In recent years, coulometric analysis has been developed and widely used as a technique for the determination of substances in solution by measuring the quantity of electricity required for the completion of a reaction in an electrolysis cell. Of two coulometric techniques, i. e., coulometric titration with a constant current and coulometric analysis with a controlled potential of a working electrode, the latter is more interesting to the author, because by this method both electrolytic separation and quantitative determination of chemical species can be achieved simultaneously. Especially, the use of a mercury cathode as the working electrode(1)) has an advantage that the optimum control potentials can easily be determinable from
polarographic data. In our laboratory, we took up the coulometry of uranium as one of the interesting themes of researches when we had an opPortunity in studying various kinds of analytical methods for uranium in accordance with the request by Japan Atomic Fuel Corporation. The coulometric determination of uranium (VI) by using
a mercury cathode at a controlled potential was studied by G. L. Booman, W. B. Holbrook and ]. E. Rein(2)). It was found to be very useful for the analysis of uranium in a highly enriched nuclear element which underwent fission to an appreciable
extent. But the instrument used by them(3)) was complicated and expensive. Therefore, a simple counter millicoulometer has been coustructed in our laboratory, a readymade potentiostat (electrical instrument for automatic controlled potential electrolysis) being tried for electrolysis. By these instruments, the author studied the fundam ental conditions of coulometry for uranium and tried the successive determinations of uranium and thallium in mixed solutions.