Chlorine Dioxide

Test Method Technical Information

TECHNICAL SPECIFICATION

Tests For: Chlorine Dioxide and other residuals in water
Tests Range: 0–10 mg/L as ClO₂, 0–25 mg/L as Cl
Reagent Chemistry Used: DPD/ Potassium Iodide
Basis of Test Method: Based on Standard Method 4500-Chlorine Dioxide-B
Method Detection Limit*: 0.02 mg/L
Limit of Quantification**: 0.02 mg/L

*The Method Detection Limit (MDL) is defined as the minimum measured concentration of a substance that can be reported with 99% confidence to be different from the method blank results.

**The Limit of Quantification (LOQ) is the smallest quantity that can be detected with reasonable certainty for a given analytical procedure.

Testing for Chlorine and Chloramines

Chlorine and chlorine release compounds are widely used for the disinfection of water. When dissolved in water chlorine forms hypochlorous acid and hypochlorite ions. Chlorine remaining in the water in this form is known as the free chlorine residual.

Chlorine does however react with ammonia and nitrogen-based species to form chloramines. These compounds are poor disinfectants and can also impart a characteristic taste or odour to the water. It is important therefore in certain applications to be able to distinguish between chlorine residual present as free chlorine and as chloramines.

The Palintest DPD Chlorine/Chloramines method provides a simple means of measuring free chlorine (HOCI/HOCl-), monochloramine (NH₂CI) and dichloramine (NHCI₂).

Reagent Chemistry

The Palintest Chlorine/Chloramines test uses the DPD method. This method is internationally recognised as the standard method of testing for chlorine and other residuals. In the Palintest method the reagents are provided in tablet form for maximum convenience and simplicity of use.

Free chlorine reacts with diethyl-p-phenylene diamine (DPD) in buffered solution to produce a pink coloration. The intensity of the colour is proportional to the free chlorine concentration. Addition of a trace amount of potassium iodide induces further reaction with any monochloramine present. The increase in colour intensity is therefore proportional to the monochloramine concentration. Subsequent addition of excess potassium iodide causes dichloramine to react in a similar manner. The increase in colour intensity is now proportional to the dichloramine concentration.

In this way it is possible to differentiate between free chlorine, monochloramine and dichloramine residuals present in the sample. The colour intensities at each stage of the test are measured using a Palintest Photometer.

EPA, Definition and Procedure for the Determination of the Method Detection Limit, Revision 2, Dec 2016.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”).