Chlorination can be carried out by adding chlorine gas to water or by adding sodium or calcium hypochlorite. Chlorine gas partly dissolves in water to form hydrogen ions (H+), chloride ions (Cl-) and hypochlorous acid (HOCl). Sodium or calcium hypochlorite partly dissolves to form hypochlorous acid and then some of the hypochlorous acid forms hypochlorite ions and hydrogen ions.
The ratio between the amount of each form of chlorine depends on pH and temperature.
Below pH 2 around half the chlorine is Cl2 and half is hypochlorous acid. Between pH 2 and pH 7.2 more than half the chlorine is present as hypochlorite. Approximately 70% is hypochlorous acid and 30% is hypochlorite at pH around 7. Above pH 7.2 hypochlorite ion starts to predominate.
Hypochlorous acid is the far better disinfectant so the water needs to be somewhere less than pH 7.2. The disinfecting capacity of the chlorine is greatly reduced as the pH goes up.
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 reacts 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.
Peroxidase method for broad range chlorine. In this method chlorine oxidizes an indicator in the presence of a peroxidase enzyme. This method has been proposed for use in chlorine indicator strips. The result is reported as free chlorine Cl2 in ppm. Test strips using this method can also be used to test for peroxides. If the sample contains chloramines then these can be converted to free chlorine under acid conditions in the presence of iodide ion and a peroxidase catalyst. The result is then total chlorine. Methods have been proposed to convert chloramines on one part of a test strip so that a result for free chlorine, total chlorine and hence by subtraction combined chlorine can be obtained.