METHOD AND APPARATUS FOR MEASURING ION CONCENTRATION

PROBLEM TO BE SOLVED: To obtain a stable measured result having good reproducibility. SOLUTION: A working electrode 22, a paired electrode 23 and a reference electrode 24 made of gold electrodes disposed mutualy nearly and dipped in a test water 2 constitute an electrode unit 3. A short circuit 8 short-circuits these electrodes to stabilize the potentials of the electrode surface layers of the electrodes 22, 23 and 24 during a non-measuring period. An applying voltage generator 5 applies a measuring sweep voltage of a predetermined range between the electrodes 22 and 23. An arithmetic controller 4 obtains an E-i curve between the electrodes 22 and 23 from a current value flowing between the electrodes 22 and 23, applies the curve to a predetermined function and calculates a free residual chlorine concentration from a plurality of parameters of the function.