV) modified McDevit-Long. For larger nonelectrolyte molecules equation (1) of the modifited McDevit-Long. theory (IV) gives results in better agreement with the experiment than those of (I), (II) and (III).