The BOILING POINT of the CON-
STANT-BOILING MIXTURE HC1-H20 A Correction of
an Error Occurring in H a n y Textbook8
WILLIAM E. CADBURY, JR. Haverford College, Haverford, Pennsylvania
An error i n recording the boiling point of the azeotropic mixture HCC-H20is pointed out. A critical examination of this boiling point is made, both experimentally and through the literature. It i s suggested that the value 110°, frequently appearing i n textbooks, be replaced by the value 108.584O.
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N EXPERIMENT frequentSy performed in laboratory courses in elementary physical chemistry is the study of the system HCl-HzO. In most of the textbooks available to students there is a serious error in recording the boiling point of the constant-boiling mixture. Several well-known writers on physical chemistry record the boiling point of this mixture as 110' a t 760 mm. pressure; among these are Cetman and Daniels (I), Friend (2), Washbum (3), Rodebush and Rodehush (4), and Walker (5). The International Critical Tables (6) also give this value. The value recorded in Taylor's Treatise (7) is 108.5'. and in Chapin (8) is 108.4'. According to Bonner and Wallace (9) the boiling point of this mixture is 108.584° * 0.004° a t 760 mm. The first work on this subject was done by Roscoe and Dittmar (10) who give the boiling point as 110' a t one atmosphere. Hulett and Bonner (11) report 108.54' a t 763 mm. CarriPre anCArnaud (12) measured the boiling point of various mixtures of HCl and water a t 760 mm. pressure and report the boilmg point of the constant-boiling mixture as 110"; they make no mention of calibration of the thermometer used. The boiling point of the constant-boiling mixture
was determined in this laboratory by MI. Allen W. Stokes asfollows: A mixture of HCI, C.P. concentrated, and water was diluted to a specific gravity 1.1 and distilled. The first two-thirds of the distillate was discarded, and all of the remainder but a small residue was collected. This was redistilled through a long fractionating column, the thermometer bulb and mercury column being immersed in the vapor. Insulation of the column against heat loss by radiation was accomplished by wrapping the column with asbestos paper. Superheating was practically eliminated by the use of platinum tetrahedra in the bottom of the boiling flask. The boiling point and barometric pressure were read after about half of the solution had been redistilled, readings being taken every minute for a period of fifteen minutes to be sure that a constant temperature had been reached. Data were obtained a t various pressures in the neighborhood of 760 mm., hoiling point was plotted against pressure, and the boiling point a t 760 mm. was determined from the curve thus obtained. The thermometer was.+ecked against boiling water in the same apparatus. The boiling point of this constauthoiling mixture was found to be 108.58" + 0.05'at 760 mm. pressure. This value agrees, well within the limit of experimental error, with the v$ue given by Bonner and Wallace (9). Since the w o ~ kof Bonner and Wallace appears to be of the highest accuracy of any of the researches on this subject reported in the literature, it is of considerable importance that the erroneous value appearing in so many textbooks be replaced by the value determined by these authors.
LITERATURE CITED
(1) GETMAN,, F. Hi AND DANIELS.F., "Outlines of theoretical ehem~stry. 5th ed.. John Wiley & Sons, New York City. 1931, p. 161. FXIEND,J. N., "Texthook of physical chemistry," 1. B. Lippincott Ca.,Philadelphia, 1933, Vol. I, p. 416. WASHBURN, E. W.. "Principles of physical chemistry," 1st ed.. McGraw-Hill Book Co., New York. 1915, p. 181. RODEBU~R, W. H. AND RODEBUSH, E. K.,"An mtroductory course in physical chemistry," D. Van Nostrand Co.. New Yark City, 1932, p. 151. WALKER, J., "Introduction to physical chemistry," 9th ed., Mamullan and Co.. London, 1922. Inlernalional Critical Tables, Vol. 111, p. 323. Tnnon, H. S., "A treatise on physical chemistry." 2nd ed., D. Van Nostrand Co., New York City, 1931, Vol. I, p. 524.
(8) CHAPIN, W. H., "Second year college chemistry." 3rd ed., John Wiley & Sons, New York City, 1933, p. 30. W. D. AND WALLACE, R. E., boiling points (9) BONNER, of constant-boiling hydrochloric acids, J. Am. C h . Soc., 52, 1747-50 (May, 1930). H. E. UND DITTW, W., " ~ b e die r Absorption des (lo) ROSCOE, Chlorwasserstoffs und des Ammoniaks in Wasser," Ann., 112, 339 (1859). (11) H a E n . G. A. nm, BONNER,W. D.,"A method for preparing standard hydrochloric acid solutions," J. Am. C k m . Soc., 31,3903 (March, 1909). E. ET ARNAUD. "D6terminatiou des conrbes (12) CARRI~RE, #&bullitionet de ros& des maanges d'acide chlarhydriqne et d'eau sous la pression de 760 mm.," Compt. rend.. 179, 1265 (Dec., 1924).
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