Coulometry

Introduction

Coulometry is the name given to a group of electroanalytical chemistry techniques that determine the amount of matter transformed during electrolysis by measuring the amount of electricity (in coulombs) required to perform the electrolysis. There are two basic techniques: potentiostatic coulometry and coulometric titration or amperostatic coulometry. Since these are electroanalytical techniques they use an electrochemical cell. Coulometric techniques are usually as accurate as other analytical techniques used to carry out similar analysis and are quicker and more convenient. The reagent is best generated as opposed to allowing it to directly transform in the reaction vessel to avoid the chance of sample polarization.

Coulometric methods are required to run at 100% efficiency with regards to the current, upholding Faraday's law of electrolysis. This means that a stoichiometric relationship is present for the amount of electricity used and the material being converted. For this reason standardization or calibration curves are not required because the amount of current can be determined theoretically from the equation: Q = I * t, where Q is the amount of electricity in coulombs, I is the current in amperes and t is the time in seconds. The interaction may happen indirectly and often occurs by adding another material to the reaction that will reduce before water, therefore maintaining the rule.

Potentiostatic coulometry

Potentiostatic coulometry involves keeping the electrochemical potential at the working electrode (the electrode involved in the reaction) constant, which results in an exponential decrease in current as the reaction proceeds. However, a key component is that it does not affect the other components in the solution. This type of analysis does not require a plated solid or other weighable product making it more favorable than electrogravimetry. An instance where this may be used is determining arsenic from the electrolysis of arsenous acid (H₃AsO₃) to arsenic acid (H₃AsO₄). It is necessary though to choose an appropriate electrode potential. The cathode of a potentiostatic coulometry experiment is usually a platinum electrode with a large surface area or a mercury pool, although other easily attackable electrodes can be used in specific circumstances.

Coulometric titration

Coulometric titrations use a constant current that is measured until an indicator signals equilibrium. The amount of electricity is determined through a relationship between the amount of current and the reaction time.