Conductivities of the hydrogen and hydroxyl ions

The transport of water by the ions was first measured by Washburn. Using the Hittorf method, a reference substance such as sugar or urea is added to the solution. Presumably the reference substance does not move in the field, and the transport of the solvent can be calculated from the analysis of the solution in the three compartments. If a value is assumed f or the number of water molecules attached to one ion, a value f or the number attached to the other ion can be calculated. Presently other methods for evaluation of hydration numbers are preferred—from measurements of the partial molar volume of the salt in the solution, for example. The different methods are internally consistent but often do not agree well with each other. It is generally assumed that the negative ions are not hydrated. Then the hydration numbers are, approximately Li , 6 Na , 4 K , 2 Rb , 1. 

The process of proton transfer results in a more rapid transfer of positive charge from one region of the solution to another than would be possible if the ion H3O has to push its way through the solution as other ions must. For this reason also the conductivities of and OH ions are not related to the viscosity of the solution. 

The ion conductivities increase markedly with increase in the temperature. For ions other than H and OH this increase is principally the consequence of the decrease in the viscosity of the medium. In water solutions in the range from 0 to 100 °C the change in molar conductivities for ions other than H and OH averages about 2% per degree. The conductivities of and OH have larger temperature coefficients (about 14% and 16%, respectively) because of the difference in the conduction mechanism. The decrease in ion conductivities with increase in pressure is also mainly a result of the increase in viscosity with pressure. 

If the solution of electrolyte is not infinitely dilute, the ion is retarded in its motion because of the electrical attraction between ions of opposite sign (asymmetry effect), and because the positive and negative ions are moving in opposite directions each carrying some solvent (electrophoretic effect). Both of these effects are intensified as the concentration of the electrolyte increases so that the retarding forces increase and the conductivity decreases. 

The electrophoretic effect arises from the motion of the atmosphere in the direction opposite to that of the ion. Both the atmosphere and the ion pull solvent with them and each is, in effect, swimming upstream against the solvent pulled along by the motion of the other. This retardation is less in very viscous solvents because the motion of both the atmosphere and the ion is slowed down. The expression for the electrophoretic retardation has the form,foruni-univalentelectroIytes,Ac whereA. = 8.249 x where jisthe 

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Calculations based on the known ionization product of water and the conductances of the hydrogen and hydroxyl ions at infinite dilution (see p. 340) show that the specific conductance of perfectly pure water should be 0.038 X 10 ohm" cm." at 18 . 

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