How to determine free chlorine

Free Chlorine, DPD Photometric Method (0-4 ppm)
The test for chlorine is a test for the hydrolysis products of chlorine gas in water. Chlorine gas is soluble in water, forming hypochlorite ion and hypochlorous acid. It is in the latter form that chlorine exerts its disinfecting and oxidizing properties.
In industrial water conditioning, chlorination is used primarily for the control of slime and algae, although chlorine may also be used to assist in coagulation, taste odor, color and iron removal problems.
There is a marked difference in the germicidal properties of a chlorine residual, depending on the form in which chlorine exists in the treated water. The free available chlorine residual is defined as that portion of the total residual chlorine which will react chemically and biologically as hypochlorous acid. It is in this form that chlorine exerts the most potent bactericidal effect.
The combined available chlorine residual is defined as that portion of the total residual chlorine, which will react chemically and biologically as chloramines or organic chloramines. In these forms, chlorine is a relatively mild bactericide and oxidizing agent.
Theory of Test
Free available chlorine reacts instantly with N,N diethyl -p phenylenediamine (DPD) indicator to produce a red color. The red color is proportional to the chlorine content of the sample and is measured on a photometer.
Apparatus Required
Filter photometer complete with assorted laboratory glassware.
Chemicals Required
DPD Indicator
Phosphate Buffer Solution
Potassium Permanganate Solution
Procedure for Test
This procedure uses a wavelength of 515 nm and a cell with a light path of approximately 10-mm. Prepare a calibration curve for the photometer using successive dilutions of the potassium permanganate solution to cover the anticipated range of free chlorine in the samples to be tested. When this solution is diluted 1,000:l a chlorine equivalent of 1.0 ppm is produced in the DPD reaction.
Treatment of the standards should be in exactly the same manner as that shown below for analysis of the water samples. The absorbance values of the standards are then plotted vs. concentration to obtain the calibration curve.
To a beaker, add 1.25 ml each of phosphate buffer solution and DPD indicator solution. Add 25 ml of sample, mix, transfer the solution to a photometer cell and read the absorbance due to color immediately.
Calculation of Results
The free chlorine in parts per million as Cl2 is obtained by reference to the chlorine calibration curve.
Limitations of Test
The method measures only free chlorine and does not measure combined chlorine. Chlorine dioxide, bromine, bromamine and iodine react with DPD indicator and appear as free chlorine.
Oxidized manganese also gives a positive interference. Interference by copper up to approximately 10 ppm Cu is overcome by the EDTA incorporated in the reagents.
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