DPD Spectrophotometry for Total Chlorine Determination in Water

Total chlorine—comprising free chlorine (hypochlorous acid, hypochlorite, and dissolved elemental chlorine) and combined chlorine (chloramines and organic chloramines)—is a critical parameter in water quality assessment. It serves as a key indicator of disinfection efficacy in drinking water treatment, wastewater management, and various industrial processes. 

Among the available analytical techniques, the N,N-diethyl-1,4-phenylenediamine (DPD) spectrophotometric method stands out as the most widely adopted standard, recognized by ISO 7393-2, US EPA Method 334.0, and Chinese standard HJ 586-2010. This article outlines the principle, procedure, and practical considerations of this method.

Principle of the Method

The DPD method is rooted in a simple yet robust colorimetric reaction. Under buffered conditions at pH 6.2–6.5, total chlorine species oxidize potassium iodide (KI) added to the sample, liberating iodine. The released iodine then oxidizes the colorless DPD indicator, forming a stable magenta-colored radical cation known as a Würster dye. The intensity of this red/magenta color is directly proportional to the total chlorine concentration in the sample, following the Beer-Lambert law. Absorbance is typically measured at a wavelength of 510–515 nm using a spectrophotometer.

Analytical Procedure

The determination follows a straightforward sequence. Water samples should be analysed immediately after collection, protected from strong light, vigorous shaking, and high temperatures. An appropriate volume of sample is transferred to a vessel, followed by the addition of phosphate buffer solution to adjust the pH to 6.2–6.5. 

Excess potassium iodide solution is then introduced, along with the DPD indicator reagent. After thorough mixing and a brief reaction period to allow complete colour development, the absorbance of the solution is measured at the specified wavelength against a reagent blank. The total chlorine concentration is determined by referring to a calibration curve prepared from chlorine standards of known concentrations. For samples exceeding the upper limit of the method, appropriate dilution is required prior to analysis.

Interferences and Mitigation

While the DPD method is highly reliable, several substances can interfere with accurate measurement. Colour and turbidity in the sample can affect light transmittance and cause elevated absorbance readings; the method is generally not recommended for highly coloured or turbid waters. 

Oxidised manganese, a common interferent, reacts with DPD similarly to chlorine, producing a positive bias—this can be corrected by running a separate analysis with sodium arsenite to distinguish the manganese contribution. Other oxidising agents such as bromine, iodine, ozone, and chlorine dioxide also react with DPD and may cause false positives. 

At very high chlorine concentrations (above approximately 10 mg/L), the oxidant can bleach the DPD indicator, leading to falsely low results. Analysts must be aware of these potential pitfalls and apply appropriate corrective procedures when necessary.