In order to characterize the organic matter present on the water (potable or wastewater) from a quick way and without chemical reagents (as the conventional method spends a lot of time and involves the use of permanent reagents) in this paper we obtained absorption spectrums of the phthalate potassium in function of oxygen concentration consumed by the phthalate in the conventional test (chemical oxygen demand (COD)). From the ratio of the absorbance and COD we obtained a calibration curve to measure the level of organic matter