Spectrophotometric Determination of Residual Chlorine in Water

Residual chlorine is a critical parameter in drinking water treatment, swimming pool sanitation, and industrial water systems. It ensures effective disinfection while preventing microbial regrowth in distribution networks. Among the various analytical techniques available, spectrophotometric methods are the most widely adopted due to their simplicity, sensitivity, and reliability. 

The DPD (N,N-diethyl-p-phenylenediamine) method, in particular, has been established as the standard approach in many national and international regulations. This article outlines the principle, procedure, and key considerations for determining residual chlorine using spectrophotometry.

Principle of the DPD Method

The DPD method is based on a selective colorimetric reaction. Under controlled pH conditions (typically between 6.2 and 6.5), free chlorine species—primarily hypochlorous acid and hypochlorite ion—oxidise the colourless DPD indicator to a pinkish-red compound. 

The intensity of this colour is directly proportional to the concentration of free chlorine present in the sample. The coloured compound exhibits maximum light absorption at a wavelength of 515 nm. By measuring the absorbance of the reacted solution with a spectrophotometer and comparing it against a calibration curve prepared from standard chlorine solutions, the residual chlorine concentration can be accurately quantified.

For the determination of total residual chlorine—which includes both free chlorine and combined chlorine (chloramines)—a small amount of potassium iodide is added to the sample along with the DPD reagent. The iodide acts as a catalyst, enabling the combined chlorine species to react with DPD as well. The difference between total chlorine and free chlorine measurements gives the concentration of combined chlorine.

Analytical Procedure

The analytical workflow for the DPD spectrophotometric method follows a straightforward sequence. A representative water sample is collected in a clean glass or plastic container. An aliquot of the sample, typically 5 to 10 mL, is transferred to a spectrophotometer cell or cuvette. 

A DPD reagent—either in powder, tablet, or liquid form—is added to the sample and mixed thoroughly. The solution is allowed to stand for a defined reaction time: for free chlorine determination, the absorbance reading should be taken within one minute of reagent addition to minimise interference from combined chlorine species. For total chlorine measurement, a longer reaction time of three to six minutes is required to ensure complete conversion. The absorbance is then measured at 515 nm using a spectrophotometer or colorimeter. The measured absorbance is converted to concentration using a pre-established calibration curve or a direct-reading instrument mode.