Prepare two cuvettes from the same production batch. If the expected concentration is unknown, start with the medium concentration cuvettes (COD MR 0-1500 mg / L). Otherwise,use the respective concentration range CSB LR 0-150 mg /l, COD MR 0-1500 mg /l or COD HR 0-15000 mg /l.Mark one of the two cuvettes as a reference. Add 2 ml of distilled water to the reference cuvette and 2 ml of sample to the sample cuvette. Then the cuvettes are gently shaken.Take note of the heat development. Place the cuvettes in the preheated thermoreactor for120 minutes at 150 °C. Then remove the cuvettes from the thermoreactor and allow cooling to approx. 60 °C. Note that the cuvettes in the thermoreactor are hot. Shake the cuvettes and let them cool to room temperature.After cooling, place the reference cuvette in the MD 200 photometer and press the ZERO button. Then place the sample cuvette in the photometer and press the TEST / START button.The result is displayed in mg/l COD.