How
to test for chlorine in water.
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In broad terms, regardless of the type of chlorine initially used in disinfection, either chlorine gas or sodium or calcium hypochlorite, when mixed with water it will end up as hypochlorous acid (HOCl) and hypochlorite ions (OCl-). Hypochlorous acid partly dissociates into hypochlorite ions plus hydrogen ions. The ratio between the amount of each depends on pH. Hypochlorous acid is the far better disinfectant. Approximately 80% is hypochlorous acid at pH around 7 and 40% is hypochlorous acid at pH around 8. So the disinfecting capacity of the chlorine is greatly reduced as the pH goes up. Quoting Harp 2002, "The two chemical species formed by chlorine in water, hypochlorous acid and hypochlorite ion, are commonly referred to as “free available” chlorine. The free chlorine that is tested by test strips or a test kit is made up of 2 parts, hypochlorous acid and hypochlorite ion. So when testing for chlorine, the pH of the water needs to be tested to determine the disinfecting efficiency. The rule of thumb to apply is .. have the pH at 7.2 or below for good disinfection. Usually not all chlorine exists as free chlorine. It depends how much organic matter, particularly nitrogenous organic matter, is present. The chlorine can react with ammonia or organic amines to form chloramines. This chlorine is called combined chlorine. Chloramines have lower disinfecting power than free chlorine but have the advantage of being more persistent. Finding a value for combined chlorine is usually carried out indirectly by measuring free then total chlorine. Combined chlorine is found by subtracting the free chlorine value from the total chlorine value. There might be a high total value but a low free reading. That means a lot of the chlorine is tied up as combined chlorine and is therefore less effective for disinfection. DPD method. The most common method for free chlorine is the DPD method. At near neutral pH chlorine oxidises the DPD to form a magenta coloured compound. However this method only works for low chlorine levels up to approx 10 ppm. At higher chlorine levels the reaction proceeds beyond the magenta coloured compounds with a 'bleaching out' of the deeper colour. Normally chloramines only react slowly with DPD. To include chloramines in tests using DPD some iodide is added before the DPD. The iodide reacts with the chloramines to form a compound that will react with DPD to form a magenta colour. So after iodide is added to the test solution and then after DPD is added, the final colour will represent free and combined chlorine. This is called Total chlorine. So to find combined chorine alone, two tests are carried out, one with iodide and the other without iodide added. In a variation of the DPD test, rather than rely on visual or instrument measurement of intensity of the magenta colour, some kits use a titration. This works for both the free and combined components. The titrant is a reducing agent that converts the magenta coloured compound to form a colourless solution. Chlorine is calculated from the amount of titrant used. Iodometric method. For higher levels of chlorine an iodometric titration is sometimes used. This method tests for total chlorine. The chlorine in the test solution oxidizes some iodide that has been added. This forms an iodine complex that then reacts with an indicator like starch. The solution is titrated with a reducing agent back to a clear colour. This method is subject to interferences from other oxidizing agents that may be present apart from chlorine. It is similar to the method used to test for peracetic acid in the presence of hydrogen peroxide. Reference: Danial L. Harp. 2002. Current Technology of Chlorine Analysis for Water and Wastewater. Technical Information Series — Booklet No.17, Hach Company.
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