March, 1961.
THERMODYNAMIC~STUDY OF THE SYSTEM AMMONIUM FLUORIDE-WATER
on many aspects of the problem. The assistance of Mr. C. A. Seils and Mr. R. M. Clarke, who per-
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formed the chemical analyses, is also gratefully acknowledged.
A THERMODYNAMIC STUDY OF THE SYSTEM AMMONIUM FLUORIDEWATER. I. THE HEAT CAPACITY AND THERMODYNAMIC FUNCTIONS OF AMMONIUM FLUORIDE MONOHYDRATE BY LEONARD C. LABOWITZ AND EDGAR F. WESTRUM,JR. Department of Chemistry, University of Michigan, Ann Arbor, Michigan Received July 8 , 1960
Thermal analysis and solubility determinations on the NH4F-rich side of the NH~F-HzOeutectic indicate a peritectic temperature of 246.0"K. and a eutectic temperature of 244.15"K., in contrast with 256.4 and 246.7"K., respectively, reported by Yatlov and Polyakova. The existence of the compound N€LF.H*O a t 245.7"K. was established by application of the method of wet residues to the system NH4F-NHaI-H20 a t that temperature. The heat capacity of the monohydrate was determined from 5 to 323°K. The values of the saturation heat capacity, C., and the derived thermodynamic functions So, HO - Hoe, and (Fa - HoO))/Ta t 298.15"K. are 37.22 cal./(deg. mole), 34.92 cal.!(deg. mole), 6146 cal./mole, and -14.30 tal./( deg. mole), respectively. The enthalpy increment of the peritectic reaction is 1360 cal./mole at 246°K.
Introduction In the course of investigating the range of existence and the therinophysical properties of the solid solution of ammonium fluoride (NHIF) in ice reported by Brill and Zaromb, l v 2 thermal analysis measurements were carried out from 230 to 290°K. over the entire composition range of the system NHIF-H~O. The results of these measurements in the monohydrate region of the system indicate that the eutectic and peritectic temperatures are so much closer together than the corresponding temperatures found by Yatlov and Polyakova3 that it was not immediately evident whether the incongruently melting compound ammonium fluoride monohydrate (NHIFeH20) reported by them really exists or whether the observed thermal halt was due to the eutectic transition alone. The present investigation involving equilibrium heat capacity determjnation by the adiabatic technique, Schreinemrakers' wet residues analyses, and solubility determinations, was therefore undertaken. The existence of the compound ammonium fluoride monohydrate was established and its thermodynamic properties were determined. Experimental Preparation and Purity of Ammonium Fluoride.-Ammonium fluoride wai3 prepared by treating reagent hydrofluoric acid (48 wt. '% aqueous H F ) in a silver beaker with an excess of ammonia gas. The warm, stron ly ammoniacal solution of ammonium fluoride obtained in tfis manner was transferred into a polyethylene bottle through a polyethylene funnel and a filter paper leached with hydrofluoric acid. Boiled distillled water was added t: the filtrate to bring all of the crystals into solution a t 40 . The bottle was then sealed and submerged in a large and well-insulated bath and allowed to cool gradually from 40 to 0" over a period of four days. Large clusters of acicular crystals resulted from this slow cooling-process. The crystals were separated from the mother liquor on a polyethylene Biichner funnel, rinsed with cold ammonia-saturated methanol, and packed into a polyethylene drying column. A stream of anhydrous ammonia gas was passed over the material for two days a t room temperature. The dry crystals were then transferred to a silver beaker and evacuated for an hour in a vacuum desiccator to __ (1) R . Brill and S. Zaromb, Nature, 173, 316 (1954). (2) 6. Zaromb a n d R . Brill, J. Chem. Phys., 24, 895 (1956). ( 3 ) V.S.Yatlov and E:. AI. Polyakova, Zhur. Obshchei Khzm., 16, 724
(1945).
remove excess ammonia gas. The product was stored in closed polyethylene bottles under an atmosphere of ammonia gas in a desiccator lined with paraffin and charged with calcium oxide. Analysis for NHa+ by the Kjeldahl method4 gave 48.70 0.04 (volumetric buret) and 48.72 f 0.02 (weight buret) wt. % (theoretical: 48.70). Analysis for F- by the PbFCl gravimetric method6 using the modified precipitation procedure of Kapfenbergel.6 indicated 51.32 & 0.03.wt. .% F- (theoretical: 51.30). Interference of ammonium ion with the PbFCl precipitation' was eliminated by boiling the sample with dilute sodium hydroxide solution until the vapors evolved had no noticeable effect on the color of moist red litmus paper. The remaining alkaline solution was then neutralized with dilute hydrochloric acid in the presence of methyl orange indicator. To ascertain if the high chloride concentration resulting from this neutralization would interfere, the precipitation was carried out on a standard sample of sodium fluoride in the presence of a comparable amount of sodium chloride; the results were 99.73%, of theoretical. The determination of water by the Karl Fischer methods gave 0.004 wt. % HzOwith the "dead stop" end-point technique. Using the procedure outlined for ammonium fluoride by Rosin,g none of the following contaminants were detected (the figures represent the sensitivity limits of the tests in wt. 70): ignition residue
