A Potentiometric Method of Detecting Moving Boundaries in the Determination of Transference Numbers

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Abstract
A new method is described by which transference numbers for both leading and following ions can be determined in a moving boundary experiment. The boundary is detected by the change in potential accompanying its passage into the volume between two microprobes sealed into the moving boundary tube. With the aid of a volume calibration, the transference number of the leading ion can thereby be determined. A method of measuring ionic mobilities directly is described in which the same microprobes are used to measure both the potential gradient and the boundary velocity. Transference numbers for the following ion can also be measured by the indirect method, which makes use of the Kohlrausch relationship. The microprobes are used in this case to measure the conductance ratio between leading and following solutions. The detector is described in detail along with a suitable power supply. A sheared cell is introduced in which the boundary is formed initially with the aid of a pipet and a flowing system. The apparatus is tested thoroughly on systems whose transference numbers are known.