Below the Surface: Measuring Organic Sum Parameters in Water

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The electrochemical determination of COD by OH–radicals can be used as an alternative. The advantages of this method are a short response time combined with relatively high recovery rates while using completely harmless reagents. To reach the necessary stable conductivity, the sample is normally diluted in an electrolyte of sodium sulfate (a normal neutral salt). For the real COD determination, the analyzers have to be correlated with a standard method. The response time of this method is approx. one minute and the frequency of determinations can be as low as two minutes. Nearly no additional reagents are required, making this method quite suitable for environmental monitoring.

TOD and TOC Determination

TOD measurement is based on the same idea as COD: All organic compounds are oxidized completely to determine the required oxygen demand. However, using thermal oxi-dation at 1,200 °C, a complete consumption of all organics is guaranteed and no chloride disturbances occurs (as with the di–chromate method). For this method, the sample is thermally oxidized in an inert reactor with the oxygen consumption measured directly in the gas phase. A similar measurement is used for TOC analysis, where CO₂ is detected instead of O₂ by an NDIR detector.

In the furnace, the water of the injected sample is evaporated immediately while organic compounds are oxidized. Currently, there are several so called high–temperature methods available, with LAR's oxidation at 1,200 °C among the most efficient ones. Unlike other approaches (such as the catalytic high-temperature methods which usually do not reach a higher temperature than 900 °C) the LAR test does not need catalysts.

This is not only easier, but also eliminates the risk of catalyst poisoning, which may cause a malfunction of the whole pro-cess. With the non–catalytic high–temperature method at 1,200°C, the process takes only about one or two minutes, so that measurements can be carried out as frequent as every three to five minutes.

The Second Best Option for Water Analysis?

The TOD method may be considered as the second best option for river water monitoring, as it provides 100 % recovery rates. As no use of reagents is required, this method is well suited for environmental monitoring. In accordance to the US standard by ASTM D6238, LAR is currently the only company manufacturing a reliable online analyzer for TOD which is successfully in operation throughout the world.

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