A New Method of Determining Residual Chlorine

Abstract

M OST of the limitations of known methods of determining residual chlorine in water result from the fact that the solution is acidified during the determination. These errors, due to acidification, have been investigated by Scott and others, (1, 2, 3, -1) who show the desirability of determining residual chlorine in neutral solution. Determinations in neutral solution are often made by titration with sodium thiosulfate. Since there are several possible reactions between sodium thiosulfate and chlorine in neutral solution (5, 6), this titration cannot be classed as a quantitative determination of residual chlorine.The titration of chlorine with sodium arsenite in neutral solution has been investigated and found to proceed according to a single quantitative reaction (7). Thus, residual chlorine can be titrated accurately with sodium arsenite in neutral solution if there is available a means of indicating the end-point of the titration. The starch iodide end-point may be used if a known amount of standard sodium arsenite solution is added to the sample and the amount not consumed by the chlorine is determined by titrating with standard iodine solution (8). This method, however, is inconvenient and the starch iodide end-point is not sensitive enough to permit the accurate determination of low residuals. The end-point of certain titrations may be determined by means of the dropping mercury electrode and in certain cases by means of a very small platinum electrode (9), by a modification of the polarographic method. Kolthoff and Pan have called such determinations amperometric titrations (10). The method here presented is a type of amperometric titration employing electrodes especially designed to permit the convenient determination of residual chlorine in water. An amperometric titration differs from a conductometric and a potentiometric titration in that it utilizes the phenomenon of electrode polarization.

Development of Instrumentation in Chlorination

“…The accuracy of the OTA method is dependent to a considerable degree upon the accuracy of the original orthotolidine test. These weaknesses led to the development of the amperometric titration method (7).…”

Iron and Manganese Removal by Free Residual Chlorination

“…It was noted that very little oxidation of manganese took place until all of the ammonia had been removed by the chlorine. This was later borne out by measurements of chlorine and chloramine residuals with a Marks' electrometric titrator (5).…”

Some Chemical Aspects of the Ammonia‐Chlorine Treatment of Water

“…The results obtained by difference between acid ortho-tolidine and the flash paminodimethy laniline test were considered preferable to any of the above for chloramine chlorine. In addition, an apparatus was constructed using a Marks (14) titrator cell for determining chloramine. This special cell was loaned to the laboratory by Wallace and Tiernan Co., Inc. Titration of the neutral sample using standard arsenious acid, followed by titration after the addition of potassium iodide with this apparatus, was found to be of great value in the determination of free chlorine and chloramine.…”