Determination of Residual Chlorine by DPD Spectrophotometry

Spectrophotometry is the most widely used technique for determining residual chlorine in water. Among the various colorimetric methods, the DPD method is the most established and is specified in many national and international standards. The method is based on a simple chemical reaction: in a buffered solution at pH 6.2 to 6.5, free chlorine oxidises N,N‑diethyl‑p‑phenylenediamine to a purple‑red compound. 

The intensity of this colour is proportional to the chlorine concentration and is measured at a wavelength of 515 nm. The method is sensitive, rapid, and suitable for both free and combined chlorine measurement.

Principle of Free Chlorine Measurement

When DPD reagent is added to a water sample containing free chlorine—mainly hypochlorous acid and hypochlorite ion—a rapid oxidation reaction occurs. The colourless DPD compound is converted to a purple‑red product, and the absorbance at 515 nm increases in direct proportion to the free chlorine concentration. The reaction is complete within seconds, and the colour is stable for several minutes, making the method convenient for routine analysis. The concentration is determined by comparing the sample absorbance to a calibration curve prepared from standard solutions.

Determination of Total Chlorine

For total chlorine measurement—which includes both free chlorine and combined chlorine (chloramines)—a small amount of potassium iodide is added along with the DPD reagent. The iodide catalyses the reaction between combined chlorine species and DPD. Chloramines oxidise iodide to iodine, which in turn oxidises DPD to the same purple‑red product. The measurement therefore reflects both free and combined chlorine. The combined chlorine concentration is obtained by subtracting the free chlorine reading from the total chlorine reading.

Analytical Procedure

The procedure follows a straightforward sequence. A water sample is collected in a clean container and measured immediately to avoid chlorine loss. A measured volume of the sample is transferred to a cuvette or colour‑comparison tube. DPD reagent is added, either as a powder, tablet, or liquid. 

For free chlorine, the absorbance is read within one minute of reagent addition. For total chlorine, potassium iodide is added together with the DPD reagent, and a reaction time of three to six minutes is allowed for complete colour development. The absorbance is then measured at 515 nm against a reagent blank. The result is obtained from the calibration curve.

Interferences and Precautions

Several substances can interfere with the DPD method. Oxidising agents such as ozone, chlorine dioxide, bromine, and iodine produce positive interference because they also oxidise DPD. Strong reducing agents, including sulphite and thiosulphate, consume free chlorine and cause negative interference. 

Sample turbidity and colour can affect the absorbance reading; if present, they should be compensated for by using a sample blank. The pH of the sample must be within the specified range; if the pH is too high or too low, the colour development is incomplete. Temperature also affects the reaction rate, so samples should be analysed at room temperature.