T. J. COLLOPT AND J. I?. BLUM
1324
298’K. (’an be accomplished by using heat content data from Stull and Sinke.6 TABLE I1 VAPORPRESSURE DATAON PALLADIUM METAL Run
1’. “ K .
1 2 3 4 5 6
1640
n
8 9 10 11 12 13 14 15 16
1640
1640 1338 1340 1340 1220 1363 1363 1363 1501 1562 1578 1484 1470 14G7
P , mm.
9 28 x I .i8 x 1.62 X 3 58 X 3.99 x 3.54 x 1 93 x 8.24 X 6.49 X 7.91 X 1 . 9 x~ 5.14 x 6.79 x 5.32 x 4 95 x 3.40 x
10-4 10-3
10” 106 10” 10-7 lo* 10” 1010-4 10-4 10-4 10-5
10-5 10-6
A H m (kcal./mole)
92.6 91.8 90.8 90.7 90.6 90.9 90.0 90.2 90.9 90.3 89.7 90.2 90.2 92.3 91.8 92.7
4v. AH0298= 91 0 i 0 . 8 kcal./mole The log P vs. l / l ’ plot gives a A H = 87.9 a t T a v . = 1468 which corrected to 298°K. using Stull and Sinke’se data gives a heat AH0298 = 90.0 zt 2 kcal./mole.
The hiqh temperature vapor pressure measurements in Table I on Pt confirm the previously reported data summarized by Honig3 and establish the reliability of the apparatus and technique. The third lam heat is slightly higher than reported by Jones, Langmuir and ilfackay’ but is well within their experimental error. Surprisingly, few reports of the vapor pressure of Pt exist in the literature.
T’ol. 64
The only reports on palladium are in wide disagreement. Dnane and Haefling have reported Knudsen effusion measurements on palladium contained in a cell constructed partly of Ta and partly of graphite.2a This two-material construction was needed because of the reacti\-ity of Pd(s) with graphite and of Pd(g) with Ta. Walker, Efimenko and Lofgren have reported measurements on Pd done by the Langmuir technique.2b Their measured pressures differ by several orders of magnitude from those of Haefling and Daane. The results reported here for Pd fall in between those of Daane, et a1.,2aand of Walker, et aLZb The heat of sublimation is almost exactly an average. Since both previous investigators are experienced in the vapor pressure field, one can only suggeqt that perhaps the reactions of Pd with Ta and C n-hich Dame and Haefling were trying to avoid were still significant and that some more volatile impurity was sublimed in addition to the Pd. With graphite crucibles and lids, it is often possible to have cracks or holes through which effusion occurs besides the regular effusion orifice. Elimination of this possibility can only be accomplished by a calibration experiment with the same cell. In the work of Walker, et aZ.,*” the w e of an emissivity of 0.33 for palladium in the determination of the temperature of the sample may be the source of the error. Acknowledgment.-The authors are pleased to acknowledge the financial support of the ITisconsin Slumni Research Foundation and of the Atomic Energy Commission. The high purity Pd wire was generously provided by the International Kickel Company through the courtesy of‘ E. )I. \Ti se .
SPECTROPHOTOMETRIC EVIDEYCE FOR COMPLEX FORMATIOX IS THE TRI-TZ-BUTYLPHOSPHATE-WATER-NITRIC ACID SYSTEM BY T. J. COLLOPY .4ND J. F. BLUM Technical Division, National Lead Cornpanil of Ohio, Cincinnati, Ohio Receiied A p r i l 29, 1960
Tri-(normal)-butyl phosphate solutions containing varying concentrations of nitric acid were investigated to determine the stoichiometry and stability of the complexes formed in the reaction between the tlvo. Spectral data in the range 260 to 320 mp have shown that tri-n-butyl phosphate and associated nitric acid react to form a stable equimolar complex. Evidence is presented which indicates that additional stable complexes having different mole ratios are not formed. This work has demonstrated the complex which is present in the tri-n-butyl phosphate phase, and has enabled the authors t o interpret data that have been obtained for the two phase tri-n-butyl phosphate-water-nitric acid system. The significance of this complex can be found in its direct application to metal extraction processes, and an understanding of the fundamental chemistry underlying these processes.
Introduction Several investigators have determined the equilibrium distribution of nitric acid between tri-nbutyl phosphate and water. Three of these in~estigators’-~ have indicated, from an interpre(1) R. L. Moore. ” T h e Extraction of Uranium in the Tributyl Phos. phate Metal Recovery Process,” HW-15230, September 1, 1949. (2) K. Alcock, S. S.Grimley, T. V. Healy, J. Kennedy and H. A. C. hlcKay, Trana. Faraday Soe., 62, 39 (1956). (3) A. T. Gresky, M. R. Bennett, S. S. Brrtndt. W. T. M c D u 5 e and J. E. Savolianen, Progress Report on Laboratory Development of the Thorex Process, USAEC Report ORNL-1367, January 2, 1953 (Secret).
tation of distribution data, that equimolar complex formation does occur in the tri-n-butyl phosphate phase between tri-n-butyl phosphate and nitric acid. Other investigators4 have interpreted their distribution data as indicating the existence of a 1: 1 complex and a 1 : 2 complex between trin-butyl phosphate and nitric acid in the organic phase. During an investigation of the chemistry of the tri-n-butyl phosphate-uranyl nitrate solvent ex(4) R. J. Allen and
RI. A. DeSesa, .Vucleonics, 16, (IO) 88 (1957).
Sept., 1960
CoiuPLz.x
FORMATIOX IN TIU-n-BCTYL PHOSPHATE-WATER-~ITRIC ACID
1325
traction process,s it became necessary to determine (1) the mole ratios of tri-n-butyl phosphate to nitric acid in the complexes formed, and (2) the stability of the complexes formed. The authors believed that complex formation could be demonstrated most effectively by conducting this investigation in a single phase organic solution containing purified tri-n-butyl phosphate and pure nitric acid (100% nitric acid is completely miscible with tri-n-butyl phosphate). The data obtained in this investigation could then be used in conjunction with data on the composition of aqueous nitric acid solutions to better explain complex formation in the two-phase tri-n-butyl phosphatewater-nitric acid system.
were determined by titration with standard alkali to a phenolphthalein end-point. The nitric acid equilibrium distribution data were obtained by mixing the aqueous nitric acid solutions with tri-n-butyl phosphate for a period of three minutes at constant temperature (25"). The mixture was allowed to separate, and the phases were sampled and analyzed. The acidity of both the aqueous and organic phases was determined by titration with standard alkali t o a phenolphthalein end-point. The organic aliquot was added to an excess of distilled water and the titration was made directly on this mixture.
( 5 ) T. J. Collopy, "The Tributyl Phosphate-Nitric Acid Complex and Its Role in Uranium Extraction," NLCO-749, April 23, 1958. (6) J. Kennedy and S. S. Grimley, Tri-n-butyl Phosphate Studies. Parts I, I1 and 111, AERE-CE/R-968, October 6 1952. 17) R. S. Jones and G . D. Thorn, Research, 2TB, 580 (1949). (8) P. Job, Con& rend., 184, 204 (1927). (9) W. C. Vosburgh and G. R. Cooper, J. A m . Chem. Soc., 63, 437 (1941). (10) A. E. Harvey. Jr., and D. L. Manning, tbid., 7 2 , 4488 (1950). 111) J. H. Yoe and A. L. Jones, Ind. Enp. Chem., Anal. Ed., 16, 111 (1944).
(12) 0. Redlich, Chem. Revs,39, 333 (1946). (13) 0. Redlich and J. Bigeleisen, J . A m . Chem. Soc., 6 5 , 1883 (1943). (14) N. R. Rao, Indian J. Phys., 15, 185 (1941). (15) W. F. K. Wynne-Jones, J . Chem. Soc., 1064 (1930). (1Gf A. Kossiakoff and D. Harker, J. A m . Chem. Soc., 60, 2047 (1938). (17) 0. Redlich, Z. physzk. Chem., A182, 42 (1938). (18) H. Yon Halban and J. Elsenbrand, *bid.. 132, 401 (1928). (19) G. C. Hood, 0. Redlich and C. A. Reilly, J. Chem. Phys.. 22, 2067 (1954).
Results and Discussion Aqueous nitric acid solutions have been shown by numerous investigator^^^-^^ to contain associated nitric acid molecules (HKOa). The degree of association has been shown to increase rapidly with increasing concentration of the acid. In a two-phase system of tri-n-butyl phosExperimental Materials.-For the spectral study, commercial-grade phate and aqueous nitric acid (associated nitric tri-n-butyl phosphate was purified employing a method of acid and tri-n-butyl phosphate being completely distillation over alkali .6 The pure associated nitric acid miscible), it has been shown that hydrogen ions (100% nitric acid was assumed to be completely associated) or nitrate ions do not exist appreciably in the triwas prepared by vacuum distillation of the acid from a mixture of concentrated sulfuric acid, 70% nitric acid and n-butyl phosphate phase.2 If associated nitric solid sodium nitrate. The acid obtained was colorless and acid (HiT03) were present in the aqueous phase, gave analysis for greater than 99% nitric acid. The absorp- however, it might be expected that the polar trition spectrum of the acid was the same as that reported by n-butyl phosphate would combine with the polar other investigators for associated nitric acid molecules? The associated acid was frozen except when removing aliquots for associated nitric acid through hydrogen bonding. experimental purposes, so that discoloration or decomposi- The data presented by Redlich and Bigeleisen, tion of the stock acid did not occur. The solvent used in illustrating the presence of associated nitric acid in this work was reagent-grade carbon tetrachloride. The aqueous nitric acid solutions, plus the above conmaterials used to obtain the distribution curve were reagent- siderations, led the authors to beliere that any grade nitric acid and commercial-grade tri-n-butyl phosphate (produced by Ohio Apex Division of the Food and Machinery complex formation in the system would be beChemical Company). tween the as2:ociated nitric acid molecules and the Apparatus -The absorption spectra of tri-n-butyl phos- tri-n-butyl phosphate molecules. A spectrophotophate (TBI'), associated nitric acid (HNOJ), and the 1:l TBP.HN03 complex in the wave length range, 260 to 320 metric study was conducted employing tri-nmp were obtained with a Beckman Model DK-2 spectro- butyl phosphate and associated nitric. acid in the photometer employing matched silica cells of 1-cm. path organic solvent, carbon tetrachloride, to determine length. Measurements a t fixed wave lengths were made the mole ratio and stability of any significant comwith a Beckman hlodel DU spectrophotometer using plexes which are formed. Carbon tetrachloride was matched 1-em. silica cells. The apparatus used to determine the distribution curve was a separatory funnel equipped chosen as the solvent for this study because of its with an interfacial stirrer. inertness toward the reactants under investigaProcedure.-The method of continuous variation*se tion. was employed in determining TBP.Hh'O3 complex formation The spectra of O.OG2.3 41 tri-n-butyl phosphate, in the solvent carbon tetrachloride using individual stock solutions containing 0.125 ill concentrations of tri-n-butyl 0.0625 M associated nitric acid, and a 1:1 mixture phosphate and of associated nitric acid. The molar ratio (0.0623 Jf tri-n-butyl phosphate-0.0625 JP asmethodlOJ1 was employed to determine the stability of the sociated nitric acid) of the two in the wave length 1 : 1 complex. The concentration of associated acid was held constant (0.0625 M in the carbon tetrachloride solvent), range, 260 to 320 mp, are presented in Fig. 1. and the tri-n-butyl phosphate concentrations was varied. Wave lengths lower than 260 mp could not be The total volume of each solution also was held constant by investigated because of the high absorption of the dilution with carbon tetrachloride to a final volume of 50 ml. carbon tetrachloride solvent helow 260 nip. The The possible formation of a 1: 2 tri-n-butyl phosphatenitric acid romples in carbon tetrachloride also was studied absorbance of the nusture at aiiy wave leiigth employing the molar ratio method. In this case, the con- should he eqiial t o the sum of the :il)sorbaiice of rentration of tri-n-butyl phosphate was held constant and each component a t the corresponding wave length, t h nitiic ~ acid roncentration was varied until a maximum it' no reaction occurs in the niixturr. The sun^ concentration ratio of 0 16 $1 nitric arid to 0.04 M tri-n- of the absorbance of A and 33 in Fig. I a t any wave butyl phosphate was obtained in carbon tetrachloride. Aqueous nitric acid solutions of varying molarities were length over the range investigated does not equal prepared for the partition study by adding the required the absorbance of C a t the corresponding waw roncentration of reagent-grade acid (700j0)to distilled water. length. The difference of the observed results of The molarities of the initial aqueous nitric acid solutions C from the calculated results is attrilluted t o thc
T.J. COLLOPY AXD J. F. BLUM
1326
Yol. 61
Data a150 were obtained which showed a 1:l TBP "03 complex in the solvent chloroform and in a mixed solvent, 9570 isooctane by volume and .5y0 diethyl ether by volume. The fact that the maxima 11 ere reproducible at each wave length was attributed to the formation of only one complex in the systemgat the concentration ranges investigated. The continuous irariation curves were blightly unsymmetrical with the absorbance difference being higher on the excess acid side than on the exces? T B P side. These data might indicate formation of another complex species in solution. Further study was conducted in excess acid \ medium, the results are presented in the section on the investigation of higher complexes. Figure 8 illustrates the method oi molar ratiowhich was employed to show that the 1 . 1 T B P HN03 complex was very stable. According to theory, for a very stable complex, a plot of absorbance of the complex against the molar ratio of the reagents, which react t o form the coniplex (the concentration of one of the reagents remaining constant), would rise from the origin as a straight line and break sharply to constant absorbance at the molar ratio of the components in the complex. The plot of a complex that dissociates appreciably would not show a sharp break, but would give a 200 2i0 280 290 300 310 320 continuous curve and approach constant absorb\Tave length ( m f i ) . Fig. 1.--dhsorption spectra curve A, 0.0625 31 TBP in ance only at large excesses of the variable compocarbon tetrachloridp, curve R , 0.0625 111 associated nitric nent. Curve A (Fig. 3) is a plot of the solution abacid in carbon tetrachloride; curve C, mixture containing sorbance versus molar ratio (TBP/HKOJ in carbon 0.0625 -Ifassociated nitric acid and 0.0625 ,M TBP in carbon tetrachloride, associated nitric acid concentratptrachloridr tion 0.0625 M ) at a wave length of 308 nip. The plot rises from the origin as a straight line (since associated nitric acid absorbs at this wave length, 0.20 the origin in this case is the absorbance of 0.0625 0.18 JI associated nitric acid) and breaks sharply at ii mole ratio of 1:1, as predicted for a strong complex. With increasing excess tri-n-butyl phos+ 0.16 phate concentration over that required for com5 0.14 plexing, the curve does not achieve constant density but continues to rise linearly. This linear increaw 8 0.12 of absorbance is due to the absorbance oi thc 2 0.10 excess tri-n-butyl phosphate. Line B is the c d 5 ( d a t e d curve obtained by subtractiiig the exre+ 0.080 tri-n-butyl phosphate absorbance from the ah2 .orbance found experimentally. The actual coni0.060 ples curve then rises from the origin as a straight 0.010 line and breaks sharply to constant density. The Investigation of Higher Complex Equi0.020 libria.--Figure 4 again illustrates the met hod 0 of molar ratio. used in this case t o detrrminc 0 0.2 0.4 O.(i 0.8 1.0 whether any evidence of higher nitric. acid m m 1101~.fnictioti of H S ( ) $ . plexeh [TBP (HSOJ)n. where I I > 11 i i .hm\.ii Pig. 2.-. Coritinuoiis variation plots for t Ii(3 TBI'-HS( ) : in solutions c*oiitaiiiiiig excess asiociated iiitri(L roinplex in carbon tctrachloritio: ( v r w .I, 208 iup; curve B, acid. C'urve5 .1 and 13 are plots of ioliitioir wl)2886 mp. sorbaiiw 1 erszis aiwcxitecl iiitric acid c~oiireiitrnt1011 ---SO2group baiid shift, u i i d iiiteiisity chaiiges with tri-ri-butyl pho-phate present. T h e tri+upon reaction of the acid with tri-n-butyl phos- butyl phosphate coiiceiitration wai niaintaiiied :it phate to form the complex. 0.04 JI. Curves C and D are plots of solutioii The Determination of the 1: 1 Complex.-Thr: absorbance of nitric acid alone. Curves A and J3 method of cont,inuous wriation was employed to rise from the origin a$ straight line- :tnd break examine complex formation iii the TBP.HNO3 appreciably at a nitric acid cwic'riitration q u a l to system. Contiiiuous variation data for TBP. 0.04 N . Onw again the break owurwd at the H S O a mixtures show the existence of a I : 1 TRT'. point uidicatiiig w I I romplm A 4 a r p hreah IISO:; coniyles ill carbon tetruchluridc (Fig. 2 ) . N ould not Ix cxpcc tcd 11 higher coiiiple\ ccluilihrin
1-
-
L
L
-
4
-
i--
COMPLEX FORMATION IN TRI-~-BUTYL PHOSPHATE-WATER-NITRIC ACID
Sept., 1960
1327
exist, but rather, a gradual change of slope. Furthermore, if only one complex equilibrium is operating and, as proved earlier, if the complex is 0.14 extremely stable then the absorbance of the excess acid should be a function of the concentration of the excess acid; that is, absorbance is equal to a 0.12 constant times the concentration of excess acid g ( A = KC). The constant obtained by plotting A $ versus excess C in the above equation should be -E: 0.10 equal to the constant obtained by plotting A versus 8 associated nitric acid when no tri-n-butyl phos% phate is present a t any wave length investigated. Curves A and B (excess acid) are parallel within experimental accuracy to curves C and D for the absorbance of associated nitric acid a t the wave lengths 290 and 295 mp, respectively. The same results indicative of no second complex also were 0.040 L j L ' U i l l l found to exist at six other ware lengths investigated 0 2 4 6 8 10 12 in the range of 275 to 305 mp. Molar ratio TBP/HNOD (HYOa constant). Since it has been established that (1) associated Fig. 3.-The stability of the 1:l TBP-HNOa complex nitric acid exists in aqueous nitric acid solution, in carbon tetrachloride: curve A, experimental curve; and (2) that a stable, equimolar complex forms line B, experimental curve-correction for TBP absorption between tri-n-butyl phosphate and associated 308 mp. nitric acid, the equilibrium distribution curve for 1.0 1 1 nitric acid between tri-n-butyl phosphate and water was interpreted in terms of complex formation between tri-n-butyl phosphate and associated nitric 0.8 acid in the organic phase. Figure 5 presents the AND *NO, 6 equilibrium curve up to an aqueous nitric acid 2 0.6 concentration of 12 i ? . The associated acid values for each aqueous solution (based on data of Redlich and Bigelei~en'~) 4m 0.4 are also indicated. Figure 5 shows that as the 0.2 associated acid increases, the amount of complex formed in the tri-n-butyl phosphate phase increases up to the value for the 1 : 1 complex as would be 0 predicted. The linearity of the equilibrium curve 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 beyond the 1:1 complex and the results of the specAssociated nitric acid concn. tral data indicate that the distribution of associ- Fig. 4.-The absorbance of the 1: 1 complex in the presence of associated nitric acid. ated nitric acid into the organic is a function of its solubility in the 1:l TBP.HSO8 complex and Associated HNO, concn. ( M ) (as.). not of further complex formation. The fact that 1 2 3 4 5 6 ' 7 8 associated nitric acid distributes very strongly in favor of the organic phase even a t low acid concentrations (and low associated acid concentration) was attributed to the associated nitric acid reaction with tri-n-butyl phosphate shifting the associated nitric acid equilibrium in the aqueous nitric acid solution. The results reported herein showed the formation of a stable, equimolar complex between tri-nbutyl phosphate and associated nitric acid in the organic phase. It was concluded from these results and from the previously reported data on the pre :ence of associated nitric acid in aqueous nitric acid solutions that the reaction of nitric acid and tri-n-butyl phosphate to produce a complex is coiltrolled by the associated nitric acid concentration 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 9 in the two-phase system under study. Total acid H [H+ HNO,] concn. ( M ) (as.). Acknowledgment.-The authors wish to acknowFig. 5.-Equilibrium distribution of nitric acid between ledge assistance by members of the Technical Di- TBP and water a t various nitric acid concentrations vision. - . emeciallv Mr. W. C. Manser. (25").
A
CURVE5 A
8. ASSOCIATED
g-
+
>
I
