The primary difference between the two conductometric types is that the direct detector is susceptible to interference from ionic contamination, acids, bases, and halogenated organics. In the membrane-based conductometric method, the membrane is a protective barrier to interfering ions, enabling the analysis of CO 2 only.
The result is a more accurate TOC reading. Recommended Articles. More Articles. Send this to a friend. Send Cancel. The table below gives an indication of Total Organic Carbon levels in various types of water. Levels vary widely within each type, but, broadly, they range from sub-ppb levels in ultra-pure water for laboratory and microelectronic applications up to hundreds of ppm in effluents and process streams.
For many of these systems, the TOC alone does not provide enough information. The carbon-containing compounds can be present in different forms and the proportions of each can be critical. A breakdown of some of these divisions is shown in the above diagram. Dissolved organic carbon DOC is generally taken as that which will pass through a 0. TOC is universally measured by oxidizing the organic compounds present to forms which can be quantified.
A variety of oxidation and detection methods are used depending on the nature and concentration of TOC being measured and the analytical requirements e. Persulphate oxidation, enhanced by UV or heat, is widely used for laboratory TOC determination in many types of water from potable to pharmaceutical and electronic grades.
The CO2 produced is usually measured by NDIR or by the change of conductivity that it produces when dissolved in a separate stream of pure water. To exclude the effect of other oxidation products the gas may be passed through a membrane. Good oxidation can be achieved but a compensation method is needed to account for the blank from the reagent.
TOC at ppb levels can be detected. IC can be removed by acidifying the sample to a pH value of two or less to release IC as CO2 which can be measured or vented to waste. The situation is somewhat different for high purity waters with a low conductivity.
Water with a sufficiently low conductivity resistivity approaching As discussed before, all such trace-level measurements must be done on-line. A number of TOC monitors have evolved to meet the need for rapid monitoring of low ppb TOC levels in high purity laboratory water systems. The carbon dioxide generated by oxidation is detected using an infrared gas analyzer NDIR.
The sample is delivered to the combustion furnace, which is supplied with purified air. It decomposes and is converted to carbon dioxide. The carbon dioxide generated is cooled and dehumidified, and then detected by the NDIR.
The concentration of TC total carbon in the sample is obtained through comparison with a calibration curve formula.
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