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.

✓ Measures free and total chlorine

X Only low range direct measurement for free chlorine, up to 10 mg/L

✓ Cost effective method per test

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 total chlorine in the sample.

Key Points

X Measures total chlorine only

✓ High range total chlorine measurement from ~0.5 mg/L and above

✓ Cost effective method per test

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:

• This is only a semi-quantitative analysis method. The result you get will be within a range rather than a specific number.
• Colour is perceived differently person to person, and so the reading is very subjective.
• Varying light conditions will also affect the perception of colour.

Key Points

✓ Measures free and total chlorine

✓ High range free chlorine measurement up to ~750 mg/L

X Does not give a specific, accurate result

•  X Subjective test method

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.

Key Points

✓ Measures free and total chlorine

✓ High range free chlorine measurement up to ~200 mg/L

X Requires frequent recalibration

•  X Fixed unit not suitable for 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. 

This becomes particularly important when absence of chlorine is to be analysed, Kemio can accurately measure down to 0.02 mg/L for free and total chlorine. Because the results do not get impacted by turbidity, you can have confidence in your test results with reliable analysis. Moreover, there are no reagents, tablet crushing, or other complex steps involved hence it eliminates human error and guarantees superior performance.

To find out more about how Kemio™ works read our article How Does Kemio™ Work? The Technical Summary.

Key Points

✓ Measures free and total chlorine

✓ High range measurement up to 100 mg/L free chlorine

✓ Digital data log, ideal for audits and complete traceability

✓ Portable instrument suitable for use in-field

✓ Not affected by sample colour or turbidity