There are multiple testing options currently on the market for measuring chlorine in water samples. Most in-field testing options will measure high concentrations of total chlorine but only low range free chlorine, therefore dilution is often required to increase the range.
In this article we will review the main chlorine testing options currently on the market.
DPD is historically the most common method used to measure chlorine in water samples. The DPD method is a colorimetric method used to determine free and total chlorine concentration. To help you understand the difference between free and total chlorine we have written a support article which can be viewed here.
DPD is a colourless compound which turns a magenta pink colour in the presence of oxidising agents such as chlorine. Potassium iodide can be added to the reaction in a second step to determine the amount of combined chlorine present. DPD is the only colorimetric method which allows differentiation of the various forms of combined chlorine. The DPD method is compatible with both visual and photometric testing.
Extended range measurement can be carried out using reagents formulated with larger amounts DPD in a smaller sample volume, however this will still only measure up to around 10 mg/L free chlorine. Measuring a higher range of free chlorine using DPD would require sample dilution, which will increase testing time, complexity and affect result accuracy.
Potassium iodide reagents can be used to measure higher range chlorine levels in water samples. Chlorine will react with potassium iodide in an acidic solution to release iodine which turns the solution brown. The colour of the solution can be used to determine the chlorine concentration using visual or photometric methods.
This method can be used to measure very high chlorine concentrations from around 0.5 mg/L chlorine and above; however, it is not specific for free chlorine and will only give a measurement for the amount of total chlorine in the sample.
Test strips are a semi-quantitative, visual analysis method which can be used to measure free and total chlorine. Test strips work using an indicator dosed onto the strip which will change colour if the desired parameter is present. Common substrates used on free chlorine test strips include tetramethylbenzidine (TMB) and syringaldazine (FACTS).
Test strips can measure both free and total chlorine to a very high range. However, using test strips to determine chlorine concentration does suffer some disadvantages:
Amperometry involves measuring the change in electrical current caused by chemical reactions. Online analysers are designed for continuous measurement and are usually calibrated to work around a specific point.
Amperometric sensors can be used to measure both free and total chlorine, and will measure free chlorine up to ~200 mg/L. However, online meters using amperometric sensors require very frequent recalibration to continue to give reliable results. They are also fixed units, making them unsuitable for in-field testing.
Kemio™, our next generation measurement platform, can now measure free chlorine up to 100 mg/L without the need for dilution. The high range free chlorine sensor is specific to free chlorine, making it a unique option for free chlorine measurement. As well as free chlorine, Kemio™ can also measure total chlorine up to 500 mg/L along with other disinfection parameters.
Kemio™ works by using a technique known as chronoamperometry. This involves applying a fixed potential to the sensor in solution and measuring the resulting current. As this is an electrochemical technique, Kemio is unaffected by floating particles, sample colour, or turbidity, making it the perfect solution for testing in applications where coloured, turbid samples are common. To find out more about how Kemio™ works read our article How Does Kemio™ Work? The Technical Summary.
To find out more about Kemio™ Disinfection or to enquire about a trial, please contact our team below.
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