We have found it difficult to implement coulometry on small volumes and yet react the sample quantitatively in a reasonable time. We describe here a simple and satisfactory solution, making use of a commercial ring-disk electrode system.1A small cup-like cell is constructed with dimensions such that the ring-disk assembly can rotate freely inside it at a small distance from the bottom, as shown in the figure. The ring is used as the counter-electrode, and appears to be acceptable in spite of a much smaller area than the working electrode (the disk). The reference electrode is provided by a capillary connection at the bottom. The capillary is filled with a solution of supporting electrolyte and agar, which in turn is connected through a salt-bridge (containing electrolyte only) to a calomel electrode. The level of the electrolyte in the reference arm is higher than that in the cell, so that a very small positive flow is ensured to prevent contamination of the agar with the sample. The very smooth rotation of the ring-disk system permits speeds up to about 2000 RPM. It was found that for electrolysis of metals such as Cd and Pb on a mercury-coated glassy carbon electrode, the half-time for electrolysis is about 2.5 min at the optimum rotation speed of 800 RPM, with a sample volume of 10 μL.