PROPERTIES OF STRONTIC BROMIDE. The properties of the bromide of strontium resemble very closely those of the corresponding salt of barium. As is well known, however, the strontium salt usually crystallizes with six instead of with two molecules of water. The crystals, unlike those of the barium salt, are noticeably hygroscopic in ordinary air, so that they cannot be weighed with great accuracy; they melt easily in their own water of crystallization at about 100°. This latter fact renders more difficult the quantitative drying of the salts; indeed, in the few cases where the water of crystallization was determined, it was necessary to allow the crystals slowly to lose their water in a desiccator before ignition. Thus, it was found in the following experiment that five molecules of water were given off, the sixth having very little, if any, tension at ordinary temperatures. A week's standing in the air of the laboratory sufficed to supply again all the water which had been lost. These results point without doubt to the existence of a definite substance having the formula SrBr2. H2O, which is hygroscopic in the air and corresponds to the compound BaBr. H2O, obtained in a similar way.* The existence of this substance has already been inferred by Lescœur † from observations of the vapor tension of the crystal water. Anhydrous strontic bromide is perhaps even more hygroscopic than the corresponding salt of barium. Strontic bromide melts to a transparent liquid at 630° (Carnelly), losing bromine in noticeable quantities if exposed to the air for some time at this temperature. Fused in a current of dry hydrobromic acid the salt soon recovers this lost bromine, and upon subsequent solution in water shows itself to be wholly neutral both to phenol phthalein and methyl orange. It will be seen that this fact is of the *These Proceedings, XXVIII. 12, foot-note. † Ann. de Chim. et de Phys., [6.], XIX. 553 (1890). utmost significance. The cold fused transparent or translucent mass is much less hygroscopic than the powder from which it was made. The importance of driving out every trace of water from the salt before weighing cannot be overestimated. Systematic experiments * with baric bromide and chloride led to the conclusion that probably neither of these salts retains water at a red heat, and it was to be expected that the same fact might be true of the substance in hand. In order to test the point, four grams of very pure strontic bromide dried at about 400° were fused in a stream of hydrogen bromide. The mass gained nearly six milligrams in weight, showing that the loss of bromine in the air at 400° much more than counterbalanced a possible trace of water. Again, 11.2610 grams of the same specimen, dried at 305° until constant in weight, were found to weigh 11.2630 grams after fusion as before. Since these gains corresponded closely with losses of bromine found alkalimetrically in similarly heated but unfused samples, it is evident that very little if any water can be held by the dried salt. It has already been pointed out that no absolute proof of such a fact is possible; † and these experiments, together with the analogy furnished by the more manageable barium salts, seem to be the last resort. The apparatus used for these experiments will be described under the heading "Method of Analysis." The specific gravity of anhydrous strontic bromide has been found by Bödeker to be 3.96. Since no more recent data regarding this constant could be found, another determination, described below, seemed to be needed. 3.2560 grams of a pure specimen which had been fused in the air and dried at 200° in the pycnometer were found to displace 0.6678 gram of toluol at 24°. Since the specific gravity of the toluol under these conditions, referred to water at 4°, was found to be 0.8618, that of the strontic bromide referred to the same standard must be 4.203. Again, 2.3065 grams of strontic bromide which had been fused in a stream of hydrobromic acid displaced 0.4699 gram of toluol, thus having a specific gravity of 4.229. The mean of these determinations, 4.216, was adopted as the basis of the reduction of the weighings to the vacuum standard. Strontic bromide, like baric bromide and chloride, may be evaporated to apparent dryness over a free flame in a platinum dish without losing a trace of halogen. Experiment showed that, upon mixing pure bromide of strontium with small quantities of bromide of calcium and *These Proceedings, XXVIII. 12; XXIX. 58. †These Proceedings, XXVIII. 14. barium and crystallizing the mixture, both impurities tended toward the mother liquors. Hence simple crystallization affords a method of eliminating the two most likely impurities. The other properties of strontic bromide do not pertain especially to the present work. PREPARATION OF MATERIALS. Strontic Bromide. Six different specimens of the salt were analyzed, in order to establish the presence or absence of accidental impurities. In the first place, five hundred grams of the purest strontic nitrate of commerce were dissolved in two litres of pure water, and four times in succession a cubic centimeter of pure sulphuric acid, diluted with much water, was added to the solution. Each time only a small amount of precipitate appeared at once, the rest appearing slowly. After waiting in each case three or four days, the clear liquid was decanted. No barium could be found even in the first precipitate of strontic sulphate; but it is true that the spectroscope is not a very satisfactory means for the detection of barium under these circumstances. The acid solution of strontic nitrate, which had been thus almost if not quite freed from a possible trace of barium, was evaporated to small bulk, filtered from the precipitated strontic sulphate, and twice successively brought to crystallization. Each mass of crystals was washed three times with alcohol upon the filter pump, to free it from the mother liquor, which might contain calcium or magnesium. After having been converted into pure carbonate by precipitation with ammonic carbonate and long continued washing the strontium was combined with bromine. For this purpose hydrobromic acid remaining from the barium work, obtained by repeated fractional distillation of the common acid, was used. The strontic bromide was evaporated in a platinum dish. This was slightly attacked, bromine having been set free by a little occluded strontic nitrate in the carbonate. After evaporation to dryness the bromide was fused at a bright red heat in platinum. The alkaline solution of the fused cake was treated with hydric sulphide, filtered, acidified with hydrobromic acid, warmed, filtered from the platinic sulphide, boiled to free it from sulphuretted hydrogen, again filtered, and crystallized twice from water. The crystals were washed with alcohol, and the strontic bromide thus obtained is numbered I. below; it was used for the three preliminary experiments, as well as for Analysis 13. The second sample of strontic bromide was prepared from similarly treated strontic nitrate which had been recrystallized four times instead of twice. The nitrate was converted into oxide by ignition in a nickel crucible; and the dissolved residue was filtered to get rid of a small amount of nickel. Ammonic sulphydrate gave no trace of coloration to a portion of the filtrate. Two recrystallizations in a platinum bottle sufficed to free the strontic hydrate from a trace of undecomposed oxides of nitrogen, and the last crystals dissolved to form an absolutely clear solution in pure hydrobromic acid.* The solution of strontic bromide was evaporated to crystallization, the crystals were dehydrated, and the anhydrous salt was fused; finally, after solution, standing, and filtration, a fresh crop of crystals was obtained. This sample, labelled No. II., was used for Analysis 14. Among several different methods for obtaining pure strontic salts, that recommended by Barthe and Falières † seemed to promise well and accordingly the third preparation was based upon their work The so called "pure" strontic chloride of commerce was dissolved in water, treated with ammonic hydrate and a little carbonate, and filtered from the precipitate containing iron, aluminum, and so forth. To the filtrate was added an excess of sulphuric acid, and the precipitated strontic sulphate was thoroughly washed with dilute sulphuric acid and then with pure water, in the hope of freeing it from magnesium and calcium. When the wash water became neutral to methyl orange the precipitate was treated with enough ammonic carbonate solution to convert about half of it into carbonate, and the mixed precipitate was then washed with water by decantation until only a very small constant trace of sulphuric acid (due to strontic sulphate) was found in the decantate. The carbonate was then decomposed by pure hydrochloric acid, and the solution was allowed to stand in a glass flask for nine months over the undecomposed sulphate, with occasional shaking. The strontic chloride was decanted, the sulphate was washed once with water, and the filtered decanted liquid was evaporated in a platinum dish until most of the free hydrochloric acid had been expelled. The dissolved residue was neutralized with ammonia, shaken with a little ammonic carbonate, and then filtered. To the greatly diluted filtrate was added an excess of pure ammonic carbonate, and the precipitate was washed until the wash water was free from chlorine. The strontic carbonate was dissolved in nitric acid which had been * See these Proceedings, XXVIII. 17, bottom of page. † Journ. Chem. Soc., Abs. 1892, p. 1277. Bull. Soc. Chim., [3.], VII. 104. twice distilled in platinum, and the nitrate was crystallized twice successively in a platinum dish. Each quantity of crystals was washed with small quantities of water and three or four additions of alcohol. The first mother liquor, upon being fractionally precipitated by means of alcohol, showed distinct traces of calcium in the extreme solution; thus Barthe and Falières's method was not capable of freeing the substance wholly from calcium. The second mother liquor showed no trace of calcium upon the most careful scrutiny. Two hundred grams of the purest crystals, after having been dried at 130°, were dissolved in about a litre of the purest water and filtered into a large platinum dish, into which was passed first pure ammonia gas and then pure carbon dioxide through a platinum tube.* The pure strontic carbonate was washed by decantation eight or ten times, dried on the steam bath, and ignited in a double platinum crucible over a spirit lamp. Part of this carbonate was converted into bromide by means of the purest hydrobromic acid,† and the product was digested for a long time with a considerable excess of carbonate. After filtration and evaporation the strontic bromide was fused in a platinum dish over the spirit lamp, the salt being perfectly clear while liquid. The translucent cake was dissolved, allowed to stand, filtered, faintly acidified with hydrobromic acid, and crystallized twice from water. Each time the crystals were washed with the purest alcohol. The resulting bromide of strontium was used for Analyses 1, 2, 3, 5, 6, 7, 12, 15, 16, 17, and 18. The next sample was prepared from the strontic carbonate which had been digested with the strontic bromide just described. It was dissolved in the purest hydrobromic acid and purified much as before, except that the salt was fused twice with intermediate crystallizations, instead of only once. This fourth preparation was used for Analysis 9. The fifth sample was made by the repeated crystallization of the combined mother liquors obtained from the four previous preparations. It was used for Analyses 4, 8, and 19. The sixth preparation of strontic bromide was made from the strontic sulphate remaining from the third. This residue was treated with enough ammonic carbonate to convert all but about twenty grams of * See page 379. † Prepared from pure baric bromide and redistilled many times. See these Proceedings, XXVIII. 17. |