Alternate Reagents for the Amperometric Analysis of Residual Chlorine II. Laboratory Results and Discussion

The determination of residual chlorine by amperometric analysis has been practiced since the 1950's. With the exception of engineering design updates, the fundamental technique has changed little in this time. One of the operational requirements of this method has traditionally been pH adjustment of the sample to ∼ 4.0–5.0. Historically, this has been accomplished with either acetate or phosphate based reagents. However, in the first paper in this series1 a novel theory was introduced which allows a different set of criteria to be used for reagent selection. The alternative reagents that meet these requirements offer several advantages over the traditional ones.In this paper, the selection criteria for alternative reagents for use in amperometric chlorine analyzers are first reviewed in light of the proposed theory. Primarily, the use of a compound with proton donor capabilities is required, but a low pH is not necessary. Other selection criteria include high solubility, chemical stability, environmental inertness, non-nutritive for biological systems, low cost and readily available. Candidate reagents are selected, their compositions optimized for maximum H+ availability. Thus novel reagent compositions and a reagent delivery system, which obviates the need for solution preparation, are evaluated.A test facility was built which allowed automatic and continuous control of up to three desired chlorine levels and simultaneous testing of up to fifteen analyzers. The use of three separate tanks and pumping systems facilitated parallel experiments. Tests were carried out on amperometric chlorine analyzers operated in either two electrode or three electrode mode. Chlorine concentrations ranging from 0 to >100 ppm were used, though the majority of the testing was at the level of 10 ppm or less. Reagent performance was assessed in terms of the analyzer's linearity of response and sensitivity (!A/ppm of chlorine). Long term stability was also monitored. The reagents studied included the standard acetate buffer (for the establishment of baseline performance), near-saturated solutions of potassium and sodium bicarbonate, and a saturated solution of borax (sodium tetraborate decahydrate) at a pH of 6.8. Experimental data are presented, along with detailed analyses. The theory that proton availability is the critical parameter, not pH, is supported, and it is shown that accurate chlorine measurements can be made amperometrically with reagents at pH values as high as 9.0, provided the proper criteria are met.