Attempting to use ultrafiltration as a method of preparing colloidal suspensions of known composition for use in verifying electrokinetic theories requires care in the determination of the bulk suspending electrolyte concentration even at low particle volume fraction. A model of such a system is proposed to calculate the actual bulk concentrations. The work is based on the supposition that the application of the pressure difference across the colloidal suspension in the ultrafiltration process creates a volume fraction profile within the system. When removed from the applied pressure, the system returns to a uniform state, but with an equivalent bulk electrolyte concentration that depends on the total particle concentration and the composition of the filtrate. This model predicts that the bulk concentration varies with volume fraction, which is qualitatively similar to experimental observation. However, the magnitude of the changes calculated were not large enough to fully explain the experimental results; this suggests some other factor may also be contributing to the observed phenomena.