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The chemistry of solutions of hydrogen fluoride in trinonylamine
J. inorg,nucLChem., 1967,Vol.29, pp. 1119to 1131. PergamonPress Ltd. Printedin NorthernIreland
THE CHEMISTRY OF SOLUTIONS OF HYDROGEN FLUORIDE IN TRINONYLAMINE P. D. BLUNDY, W. H. HARDWICK, M. P. SIMPSON, J. R. STEVENS and P. F. WACE Chemical Engineering & Process Technology Division, A.E.R.E., Harwell, Berks. (Receiced 18 October 1966)
Abstract---An account is given of the solubility relationship of water, hydrogen fluoride and trinonylamine. The amine and acid are completely miscible if the water content of the system is sufficiently low. In general, however, the acid is divided between an aqueous and single organic phase, but a second organic phase can be present at low HF concentrations. The vapour pressures of the three constituents, together with other physical properties of the system show that an amine bifluoride is readily formed to which further molecules of HF may be attached by hydrogen bonding. The chains are terminated by a water molecule, although in the presence of diluents such as xylene less water is present in the organic phase. INTRODUCTION A PROCESSfor the separation of anhydrous hydrofluoric acid f r o m aqueous solutions, by extraction with a tertiary alkyl amine and subsequent heating o f the extract to release H F , has been developed and successfully operated in a pilot plant31~ The development of the process necessitated gathering m a n y data on the physical and chemical properties o f the particular system that was chosen, H F - H ~ O - t r i n o n y l a m i n e (TNA), and these are presented here with some tentative conclusions as to the composition of the organic phases in equilibrium with aqueous H F . The extraction of acids f r o m aqueous solutions by long chain alkylamines was first described by SMITH and PAGE. (1) Their observations were extended to include H F by MOORE, (a) WILSON and WOGMAN(4) and other authors. ~5-7) These studies, in c o m m o n with other published work on the extraction o f acids and amine complexes o f metal salts by amines, relate to organic phases consisting of amine dissolved in a liquid such as c a r b o n tetrachloride or a hydrocarbon. Such diluents improve the separation properties o f the organic phase, for the amines are viscous liquids whose acid addition complexes tend to be highly viscous liquids or solids. Also, properties such as the density difference between phases and the interfacial tension can be improved by dilution with inert solvents. In m u c h o f the work described below the amine was not diluted, since the decomposition o f an amine complex by heating to release H F would be complicated by the presence of a diluent. The use o f undiluted amine is possible in this particular system because of the relatively low viscosity of the a m i n e / H F phase. Other hydrohalic acids f o r m essentially solid addition complexes. W. I-L HARDWICKand P. F. WACE, Chem. Proc. Engng 46, 283 (1965). ~ E. L. SMrrH and J. E. PAGE,J. Soe. chem. Ind. 67, 48 (1948). tal F:. L. MOORE,Analyt. Chem. 29, 1660 (1957). ~a' A. S. WILSONand N. A. WOGMAt%HW-SA-2211, U.S.A.E.C. Report. l:,~ V. M. VDOVENKO,Z. V. KOVALEVAand M. A. RYAZONOV,Radiokhimiya 4, 609 (1962). (~ J. J. BULLOCKet al. J. phys. Chem. 68, 9 (1964). (7~ U. BERTOCCIand G. ROLANDI,J. inorc~7,llucl. Chem. 23, 323 (1961). 1119 tl,
1120
P . D . BLUNDY et aL
VDOVENKO et al. ~5~have p u b l i s h e d s o m e d a t a for the w a t e r c o n t e n t o f T N A - b e n z e n e s o l u t i o n s c o n t a i n i n g H F ; so also have BERTOCCI a n d ROLANDI. tT) EXPERIMENTAL The trinonylamine used in this work was supplied by ICI; its physical properties are given in Ref. (1). It was further purified by a method based on the formation of two organic phases when the amine is contacted with dilute HF, when secondary amines concentrate in the HF-rich phase while the tertiary amine and non-basic substances concentrate in the HF-deficient phase. ~s~ Trinonylamine of > 95 purity was obtained in this way. Water in the organic phase was determined by a modification of the Karl Fischer Method where pyridine was used to neutralize the HF present in the system. Amine in aqueous phases was determined by an adaptation of WASrmROOK'Smethod ~9~based on the extraction of the amine as a coloured cobalt thiocyanate complex. Owing to the low boiling point of t-IF it was necessary to obtain equilibrium data for high HF concentrations at 0°C in order to work at atmospheric pressure. This was reasonable in view of the apparent insensitivity of the equilibrium to temperatures for the low HF concentrations. The technique is described elsewhere. ~1°~ The vapour pressure of HF over amine solutions was determined by MORRIS~11~using a gas saturation method. Nitrogen was passed through three vessels in series, each containing amine/HF solutions, and the amine and HF in the effluent nitrogen determined. Some of the points were obtained using anhydrous solutions prepared by absorbing anhydrous HF in amine, while other amine solutions were made by extraction of HF from aqueous solutions. The vapour pressure of water over HF solutions was obtained in a similar manner by passing nitrogen saturated with water vapour at known temperatures over three vessels in series containing solutions of HF in amine held at constant temperature and analysing the equilibrium liquid phases obtained. Heats of dilution of HF in TNA were measured by contacting amine with aqueous solutions of H F of different strengths in a calorimeter.~12~ RESULTS T h e p a r t i t i o n o f H F between w a t e r a n d u n d i l u t e d T N A was studied over the t e m p e r a t u r e range 2-60°C. The d a t a p l o t t e d in Fig. 1 are for 25°C, a l t h o u g h as can be seen f r o m T a b l e 1 no v a r i a t i o n in p a r t i t i o n with t e m p e r a t u r e was detectable. D a t a f o r three o t h e r amines, tri-n-octyl, tri-iso-octyl a n d tri-iso-decyl, were n o t significantly different when expressed on the s a m e m o l e r a t i o basis. F r o m the H F p a r t i t i o n curve it is seen t h a t a t a b o u t 0.05 M in the a q u e o u s p h a s e the affinity o f T N A for H F increases very rapidly, b u t falls again after 2 m o l e o f H F have entered the a m i n e phase. T h e b r o k e n p a r t o f the curve indicates t h a t over this region, viz. f r o m 0.055 to 0.07 a q u e o u s m o l a r i t y , the organic p h a s e splits into two, one p h a s e being a l m o s t p u r e amine. I f the a m i n e is diluted with solvents such as benzene n o s e p a r a t i o n occurs a n d the p a r t i t i o n curves are continuous, even when the a m i n e c o n c e n t r a t i o n is as high as 80 ~o b y weight. Ternary system T N A - H F - H 2 0 A c o m p r e h e n s i v e s t u d y o f the t e r n a r y system tl°~ has resulted in the t e r n a r y d i a g r a m s h o w n in Fig. 2, with a n e n l a r g e d p o r t i o n for the l o w H F c o n c e n t r a t i o n ts~ j. R. STEVENSand M. P. SIMPSON,U.K. Patent 1020513. 19~A. W. WASnBROO~C,Analyst 84, 177 (1959). ~0~ M. P. SIMPSON,J. R. STEVENSand P. F. WACE, A.E.R.E.-M.1112. t11~W. H. HARDWICK,D. R. MORRIS,P. D. BLUNDVand M. P. SIMPSON, A.E.R.E.-R. 3296 (1959). cla~ W. H. HARDWICKet al. A.E.R.E.-R. 4399 (1963).
The chemistry of solutions of hydrogen fluoride in trinonylamine
112!
region shown in Fig. 3. The ternary diagram shows that, because of the low solubility of amine in water, H F is largely divided between an amine phase, containing approximately 5 700of H20, and an aqueous solution. The water content of the amine phase for H F concentrations greater than 6 700 remains fairly constant. The molar ratio of HzO/TNA is about unity, and many determinations of the water content of the organic phase were carried out to establish the ratio accurately. The results are shown in Fig. 4. The apparent maximum at about 2 mole H F to one of amine at 25°C may be associated with the very high viscosity of the amine phase at this
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FIG. 1.--Distribution of acids between trinonylamine and water at 25°C. HF concentration. Phase separation is more difficult, and excess water may be physically entrained in the organic phase. The result at 60°C (Fig. 5), where viscosity is lower, do not show this maximum and the molar ratio HzO/TNA is nearly unity. The increase in the water content of the amine phase above HF/TNA molar ratio of 4 is not explained. The vapour pressures of both H F and water are quite high at 60°C, and higher still at 90°C, and this makes for experimental difficulties. The effect is nevertheless probably a real one. The solubility of water in pure amine was measured as 0.12 70 w/w at room temperature. Below 6 70 H F the phase relationships of undiluted TNA are more complex and the addition of water will cause a single amine hydrogen fluoride solution to split into two organic phases. One of these is almost pure amine containing only 0.68 ~o H F and 0.1 70 H20; the other is much richer in H F and contains 5.7270 HF, 5-01 70 water. If the residue is all TNA this gives a mole ratio of HF:H~O:TNA of 1-26 1.23:1.00 for the HF-rich phase. Further addition of water results in the separation 17
P . D . BLUNDY et al.
1122
TABLE 1.--PARTITION OF HF BE~_~N TNA AND WATER AT 2 °, 25 ° ~ 60°C Molarity of H F in aqueous phase
g H F per g amine
Moles I-IF per mole amine
2°C 1"75 6"48 12"75 18"80 14"30 22.00
0.130 0"175 0"201 0"229 0"215 0-245
0.32 1"70 4"25 6"50 7"45 10"3 13"7 15"75 16"7 22"1 32"2 39"2 43"1 48"2 48"5 49-5 0"054 0-038 0"105 0"072
0-120 0"140 0'162 0"175 0"179 0"193 0.201 0.217 0"219 0"241 0'309 0"372 0-395 0-480 0"493 0"520 0"00181 0.00100 0"0913 0"0658
1"67 6"53 10-35 12.78 18.82 22"20
0-142 0"174 0.192 0.202 0-228 0.248
2"56 3"46 3"97 4"51 4"25 4"84
25°C 2"37 2.76 3"20 3"45 3"54 3"81 3"98 4-29 4"31 4"78 6"10 7"35 7"80 9"49 9"75 10"25 0"036 0.020 0.181 0"130
60°C 2"81 3"44 3"80 4.00 4"50 4"90
o f a third, a q u e o u s p h a s e c o n t a i n i n g a b o u t 0"1 7o H F . T h e c o m p o s i t i o n o f these p h a s e s r e m a i n s constant, b u t their relative v o l u m e s v a r y within this r e g i o n o f the diagram. T h e d i a g r a m also shows t h a t the a m i n e a n d H F a r e c o m p l e t e l y miscible i f the w a t e r c o n c e n t r a t i o n is sufficiently low.
Effects o f diluents T h e effect o f a diluent, benzene, o n the w a t e r / a m i n e r a t i o is s h o w n in Fig. 4. T h e scatter o f results reflects the difficulty o f the a n a l y t i c a l m e t h o d , b u t the general effect o f diluting the a m i n e is to l o w e r the solubility o f w a t e r in the a m i n e - H F complex. This agrees with the results o f VDOVL~KO et al. C5)
The chemistry of solutions of hydrogen fluoride in trinonylamine •
AMINE
Determinations
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in Ref. 25 in Ref. 27
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The determinations of water in diluted amine solutions given in Table 2 were made primarily to investigate the ratio of H~O to HF over the range where undiluted amine-HF solutions split into two phases; there are also data for solutions stronger in HF. The more dilute amine solutions give an H~O to HF ratio of 0.5 for 1 mole of HF in amine suggesting that the species RaN(HF)2H20 is present together with free amine. At higher amine concentrations, and particularly with neat amine, the affinity for both water and HF is increased until the bifluoride composition is reached but, as has already been said, the entrainment of aqueous phase in the very high
The chemistry of solutions of hydrogen fluoride in trinonylamine
1125
TABLE2.--EXTRACTIONOF HF AND HIO BY TNA DILUTEDWITH BENZENE; AND Be 100~o TNA
Aqueous phase HF(M)
TNA in benzene (w/w)
0.097 0.155 1"16 10.38 22'5 0"079 0-097 0.800 9.80 0-074 0"115 0'365 10.15 21"5 0"0755 0.094 0"175 10-22 21"0
20'1 20.0 20-5 19.6 19.7 37'7 35"8 37"6 36"7 62-3 62"4 62'5 62"4 63"2 79.7 79-1 79.8 76'9 79"6
Organic phase 100 % TNA Concentrations, in mole ratios HF H20 (HF + FI20) HF H~O HF + H~O TNA TNA TNA TNA TNA TNA 0'95 1"9 3-02 4"4 5"0 0"92 1"3 3"2 4"4 0"79 1"5 2-7 4"3 5-3 0"82 1"5 2"2 4-02 5.1
0"5 0"83 0"7 0.73 0.75 0"54 0"68 0.71 0-73 0"5 0'93 0'94 1.00 1'10 0"56 1"1 1'25 1.05 1'2
1.00 2"73 3"72 5"13 5"75 1'46 1"98 3"91 5"13 1"29 2"43 3-64 5"30 6.40 1-38 2-60 3"45 5-07 6"3
1"7 2-1 2.67 3"8 4"37 1"45 1"70 2.60 3.78 1'30 1.80 2'45 3"83 4-80 1"35 1"67 2.17 3"80 4.70
1"4 1"35 1'2 1"2 1'2 1"2 1"4 1"2 1'2 1"2 1"4 1"2 1-2 1-2 1"15 1"4 1"3 1"2 1"2
3"1 3'45 3-87 5.00 5"57 3'65 3.10 3'80 4-98 4'5 3.2 3"65 5"03 6.00 3-5 3.07 3-47 5.00 5-90
viscosity organic phase m a y give anomalously high water analyses. Beyond the amine bifluoride composition the mole ratio ( H F + H 2 0 ) / T N A in the equilibrium amine phase appears to be constant at constant aqueous H F molarity (Table 2) with H F , unexpectedly, replacing H 2 0 on an equimolar basis as the amine is diluted with benzene. This enhancement o f H F extraction by the addition o f diluents is shown in Fig. 6. H F vapour pressure The results are plotted in Fig. 7, to show the v a p o u r pressures o f H F for different concentrations o f H F at different temperatures. The curves show that after an initial rapid rise, the v a p o u r pressure o f H F remains almost constant until the organic phase contains 2 mole o f H F , when it again begins to rise rapidly. The data m a y be used to calculate the latent heat o f vaporization using the Clausius-Clapeyron equation. A t 3 mole o f H F / m o l e amine the latent heat of vaporization o f H F is 11.2 -4- 0.15 kcal/mole, a value very similar to literature values for the latent heat o f vaporization o f H F f r o m water. BROSHEER et al. tla~ f o u n d 11-18-10.51 kcal/mole, while BICHOWSKY and ROSSINI give 11"11-11.56 kcal/ mole. t14) W h e n less than 2 mole o f H F are present the latent heat o f vaporization is 14.4 zk 0-3kcal/mole f r o m the v a p o u r pressure values. This implies that the first two moles o f H F are more strongly b o n d e d to the amine than are subsequent additions. (xs~ j. C. BROSHEER,F. A. LENFESTYand K. L. ELMORE,Ind. Enffng. Chem. 39, 423 (1947). ~1,~F. R. BICHOWSKYand F. D. ROSSXNI,Thermochemistry of the Chemical Substances. Reinhold, New York (1936).
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7.--Vapour pressure HF vs. HF-TNA mole ratio. 1126
100
The chemistry of solutions of hydrogen fluoride in trinonylamine
1127
The vapour-liquid diagram for the HF-amine system has been estimated by extrapolation from the data given above together with amine vapour pressures, within the obvious limitations of 180°C and 300 mm total pressure. Equilibrium still measurements were made with some water present and pot distillation also provided data3 lz) These are summarized in Fig. 8.
Vapour pressure of water The addition of water to the system makes the vapour pressure relationships very complex, especially as reducing the H F in the amine below about 6 per cent leads to the separation of two organic phases. The vapour pressure of water over organic phases containing of the order of 1 mole of water is quite high. For example, over a phase containing 1 mole of water and 2.5 mole of HF per mole of amine the H20 vapour pressure is 289 mm at 81°C (Fig. 9), while from Fig. 7 it is seen that the HF vapour pressure is only of the order of 1 mm at this temperature. As richer HF solutions are contacted with amine, the H F to amine ratio in the organic phase increases, while the water content remains approximately constant at 1 mole. Consequently, the vapour pressure of H F rises quite rapidly as the HF/amine mole ratio is increased beyond two and its partial pressure eventually becomes comparable with that of water. This is illustrated by considering the organic phase in equilibrium with the aqueous azeotrope. The amine phase in equilibrium with the azeotrope at room temperature contains 3"85 mole HF/mole amine and about 1 mole H~O/mole amine, and the partial pressures over this phase must be the same as over the azeotrope, which will of course contain a small amount of dissolved amine (0.01 ~ by weight). The partial pressures of H F and water will be (approximately) in the same ratio as the mole ratio in the azeotrope aqueous phase, which is 1.0: 1.8. The vapour pressure of water then will be nearly twice that of HF. On heating this liquid, water and HF will both be lost, but the rate of loss of water will be greater than that of HF, and also the partial pressure of H F will decrease rapidly relatively to that of water in the residual liquid.
Specific gravity and viscosity A plot of the change of specific gravity with increasing HF content of a TNA phase at 25°C is shown in Fig. 10. It changes little until 2 mole of HF have been added and then begins to rise sharply. The viscosity (also plotted) in Fig. 10, shows a maximum at about the same concentration.
Heat of dilution The heat of dilution of H F in TNA, like the partial pressure of H F over the solutions, is a measure of the thermodynamic affinity of H F for TNA, and a heat of dilution curve gives the variation of this property with concentration. The dilution curve of Fig. 11 thus exhibits the same general shape as the boiling point curve of Fig. 8. It provides useful confirmatory evidence of the rather difficult vapour pressure measurements, particularly in the region where the affinity is changing rapidly, and it extends the information to higher HF/TNA ratios. For example, the "plateau" value of 11.6 kcal/mole for the heat of dilution in the region of two organic phases
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1130
P.D.
BLUNDY et aL
may be added to the (disputable) value of 1.9 kcal/mole for the latent heat of evaporation of H F from H F to obtain 13.5 kcal/mole for the heat of formation of the bifluoride complex TNA-2HF-HzO. Again, the general trend of the curve shows how the latent heat of evaporation of H F from increasingly acid organic phases falls towards that for pure HF. DISCUSSION
The extraction of HF by TNA follows a similar pattern to that exhibited by other halogen acids and strong acids like HNOz, but there are some important differences which are illustrated in Fig. 1. The results plotted in Fig. 1 are for HF extraction by undiluted amine, but as Fig. 6 shows, the differences introduced by dilution with benzene or other hydrocarbons are relatively small. The first observation is that hydrochloric acid, for example, shows a much stronger affinity for TNA at low concentration than does HF. However, the extraction of H F rises very rapidly between 0.06 M and 0.1 M to the composition of the amine bifluoride, RNH(HF,), and a further molecule is added by the time the aqueous phase has reached 2 M, while HC1 shows little tendency to increase the acid to amine ratio above unity over the same range. The difference between HF and HC1 is probably due to the small degree of dissociation of HF at low concentrations, while phosphoric acid, which is intermediate in strength, shows an intermediate pattern. To form the amine hydrochloride by extraction from aqueous solution it is necessary to remove a proton from the 1-130+ ion and transfer it to the nitrogen of the amine; then, with addition of a chloride ion, RaNHCI is formed. This compound, or ion pair, is readily soluble in organic liquids, including the amine itself. This mechanism applies to the other halogen acids, to nitric acid and to perchloric acid, all of which readily form these simple amine salts. The shape of the partition curves will depend firstly on the acid strength and secondly on the solubilities of the amine salts in the amine phase or solution of amine in a diluent. Hydrofluoric acid, being a weak acid, does not as easily provide protons for amine salt formation at low concentrations. According to measurements of the activities of HF solutions by BROENEand DE VRmS{1~ there is a rapid increase in both the H + and H F 2- ion concentrations between 0"01 and 0"1 M total H F molarity. The amine is then able to form the complex R3NH(HF2). The change in physical properties such as density and viscosity support this view and this complex appears to be readily soluble in organic media. Additional H F molecules may be attached to the bifluoride by hydrogen bonding until the liquid becomes in fact a solution of amine in liquid HF, and in fact the two compounds are completely miscible if the water content is sufficiently low (Fig. 2). The values of the latent heat of vaporization and heat of mixing where less than 2 moles of H F are present are of the same order as VERSTEGEN'Svalue tle) of 14.9 kcal/ mole for AH, the heat of formation oftri-n-octyl amine nitrate. We did not observe a change in extraction with temperature over the range 2-60°C, and therefore could not determine a value for AH from these measurements. As more HF molecules are added to the amine the value for the heat of vaporization is reduced to that observed for the latent heat of vaporization of H F itself. The water extracted into the amine phase shows two characteristics. One is that C1~ BROENE and DE VRms, 3.. Am. chem. Soc. 69, 2644 (1947). txe~ j. M. P. J. VERSTE~3EN, Trans. Faraday Soc. 55, 1878 (1962).
The chemistry of solutions of hydrogen fluoride in trinonylamine
l 131
its vapour pressure tends to be much higher than the H F vapour pressure. The other is that when 2 mole or more of H F are present the water to amine mole ratio is very nearly unity, although a somewhat lower value is observed in the presence of diluents. These two facts suggest a model in which the hydrogen fluoride forms a chain which is attached to the amine nitrogen at one end, while a water molecule terminates the chain at the other, viz. R a N H - F - ( H F ) ~ - H - F H-O-H. At higher temperature with increasing concentrations of H F it is probable that the bonding of the H F chain would be weakened and that water molecules would enter as "chain breakers".
THE CHEMISTRY OF SOLUTIONS OF HYDROGEN FLUORIDE IN TRINONYLAMINE P. D. BLUNDY, W. H. HARDWICK, M. P. SIMPSON, J. R. STEVENS and P. F. WACE Chemical Engineering & Process Technology Division, A.E.R.E., Harwell, Berks. (Receiced 18 October 1966)
Abstract---An account is given of the solubility relationship of water, hydrogen fluoride and trinonylamine. The amine and acid are completely miscible if the water content of the system is sufficiently low. In general, however, the acid is divided between an aqueous and single organic phase, but a second organic phase can be present at low HF concentrations. The vapour pressures of the three constituents, together with other physical properties of the system show that an amine bifluoride is readily formed to which further molecules of HF may be attached by hydrogen bonding. The chains are terminated by a water molecule, although in the presence of diluents such as xylene less water is present in the organic phase. INTRODUCTION A PROCESSfor the separation of anhydrous hydrofluoric acid f r o m aqueous solutions, by extraction with a tertiary alkyl amine and subsequent heating o f the extract to release H F , has been developed and successfully operated in a pilot plant31~ The development of the process necessitated gathering m a n y data on the physical and chemical properties o f the particular system that was chosen, H F - H ~ O - t r i n o n y l a m i n e (TNA), and these are presented here with some tentative conclusions as to the composition of the organic phases in equilibrium with aqueous H F . The extraction of acids f r o m aqueous solutions by long chain alkylamines was first described by SMITH and PAGE. (1) Their observations were extended to include H F by MOORE, (a) WILSON and WOGMAN(4) and other authors. ~5-7) These studies, in c o m m o n with other published work on the extraction o f acids and amine complexes o f metal salts by amines, relate to organic phases consisting of amine dissolved in a liquid such as c a r b o n tetrachloride or a hydrocarbon. Such diluents improve the separation properties o f the organic phase, for the amines are viscous liquids whose acid addition complexes tend to be highly viscous liquids or solids. Also, properties such as the density difference between phases and the interfacial tension can be improved by dilution with inert solvents. In m u c h o f the work described below the amine was not diluted, since the decomposition o f an amine complex by heating to release H F would be complicated by the presence of a diluent. The use o f undiluted amine is possible in this particular system because of the relatively low viscosity of the a m i n e / H F phase. Other hydrohalic acids f o r m essentially solid addition complexes. W. I-L HARDWICKand P. F. WACE, Chem. Proc. Engng 46, 283 (1965). ~ E. L. SMrrH and J. E. PAGE,J. Soe. chem. Ind. 67, 48 (1948). tal F:. L. MOORE,Analyt. Chem. 29, 1660 (1957). ~a' A. S. WILSONand N. A. WOGMAt%HW-SA-2211, U.S.A.E.C. Report. l:,~ V. M. VDOVENKO,Z. V. KOVALEVAand M. A. RYAZONOV,Radiokhimiya 4, 609 (1962). (~ J. J. BULLOCKet al. J. phys. Chem. 68, 9 (1964). (7~ U. BERTOCCIand G. ROLANDI,J. inorc~7,llucl. Chem. 23, 323 (1961). 1119 tl,
1120
P . D . BLUNDY et aL
VDOVENKO et al. ~5~have p u b l i s h e d s o m e d a t a for the w a t e r c o n t e n t o f T N A - b e n z e n e s o l u t i o n s c o n t a i n i n g H F ; so also have BERTOCCI a n d ROLANDI. tT) EXPERIMENTAL The trinonylamine used in this work was supplied by ICI; its physical properties are given in Ref. (1). It was further purified by a method based on the formation of two organic phases when the amine is contacted with dilute HF, when secondary amines concentrate in the HF-rich phase while the tertiary amine and non-basic substances concentrate in the HF-deficient phase. ~s~ Trinonylamine of > 95 purity was obtained in this way. Water in the organic phase was determined by a modification of the Karl Fischer Method where pyridine was used to neutralize the HF present in the system. Amine in aqueous phases was determined by an adaptation of WASrmROOK'Smethod ~9~based on the extraction of the amine as a coloured cobalt thiocyanate complex. Owing to the low boiling point of t-IF it was necessary to obtain equilibrium data for high HF concentrations at 0°C in order to work at atmospheric pressure. This was reasonable in view of the apparent insensitivity of the equilibrium to temperatures for the low HF concentrations. The technique is described elsewhere. ~1°~ The vapour pressure of HF over amine solutions was determined by MORRIS~11~using a gas saturation method. Nitrogen was passed through three vessels in series, each containing amine/HF solutions, and the amine and HF in the effluent nitrogen determined. Some of the points were obtained using anhydrous solutions prepared by absorbing anhydrous HF in amine, while other amine solutions were made by extraction of HF from aqueous solutions. The vapour pressure of water over HF solutions was obtained in a similar manner by passing nitrogen saturated with water vapour at known temperatures over three vessels in series containing solutions of HF in amine held at constant temperature and analysing the equilibrium liquid phases obtained. Heats of dilution of HF in TNA were measured by contacting amine with aqueous solutions of H F of different strengths in a calorimeter.~12~ RESULTS T h e p a r t i t i o n o f H F between w a t e r a n d u n d i l u t e d T N A was studied over the t e m p e r a t u r e range 2-60°C. The d a t a p l o t t e d in Fig. 1 are for 25°C, a l t h o u g h as can be seen f r o m T a b l e 1 no v a r i a t i o n in p a r t i t i o n with t e m p e r a t u r e was detectable. D a t a f o r three o t h e r amines, tri-n-octyl, tri-iso-octyl a n d tri-iso-decyl, were n o t significantly different when expressed on the s a m e m o l e r a t i o basis. F r o m the H F p a r t i t i o n curve it is seen t h a t a t a b o u t 0.05 M in the a q u e o u s p h a s e the affinity o f T N A for H F increases very rapidly, b u t falls again after 2 m o l e o f H F have entered the a m i n e phase. T h e b r o k e n p a r t o f the curve indicates t h a t over this region, viz. f r o m 0.055 to 0.07 a q u e o u s m o l a r i t y , the organic p h a s e splits into two, one p h a s e being a l m o s t p u r e amine. I f the a m i n e is diluted with solvents such as benzene n o s e p a r a t i o n occurs a n d the p a r t i t i o n curves are continuous, even when the a m i n e c o n c e n t r a t i o n is as high as 80 ~o b y weight. Ternary system T N A - H F - H 2 0 A c o m p r e h e n s i v e s t u d y o f the t e r n a r y system tl°~ has resulted in the t e r n a r y d i a g r a m s h o w n in Fig. 2, with a n e n l a r g e d p o r t i o n for the l o w H F c o n c e n t r a t i o n ts~ j. R. STEVENSand M. P. SIMPSON,U.K. Patent 1020513. 19~A. W. WASnBROO~C,Analyst 84, 177 (1959). ~0~ M. P. SIMPSON,J. R. STEVENSand P. F. WACE, A.E.R.E.-M.1112. t11~W. H. HARDWICK,D. R. MORRIS,P. D. BLUNDVand M. P. SIMPSON, A.E.R.E.-R. 3296 (1959). cla~ W. H. HARDWICKet al. A.E.R.E.-R. 4399 (1963).
The chemistry of solutions of hydrogen fluoride in trinonylamine
112!
region shown in Fig. 3. The ternary diagram shows that, because of the low solubility of amine in water, H F is largely divided between an amine phase, containing approximately 5 700of H20, and an aqueous solution. The water content of the amine phase for H F concentrations greater than 6 700 remains fairly constant. The molar ratio of HzO/TNA is about unity, and many determinations of the water content of the organic phase were carried out to establish the ratio accurately. The results are shown in Fig. 4. The apparent maximum at about 2 mole H F to one of amine at 25°C may be associated with the very high viscosity of the amine phase at this
100~-L-
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FIG. 1.--Distribution of acids between trinonylamine and water at 25°C. HF concentration. Phase separation is more difficult, and excess water may be physically entrained in the organic phase. The result at 60°C (Fig. 5), where viscosity is lower, do not show this maximum and the molar ratio HzO/TNA is nearly unity. The increase in the water content of the amine phase above HF/TNA molar ratio of 4 is not explained. The vapour pressures of both H F and water are quite high at 60°C, and higher still at 90°C, and this makes for experimental difficulties. The effect is nevertheless probably a real one. The solubility of water in pure amine was measured as 0.12 70 w/w at room temperature. Below 6 70 H F the phase relationships of undiluted TNA are more complex and the addition of water will cause a single amine hydrogen fluoride solution to split into two organic phases. One of these is almost pure amine containing only 0.68 ~o H F and 0.1 70 H20; the other is much richer in H F and contains 5.7270 HF, 5-01 70 water. If the residue is all TNA this gives a mole ratio of HF:H~O:TNA of 1-26 1.23:1.00 for the HF-rich phase. Further addition of water results in the separation 17
P . D . BLUNDY et al.
1122
TABLE 1.--PARTITION OF HF BE~_~N TNA AND WATER AT 2 °, 25 ° ~ 60°C Molarity of H F in aqueous phase
g H F per g amine
Moles I-IF per mole amine
2°C 1"75 6"48 12"75 18"80 14"30 22.00
0.130 0"175 0"201 0"229 0"215 0-245
0.32 1"70 4"25 6"50 7"45 10"3 13"7 15"75 16"7 22"1 32"2 39"2 43"1 48"2 48"5 49-5 0"054 0-038 0"105 0"072
0-120 0"140 0'162 0"175 0"179 0"193 0.201 0.217 0"219 0"241 0'309 0"372 0-395 0-480 0"493 0"520 0"00181 0.00100 0"0913 0"0658
1"67 6"53 10-35 12.78 18.82 22"20
0-142 0"174 0.192 0.202 0-228 0.248
2"56 3"46 3"97 4"51 4"25 4"84
25°C 2"37 2.76 3"20 3"45 3"54 3"81 3"98 4-29 4"31 4"78 6"10 7"35 7"80 9"49 9"75 10"25 0"036 0.020 0.181 0"130
60°C 2"81 3"44 3"80 4.00 4"50 4"90
o f a third, a q u e o u s p h a s e c o n t a i n i n g a b o u t 0"1 7o H F . T h e c o m p o s i t i o n o f these p h a s e s r e m a i n s constant, b u t their relative v o l u m e s v a r y within this r e g i o n o f the diagram. T h e d i a g r a m also shows t h a t the a m i n e a n d H F a r e c o m p l e t e l y miscible i f the w a t e r c o n c e n t r a t i o n is sufficiently low.
Effects o f diluents T h e effect o f a diluent, benzene, o n the w a t e r / a m i n e r a t i o is s h o w n in Fig. 4. T h e scatter o f results reflects the difficulty o f the a n a l y t i c a l m e t h o d , b u t the general effect o f diluting the a m i n e is to l o w e r the solubility o f w a t e r in the a m i n e - H F complex. This agrees with the results o f VDOVL~KO et al. C5)
The chemistry of solutions of hydrogen fluoride in trinonylamine •
AMINE
Determinations
•
--
1123
in Ref. 25 in Ref. 27
No, of p h a s e s p r e s e n t . . . . . .
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FIG. 3.--Enlaxged ternary diagram showing effect of low concentrations of water and hydrogen fluoride in trinonylamine.
1124
P.D. BLUNDYet
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The determinations of water in diluted amine solutions given in Table 2 were made primarily to investigate the ratio of H~O to HF over the range where undiluted amine-HF solutions split into two phases; there are also data for solutions stronger in HF. The more dilute amine solutions give an H~O to HF ratio of 0.5 for 1 mole of HF in amine suggesting that the species RaN(HF)2H20 is present together with free amine. At higher amine concentrations, and particularly with neat amine, the affinity for both water and HF is increased until the bifluoride composition is reached but, as has already been said, the entrainment of aqueous phase in the very high
The chemistry of solutions of hydrogen fluoride in trinonylamine
1125
TABLE2.--EXTRACTIONOF HF AND HIO BY TNA DILUTEDWITH BENZENE; AND Be 100~o TNA
Aqueous phase HF(M)
TNA in benzene (w/w)
0.097 0.155 1"16 10.38 22'5 0"079 0-097 0.800 9.80 0-074 0"115 0'365 10.15 21"5 0"0755 0.094 0"175 10-22 21"0
20'1 20.0 20-5 19.6 19.7 37'7 35"8 37"6 36"7 62-3 62"4 62'5 62"4 63"2 79.7 79-1 79.8 76'9 79"6
Organic phase 100 % TNA Concentrations, in mole ratios HF H20 (HF + FI20) HF H~O HF + H~O TNA TNA TNA TNA TNA TNA 0'95 1"9 3-02 4"4 5"0 0"92 1"3 3"2 4"4 0"79 1"5 2-7 4"3 5-3 0"82 1"5 2"2 4-02 5.1
0"5 0"83 0"7 0.73 0.75 0"54 0"68 0.71 0-73 0"5 0'93 0'94 1.00 1'10 0"56 1"1 1'25 1.05 1'2
1.00 2"73 3"72 5"13 5"75 1'46 1"98 3"91 5"13 1"29 2"43 3-64 5"30 6.40 1-38 2-60 3"45 5-07 6"3
1"7 2-1 2.67 3"8 4"37 1"45 1"70 2.60 3.78 1'30 1.80 2'45 3"83 4-80 1"35 1"67 2.17 3"80 4.70
1"4 1"35 1'2 1"2 1'2 1"2 1"4 1"2 1'2 1"2 1"4 1"2 1-2 1-2 1"15 1"4 1"3 1"2 1"2
3"1 3'45 3-87 5.00 5"57 3'65 3.10 3'80 4-98 4'5 3.2 3"65 5"03 6.00 3-5 3.07 3-47 5.00 5-90
viscosity organic phase m a y give anomalously high water analyses. Beyond the amine bifluoride composition the mole ratio ( H F + H 2 0 ) / T N A in the equilibrium amine phase appears to be constant at constant aqueous H F molarity (Table 2) with H F , unexpectedly, replacing H 2 0 on an equimolar basis as the amine is diluted with benzene. This enhancement o f H F extraction by the addition o f diluents is shown in Fig. 6. H F vapour pressure The results are plotted in Fig. 7, to show the v a p o u r pressures o f H F for different concentrations o f H F at different temperatures. The curves show that after an initial rapid rise, the v a p o u r pressure o f H F remains almost constant until the organic phase contains 2 mole o f H F , when it again begins to rise rapidly. The data m a y be used to calculate the latent heat o f vaporization using the Clausius-Clapeyron equation. A t 3 mole o f H F / m o l e amine the latent heat of vaporization o f H F is 11.2 -4- 0.15 kcal/mole, a value very similar to literature values for the latent heat o f vaporization o f H F f r o m water. BROSHEER et al. tla~ f o u n d 11-18-10.51 kcal/mole, while BICHOWSKY and ROSSINI give 11"11-11.56 kcal/ mole. t14) W h e n less than 2 mole o f H F are present the latent heat o f vaporization is 14.4 zk 0-3kcal/mole f r o m the v a p o u r pressure values. This implies that the first two moles o f H F are more strongly b o n d e d to the amine than are subsequent additions. (xs~ j. C. BROSHEER,F. A. LENFESTYand K. L. ELMORE,Ind. Enffng. Chem. 39, 423 (1947). ~1,~F. R. BICHOWSKYand F. D. ROSSXNI,Thermochemistry of the Chemical Substances. Reinhold, New York (1936).
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FIG. 6.--Distribution of HF between water and TNA diluted with benzene. 1000
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7.--Vapour pressure HF vs. HF-TNA mole ratio. 1126
100
The chemistry of solutions of hydrogen fluoride in trinonylamine
1127
The vapour-liquid diagram for the HF-amine system has been estimated by extrapolation from the data given above together with amine vapour pressures, within the obvious limitations of 180°C and 300 mm total pressure. Equilibrium still measurements were made with some water present and pot distillation also provided data3 lz) These are summarized in Fig. 8.
Vapour pressure of water The addition of water to the system makes the vapour pressure relationships very complex, especially as reducing the H F in the amine below about 6 per cent leads to the separation of two organic phases. The vapour pressure of water over organic phases containing of the order of 1 mole of water is quite high. For example, over a phase containing 1 mole of water and 2.5 mole of HF per mole of amine the H20 vapour pressure is 289 mm at 81°C (Fig. 9), while from Fig. 7 it is seen that the HF vapour pressure is only of the order of 1 mm at this temperature. As richer HF solutions are contacted with amine, the H F to amine ratio in the organic phase increases, while the water content remains approximately constant at 1 mole. Consequently, the vapour pressure of H F rises quite rapidly as the HF/amine mole ratio is increased beyond two and its partial pressure eventually becomes comparable with that of water. This is illustrated by considering the organic phase in equilibrium with the aqueous azeotrope. The amine phase in equilibrium with the azeotrope at room temperature contains 3"85 mole HF/mole amine and about 1 mole H~O/mole amine, and the partial pressures over this phase must be the same as over the azeotrope, which will of course contain a small amount of dissolved amine (0.01 ~ by weight). The partial pressures of H F and water will be (approximately) in the same ratio as the mole ratio in the azeotrope aqueous phase, which is 1.0: 1.8. The vapour pressure of water then will be nearly twice that of HF. On heating this liquid, water and HF will both be lost, but the rate of loss of water will be greater than that of HF, and also the partial pressure of H F will decrease rapidly relatively to that of water in the residual liquid.
Specific gravity and viscosity A plot of the change of specific gravity with increasing HF content of a TNA phase at 25°C is shown in Fig. 10. It changes little until 2 mole of HF have been added and then begins to rise sharply. The viscosity (also plotted) in Fig. 10, shows a maximum at about the same concentration.
Heat of dilution The heat of dilution of H F in TNA, like the partial pressure of H F over the solutions, is a measure of the thermodynamic affinity of H F for TNA, and a heat of dilution curve gives the variation of this property with concentration. The dilution curve of Fig. 11 thus exhibits the same general shape as the boiling point curve of Fig. 8. It provides useful confirmatory evidence of the rather difficult vapour pressure measurements, particularly in the region where the affinity is changing rapidly, and it extends the information to higher HF/TNA ratios. For example, the "plateau" value of 11.6 kcal/mole for the heat of dilution in the region of two organic phases
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240
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Determinations
using 35*/, aqueous HF
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Determinations
using 60% aqueous HF
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1130
P.D.
BLUNDY et aL
may be added to the (disputable) value of 1.9 kcal/mole for the latent heat of evaporation of H F from H F to obtain 13.5 kcal/mole for the heat of formation of the bifluoride complex TNA-2HF-HzO. Again, the general trend of the curve shows how the latent heat of evaporation of H F from increasingly acid organic phases falls towards that for pure HF. DISCUSSION
The extraction of HF by TNA follows a similar pattern to that exhibited by other halogen acids and strong acids like HNOz, but there are some important differences which are illustrated in Fig. 1. The results plotted in Fig. 1 are for HF extraction by undiluted amine, but as Fig. 6 shows, the differences introduced by dilution with benzene or other hydrocarbons are relatively small. The first observation is that hydrochloric acid, for example, shows a much stronger affinity for TNA at low concentration than does HF. However, the extraction of H F rises very rapidly between 0.06 M and 0.1 M to the composition of the amine bifluoride, RNH(HF,), and a further molecule is added by the time the aqueous phase has reached 2 M, while HC1 shows little tendency to increase the acid to amine ratio above unity over the same range. The difference between HF and HC1 is probably due to the small degree of dissociation of HF at low concentrations, while phosphoric acid, which is intermediate in strength, shows an intermediate pattern. To form the amine hydrochloride by extraction from aqueous solution it is necessary to remove a proton from the 1-130+ ion and transfer it to the nitrogen of the amine; then, with addition of a chloride ion, RaNHCI is formed. This compound, or ion pair, is readily soluble in organic liquids, including the amine itself. This mechanism applies to the other halogen acids, to nitric acid and to perchloric acid, all of which readily form these simple amine salts. The shape of the partition curves will depend firstly on the acid strength and secondly on the solubilities of the amine salts in the amine phase or solution of amine in a diluent. Hydrofluoric acid, being a weak acid, does not as easily provide protons for amine salt formation at low concentrations. According to measurements of the activities of HF solutions by BROENEand DE VRmS{1~ there is a rapid increase in both the H + and H F 2- ion concentrations between 0"01 and 0"1 M total H F molarity. The amine is then able to form the complex R3NH(HF2). The change in physical properties such as density and viscosity support this view and this complex appears to be readily soluble in organic media. Additional H F molecules may be attached to the bifluoride by hydrogen bonding until the liquid becomes in fact a solution of amine in liquid HF, and in fact the two compounds are completely miscible if the water content is sufficiently low (Fig. 2). The values of the latent heat of vaporization and heat of mixing where less than 2 moles of H F are present are of the same order as VERSTEGEN'Svalue tle) of 14.9 kcal/ mole for AH, the heat of formation oftri-n-octyl amine nitrate. We did not observe a change in extraction with temperature over the range 2-60°C, and therefore could not determine a value for AH from these measurements. As more HF molecules are added to the amine the value for the heat of vaporization is reduced to that observed for the latent heat of vaporization of H F itself. The water extracted into the amine phase shows two characteristics. One is that C1~ BROENE and DE VRms, 3.. Am. chem. Soc. 69, 2644 (1947). txe~ j. M. P. J. VERSTE~3EN, Trans. Faraday Soc. 55, 1878 (1962).
The chemistry of solutions of hydrogen fluoride in trinonylamine
l 131
its vapour pressure tends to be much higher than the H F vapour pressure. The other is that when 2 mole or more of H F are present the water to amine mole ratio is very nearly unity, although a somewhat lower value is observed in the presence of diluents. These two facts suggest a model in which the hydrogen fluoride forms a chain which is attached to the amine nitrogen at one end, while a water molecule terminates the chain at the other, viz. R a N H - F - ( H F ) ~ - H - F H-O-H. At higher temperature with increasing concentrations of H F it is probable that the bonding of the H F chain would be weakened and that water molecules would enter as "chain breakers".