Should I measure free or total chlorine with a seawater application?


Dokument-ID TE12921


Veröffentlichungsdatum 06.01.2022
Should I measure free or total chlorine with a seawater application?
Total Chlorine Measurement in Seawater using DPD method and the CL17
When using the DPD method to measure chlorine in seawater it is best to measure total chlorine instead of free chlorine.  In seawater there is a high concentration of bromide (55 ppm on average) and chemical interactions with strong oxidants such as chlorine or ozone (specifically) can create sensible amounts of bromine. Therefore, in seawater, bromine, being a product of chemical reaction, may be present in sizable amounts along with residual concentrations of the original oxidants, e.g. ozone or chlorine. Thus, there is more than one oxidant in sweater and any analysis will measure it non-specifically.  Essentially, there is no method available at this time which will be able to differentiate free chlorine from other oxidants in seawater.
It is recommended to measure TRO = total residual oxidant concentration as it is a sum of all residuals.
This TRO concentration can be expressed in the units of mg/l of Cl2 just displaying strength of the oxidant as compared to chlorine. Most importantly, to capture all TRO, one should use either iodometric titration or total chlorine DPD method.  Both methods were designed to measure all chlorine species and therefore the best suited for TRO measurements.
Our research indicated that CL17 chemistry produces the best results and is very comparable with iodometric titration.
Attached is a letter from Hach stating that Total Chlorine measurement is recommended.

See the following articles for more information on Chlorine measurement in seawater

Can the CL17 Chlorine Analyzer be used with seawater samples?
Does chloride interfere with the DPD chemistry?

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