Comparisons of the mobilities of salt ions obtained by the moving boundary method and two empirical equations in capillary electrophoresis

The actual mobilities of some small salt ions with high charge intensities, such as Cl-, K+, Na+, Ba2+, SO42- and La3+, calculated using two empirical equation, mu(act) = mu(o) exp (-0.77 root zI) [valid for z greater than or equal to 2 and 0.001 mol/l less than or equal to l less than or equal to 0.1 mol/l] and mu(act) = mu(o) exp(-0.5z(1.78)root I) [ valid for I less than or equal to 0.001 mol/l], are compared with the actual mobilities determined by the moving boundary method (here, mu(act) and mu(o) are the actual and absolute mobilities, respectively, z is the charge of the ion, I the ironic strength). The results show that, (i) under the given conditions both of the empirical equations are valid for small ions with high charge intensities as well as large ions with low charge intensities, (ii) the former is also valid for multivalent ions of ionic strength I less than or equal to 0.001 mol/l, (iii) the latter also applies to monovalent ions of ionic strength 0.001 mol/l less than or equal to I less than or equal to 0.1 mol/l. Thus, the two equations can be expressed as a single equation: mu(act) = mu(o) exp(-eta root zI) [used for I less than or equal to 0.1 mol/l, and if z=1, eta=0.5, or if z greater than or equal to 2, eta=0.77].