Spectrophotometric Determination of Chlorine Dioxide in Water

Spectrophotometry offers a practical way to detect chlorine dioxide (ClO₂) in water. The method relies on the reaction between chlorine dioxide and a chosen reagent, producing a colored compound whose intensity directly relates to the disinfectant concentration.

Basic principle

When chlorine dioxide reacts with a selective indicator—such as DPD (N,N‑diethyl‑p‑phenylenediamine) under controlled pH, or other chromogenic reagents—a pink, red, or violet color develops. The deeper the color, the higher the chlorine dioxide level. A spectrophotometer measures how much light of a specific wavelength is absorbed by the colored solution. This absorbance value is then compared to a calibration curve to determine the exact concentration.

General procedure

Collect a water sample – Use a clean, colorless cuvette or glass cell. Avoid prolonged exposure to air, as chlorine dioxide can escape.

Add the color‑forming reagent – A measured volume of reagent is mixed with the sample. The solution quickly changes color if chlorine dioxide is present.

Wait for full development – Allow the reaction to complete within a short, fixed time window.

Measure absorbance – Place the cuvette into the spectrophotometer, set the instrument to the optimal wavelength (commonly around 510–550 nm depending on the reagent), and record the absorbance value.

Determine the result – Using a pre‑prepared calibration curve (constructed from known standard solutions), convert the absorbance reading into a concentration value.

Important tips

Use fresh samples and reagents for accuracy.

Eliminate interferences from other oxidants (e.g., free chlorine, chloramines) by choosing a method specific to chlorine dioxide or by applying masking agents.

Perform a blank measurement with distilled water and reagent to zero the instrument before reading samples.

This spectrophotometric approach is fast, sensitive, and suitable for both laboratory and field applications when a portable spectrophotometer is available.