Theory and Measurement of the suspension effect

Abstract

The suspension effect is shown to arise from unequal concentrations of cations and anions in the neighbourhood of the salt bridge tip. Therefore it is a liquid junction potential. A theoretical treatment of the suspension effect (Δψ) is based on the principle of zero electrical current. The method is analogous to models of membrane transport. If the potential of the suspended particles with respect to the bulk liquid is a constant (e.g. AgI), the expression for Δψ reads:Δψ=(kTe)1Aln⁡[1+BAκa⋅12φ(1−φ)⋅γ(1−γ2)]. Here 1/ϰ is the double layer thickness and γ=tanh (eχ0/4kT). χ0 is the surface potential, a the average radius and the volume fraction of the suspended particles. The constants A and B depend on γ and the model chosen for the mobilities in the double layer. If, on the other hand, the charge of the particles is a constant (e.g resins), the expression for Δψ reads:Δψ=(kTe)1A(n)ln⁡[1+BA(n)np2n]. Here np is the equivalent concentration of the suspended particles and n the electrolyte concentration, both referring to the liquid part of the solution. A is now a function of n, but B is still a constant, therefore only at small values of Δψ does the above expression take a simple form. Experiments were carried out with AgI-suspensions, where we, varied γ, ϰ and (the latter by centrifuging). The equation for constant potential was found to describe the experimental findings quite well, except that we found a break in the curve of Δψ vs. at low and high pAg. A could not be determined with any precision, but B showed good agreement with a model where the mobility in the diffuse double layer is given by u=u0/cosh (eχ/2kT), where u0 is the mobility outside the double layer and χ the electrical potential in the double layer with respect to the bulk liquid.