Abstract: The apparatus for moving-boundary method is upgraded by digital technique. The conductivity-time curve is measured, and the moving velocity of the boundary through two pairs of conductive electrodes can be determined precisely and objectively. This method eliminates the subjective deviation from manual observation. No indicator is needed to mark the boundary, and the problem of unobservable boundary at low concentration is therefore solved. The experimental object is also extended from traditional acidic electrolyte solution (e.g., aqueous HCl) to neutral electrolyte solution (e.g., aqueous NaCl, KCl, etc.). In the range of experimental concentrations (approx. 0.01 mol?L^?1 to 0.1 mol?L^?1), the transference number of H⁺ can be obtained with a reasonable deviation (0.8230–0.8350) by this method. The improved apparatus can be used to determine the transference numbers and mobilities of electrolytes, which provides a new way for physical chemistry experiment teaching.

Key words: Improvement for experimental technique and apparatus, Moving-boundary method, Conductivity, Transference number, Mobility