The Three-Component System Sodium Bromide-Hydrogen Bromide-Water An Experiment i n Elementary Physical Chemistry , S. JAMES O'BRIEN, CHRISTOPHER L. KENNY, a n d RAYMOND J. FUXA The Creighton University, Omaha, Nebraska
I.
N A previous paper ( I ) , the system potassium chlo-
nde-hydrogen chloride-water was suggested as an elementary physical chemistry experiment on a three-component system containing two electrolytes with an ion in common. It was shown that by means of this experiment the wet residue method (2) of determining the composition of the solid phase in equilibrium with saturated solutions could be demonstrated. However, the fact that the solid phase in equilibrium with the saturated solutions in this system is so obvious makes the experiment somewhat lacking in interest. Consequently, it seemed desirable to work out an experiment for the introductory physical chemistry course, on a system of this kind, in which the composition of the solid phase is not so evident. The system sodium bromide-hydrogen bromide-water suggested itself as a possibility, since it has been shown in several studies (3, 4, 5, 6) that sodium bromide exists as the dihydrate in aqueous solutions of hydrogen bromide a t lower concentrations, and that a t higher concentrations of the acid the solid phase becomes the anhydrous salt. A procedure suitable for an elementary physical chemistry class and the results of an experiment following this procedure are outlined below. EXPERIMENT
Afiaratus and Materials.4lass-stoppered bottles, constant-temperature bath, sodium bromide, hydro-
bromic acid, 0.2 N silver nitrate, 0.1 N sodium hydroxide, phenolphthalein, potassium chromate. Procedure.-A series of hydrobromic acid solutions are prepared by diluting a concentrated solution. About 50 cc. of each of these solutions are placed in glass-stoppered bottles, and slightly more sodium bromide than is required to saturate the solutions a t about 30' is added. The bottles are then placed in a constant-temperature bath regulated a t 25' and allowed to remain there for a t least 24 hours in order to establish equilibrium between the solid and solution phases. Longer periods of time increase .the probability of obtaining good results. It is suggested that the solutions be prepared a t the beginning of one laboratory period and be left in the constant-temperature bath until the next period. The solutions should be shaken vigorously from time to time. Automatic stirring or shaking is, of course, desirable, but for the purpose of this experiment it is not essential if the 24-hour period is taken. When equilfbrinm is established a t 25', approximately one-gram samples of the solution and residue are taken from each bottle and analyzed for hydrobromic acid by titration with sodium hydroxide, and for total bromide by titration with silver nitrate (1). From these titrations the per cent .by weight of hydrogen bromide and of potassium bromide in the solutions is calculated. 1
TYPICAL RESULTS
In Table 1 are given the results of an experiment in which the above procedure was followed. The hydrobromic acid solutions were obtained by diluting a constant-boilmg mixture prepared in the laboratory, the hydrogen bromide being made by the catalytic union of the elements. The two most concentrated solutions became slightly yellow when the sodium bromide was added (5); the others remained colorless. The sodium bromide was of U.S.P. quality. The constant-temperature bath was regulated a t 25' + 0.02°. TABLE 1
C ~ m p o ~ t i o nSolulionr of Composition of R~siduss
% NaBr % HBr
% N ~ B I% HB?
0.00 3.79 7.39 14.08 38.29 43.38
64.1 1.6 59.48 3.23 60.92 4.86 62.9 15.96 20.02 60.07
48.63
44.41 37.37 32.49 8.75 4.95
Solid Phorr NnBr.2HzO NaBr.ZH,O NaBv2H.O NaBr.ZH,O NaBr NaBr
These results are also shown graphically in the figure. not always remember that sodium bromide forms the The data of Nicolaev and Ravich (4) on the composi- dihydrate, it is believed that it provides more interest tion of the solutions are also included in this fignre as a than the previously mentioned experiment, and that it means of comparison. It is evident that the results are illustrates more fully the value of the wet-residue sufficientlyaccurate for the purpose. The experiment method. has also been performed using commercial hydrobromic acid of various degrees of purity with fairly good results; LITERATURE CITED even with one shelf-worn sample of the acid that had become brown in color the results were passable. Ex- (1) O'BRIENAND ~ N N Y J. . &EM. EDUC., 14, 5 3 3 4 (1937). cellent results were obtained in another experiment in (2) See ref. (2) m the paper cited above. "A comprehensive treatise on inorganic and which this brown solution of acid was purified by distil- (3) MELLOR. theoretical chemistry." Longmans. Green and Co., New lation from anthracene. York City, 1922, Vol. 11, p. 532. AND RhvICH, J. Gen. Chnn. (U.S.S. R.), 1, 78E-91 Since the experiment indicates the transition from (4) NICOLAEV 11933). ~-..-,. one solid phase to another, and since the average stu- (5) RICCI, J. Am. Cha.Soc., 56,290-9,29%303 (1934). dent in the beginning physical chemistry course does (6) ScOTI AND DURHAM. 1.Phys. Chm..34,531-7 (1930).
