METHOD FOR MEASUING LIQUID CONCENTRATION

PROBLEM TO BE SOLVED: To regulate the injection quantity of a reagent so as to optimize the resolution of a hue by injecting the reagent of the injection quantity determined on the basis of a reference concentration detected by test injection thereof to a solution to be measured and then measuring the concentration of the solution to be measured. SOLUTION: In a hardness measuring device for water, for example, the detected reference concentration is inputted to a judgment data to determine the injection rate of a liquid reagent. A controller outputs an output signal for injecting a prescribed quantity of the liquid reagent to a chemical injection pump 17 and turns ON a stirring device 4 to inject the liquid reagent to a solution to be measured under stirring from a tank 16. The stirring device 4 is then OFF, and the transmitted light intensity of a first light source formed of an emitter 11 and a light receiving body 12 and the transmitted light intensity of a second light source formed of an emitter 19 and a light receiving body 20 are measured. The transmitted light intensity ratio (the transmitted light intensity after discoloration/transmitted light intensity before chemical injection) is calculated on the basis of the measured values, and the hardness of the solution to be measured is judged on the basis of this calculated value. The judgment result is notified to the outside through the controller, whereby a series of hardness measurement for the solution to be measured is ended.