METHOD FOR MEASURING LIQUID CONCENTRATION

PROBLEM TO BE SOLVED: To improve the measuring precision by stopping the injection of a reagent when the hue change detected value reaches a preset numerical value to measure the concentration. SOLUTION: In the hardness measurement of a solution to be measured, a stirring device 4 and a chemical injection pump 17 are ON, a liquid reagent is gradually injected from a tank 16 to the solution to be measured under stirring, and the transmitted light intensity of the solution to be measured is continuously measured by use of emitters 11, 19 and light receiving bodies 12, 20. The transmitted light intensity ratio (the transmitted light intensity after discoloration/the transmitted light intensity before chemical injection) is calculated on the basis of the measured value, and when the measured value reaches the numerical value of transmitted intensity ratio in each hardness preset in a judgment table, the chemical injection pump 17 and the stirring device 4 are OFF to stop the reagent injection. Accordingly, the hardness measurement of the solution to be measured is completed when the transmitted light intensity ratio reaches the numerical value set in the judgment table, and the measurement result is notified to the outside through a controller, whereby a series of hardness measurement of the solution to be measured is ended.