Measuring the conductivity of very dilute electrolyte solutions, drop by drop

The study of the conductivity of electrolyte solutions is important for practical applications and for the understanding of ion mobility. Because of that, undergraduate experiments on ionic conductivity are common practice in first year general chemistry or more advanced physical chemisdegreetry laboratories. Often, the conductivities are measured for solutions prepared for various salts, in a range of concentrations, and the relationship between solution conductivity and concentration is interpreted in terms of the Kohlrausch law. Extrapolation of the molar conductivities to infinite dilution allows the study of the individual ionic conductivities. In practice, the preparation of dilute solutions for these experiments can be cumbersome, because small electrolyte contaminations can dominate the conductivity of the solutions. Additionally, significant amounts of reactants, particularly deionized water, must be used. Here, a simple experimental procedure is proposed to obtain the concentration dependence of ionic conductivities for very dilute (sub-millimolar) electrolyte solutions. The experiment consists in measuring the conductivity of solutions of increasing concentration prepared by dropping the electrolyte solution into a single initial vessel of deionized water. The range of concentrations achieved is one in which the conductivities vary linearly with the concentrations, such that the molar conductivities can be obtained directly without the use of the Kohlrausch equation. The simplicity of the experimental procedure leads the students to obtain very good quality results using minimal amounts of materials. Examples are presented for the conductivities of various strong electrolytes, and for weak acetic acid electrolyte, for which the conductivity is dependent on the degree of dissociation even at very low concentration