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Chlorine analysis: colorimetric versus amperometric

Chlorine analysis is a key parameter of monitoring at wastewater, drinking water and recycled water treatment plants, and typically represents a critical control point (CCP) parameter. It is essential that it is monitored accurately and in a timely manner.

At around $50,000 per system installation, it is important to understand the available types of online chlorine analysers, and their advantages and limitations, to ensure an appropriate and cost-effective solution is selected for the specific application.

Amperometric analysers

Amperometric analysers rely on the concentration of hypochlorous acid (HOCl) to provide a milli-amps amperometric signal. However, as pH increases, HOCl dissociates to form hypochlorite ions (OCl-) and hydrogen ions (H+) causing the amounts of individual component of effective free chlorine to change with pH value. For this type of system, a pH reference is required to enable an accurate result to be obtained when analysing samples with very high or low pH.

Colormetric analysers

Colorimetric analysis is based on N,N-Diethyl-p-Phenylenediamine (DPD), which reacts with the free chlorine to form a coloured product that is read photometrically to determine the amount of chlorine present. This type of analyser requires a system that delivers specific amounts of sample water, DPD reagent and buffer solution to the measuring cell.

What to consider when choosing a system

Both these chlorine analysis methods can offer accurate readings, but the method of analysis is significantly different. Factors such as sample matrix characteristics or chemistry (variability of pH, flow rate, pressure, turbidity); speed of response (sampling frequency); required system maintenance and operator intervention; and whole-of-life cost should be considered when choosing a system.

From the perspective and experience of operators of online chlorine analysers, the following table presents a summary of general observations and considerations as well as a list of advantages and disadvantages for colorimetric and amperometric analyser systems.

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This article is from the AWA Water Quality Monitoring and Analysis Specialist Network’s August 2020 newsletter. To join the WQMA network, you'll need to update your member profile.