Residual chlorine measuring apparatus and method of measuring residual chlorine

A residual chlorine measuring apparatus comprising:

• a detection electrode that is immersed in a sample solution containing one selected from the group consisting of benzotriazole and benzotriazole salt, the detection electrode being made of gold or platinum;
• a counter electrode that is immersed in the sample solution, the counter electrode being made of silver/silver chloride;
• a voltage applying device that applies an applied voltage between the detection electrode and the counter electrode;
• an ammeter that measures an oxidation-reduction current flowing between the detection electrode and the counter electrode;
• and
• an arithmetic controller that controls the voltage applying device, acquires the oxidation-reduction current measured by the ammeter, and stores the oxidation-reduction current,
• wherein the arithmetic controller is configured to perform
• a step A1 of applying an applied voltage Va1 in a range of −0.5 to −2.0 V between the detection electrode and the counter electrode for a period of time Ta1,
• a step A2 of applying an applied voltage Va2 in a range of 0.5 to 2.0 V between the detection electrode and the counter electrode for a period of time Ta2,
• a step A3 of applying an applied voltage Va3 in a range of −0.5 to −2.0 V, after the step A2, between the detection electrode and the counter electrode for a period of time Ta3,
• a step B1 of applying an applied voltage Vb1 in a range of 0.5 to 2.0 V between the detection electrode and the counter electrode for a period of time Tb1 that satisfies a condition of Tb1≤Ta2,
• a step B2 of applying an applied voltage Vb2 that satisfies conditions of Vb2 =0 to −2.0 V and Vb2 ≠0 V, after the step B1, between the detection electrode and the counter electrode for a period of time Tb2 that satisfies conditions of Tb2 ≤Ta1 and Tb2 ≤Ta3, and
• a step B3 of applying an applied voltage Vb3 in a range of −0.5 to 0.2 V, after the step B2, between the detection electrode and the counter electrode while a voltage is maintained to be constant,
• wherein:
• the arithmetic controller is configured to control the voltage applying device such that the voltage applying device performs the step A2 and the step A3 one or more times after the step A1 as a preprocessing step to remove an oxide coating made from said one selected from the group consisting of benzotriazole and benzotriazole salt which are formed on the detection electrode,
• the arithmetic controller is configured to control the voltage applying devices such that step B1 to the step B3 are performed one or more times after performing the step A2 and the step A3 one or more times, and
• the arithmetic controller is configured to acquire an oxidation-reduction current obtained by the ammeter in the step B3 as a measurement current, and to calculate a total residual chlorine concentration of the sample solution, according to the measurement current,
• the arithmetic controller is configured to control an absolute value of the applied voltage Vb2 to be equal to or smaller than an absolute value of the applied voltage Va1, and
• the arithmetic controller is configured to control the absolute value of the applied voltage Vb2 to be equal to or smaller than an absolute value of the applied voltage Va3.