PURPOSE: To enable measurement of an acid concn. with good accuracy by providing zirconia film type hydrogen ion densitometers to a valency adjusting device and separating device of an apparatus which adjusts the valency of the specific metal in a strong acid soln. of plural kinds of metals to the same valency and separates the respective metals.

CONSTITUTION: Zirconia film type hydrogen ion densitometers ZM1, ZM2...ZM4 are respectively installed to an adjusting vessel 5 of a valency adjusting device II which reduces the hexavalent Pu in a fuel soln. from a device I for cleaning up a nitric acid soln. to quadrivalent Pu, the supply point of the 1st extraction pulse column 7, the two sampling points in the part lower than the supply point of the column 7 with an apparatus for separating and extracting fission products Pu, U from a strong acid soln. of plural kinds of metals, for example, a concd. acid soln. of the used nuclear fuel from a light-water type power reactor, etc. The above-described hydrogen ion densitometers ZM5WZM12 are further installed respectively to the specific intermediate stage of the 1st scrubbling pulse column 11 of a separator III, the upper and lower supply ports and specific stage of a valency adjusting pulse column 13, the specific stage of a separating pulse column 14 and the specific stage of a stripping pulse column 15. The H+ in each part of the apparatus is thus measured with good accuracy and the automatic operation is made possible.