Effect of Electrolyte Environment on Divalent Tartrate Ion Mobility

Abstract

The variation of the electrophoretic mobility, ZLT, of divalent tartrate ion with changing electrolyte environment has been studied. Mobilities were determined from boundary velocity ~neasurernents a t 1.00 "C in the Tiselius electrophoresis apparatus. The solutions used, which were adjusted to pH 8.3-8.5, \\.ere principally either 0.04667 ill CJ or 0.01 ill C?T -0.11 i l l Cry, where C+, T-, and A-are, respectively, a univalent cation, divalent tartrate ion, and a univalent anion. Iielative viscosities, a,,l, of a number of these solutions were also obtained. For a given anion environment, the variation of U T with clla~lging cation is characterized by a minimum a t sodium and a maximum a t tetra~nethylam~noilium for a series of six alkali metal and tetraalkylammonium ions. For the alBali metal cations and for tetrapropylammoniu~u ion, Z I T undergoes a distinct decrease \\.hen the solution is cha~iged froin 0.04667 i l l C?T to 0.01 ild C?T -0.11 M CC1. For all solutions containing an anion A-, ZIT decreases with illcreasing hydrocarbon content of A-for a given cation. Possible ion association, obstruction, and ion-solvent interaction effects are considered.In Dole's development of the steady state moving boundary theory for strong electrolytes (I), it is assumed that the mobilities of the ions in solutions of electrolytes vary by the same proportional factor with change in composition, that is, the relative mobilities are constant. This assunlption makes possible an elegant mathematical treatinent for the general case of n ions. Only the relative mobilities appear in the equations, since the ion mobilities themselves always occur in the fundamental moving boundary relationship as ratios. Thus, the inuch poorer assuinption of constant ion inobilities is avoided. The assumed constancy of relative inobilities is justified to the extent that the final results of a moving boundary experiment (boundary velocities and refractive index gradients), predicted froin the initial conditions by the Dole theory, have been found to agree to within lye or less of the observed values for uni-univalent electrolytes (2). Also, for simple systems involving weak electrolytes, the application of moving boundary theory, which includes the assumption that the relative mobilities of the ionic subspecies are constant, yields predictions in accord with observed results to within about lyo (3). Nevertheless, especially in experiments with strong electrolyte solutions containing inore highly charged ions, pronounced mobility changes can occur. For example, we have observed (Table 111) that the decrease of divalent tartrate ion mobility with increasing ionic strength varies markedly with the nature of the solutions in which it is measured. The value in 0.04667 i l l disodium tartrate is about 4y0 lower than that in 0.03333 116disodiuin tartrate, while in a n~ixture of 0.11 M sodium acetate and 0.01 M disodium tartrate, the decrease is about 13y0, although the change in ionic strength, 0.04 units, is the same in both cases. These