MEASURING THE CONDUCTIVITY OF VERY DILUTE ELECTROLYTE SOLUTIONS, DROP BY DROP

Undergraduate experiments on ionic conductivity are common practice in general chemistry and advanced physical chemistry 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. Contaminations can dominate the conductivity of the solutions such that students might obtain unsatisfactory results for analysis. Here, the experience of using a simple experimental procedure to obtain the concentration dependence of ionic conductivities for very dilute (sub-millimolar) electrolyte solutions in undergraduate laboratories is described. The experiment consists of measuring the conductivity of solutions of increasing concentration prepared by dropping the electrolyte solution into an 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 good quality results using minimal amounts of materials. Examples are presented for the conductivities of strong electrolytes, and for the weak acetic acid electrolyte, for which the conductivity is dependent on the degree of dissociation also for dilute solutions.离子电导率（ionic conductivity）测定实验是普通化学与高等物理化学实验室的常规本科教学内容。传统实验通常会配制不同盐类的系列浓度溶液以测量其电导率，并借助科尔劳施定律（Kohlrausch Law）解析溶液电导率与浓度的关联，但溶液中的杂质往往会主导电导率的测量结果，导致学生得到不理想的分析结果。本文介绍了一种简易实验流程，用于在本科实验室中测定极稀（亚毫摩尔级）电解质溶液的电导率随浓度的变化关系，该实验的操作步骤为：将电解质溶液滴入盛有去离子水的初始容器中，配制出一系列浓度递增的溶液并测量其电导率。由于所覆盖的浓度范围内，电导率与浓度呈线性关系，因此无需借助科尔劳施方程（Kohlrausch equation）即可直接求得摩尔电导率，该实验流程操作简便，学生仅需少量实验材料即可获得高质量的实验结果。本文还给出了强电解质以及弱电解质乙酸的电导率测量实例，对于乙酸这类弱电解质，即便在稀溶液中，其电导率也与解离度密切相关。