The USE of SODIUM HYPOCHLORITE as a QUANTITATIVE SAMPLE STANCIL S. COOPER Washington University, Saint Louis, Miasoui
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VER a period of several years the determination of the available chlorine in bleaching powder was given in this laboratory as a routine experiment to students in first semester quantitative analysis. Good results were seldom obtained with these samples. Two characteristics of the samples, namely, their lack of homogeneity and their instability, were found to be responsible for this situation. The instructors themselves frequently did not know the actual composition of the samples issued. In a course in quantitative analysis the aim of the instructor should be the teaching of the technic and theoretical principles of analyses. Students should learn to appreciate the accuracy with which determinations can be made. Those who have no faith in their analyses in all probability will accomplish very little. They quickly lose confidence in their analyses if they are given samples which do not yield consistent results. They lose faith in the method, in their own ability as analysts, and, consequently, in quantitative
work as a whole. Reliable results can be obtained only when a good method is applied to a good sample. Two years ago we abandoned the use of bleaching powder and substituted the commercial household bleach, sodium hypochlorite. These bleaches are homogeneous, give good results in the hands of the beginning student, and can be issued by accurate dilution from a stock solution. During the past semester ninety-five determinations of the grams of chlorine per 100 ml. of sample were made by students in !&st semester quantitative analysis. The strength of the samples issued varied from one part of the stock solution in five parts of water to the stock solution itself. Dilutions were made with pipets and burets., Four diierent stock solutions were used. The student diluted 25 ml. of his sample to 250 ml., removed a 25 ml. aliquot, and analyzed it by the iodometric method. These reports were compiled and the values computed to the common basis of five grams available chlorine per 100 ml. of stock solution in order that the results on
all four stock solutions could be compared. The results are shown by the frequency distribution curve in Figure 1. The probable error in a single observation was found to be *0.03, giving a mean of 5.00 * 0.03 g.
cent. fall outside three times this error (which is the usual criterion for the rejection of observations). The sodium tbiosulfate used was standardized against potassium dichromate which had been standardized against iron wire. The cause of most of the poor results was traced directly to an incorrectly standardPer cent. error. -3.2 -1.6 0 fl.8 f 3.2 ized thiosulfate. Perhaps the direct arsenite method would yield better results. The sodium hypochlorite samples lose chlorine very slowly. One sample, analyzed over a period of two hundred fifty days (which included the summer months), showed an average loss of 0.085 per cent. of the original chlorine content per day. This sample, however, was seldom opened and was not given the same treatment as a stock solution from which samples were being drawn from day to day. One of the stock solutions from which student samples were being taken was tested after three, five, eight, and thirteen days. The rate of loss of chlorine from this solution was 0.090 per cent. of its original value (5.60 g. per 100 ml.) per day. This rate was practically constant. Not only can these sodium hypochlorite solutions be accurately sampled, so that good results can be obtained by beginners, but they are so stable and their rate of decomposition may be so accurately determined that, over the short period of time which they are used, the actual composition of the sample can be known. Such commeraal bleaches as "Chlorox," "Purex," etc., can which to determine chlorine Der 100 ml. Of the total number of re~orts, be recommended as aood sam~leson 53.7 per-cent. fall within this error and only 5:3 pe; available chlorine.
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