30” C. and 100-ml. volume-the amount of methylene blue perchlorate dissolved is equivalent to 8.3 mg. of the perchlorate correction. The solubility of the methylene blue perchlorate is independent of the amount of methylene blue present in excess after precipitation, as well as of the presence in solution of potassium chloride and potassium chlorate. The presence of these salts had no influence on the absorbance of methylene blue solutions. As a result of the above experiments, the following method for the quantitative estimation of the perchlorate ion in solution is recommended. Methylene blue hydrochloride is purified by recrystallization from a 1 to 1 ethyl alcohol solution. Its purity is determined by one of the methods mentioned earlier. The absorbance concentration curve for the purified methylene blue is obtained. The sample containing a soluble perchlorate is accurately weighed and dissolved in water in a 100-ml. flask so as to contain approximately 2 mg. of perchlorate per milliliter. Of this solution 25 ml. are taken together with a knorvn quantity (20 mg.) of pure potassium perchlorate and transferred to another 100-ml. flask (il). I n a third flask (B) only 20 mg. of pure potassium perchlorate are placed. RIethylene blue (300 mg., accurately weighed) is dissolved in 50 ml. of water using slight heating if necessary. The solution is added t o each of flasks A and B. The flasks are shaken intermittently for 10 minutes to complete the precipitation, made up to volume, allowed to stand for half an hour, and filtered through a dry sintered-glass filter of G.4 grade. The first 20 to 25 ml. of the solution are rejected. The excess methylene blue in the filtrate so collected is determined by proper dilution of the solution using a colorimeter. The methylene blue used up and the amount of perchlorate present can be calculated. If the amounts of perchlorate
Table 111. Influence of Excess of Methylene Blue on Precipitation of Methylene Blue Perchlorate
KC10, Methyl- Found by ene ColoriKC10, Blue metric DifferAdded, Added, Method, ence C, A - 6, *4J B, Mg. bfg. Ng. Mg. 8.5 60 200 51.5 8.6 60 300 51.4 51.6 8.4 60 400 51.9 8.1 60 600 Mean difference = 8.4
found are a and b, respectively, in flasks ,4 and B, the perchlorate present in 25 ml. of the sample used is a - b mg., calculated as potassium perchlorate. The correction factor for solubility under the experimental conditions used is (20 - b) mg. of potassium perchlorate. Anions such as dichromate, sulfate, permanganate, molybdate, and periodate form precipitates with methylene blue and hence interfere in perchlorate estimation by t h e above method. LITERATURE CITED
W.,J . SOC.Duers Colourists 31, 183
