Converting chlorine dioxide concentration to chlorine for the water quality report

I have the data on river water quality analysis indicating chlorine dioxide concentrations of 0.6 mg/l. However, there are no water quality standards for chlorine dioxide, only chlorine. Can I calculate the concentration of chlorine in the water from the concentration of chlorine dioxide?




You can, in fact, calculate the chlorine concentration from the concentration of chlorine dioxide. Chlorine will normally be in some compound, so when concentration of chlorine is given, it is usually concentration of chlorine compounds.


The concentration of chlorine dioxide is 0.6 mg/L. First convert the mg to g. this results in .0006 g. Divide that value by the molar mass of chlorine dioxide (67.45 grams/mole) to get .000008895 moles of chlorine dioxide, or .000008895 moles of chlorine. Multiply this by the molar mass of chlorine (35.45 grams/mole) to get .0003153 grams of chlorine. Convert this to milligrams to get 0.3 mg (one significant figure), so the concentration of chlorine from chlorine dioxide is 0.3 mg/L.


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Be aware: I think you are talking about two different things when you say “chlorine”! 


The molecular weight of 35.45 g/mol that Hunter uses refers to a single chloride atom or ion. But usually when talking about “chlorine” in context with water quality (or in processes like bleaching or disinfection), it is actually hypochlorite that is meant. The formula for Hypochlorite is OCl- and the molecular weight is 51.5 g/mol.


And again, you should be sure if the limit stated is in ppm hypochlorite-ion or in ppm Sodium Hypochlorite (Mw = 74.4 g/mol). Either case, you can use Hunters calculation with the right molecular weight.


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