Residual chlorine in water

January 2005 Analytical Instrumentation & Environmental Monitoring

It is important to understand that although the term 'free or residual chlorine' is used commonly, chlorine as such, is not present in water. Except at very low pH values, as in very strong acidic conditions, chlorine exists in water as hypochlorous acid or as the hypochlorite ion. In both cases the OCI group is present.

Free or total or 'available' chlorine may be determined by titration or by colorimetric analysis. In the titrimetric procedure, using Tytronics' FPA 414, the free chlorine reacts with KI in acidic conditions to liberate iodine, which is titrated with sodium thiosulfate. This method is more suitable for the determination of higher levels of 'free or available' chlorine as in bleach solutions.

Free chlorine may be measured colorimetrically using the FPA 814 process colorimeter. The most common method is to use the ortho-toluidine method. Both the titrimetric and colorimetric methods obviously require the addition of reagents and hence increase the requirements for maintenance of the analysers.

Total residual chlorine is also measured colorimetrically using the FPA 814. The first step is to obtain the release of the chlorine associated with nitrogen, as in the chloramines, using the reagent DPD to add KI and sodium arsenite, which in turn liberates the chlorine from the amines. Range of measurement of 'total residual chlorine' is normally 0 to 2,5 ppm.

The direct UV measurement, using the FPA 2014 process UV analyser is therefore most attractive as the very minimal of maintenance is required. As mentioned above, the chlorine in an aqueous solution exists as the hypochlorous acid or hypochlorite. The OCI entity exhibits a very strong characteristic absorption band in the UV region. The additional advantage of the FPA 2014 is that the measurement is continuous as the sample flows through a cell and no sample capture is required. Tytronics is represented locally by Protea Automation Solutions.

For more information contact Keith Guest, Protea Automation Solutions, 011 719 5700, keithg@protea.co.za, www.protea.co.za

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