Method for automatically measuring concentration of dissolved substance

A method for automatically measuring concentration of a dissolved substance, comprising:

• a step of making a measuring water of which a pH value is kept in a specific pH range, and which develops a specific color by a specific dissolved substance in a sample water, by adding respectively two kinds of reagents to the sample water sampled from a target water system;
• a step of selecting one acid-base indicator which can develop respectively different colors other than the specific color in a first pH range including the specific pH range, and a second pH range deviating from the first pH range;
• a step of preparing reagents in which in a case wherein one of pH values of reagent added waters, in which each one kind of the two kinds of reagents is added to the sample water, is within the second pH range, the selected acid-base indicator is added to one reagent added to the reagent added water on a side within the second pH range, and in a case wherein neither of pH values of the reagent added waters is within the second pH range, the selected acid-base indicator and a pH regulator for making a pH value of the reagent added water within the second pH range are added to one reagent added to the reagent added water having a pH value on a side close to the second pH range, and another pH regulator for neutralizing the pH regulator added to the one reagent is added to the other reagent;
• a step of calculating each of absorbances with respect to a red region component light, a green region component light, and a blue region component light which can be obtained by dividing a light in a visible light region of a transmitted light of the measuring water into nearly three parts by emitting a light to the measuring water made by respectively adding the two kinds of prepared reagents to the sample water;
• a step of judging the presence or absence of addition of the one reagent relative to the sample water using a calculated absorbance;
• and
• a step of calculating concentration of the specific dissolved substance in the sample water using a calculated absorbance.