FLOW INJECTION METHOD FOR THE RAPID-DETERMINATION OF CHEMICAL OXYGEN-DEMAND BASED ON MICROWAVE DIGESTION AND CHROMIUM SPECIATION IN FLAME ATOMIC-ABSORPTION SPECTROMETRY

The present paper describes a new flow injection method for the determ ination of Chemical Oxygen Demand (COD). This method consists of a fir st digestion step, where the sample is heated by microwave radiation, a second one where an anionic exchange resin retains the Cr(VI) that h as not been reduced by the organic matter of the sample and a third on e where Cr(VI), after being eluted, is determined by flame atomic abso rption spectrometry. The microwave power applied, the sulphuric acid c oncentration, the liquid flow in the digestion step and the sample vol ume were the variables studied. The recovery and precision obtained wi th this method are similar to those obtained using a standard semi-mic ro method, whereas the throughput is much higher (up to 50 determinati ons per hour). As regards sensitivity, by changing the sample loop vol ume and the concentration of dichromate, one can analyze samples with Chemical Oxygen Demand values between 25 and 5000 mg/l. The limit-of d etection is about 7 mg/l COD. An interesting feature of the new method , which is not shared by most other flow injection methods of Chemical Oxygen Demand determination, is that there is no matrix effect in the determination step.