PURPOSE: To reduce considerably the cost of an instrument as compared with a polarographic method by using an electrode consisting of a combination of metal and metallic oxide to measure the concn. of hydrazine in boiler water, etc.

CONSTITUTION: The electrode is constituted with a combination of metal and metallic oxide or a combination of the metal and metallic oxide in which fine metallic particles of platinum black, palladium black, nickel black, etc. are treated on the surface of the metal. For example, the electrode made by treating the surface of the metal with the platinum black, etc. is used for a hydrazine decomposing electrode 3 and the metallic oxide, for example, silver oxide is used for an electron attractive electrode 4 which is a counter electrode. The hydrazine is oxidized by the electrode 3 to nitrogen and releases electrons and the silver oxide is reduced to silver by the electrode 4 as a result of the effect of both electrodes. The electrode made by treating the surface of the metal with the platinum black, palladium black, etc. is used in this stage and therefore the current per concn. of the hydrazine is large. The continuous measurement of the concn. of the hydrazine is thus made possible by using such electrode.