PURPOSE: To realize highly accurate and quick measurement of COD (chemical oxygen demand) by performing the sampling two times so that the expected residual quantity at oxidizing agent will be more than half and less than half, respectively, analyzing the samples and determining the residual quantities of oxidizing agent, and then determining the constants of a correction formula based on both analytic values and residual quantities of oxidizing agent.

CONSTITUTION: Pure water is analyzed several times and the average consumption of potassium permanganate (oxidizing agent) is determined. A sample, for which the residual quantity of oxidizing agent of 40% is expected, is analyzed and the consumption of oxidizing agent is determined. Another sample, for which the residual quantity of oxidizing agent of 60% is expected, is then analyzed and the consumption of oxidizing agent is determined. Subsequently, a COD value Y and a residual quantity of oxidizing agent X are calculated based on these values and substituted into a correction formula Y=A+BX thus determining the constants A and B. The constants A and B are substituted into the correction formula and then X=50% is substituted thus determining a correct COD value Y.