PURPOSE: To enable automatic measurement rapidly with good accuracy, by preliminarily adding a known amont of a Ca-ion to brine in order to measure the total solidity corresponding to a minute amount of Ca and Mg-ions contained in brine in a combined form.

CONSTITUTION: For the purpose of measuring the total solidity being the sum total of both Ca2+, and Mg2+ components in high concn. brine used in preparing alkali and Cl2-gas, a known amount of a pure Ca-salt is preliminarily added to brine because it is necessary to set the total solidity in brine subjected to electrolysis and the solidity of 10ppm or less is measured and the total solidity is measured by using an automatic analytical apparatus. For example, a specimen brine supply burette 1, a reagent solution supply burette 2 and a diluting water supply burette 3 shown in the drawing are mounted and the specimen brine is mixed in a measuring container 5 and the total solidity is continuously and automatically analyzed by a potential meter having an indicator electrode 13 and a reference electrode 15. By this method, in performing ion exchange membrane electrolysis, control is enabled so as not to close the membrane by solidity components.