Calibration graphs of COD were produced by using standard
solutions of glucose. Glucose was used because it react
quantitatively in the classical COD method. The relationship
between the absorbance of Cr(III) and the concentration
of the standard solution is shown in Fig. 5. It can be
seen that the slope k of the calibration graph is much higher
for small COD values (10–100 mg l−1) than for larger COD