Abstract: A method and an apparatus for automatic measurement of a concentration of an oxidant or a reductant comprisesa solid electrode placed in an electrolytic cell in which a solution is flowed as a laminar flow;a potentiostat for measuring said concentration of an oxidant or a reductant;an electric potential sweeping device which is a continuous functional wave generator connecting to said potentiostat; andsaid continuous functional wave generator being used for washing said solid electrode by applying an alternating rectangular or pulse voltage having a frequency of 40 to 0.1 Hz.The working solid disk electrode is cleaned by alternately oscillating a potential in a range of the anode and cathode potentials for causing an electrolysis of water.

Abstract: NO.sub.x is absorbed in an alkaline aqueous solution of a permanganate and/or a manganate and the manganese compounds are reduced to form the precipitate of manganese dioxide.The manganese dioxide is dispersed in an aqueous solution of sodium or potassium hydroxide having a concentration of higher than 20% and a gas containing NO.sub.x having a molar ratio of NO.sub.2 /NO.sub.x of higher than 0.55 is contacted with the dispersion in the presence of a permanganate and/or a manganate at higher than 40.degree. C, to convert manganese dioxide to the permanganate and/or the manganate.