Evaluation of electrode methods for determining total residual chlorine in various water matrices

Abstract

Five electrode systems and procedures that are widely marketed for routine determination of total residual chlorine have been evaluated in a variety of water matrices. The performance, in terms of accuracy, precision, and sensitivity, has been determined for the Orion 97–70 Residual Chlorine Electrode, Chlortect Monitor, Lazar 136A Chlorine Electrode, the forward amperometric titration method, and the back amperometric titration method. Response times for the former too systems were comparable and ranged from 20–100 seconds for chlorine concentrations ranging from 0.3–7 ppm in all water matrices. The Lazar electrode had considerably longer response times. The Chlortect Monitor and the forward amperometric titration method gave the highest sensitivity, with values of + 0.005 and .01 ppm, respectively, at the 1 ppm level. However, the Lazar Electrode could not be completely evaluated because of inconsistencies in the marketed procedures.

Each system and method gave a higher precision in chlorine demand free water, tap water, swimmng pool water and finished drinking water than in raw river water, raw wastewater, and treated wastewater. Mean relative standard deviations in relatively clean water matrices were + 6.1, 5.2, 6.2, and 7.7% for the back titration method, the Chlortect Monitor, the forward titration method and the Orion 97–70 Electrode, respectively. However, the overall precisions in the complex water matrices were + 6,7, 7.3, 18.6, and 15.1% for the back titration method, the Chlortect Monitor, the forward titration method, and the Orion 97–70 Electrode, respectively. The back amperemetric titration method was found to be superior to the other methods in terms of accuracy when using iodate as chlorine standard, giving a mean recovery of 101%. With the exception of the Chlortect Monitor, the overall accuracy of all the methods was considerably higher in relative clean water matrices than in more complex waters.

Overall recoveries in reference to the back titration method in clean waters were 109, 94, and 112% for the Chlortect Monitor, the forward titration method, and the Orion 97–70 Electrode, respectively. In complex water matrices relative recoveries were 98, 90 and 80%, respectively. However, to achieve this level of accuracy with Chlortect Monitor a considerable amount of time was devoted to thoroughly cleaning the cell by using a high concentration of hypochlorite solution and purging the system with chlorine demand free water, following several runs of each complex matrix sample.

Key words:

• Amperometric titration methods
• Orion 97–70 Residual Chlorine Electrode
• Lazar 136A Chlorine Electrode
• Chlortect Monitor
• industrial‐domestic wastewater