Determination of organic material in surface waters

For the characterization of the organic material contents of surface waters collective parameters are in use. One of these methods, the potassium dichromate employing method for the detection of COD has undergone some changes during the past decade. The new standard method is not suitable for the measurement of COD in surface waters, because the sensitivity of the method is rather crude, ( only high concentrations of organic material can be detected ). In the new method the boiling time was changed from one hour to two hours. With a number of samples we used the old one hour boiling time and the two hours boiling time. In the results we found 8-9 % differences in COD values for the two boiling times. However, in the present practice, we keep the one hour boiling time for the samples, in order to get comparable results between the present and past observations. In the new method the concentrations of the oxidants were reduced to increase the sensitivity of detection of GOD. The other collective parameter is the total organic carbon ( TOC ). For the measurement of this parameter various types of instruments are in use, which are based on different measuring principles. Methods requiring high temperature destruction of the samples give 2-3 times greater values for TOC than methods emploing photochemical means of destruction. In our experiments we used a DC-190 ( Dohrmann Ltd. ) instrument which employs high temperature. During analysis the inorganic and total carbon are measured separately and the difference between the two gives the total organic carbon ( TOC ). The instrument measures CO(2) by an infrared detector separately for TC and TIC, and calculates TOC by the difference of the two. For calibration of the instrument only one standard is used at 50 ppm. Consequently this method is inaccurate under 15 ppm. However it is reliable between 15- and 500 ppm TC. During measurements of TOC the scatter in values was very high ( 60-70 %). In case of high TOC contents the great scatter was due to high levels of suspended materials. In the samples after filtratrion the scatter in TOC values diminished. In samples of lower TOC values the scatter was lower than in the case of filtered samples. In the former the greater scatter was inherent in the method itself.