Industry: Environmental
Instrument: Apollo 9000
Measuring Carbon in Salty Waters
The concentration of organic and inorganic carbon in sea water is of considerable interest. Dohrmann's DC-190 has been designed for straightforward analysis of such waters, with special features to avoid some of the problems found with salty samples.
The presence of the chloride ion in waters being analyzed for carbon presents problems when the method uses low temperature oxidation of carbon to carbon dioxide. Such instruments typically use persulfate ion with or without ultraviolet radiation. The chloride ion scavenges the free radicals that are the principal agents of oxidation, markedly reducing oxidation efficiency and prolonging oxidation time to an intolerable extent. Complexation of chlorine by Hg (II) ion introduced into the reaction mixture reduces the chloride effect, but presents disposal problems for the spent reactor fluid.
Further, the chloride ion is oxidized to chlorine, which can damage the instrument's detector. Therefore, the low temperature UV-persulfate oxidation TOC analyzers are not advised for these salt water applications. In high temperature combustion analyzers, as in the DC-190, chloride ion does not affect the reaction rate and chlorine gas is not formed in such a high concentration and would be the instrument of choice for this application.
Sodium and other cations do not interfere with low temperature oxidations. However, sodium ion has a divitrifying effect on quartz glassware, which worsens with increasing temperature. Sodium chloride also fuses onto catalyst at high temperatures, impairing oxidation efficiency, and is difficult to remove.
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