Measuring Residual Chlorine in Water by Spectrophotometry

Residual chlorine is widely used as a disinfectant in drinking water and swimming pools. To ensure effective disinfection without excessive chemical residues, accurate measurement of free and total chlorine is essential. Spectrophotometry offers a simple, rapid, and sensitive method suitable for laboratory and field use.

Principle

The most common spectrophotometric method for residual chlorine is the DPD (N,N-diethyl-p-phenylenediamine) method. In the presence of free chlorine at near-neutral pH, DPD is oxidized to a stable red-violet compound (Würster dye). 

The color intensity is directly proportional to the free chlorine concentration. If a small amount of iodide (KI) is added, combined chlorine (chloramines) also reacts, yielding total chlorine. The difference between total and free chlorine gives the combined chlorine value.

Reagents and Procedure

Typical steps are as follows:

Collect a water sample in a clean glass cuvette (e.g., 10 mL).

Add DPD indicator powder or liquid reagent.

For free chlorine measurement, read immediately after mixing (within 1 minute) at a wavelength of 515 nm or 530 nm using a spectrophotometer.

For total chlorine, add a small amount of potassium iodide, mix, and wait 2–3 minutes, then measure absorbance at the same wavelength.

A calibration curve is prepared using standard chlorine solutions (e.g., from potassium permanganate or commercial chlorine standards). The absorbance of the sample is converted to chlorine concentration (mg/L as Cl₂) using the curve.

Wavelength and Equipment

Most methods use 515 nm (visible range). Portable spectrophotometers or colorimeters designed for water testing often use 530 nm with LED light sources. The detection range typically is 0.05–5.0 mg/L, with a detection limit as low as 0.01 mg/L.

Spectrophotometric DPD method is approved by ISO 7393-2 and US EPA (method 330.5). It is accurate, easy to perform, and suitable for both discrete analysis and automated flow injection systems.