JOURNAL OF CHEMICAL EDUCATION
A SIMPLE PREPARATION OF PRIMARY-STANDARD SODIUM CHLORIDE LOUIS MEITES Yale University, New Haven, Connecticut
SOMETIMES a student wishes to standardize a supposedly determinate silver nitrate solution, either because he suspects an error in its preparation or because he wishes to secure an additional check on the correctness of his work. He is usually offered for this purpose a commercial reagent-grade, alkali-metal halide whose effective purity may be much less certainly known than that of the silver nitrate he used. Most directions for the purification of these salts include several recrystallizations from water. This is a time-consuming process, and, in addition, it is inherently incapable of giving a good yield of highly purified product. An alternate procedure involves precipitation of sodium chloride from its cold concentrated solution with hydrogen chloride gaa. This requires the construction of a hydrogen chloride generator, which is a nuisance. More important, however, is the fact that such a precipitation removes little, if any, of the potassium present in the original material. This paper d e scribes a method for the purification of sodium chloride which gives a 70 to 80 per cent yield of material with an effective purity of 100.00 0.01 per cent. One kilogram of reagent or c. p. sodium chloride is dissolved in about 2.8 liters of water a t 9C-10O0C., and the solution is slowly cooled to room temperature with occasional stirring. Ten ml. of concentrated hydrochloric acid is added, and the mixture is filtered through a medium-porosity, sintered glass funnel. The precipitated material, which contains the insoluble substances originally present, most of the potassium, and much of the bromide, is discarded. About 500 ml. of concentrated hydrochloric acid is added to the filtrate, which is then allowed to stand overnight, preferably with continuous stirring. The slurry is filtered as before and the solid is washed three times with 100-ml. portions of 4 M hydrochloric acid. It is thoroughly air-dried by suction, and is finally dried a t 180°C. in an oven permitting free access of air for not less than 24 hours. If desired, the product may be dried further in an electric oven a t 500'. Under these conditions, the loss in weight in 48 hours is close to 0.03 per cent, and seems to represent only water. A number thus prepared were analyzed for chloride gravimetrically after
*
drying a t 500'. All of the precautions essential in very accurate work were observed, with the single exception that the solubility of the silver chloride in the mother liquor and the wash liquids was disregarded: this could hardly have caused an error greater than about 0.03 per cent. After drying a t 300°, the weight of silver chloride was found, from the mean of eight analyses, to be 2.4522 =t0.0003 times the weight of sodium chloride taken. The excellent agreement with the theoretical value of 2.4521 is partly due to the opposing effects of solubility errors and retention of water by the silver chloride. No differences could be detected between samples prepared from material sold by three manufacturers. The product appeared to be entirely free from sulfate, calcium, magnesium, barium, bromide, and iodide according to the tests outlined by Kolthoff and Stenger.' Spectroscopic examination showed traces of lithium and zinc to be the only detectable metallic contaminants. Saturated solutions of several samples in freshly boiled distilled water of pH 6.6 had pH values between 6.3 and 6.7, and thus contained no appreciable amounts of residual hydrogen chloride. Similar solutions prepared from material dried a t 180" had pH values near 5.7, resulting from the retention of a trace of hydrogen chloride. This corresponds to, at most, mol per cent of hydrogen chloride, which is quite insignificant for nearly all purposes. Analyses of material secured by fractional precipitation with hydrochloric acid gave 100.01, 99.99, 100.02, and 100.00 per cent for the effectivepurities of the fractions prepared by the successive addition of four 125-ml. portions of acid. There thus appears to he no tendency for impurities to collect in any one fraction of the precipitated material. The original reagent-grade material corresponding to the product whose analyses were given in the last paragraph, and the material which precipitated on cooling, were analyzed and found to contain 99.83 and 99.45 per cent. sodium chloride, respectively, after drying at 500". ' KOLTHOFF, I. M.,AND V. A. STENQER, "V01~metli~ Andy&," Intersoience Publishers, Ino., New York, 1947, Vol. 11, pp. 251-2.
