The miniaturisation of the standard method based on the N,N′-diethyl-p-phenylenediamine (DPD) reagent for the determination of free or combined chlorine

A miniaturised continuous flow method for the determination of free or combined chlorine in purification plant water has been developed. The method is based on the standard batch procedure which uses a phosphate buffer for pH adjustment and N,N′-diethyl-p-phenylenediamine (DPD) as a colorimetric reagent. The sample, the reagent and the buffer solutions are continuously pumped into a miniaturised flow system. The mixing, the thermostating and the chemical reaction take place in a manifold with an engraved zigzag flow channel. A flow through detector is positioned at the flow channel outlet. The flow channel volume is about 25 μl and the detector volume is 0.25 μl. An experimental design was performed revealing that the significant variables were the total flow rate and the length of the mixing channel. Based on the results from this experimental design a total flow rate of 90 μl/min and a channel length of 60 mm were selected. The total consumption of sample and reagent is less than 3.9 l per month when the system is operated continuously. The response time is less than 2 min. When combined chlorine, in essence chloramine, is determined, potassium iodide is added to the buffer solution. The limit of detection is 3.2 μM for free chlorine and 1.9 μM for monochloramine.