A water-soluble lithium salt is added as a tracer substance along with a
water treatment chemical to water to be treated. The concentration of
lithium ions is electrochemically or optically measured using a lithium
ion sensitive substance, so as to perform concentration control of the
water treatment chemical added to water to be treated. An ion selective
electrode, ion selective field effect transistor, or ion optode may be
employed to perform the electrochemical or optical measurement using a
lithium ion sensitive substance.
