T H E CONSTANT-BOILING MIXTURE O F HYDROGEN FLUORIDE AND WATER C . W. YUEHLBERGER
In 1843,Bineaul pointed out that the hydrohalogen acids formed constantboiling mixtures and attempted to identify these azeotropic mixtures as definite hydrates of the acids. He showed that under ordinary atmospheric conditions, aqueous hydrofluoric acid of 3 5.6 to 3 7 .6y0concentration distilled a t constant composition. He reported that the constant-boiling mixture boiled a t 120' and had a density of 1.15. Roscoe2 later studied the constant boiling mixtures of aqueous solutions of the hydrohalogens and, in addition to confirming Bineau's experimental work, he showed that the composition of the equilibrium mixture was a function of the temperature a t which the equilibrium was reached. Thus, a t the ordinary boiling point of aqueous hydrofluoric acid under a pressure of one atmosphere, the equilibrium mixture was found to be 36 to 38y0 hydrogen fluoride. However, at 15', the equiBy this type of experimentation, Roscoe librium mixture was 32.5%. showed the fallacy of Bineau's hypothesis of the formation of definite hydrates. More recent work by Deussen3 showed that at atmospheric pressure the constant boiling mixture of aqueous hydrofluoric acid was 43.2% hydrogen fluoride. This mixture was said to have a density of 1.138a t 18' and a boiling point of 111' a t a pressure of 750 mm. of mercury. For these experiments he used a platinum still fitted with a thermometer well. The concentration of his solutions was determined by titration with barium hydroxide solution which had previously been standardized with crystallized potassium hydrogen fluoride, using phenolphthalein indicator. In our laboratory, distillations of pure aqueous hydrogen fluoride made in a platinum apparatus have indicated that the constant-boiling mixture is 38.18% HF ( ~ = O , I O % ) a t a pressure of 735 mm. of mercury. The density of this concentration of acid, as determined with a Westphal balance fitted with a Bakelite plummet, was found to be 1.138 at 20'. This agrees with the density reported by Deussen but is considerably higher than the density of 38.18% hydrogen fluoride given by Winteler4. The boiling point of the azeotropic mixture was determined in a lead distilling apparatus fitted with a thermometer well and was found to be 110.8' a t a pressure of 732 mm. of mercury. The concentration of the constant-boiling acid was determined by titration with I .5 normal sodium hydroxide solution using phenolphthalein as an indicator. Precautions were taken to exclude carbonates and silica from the reagents and to insure that the hydrofluoric acid used was free from impurities. 'Ann. Chim. Phys., (3) 7,257-75 (1843). Ann., 116,203-220 (1860). Z.anorg. Chem., 49, 297-300 (1906). Winteler: Z.angew. Chem., 1902, 33-34.
HYDROGEN FLUORIDE A S D WATER
1889
Constant boiling hydrochloric acid' and carefully crystallized oxalic acid2 were used as primary standards in determining the concentration of the standard alkali solution. From these results i t is apparent that the concentration of the constantboiling mixture of aqueous hydrofluoric acid reported by Deussen is too high. This error may have been due to impurities in the KHFz which he used as an acidimetric standard. The writer is greatly indebted to Prof. George Kemmerer and other members of the Chemistry Faculty of theuniversity of Wisconsin for their kindness in providing apparatus and other facilities, without which this work would have been impossible. Cnioersity of Wzswnsin, Madison, Wisconsin. Apnl 25, 1928.
Foulk and Hollingsworth: J Am. Chem. SOC., 45,
* Hill and Smith: J. 4 m . Chem. SOC.,44, 546-557
1227
(1922).
(1923).
