Dr Palin developed the DPD method of measuring free and total chlorine in the 1950’s and described at the time how the colorimetric method can suffer from interference from monochloramine during free chlorine readings.
Chloramination is a disinfectant method often used when there is a large distribution network, as it provides a more prolonged disinfection compared to free chlorine.
Chloramines are usually formed by adding ammonia to water containing free chlorine (HOCl– or OCl–, depending on the pH) in a particular ratio (usually chlorine:ammonia 5:1). When the reaction takes place, three kinds of inorganic chloramines can be formed: monochloramine, dichloramine and trichloramine. The chlorine:ammonia ratio, the pH of the solution and the contact time all affect which form of chloramine is predominant within the solution. The ideal pH to produce monochloramine in this reaction is 8.4.
Traditionally, DPD chemistry has been used to monitor free chlorine and chloramines. This can lead to confusion within the distribution network as the DPD method for determining free chlorine can suffer interference from monochloramine. This is known as ‘breakthrough’ and is when monochloramine breaks into the free chlorine fraction. The rate of breakthrough is affected by pH and temperature and is often witnessed as a slow increase in the free chlorine value, whilst not affecting the total chlorine reading.
At Palintest, DPD tablets are controlled so that they do not suffer excessive breakthrough when testing chloramine solutions, however some interference (<5%) still exists.
One of the largest providers of potable water in Spain has experienced these difficulties in controlling their chemical dosing in chloraminated water. They used an online monitoring system (HG 702) based on the DPD colorimetric method and frequently found fluctuations in their free chlorine measurements, which they suspected were related to breakthrough.
With high levels of ammonium (NH4) and free chlorine being dosed into the network, the utility expected to see a low free chlorine residual and a total chlorine response of approximately 1.5 mg/L. The on-line DPD method struggles to monitor the free chlorine residual accurately, whilst performing well when testing total chlorine.
Using Palintest’s sensor technology, the results show that breakthrough is not an issue with the chemistry used on the sensor. Free chlorine residuals are monitored as below 0.05 mg/L over the two-day testing period. The total chlorine results show that the sensor method matches the on-line analyser.
In summary, for applications where chloramination is being used, using DPD chemistry to monitor free chlorine residuals can be difficult – a skilled operator is called for. For applications where breakthrough is an issue, Kemio can provide the solution.
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