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Interactions between tributyl phosphate, phosphoric acid and water
J. Inorg. Nucl. Chem., 1962, Vol. 24, pp. 415 to 427. Pergamon Press Ltd. Printed in England
INTERACTIONS BETWEEN TRIBUTYL PHOSPHATE, PHOSPHORIC ACID A N D WATER* C. E. HIGGINS and W. H. BALDWIN Chemistry Division, Oak Ridge National Laboratory, Tennessee
(Received 2 October 1961; in revised form 1 November 1961) Alanamet--The distribution co-efficient for phosphoric acid between tributyl phosphate (TBP) and water increased with the acid concentration in the inorganic phase and correlated with the water behaviour in the organic phase. Water accompanying the acid into the TBP was salted out after the ratio 3 TBP-H3PO4"6 H20 was reached but levelled off at one water per TBP regardless of the phosphoric acid concentration over the range 1-4 moles H3PO4 per mole TBP. Evidence for a high degree of association in solutions of anhydrous H3PO4 and TBP was obtained from ebullioscopic molecular weight determinations in ether and the viscosity of undiluted solutions. The maximum viscosity occurred at the 3 H.~PO4 tO I TBP ratio and was about three times that of anhydrous, super-cooled H3PO4. AN E^RLIER investigation ~l) into the interactions between TBP, monovalent electrolytes, and water has been extended now to include the polyvalent orthophosphoric acid. With the exception of sull~huric acid(a) all of the many TBP-acid studies have been concerned with the monobasic acids, thus information gained from distribution tests with a tribasic acid should be of interest. The industrial potential of TBP for the purification of phosphoric acid has recently been shown by I~UMAS. (3) The previously noted miscibility of anhydrous phosphoric acid and TBp(4) made possible the investigation of interactions between these compounds in the absence of water. The method ofcontinuous variation was employed for ebullioscopic molecular weight determinations and refractive index and viscosity measurements. EXPERIMENTAL
Chemicals. Reagent grade orthophospboric acid (86.1 per cent H3PO4) was used to prepare the stock aqueous solutions. Removal of water by vacuum pump treatment of the 86 per cent H3PO4 gave solutions in the 86 to 100 per cent range. Anhydrous phosphoric acid--3'P was prepared by mixing 5 me H3PO4--32p in weak HCI with 1.14 g of 86.1 per cent H3PO4 (0.010 mole) in a 50 mi Pyrex round-bottomed flask, freezing, then pumping off the volatile materials until the correct weight loss had been achieved, warming no higher than 50° C during the drying operation. Solutions containing tracer 32p were then prepared over the concentration range 10-3 M to 15 M H3PO4, having specific activities of 0.25 to 12 me per mole throughout the period of test. The organic chemicals were of reagent grade or were distilled before use. TBP was purified as before. (1) * This paper is based upon work performed for the United States Atomic Energy Commission at the Oak Ridge National Laboratory operated by Union Carbide Corporation. (1) W. H. BALDWIN,C. E. HIGGINSand B. A. SOLD^NO,J. Phys .Chem. 63, 118 (1959). (2) E. HESrORD and H. A. C. McKAY, 3"~Inorg. NucL Chem. 13, 156 (1960). ~-~)B. C. DOUMAS,Purification of Phosphoric Acid by Solvent Extraction, University Microfilms, Ann Arbor, Mich. (1961). ~4) C. E. HIOG[NSand \V. H. B^Lt~WIN,J. Org. Chem. 21, 1156 (1956). 415
416
C.E. HIGGINSand W. H. BALDWIN
Distribution and solubility determinations. The distribution of phosphoric acid between water and TBP was accomplished by equilibrating the aqueous stock solutions with equal volumes (5-20 ml) of pure TBP and with TBP in various concentrations in organic diluents. The equilibration time was 1 hr in a water-bath at 25 +0.2 ° C. A sharp phase layering was obtained by centrifugation for 20-30 sec at 2,000 rev/min in an open centrifuge at a room temperature of 25 ° C without materially altering the solution equilibrium. ~5) Any volume changes were noted, following which the phases were separated and stored in the bath. The distribution co-efficient, E~a, in all cases refers to the ratio, after equilibration, of the concentration (moles per litre) of phosphoric acid in the organic phase to that in the aqueous phase. Mo and M a refer to molar concentrations of H3PO4 in the organic and aqueous phases, respectively, while mo and m a refer to molalities. Approximate amounts of tributyl phosphate dissolved in the high concentration equilibrium phosphoric acid solutions were determined by extracting duplicate 50 ml samples several times with equal volumes of C.P. carbon tetrachloride, concentrating, and pumping to constant weight at room temperature in weighed 50 mi round-bottomed flasks. The solubility of TBP in dilute phosphoric acid solutions was previously determined with TBP-32p after the manner used for the reported solubilities in monovalent electrolyte solutions:6) Dry TBP and anhydrous phosphoric acid mixed in mole ratios varying from 1:20 to 100~ 1 formed clear, homogeneous'solutions. Analysis of solutions. The phosphoric acid concentration in most solutions was determined by titration of aliquots (aqueous phases in water and organic phases in 50 per cent ethyl alcohol) with standard sodium hydroxide using a Fisher Titrimeter, others by solution counting the beta activity, correcting, when necessary, for the effect of solution density on counting rate after the manner of Dyrssen.¢7) Some equilibrium aqueous solutions were analysed by interpolation of their densiti~ on a density vs. stock phosphoric acid concentration curve. Values thus found agreed within + 1 per cent of the values found by titration. Water content in the equilibrium organic solutions (no diluent) was determined by use of the Karl Fischer reagent.* The TBP content was then found by subtracting from the weight of a known volume of ol:ganic solution the weights of water and phosphoric acid. Densities were determined at 25.0 +0.2 ° C using calibrated weight pipettes of 0.500-4 ml capacities. Solutions of TBP and anhydrous phosphoric acid in varying molar proportions were prepared for physical measurements. Refractive indices were obtained at 25.0 ~ C with an Abb6 refractometer. Molecular. weights were determined in ether by the ebullioscopic method of MENZIESand WRIGHX.18.9) An estimation of the viscosities at 25" was obtained by comparing the time to deliver ,-,0.4 ml between two marks 20 cm apart on a straight upright tube of 1.6 mm i.d. with that of glycerine (170 sec) as a standard. The viscosities were calculated from the relationship :Tz ~ = d~ ~--~ d2 t~ t2'where d~, d2 and t~. t_, are densities and delivery times, respectively, and tr2 for glycerine (d 2~ 1.255) is 954 cpoise. I iol RESULTS AND DISCUSSION The distribution o f phosphoric acid between pure TBP and water as a function of the equilibrium aqueous phase acid concentration is shown in Table I. The semi-log plot, Fig. I, shows E ° falling into three separate sections depending upon * We are indebted to W. R. LAINGand co-workers of the Analytical Chemistry Division for the water analysis. 15) C. E. HIGGINSand W. H. BALDWIN,Analyt. Chem. 32, 236 (1960). t6) C. E. HIGGINS, W. H. BALDWINand B. A. SOLDANO,J. Phys. Chem. 63, 113 (1959). ~7) D. DYRSSEN,Acta Chem. Scand. 11, 1771 (1957).
Interactions between tributyl phosphate, phosphoric acid and water
417
TABLE I.--ExTRACTION OF PHOSPHORICACID AND WATERBY 100% TBP AT 25 ° C Equilibrium aqueous phase
H3PO4
Equilibrium organic phase
H3PO4
concn, (M)
Mole fraction H3PO4
Density (g/ml)
concn, (M)
Mole fraction HaPO4
Mole fraction 820
Mole fraction TBP
E°
Density (g/ml) 0.9971 1.018 I "041 1.062 ! .066 l "127 i'193 1"319 1.445 l "546 I "615 ! "646 1 '743 1 '752
0.000 0.406 0"838 1 "26 1 "33 2-56 3"91 6"50 9"18 11 "4 12-9 13 "6 15"9 16"2
0.0000 0.0074 0"0155 0"0233 0"0250 0.0500 0"0801 0"147 0'233 0"324 0'398 0"439 0"609 0"643
0.9767 0.9780 0"9815 0.9864 0"9875 1.006 1 "029 l "068 !'110 1 "151 1-193 1 '220 1 "342 1 "360
0.0000 0"0375 0"106 0" 198 0"214 0"583 1 "02 l "84 2"71 3 "62 4"53 5 "09 7"71 8"10
0.0000 0.0055 0"0150 0 "0257 0-0255 0"0654 0".105 0"218 0.312 0"388 0"463 0"500 0"654* 0"670
0.499 0"492 0-502 0 "538 0"582 0-574 0"576 0"415 0"346 0"312 0"267 0"252 0'175 • 0"165
0.501 0.502 0"483 0 "436 0"393 0"361 0"320 0"367 0"342 0"299 0"270 0"248 0"175 • 0:165
0.0000 0-0924 0"126 0" 157 0-161 0"228 0"261 0"283 0"295 0"318 0'351 0"374 0"485 0"500
........
I
* Graphical interpolation.
I
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I
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........
I
t o SOLUTIONCOUNTING • TITRATION
/ /
~a 0.3 j
0"2 0-t _
,
• • _/
IO-3 IO-2 IO-~ 4 10 MOLARITY OF PHOSPHORIC ACID IN EQUILIBRIUMAQUEOUS PHASE
FIG. i.--The relationship between distribution c.o-efficient and the equilibrium aqueous phase H3PO4 concentration. the aqueous phase concentration. Each section also mirrors the water extraction behaviour (columns 7 and 8, Table 1 and Fig. 2). At first E ° increased linearly from 0-01 to 0.1 over the range 10-3-0-5 Ma, with the T B P - H 2 0 ratio remaining constant at one in the organic phase. In the second section E ° increased more rapidly with co-extraction o f water and H3PO 4. The m a x i m u m water to TBP ratio o f 2 in the organic phase occurred at 4 M,. E ° then began to level off as the addition o f more
418
C.E. HIoOINs and W. H. BALDWIN
phosphoric acid salted water out of the organic phase rapidly. In the last section E ,° rose rapidly from 0.3 at ~ 10 M a to 0.5 at 16 Ma. Through this range the T B P - H 2 0 ratio was that of the monohydrate regardless of the accompanying phosphoric acid concentration. When the phosphoric acid concentrations in each phase were converted to mole fractions, columns 3 and 6, Table 1, it was noted that generally the mole fraction of phosphoric acid in the organic phase was nearly the same as that in the corresponding aqueous phase.
I-"
IV '7I! t.6
/
o 4.5 t.4
o" '
~ t.2
.
~ t.t
.
t.0 0-9
0
1 2 3 MOLE RATIO) H3PO4 )o TBP
4
FIG. 2.--Water behaviour in the organic phase. The amount of TBP going into the aqueous phase was not appreciable over the range 0-15 M a H3PO4 (see Table 2). Phosphoric acid dissolved less TBP than water alone until the concentration exceeded 15 M a H3PO 4. Anhydrous H3PO4 dissolved TBP in all proportions at 25 ° C. TBP IN PHOSPHORIC ACID Equilibrium concentrations
TABLE 2 . - - S O L U B I L I T Y OF
SOLUTIONS AT 2'5 ° C
H3PO4 in queous
TBP in aqueous
(M)
(mg/l.)
0"00 0.12 0.29 0.58 1.14 12.4
414" 388* 370* 336* 290* " 110÷ 14.8 390f TBP miscible with anhydrous H3PO( * Determined using TBPJ2P. t Determined by weight recovery.
Interactions between tributyl phosphate, phosphoric acid and water
419
The water behaviour in acid systems extracted by TBP is peculiar to the acid itself. Phosphoric acid carried water into the organic phase to the extent of about five molecules per molecule of acid (initial slope, Fig. 2). Water co-extracted with the halogen acids in ratios varying from 3:1 to 4~1 ~1~. From evidence obtained by an organic volume change technique,( TM Tuck and DIAMOND(12) have recently postulated the presence of a trihydrated hydronium ion in the TBP phase after contact with aqueous HBr, HCI, and HCIO4. Small differences were found, however, and from these and other results~l, 2) the amount of water initially co-extracted by each acid appears to decrease in the order H3PO4> HI > HBr > HCI > HCIO4 > H2SO4 > HNO3. Nitric acid is a special case since it forces water from the TBP phase until the 1: 1 ratio of TBP and HNO3 is reached.el3, 1) As additional nitric acid goes into the organic phase the water content begins to increase.~13) The TBP appears to have approached its original monohydrated state when the organic phase is about 8 M HNO3. The organic phase acid concentration at which water is squeezed out by additional acid also varies with the acid. From the water-phosphoric acid curve (Fig. 2) the maximum appears to have occurred when the organic phase has the approximate composition 3 TBP.H3PO4.6 H20. If the organic solutions resulting from the extraction of perchloric and sulphuric acids by H~FORD and McKAy~2) have densities similar to those reported here for the organic solutions containing phosphoric acid and water then recalculating their data on a molal basis results in maxima at compositions of approximately 2 TBP'HCIO4"4 H20 and TBP.2 H2SO4"2 H20. The former does agree with the HCIO4 extraction data of SX~KI~tSgI and GWOZDZ.~14) Maxima for the halides(l) occur at roughly TBP.O.7HX.3 H20. The aforementioned minimum in nitric acids' water curve, for which the densities of the equilibrium organic solutions are known up to about 5 M a HNO3,~ls~ occurs wbe~ the composition of the organic phase is close to TBP'HNO3"~ H20. The exact ratio of HNO3 to TBP at the minimum water content is actually 1.2.c13) In previous work~l) most of the univalent electrolytes tested approached a 1~1 TBP--electrolyte compQsition in the organic phase, nitric acid alone exceeding this ratio. Here the polyvalent phosphoric acid to TBP ratio was ~ 1 at 9 M H3PO4 in the equilibrium aqueous phase, rose to 2~1 at 13.6 M,, and continued to increase rapidly. This indicates polymerization in the organic phase at the higher acidities. Other evidence will be given in a later section. The maximum ratio obtained here was four, at an equilibrium aqueous phase H3PO4 concentration of 16 M. Polymerization increased and the distribution ratio rose rapidly since the limiting condition in the aqueous phase was being approached, that is, anhydrous phosphoric acid which was miscible with TBP. The extraction of nitric acid by TBP up to as much as 2½ HNO3 molecules per TBP has been previously noted by ALCOCKet al.(13) TUCK(15)has postulated addition of the extra nitric acid molecules to the P-O-C oxygen atoms. KORPAKand DI'.'PTULA,{16) OH)D. G. TUCK,J. Chem. Soc. 3202 (1957). ~12~D. G. TUCKand R. M. DL~MOND,J. Phys. Chem. 65, 193 (1961). (~-~ K. ALCOCK,S. S. GRIMLEY,T. V. HEALY,J. KENNEDYand H. A. C. McKAY, Trans. Faraday Soc. 52, 39 (1956). ~4~ S. SW.KmaSKXand R. GwozDz, Nukleonika 5, 205 (1960). ~) D. G. Tucg, J. Chem. Soc. 2783 (1958). ([6) W. KORPAKand C. D~FrUL^, Nucleonika S, 63 (1960).
420
C.E. HIGOINSand W. H. BALDWIN
in presenting evidence for the formation of the complexes TBP'HNO3, TBP'2HNO3, TBP'3HNO3, and possibly TBP'4HNO3, have made the same suggestion. Such a bonding arrangement would appear to be consistent with the observed viscosities of the organic solutions. As shown by H~FORO and McKAY,~2) the viscosity increased very little, in fact beyond 1 Mo HNO3 decreasing until at 5 Mo it was the same as the viscosity of monohydrated TBP itself. The extraction of phosphoric acid by TBP is a somewhat different case in that the TBP is contacting an inorganic relative which also has a phosphoryl group. Its dative bonded P ~ O provides the opportunity for hydrogen bonding by an additional phosphoric acid molecule: at the higher acidities this results in several phosphoric acid molecules being associated with each TBP molecule. Furthermore, cross linkages must be present since the structure of liquid phosphori'c acids is known to consis[ of a complicated hydrogen bridge system. ~17) The water curve, Fig. 2, though, does lead one to suspect that b y t h e time the 1:I ratio of TBP to H3PO4 has been attained the phosphoric acid is connected to the P --, O of the TBP by a hydronium ion. The acid extraction data in Table 1 have been recalculated on a molal basis and plotted in Fig. 3. Two straight lines result, the one of steeper slope lying between I and 7 molal in the aqueous phase having the equation: mo = 0.17+0.295m a where mo and m= are moles HaPO4 per kg of TBP in the organic phase and moles HaPO4 per kg H20 in the aqueous phase, respectively. The other, above 7 m,, is linear out to nearly 100 molal and bears the relationship: mo=0.75+0.157m a I ,,
~2
I
I
I
I
,
,oO*/oTBP
=
2*/, TBP IN CYCLOHEXANE
= 2% TBP IN CARBON TETRA-
I
I
I / o
/
-
i,o _z
0
10 20 30 40 50 60 70 80 90 MOLES H3PO4 PER Kq. WATER IN AOUEOUS PHASE
1OO
FIG. 3.--Tbe distribution of H3PO4 between TBP and water.
The difference in slopes in Fig. 3 suggests that perhaps differences in extraction mechanism exist in the two regions covered. The intersection of the lines occurs at 1"8 molal HaPO4 in the organic phase, representing a mole ratio of H2PO4 to TBP of 0-5. From Fig. 2 it can be seen that this point is essentially the boundary between (17) J. R. VAN WAZER, Phosphorus and Its Compounds; Chemistry, Voi. 1, p. 489. Interscience, Ne w Y o r k (1958).
Interactions between tributyl phosphate, phosphoric acid and water
421
the region where the H3PO 4 is highly hydrated and that in which only the one water molecule per TBP is tolerated. The steeper sloped line covers the region where the multihydrated H3PO 4 is extracted. The lesser sloped line, which covers the bulk of the plot, applies to the region in which compound formation between TBP and the phosphoric acid is probable. In phosphoric acid, then, we have a specimen apparently exhibiting both the Class I and Class II properties previously mentioned/l) i.e., behaviour strongly affected by hydration (up to 1.2 m in the organic phase), and specific TBP-acid complex formation (above 3-4 m), respectively. T h e stipulation in the latter case here is that a molecule of water is involved regardless of the number of phosphoric acid molecules in the polymer, presumably as a hydronium io~ bridge between the TBP and the first phosphoric acid molecule in the chain. Additional evidence of two different extraction mechanisms was found by extracting at different temperatures solutions representative of each region in Fig. 3. When stock solutions of 2.92 M and 15 M H3PO4 were extracted with TBP at 2, 25 and 55°C the distribution co-efficient for the sample in the multi-hydrated H3PO4 region varied considerably, being 0.33, 0"23 and 0-14, respectively. In the region 2 case, on the other hand, the distribution co-efficient was totally unaffected, remaining at 0.37 over the temperature span tested. The bonds in the concentrated region are thus found to be stronger than those in the dilute range. Water analysis divulged the information that the region 2 sample likewise was unaltered from the 1:1 TBP-H20 ratio at the temperatures of the test, but the water content of the region I sample was affected greatly by temperature. At 55° the ratio was one. At 2° the water to TBP ratio was 1"30, considerably less than the ratio obtained when an equivalent amount of H3PO4 was extracted at 25° C, and not greatly different than the 1.2 H20's/TBP found for TBP saturated with water at 0° C. The water curve at the low temperature is at least displaced, while at 55° it appears to have flattened. The effect of a change in diluents on the extraction of H3PO 4 by TBP was obtained using 12 M stock H3PO4 and 2 per cent (by volume) solutions of TBP in carbon tetrachloride, hexane, cyclohexane, dibutyl ether and methyl ethyl ketone. The equilibrium organic phase acid concentrations were 0.003 M, 0.034 M and 0.036 M when the equilibrations were made with TBP in carbon tetrachloride, hexane, and cyclohexane, respectively--while the diluents alone extracted no acid. TBP in dibutyl ether extracted the most and was 0.46 M, but dibutyl ether alone accounted for over half of this value, being 0.25 M in H3PO4 when no TBP was present. Methyl ethyl ketone was miscible' with the' 12 M H3PO4. The oxygen-containing solvents such as ethanol,(ls) butanol, ethyl acetate and acetone, (19) oxygenated cycloparaffins(20) and ketone~, esters and ethers~21) have all been used as solvents in the commercial production and purification of phosphoric acid. The distribution of phosphoric acid between water and ether has also been reported.( 22. 23) A detailed survey of solvents and extraction processes for phosphoric acid has been published by DOUMAS.(24) (ts) p. j. Fox, J. Industr. Engng. Chem. 6, 828 (1914). (19) j. H. GRAVELL,U.S. Pat. 1499611 (1924). Chem. Abstr. 18, 2790 (1924). (2o) C. A. VAWA,U.S. Pat. 1968544 (1934). Chem. Abstr. 28, 5937 (1934). (2t) C. H. KELLER,U.S. Pat. 1981145 (1934). Chem. Abstr. 29, 559 (1935). (22) M. BACHELET,E. CliEYLANand J. I.~ Bp.IS, J. Chim. Phys. 44, 302 (1947). (23) B. HEL~mCtl and U. BAUMA~, Ber. Dtseh. Chem. Ges. 85, 461 (1952). (24) B. C. DOUMAS,Purification of Phosphoric Acid by Solvent Extraction, p.p. 56--64. University Microfilms Ann Arbor, Mich. (1961).
422
C.E. HIOGtNSand W. H. BALDWIN
Pronounced differences were found in the abilities of pure and diluted TBP to extract H3PO4. With a 2 per cent solution of TBP in cyclohexane E~° increased from 2 × 10- 6 at 4 M , to 5 × 10- 3 at 15 M a (Table 3). This is more than a two thousandfold increase wliereas with pure TBP E ° rose only from 0.26 to 0"37 over this same concentration range. In Fig. 3 is shown the extraction behaviour of TBP alone and diluted with cyclohexane and carbon tetrachloride. An aqueous phase molality of 16 is necessary before the 2 per cent TBP in cyclohexane begins to extract the HaPO 4 appreciably, while 27 m is required for the same concentration of TBP in CC14. The difference in pure and diluted TBP's extracting ability must be a result of the concentration change, since in a diluent the TBP molecules are much fewer and farther apart, combined with a decrease in polarity in the organic phase. The poorer extraction obtained when TBP is dissolved in carbon tetraehloride instead of cyclohexane is probably attributable to the effect of the diluent in altering the polarity of the phosphoryl group. TABLE 3.--EXTRACTION OF H j P O 4 - - 3 2 P BY 2 ~o TBP IN CYCLOHEXANE, t = 25 ° c
H3PO4 concentration (M) Aqueous phase 4.02
6.00 7.62 8'-02 9.20 10.0 I 1.3 12.5 13-1 14.9
Organic phase 8 × 10 -6 8"4 x 10 -5 3.60 x 10 -4 6.53 x 10 -4 3.59 × 10 -3 9.57 × 10 -3 2.64 x 10 -2 4.34 × 10 -2 5.05 x 10 -2 7.28 × 10 -2
E° 2 x 10-0 1"4 x 10-5 4.7 x 10 -5 8"14 × 10-5 3.90 x 10 -4 9.57 x 10 -4 2.34 x 10-3 3.47 × 10-3 3.85 × 10 -3 4.89 × 10-3
From Fig. 3 it is evident that the H3PO4:TBP ratio did not exceed 1 in the dilute TBP solutions. With cyclohexane as diluent the H3PO4 concentration levelled off at 3.64 mole per kg TBP, very close to the 1:1 complex. The extraction of TBP from a 2 per cent solution in cyclohexane by phosphoric acid solutions was negligible over the range shown in Fig. 3, although anhydrous H3PO 4 removed all the TBP from the diluents. The usual method of determining the species extracted is to obtain the slope of a log-log plot of E ° vs. TBP concentration, keeping the aqueous phase at a constant composition.(2s) Normally the plot holds good up to 5 or 10 volume per cent TBP before deviations occur. However, from Fig. 4, in which values of E ° resulting from the extraction of 13.1 M H3PO4--32p stock solution by TBP in cyclohexane are plotted over the range 0-2-100 per cent TBP, it appears that the complex here extracted depends on the amount of TBP present. The slope over the range 0.2-0-4 per cent TBP is 3.4, from 0.8 to 4 per cent it is 1.6 to 1"4, and from 9 to 100 per cent it is 1.04. The organic phase was saturated with one H3PO 4 molecule per TBP at 9.27 ~o TBP, increasing somewhat above I:i at 20 and 100~o TBP, the resultant aqueous phase concentrations being 13.0, 12.9 and 11.9 M, respectively. The other aqueous acidities remained at 13.1 M. (25) T. V. HEALYand H. A C. McKAv, Rec. Tray. Chim. 75, 730 (1956).
Interactions between tributyl phosphate, phosphoric acid and water
423
The solvation numbers for a multitude of metal-TBP complexes have been established(2e) and good evidence of 1 : 1 species of HNO3( 2s, Is), HCI, etcC2) has also been presented. The charge distribution on the cation is uniform in these instances, however, whereas phosphoric acid has three hydrogen atoms, all ionizable to different degrees. '
tC~t -
402
io -4
'
'
'''"1
~
~
'~''"1
'
'
' '"
f/
_
I t0 TBP CONCENTRATION, %
~00
Fio. 4.--Variation in distribution co-eff~ient as a function of TBP concentration in cyclohexane. Initial aqueous phase H3PO4 concentration was 13.1 M. The extraction of a higher concentration solution of H3POa-32P (15 M) as a function of TBP concentration in cyclohexane (Table 4) led to the formation of a third phase, between the aqueous and cyclohexane phases, when the TBP concentration was 4-35+ per cent. The TBP to H3PO4 ratio in the third phase over this range was 1:2, the same as in 100% TBP after contact with 15 M H3PO4. Upon raising the temperature the third phase disappeared at 40° C. Molecular weight determinations were made on anhydrous phosphoric acid-TBP mixtures of varying mole proportions in the hopes that some clue as to a preferred for example Reference 25; D. F. l~pv~,tD, J. P. FAres, P. R. GRAYand G. W. MASON, J. Phys. Chem. 57, 294 (1953); D. So,altOn.L, K. ALCOCK,J. M. FLETCHr~,E. HESIFORD and H. A. C. McKAY, J. Inorg. Nucl. Chem. 4, 304 (1957); H. A. C. McKAY and T. V. HEALY,Progress in Nuclear Energy, Series III, Process Chemistry, (Edited by F. R. BRucI~,J. M. Fl.grcnmt and H. H. H ~ N ) , Vol. 2, pp. 551-552, P~gamon Press, New York, London, Paris, Los Angeles (1958).
(2~) See
C. E. HIGG~NSand W. H. BALDWIN
424
species might be obtained. The mixtures weren't sufficiently stable in boiling cyclohexane or soluble enough" when the H3PO4 to TBP ratio was greater than one; however, when mixtures of ratios 1.5-3 were extracted with cyclohexane at temperatures of 10--55° C the material recovered from the cyclohexane phase had the composition of roughly one HaPO4 molecule per TBP. TABLE 4 . - - E X T R A C T I O N OF H 3 P ~ ) 4 - - 3 2 p FROM 15 M
H3PO4--a2P AT
VARIOUS
TBP CONCENTRATIONS IN CYCLOHEXANE t = 2 5 ° C
EquilibriumH3PO4 concentrations (M)
TBP conch, (voi. ~o)
Third phase volume (~o of total)
Organic phase
Third phase
Aqueous phase
E°
0 0.5 1.0 1.5 2-0 2.5 4.0 9.3 20.0 35.0 50.0 100
0 0 0 0 0 9 2.5* 8.1" 21" 42* 0 0
0 0.0068 0.0285 0.0514 0.0780 0-0864 0.0935 0.118 0.155 0.260 2.84 5.12
0 0 0 0 0 ~ -3.32 3.12 2.69 0 0
15.0 14.9 14.9 14.9 14.9 14.9 14.9 14.8 14.8 14.5 14.4 13.7
0 0.00046 0.00191 0.00345 0.00523 0.00580 0.00628 t 0.00797 t 0.0105 t 0.0179 t 0.197 0.374
* Third phase miscible with organic phase at 40° C. Moles H3PO4/I. in lighter org. phase -- moles HaPO4/I. in aq. phase. Ether was then used as the solvent for the molecular weight determinations by boiling point elevation. TBP, used as standard, behaved ideally over the range tested (0.2-1.5 g solute per 30 ml ether) while increasing association manifested itself as the mole per cent of phosphoric acid inca'eased. Molecular weights at the upper limit of 1.5 g of mixture per 30 ml ether increased from the 266 of the TBP to 330, 360, 420, 480, 510 and 570 for solutions of H3PO4 in TBP of 25, 33.3, 50, 66.7, 75 and 100 mole per cent H3PO4, respectively. Similar behaviour previously was noted by KOSOLAr~rr and POW~LL~27) when determining molecular weights of organophosphonic acids cryoscopically in naphthalene. They found the molecular weight tO exceed that of a hexamer and to still be rising at the higher concentrations. Dibasic organophosphoric acids polymerized in l i k e manner in n a p h t h a l e n e 3 2 S ) O u r ebullioscopically determined values, which varied directly with the concentration over the range studied, all fell within the molecular weight region of 265-300 when extrapolated to infinite dilution. It thus appeared that the phosphoric acid behaved as a trimer at infinite dilution in ether, and no specific complex.between TBP and anhydrous phosphoric acid was detected by the ebullioscopic method o f molecular weight determination. ~27)G. M. KOSOLAI'OrFand J. S. POWELL,J. Chem. Soc. 3535 (1950). ~2s) D. F. PEPPARD,J. R. FERRARO and G. W. MASON, J. Inorg. NucL, Chem. 7, 231 (1958).
Interactions between tributyl phosphate, phosphoric acid and water
425
The refractivity of solutions comprised of two substances in varying mole proportions in a diluent~29) and of mixtures of the pure substances themselves t30~ has been used to demonstrate hydrogen bonding and to indicate the resultant species. In Table 5 are listed the refractive indices of anhydrous mixtures of HaPO4 and TBP. The refractive index increased linearly from 1.4225 (1Sure TBP) to 1.4314 at 66"7~o H3PO4, then curved rapidly up to the value of 1-4508 at 1 0 0 ~ H3PO4. The maximum deviation between the observed refractive indices and those calculated from either the Gladstone and DaleOi) or Lorenz and Lorentz(31) refractivities occurred at 66.7 mole per cent H3PO 4 (Table 5). The values obtained from the empirical Gladstone and Dale mixture rule are listed because they were closer to the observed refractivities. As noted by HOLMES,(32) the maximum deviation between observed and calculated refractive indices corresponded to the greatest change in volume on mixing, in this case as shown by the difference in calculated and observed densities (Table 5, column 4). Calculating the refractivity differences by other methods, however, caused the maximum deviation in (n obs.-n calc.) to occur at different mole fractions of H3PO4. TABLE 5.--DENSITY AND REFRACTIVE INDEX OF ANHYDROUS H3PO4-TBP SOLUTIONS AT 25 ~ c i
Density (g/ml) Mole H 3PO4 0 25 "0 33 "3 50 "0 66 "7 75 '0 80 "0 84 "3 90.0 91 "4 95 "2 100.0
Observed 0"9722 •031 •057 •126 •233 •306 '367 I "424 1-540 1'680 1 "868
n~
Calc.*
Diff.
Observed
Calc. ~r
Diff.
I "026 1 "050 I -I 16 1-220 I "299 I "361
0"005 0-007 0"010 0"013
I "4225 1"4258 1 '4270 I '4287 1.4314 I "4337
I '4263 1"4277 I "4307 1-4347 I "4350
-0"0005 --0"0007 -0"0020 --0"0033 --0"0013
I '539
0"001
1-4361 1-4406
0-0010 -0-0002
I "681
--0'00 I
0'007
0.006 I "4371 1.4404 1.4415 I "4448 I "4508
I "4445
0'0003
* Calculated assuming no volume change. From the Gladstone and Dale mixture rule, i.~t~. The results of simple viscometric measurements on the anhydrous H3PO4-TBP mixtures have been plotted in Fig. 5. The viscosity increased slowly up to 33 ~ H3PO4, then rapidly increased to the maximum of 630 cpoise at 7 5 ~ H3PO 4, almost thrice t29~ F. M. ARSHID, C. H. GLEES, E. CI McLuRE, A. OGILVlE a n d T. J. ROSE, J. Chem. Sot'.
67 (1955). ~a0~N. A. PUSCHINand P. G. MATAVULJ, Z. Phys. Chem. A 158, 290 (1932); N. A. PUSHIN, P. MATAVULJ,I. 1. RIKOVSK!and M. NENADOVIC,Bull. Soc. Chim. Belgrade, 11, No. 3/4, 72(1940-46), Chem. Abstr. 42, 2167 (1948); N. A. PUSHIN,P. MATAVUUand I. I. RIKOVSKI, Bull. Soc. Chim. Belgrade, 13, 45 (1948), Chem. Abstr. 46, 4298 (1952). t3~ j. R. PARTINGTON, An Advanced Treatise on Physical Chemistry; Physico-Chemical Optics, Vol. 6, p. 72. Longmans, Green, London, New York, Toronto (1953). ~32~j. HOLMES,J. Chem. Soc. 103, 2165 (1913); 107, 1471 (1915).
426
C.E. HIOol~s and W. H. BAt.DWXN
the viscosity of liquid 100~o H3PO4 itself. The values obtained can be considered only approximate due to the type of apparatus employed and the use of a single standard in calculating viscosities of vastly different magnitude. Nonetheless, the results for TBP, dibutyl phosphoric acid, and H3PO4--3"7, 70 and 226 cpoise, respectively--are not unreasonably different than the reported results of 3.422, <33} 71 (at 20° C), <2s}and 200 cpoise (by graphical interpolation).<34~ 700 '
'
'
'
I
'
'
'
600
u
500
400
>.I-0 300 0
2O0
t00
0 TBP
J
I
0-5
I
I H3PO4
MOLE FRACTION
FIo. 5.--The viscosity of anhydrous TBP-H3PO4 solutions. Monobutyl- and dibutyl phosphates, the partial esters of phosphoric acid, were tested and compared with the dry TBP-H3PO4 solutions of 66.7 and 33.3 % H3PO4 respectively, said solutions having the same ratio of butoxy and hydroxy groups per phosphoryl group as do the corresponding partial esters. Monobutyl phosphate, C4HgOP(O)(OH)2, has a density at 25°C of 1.226 g/ml and a refractive index of 1-4309 compared with 1.240 g/ml and ng 1.4314 for the TBP-2H3PO4 solution. Dibutyl phosphate, (C4H90)2P(O)OH, at the same temperature has a density of 1.061 g/ml and n D of 1-4268 compared with 1.057 g/ml and n~ of 1-4270 for the H3PO4"2TBP solution. The viscosity found for dibutyl phosphate, 70 cpoise, was about twice that of its corresponding mixture, but monobutyl phosphate's viscosity, ~3~)D. P. EVANSand W. J. JONES,J. Chem. Soc. 985 (1932). {34~j. R. VANWAZeR,Phosphorusandits Compounds; Chemistry, Vol. 1, p, 485. Interscience, New York (1958).
Interactions between tributyl phosphate, phosphoric acid and water
427
500 cpoise, was close to that of the TBP.2H3PO4 solution, 538 cpoise. Dibutyl phosphoric acid's viscosity can be attributed to the fact that it dimedzes,¢~, 28, 35~ thus exhibiting more structure than the 2TBP'H3PO4 solution. Monobutyl phosphoric acid, on the other hand, is a dibasic acid such as has been shown to readily polymerize.t27,28~ The TBP.2H3PO 4 solution may be polymerized in such a fashion that its structure resembles that of monobutyl phosphoric acid. Associated liquids have greater viscosities because of their hydrogen bonding.~36> The most association in the system reported here occurred at one TBP per three phosphoric acid molecules (Fig. 5). Although the ratio at which a maximum is found does not necessarily agree with the ratio of a known complex in the system investigated, as shown by ADDISONand SMITH,t37~the degree of deviation of the viscosity curve is nevertheless an indication of the amount of association. FEI~RAROand pEpp^RDtaS~ have shown by the method of continuous variation, cryoscopically with benzene and by infra-red with cyclohexane, that TBP and mono-(2 ethylhexyl) phosphoric acid (H2MEHP) form associated complexes having the ratio (TBP)2/(H2MEHP)6 in benzene and (TBP)4/(H2MEHP)12 in cyclohexane. While we cannot tell the number of units of each substance involved in the undiluted solutions of TBP and H3PO4, the basic ratio of one TBP to three acid units is likewise indicated by the viscosity curve. It may be of interest to note that in X-ray diffraction structure studies on anhydrous phosphoric acid four formula weights were found per unit cell of crystals.t39 The TBP'3H3PO4 ratio (four units total) giving the maximum viscosity may be a fortuitous circumstance. From the density and viscosity results it is evident, though, that the major interactions between TBP and anhydrous H3PO4 do occur at 67-75 mole per cent H3PO 4. t35~B. J. THAMER,J. Phys. Chem. 64, 694 (1960). t36) G. C. PIMI~Wn~Land A. L. MCCLELLAN,The Hydrogen Bond, pp. 61-63. Freeman, San Francisco and London (1960). t37~C. C. ADDISONand B. C..SMrrn, J. Chem. Soc. 1783 (1960). 13s~j. R. FERRAROand D. F. PEPPARD,J. Phys. Chem. 65, 539 (1961). ~39~j. R. VANWAZER,Phosphorus andlts Compounds; Chemistry, Vol. 1, p. 486. Interscience, New York (1958).
INTERACTIONS BETWEEN TRIBUTYL PHOSPHATE, PHOSPHORIC ACID A N D WATER* C. E. HIGGINS and W. H. BALDWIN Chemistry Division, Oak Ridge National Laboratory, Tennessee
(Received 2 October 1961; in revised form 1 November 1961) Alanamet--The distribution co-efficient for phosphoric acid between tributyl phosphate (TBP) and water increased with the acid concentration in the inorganic phase and correlated with the water behaviour in the organic phase. Water accompanying the acid into the TBP was salted out after the ratio 3 TBP-H3PO4"6 H20 was reached but levelled off at one water per TBP regardless of the phosphoric acid concentration over the range 1-4 moles H3PO4 per mole TBP. Evidence for a high degree of association in solutions of anhydrous H3PO4 and TBP was obtained from ebullioscopic molecular weight determinations in ether and the viscosity of undiluted solutions. The maximum viscosity occurred at the 3 H.~PO4 tO I TBP ratio and was about three times that of anhydrous, super-cooled H3PO4. AN E^RLIER investigation ~l) into the interactions between TBP, monovalent electrolytes, and water has been extended now to include the polyvalent orthophosphoric acid. With the exception of sull~huric acid(a) all of the many TBP-acid studies have been concerned with the monobasic acids, thus information gained from distribution tests with a tribasic acid should be of interest. The industrial potential of TBP for the purification of phosphoric acid has recently been shown by I~UMAS. (3) The previously noted miscibility of anhydrous phosphoric acid and TBp(4) made possible the investigation of interactions between these compounds in the absence of water. The method ofcontinuous variation was employed for ebullioscopic molecular weight determinations and refractive index and viscosity measurements. EXPERIMENTAL
Chemicals. Reagent grade orthophospboric acid (86.1 per cent H3PO4) was used to prepare the stock aqueous solutions. Removal of water by vacuum pump treatment of the 86 per cent H3PO4 gave solutions in the 86 to 100 per cent range. Anhydrous phosphoric acid--3'P was prepared by mixing 5 me H3PO4--32p in weak HCI with 1.14 g of 86.1 per cent H3PO4 (0.010 mole) in a 50 mi Pyrex round-bottomed flask, freezing, then pumping off the volatile materials until the correct weight loss had been achieved, warming no higher than 50° C during the drying operation. Solutions containing tracer 32p were then prepared over the concentration range 10-3 M to 15 M H3PO4, having specific activities of 0.25 to 12 me per mole throughout the period of test. The organic chemicals were of reagent grade or were distilled before use. TBP was purified as before. (1) * This paper is based upon work performed for the United States Atomic Energy Commission at the Oak Ridge National Laboratory operated by Union Carbide Corporation. (1) W. H. BALDWIN,C. E. HIGGINSand B. A. SOLD^NO,J. Phys .Chem. 63, 118 (1959). (2) E. HESrORD and H. A. C. McKAY, 3"~Inorg. NucL Chem. 13, 156 (1960). ~-~)B. C. DOUMAS,Purification of Phosphoric Acid by Solvent Extraction, University Microfilms, Ann Arbor, Mich. (1961). ~4) C. E. HIOG[NSand \V. H. B^Lt~WIN,J. Org. Chem. 21, 1156 (1956). 415
416
C.E. HIGGINSand W. H. BALDWIN
Distribution and solubility determinations. The distribution of phosphoric acid between water and TBP was accomplished by equilibrating the aqueous stock solutions with equal volumes (5-20 ml) of pure TBP and with TBP in various concentrations in organic diluents. The equilibration time was 1 hr in a water-bath at 25 +0.2 ° C. A sharp phase layering was obtained by centrifugation for 20-30 sec at 2,000 rev/min in an open centrifuge at a room temperature of 25 ° C without materially altering the solution equilibrium. ~5) Any volume changes were noted, following which the phases were separated and stored in the bath. The distribution co-efficient, E~a, in all cases refers to the ratio, after equilibration, of the concentration (moles per litre) of phosphoric acid in the organic phase to that in the aqueous phase. Mo and M a refer to molar concentrations of H3PO4 in the organic and aqueous phases, respectively, while mo and m a refer to molalities. Approximate amounts of tributyl phosphate dissolved in the high concentration equilibrium phosphoric acid solutions were determined by extracting duplicate 50 ml samples several times with equal volumes of C.P. carbon tetrachloride, concentrating, and pumping to constant weight at room temperature in weighed 50 mi round-bottomed flasks. The solubility of TBP in dilute phosphoric acid solutions was previously determined with TBP-32p after the manner used for the reported solubilities in monovalent electrolyte solutions:6) Dry TBP and anhydrous phosphoric acid mixed in mole ratios varying from 1:20 to 100~ 1 formed clear, homogeneous'solutions. Analysis of solutions. The phosphoric acid concentration in most solutions was determined by titration of aliquots (aqueous phases in water and organic phases in 50 per cent ethyl alcohol) with standard sodium hydroxide using a Fisher Titrimeter, others by solution counting the beta activity, correcting, when necessary, for the effect of solution density on counting rate after the manner of Dyrssen.¢7) Some equilibrium aqueous solutions were analysed by interpolation of their densiti~ on a density vs. stock phosphoric acid concentration curve. Values thus found agreed within + 1 per cent of the values found by titration. Water content in the equilibrium organic solutions (no diluent) was determined by use of the Karl Fischer reagent.* The TBP content was then found by subtracting from the weight of a known volume of ol:ganic solution the weights of water and phosphoric acid. Densities were determined at 25.0 +0.2 ° C using calibrated weight pipettes of 0.500-4 ml capacities. Solutions of TBP and anhydrous phosphoric acid in varying molar proportions were prepared for physical measurements. Refractive indices were obtained at 25.0 ~ C with an Abb6 refractometer. Molecular. weights were determined in ether by the ebullioscopic method of MENZIESand WRIGHX.18.9) An estimation of the viscosities at 25" was obtained by comparing the time to deliver ,-,0.4 ml between two marks 20 cm apart on a straight upright tube of 1.6 mm i.d. with that of glycerine (170 sec) as a standard. The viscosities were calculated from the relationship :Tz ~ = d~ ~--~ d2 t~ t2'where d~, d2 and t~. t_, are densities and delivery times, respectively, and tr2 for glycerine (d 2~ 1.255) is 954 cpoise. I iol RESULTS AND DISCUSSION The distribution o f phosphoric acid between pure TBP and water as a function of the equilibrium aqueous phase acid concentration is shown in Table I. The semi-log plot, Fig. I, shows E ° falling into three separate sections depending upon * We are indebted to W. R. LAINGand co-workers of the Analytical Chemistry Division for the water analysis. 15) C. E. HIGGINSand W. H. BALDWIN,Analyt. Chem. 32, 236 (1960). t6) C. E. HIGGINS, W. H. BALDWINand B. A. SOLDANO,J. Phys. Chem. 63, 113 (1959). ~7) D. DYRSSEN,Acta Chem. Scand. 11, 1771 (1957).
Interactions between tributyl phosphate, phosphoric acid and water
417
TABLE I.--ExTRACTION OF PHOSPHORICACID AND WATERBY 100% TBP AT 25 ° C Equilibrium aqueous phase
H3PO4
Equilibrium organic phase
H3PO4
concn, (M)
Mole fraction H3PO4
Density (g/ml)
concn, (M)
Mole fraction HaPO4
Mole fraction 820
Mole fraction TBP
E°
Density (g/ml) 0.9971 1.018 I "041 1.062 ! .066 l "127 i'193 1"319 1.445 l "546 I "615 ! "646 1 '743 1 '752
0.000 0.406 0"838 1 "26 1 "33 2-56 3"91 6"50 9"18 11 "4 12-9 13 "6 15"9 16"2
0.0000 0.0074 0"0155 0"0233 0"0250 0.0500 0"0801 0"147 0'233 0"324 0'398 0"439 0"609 0"643
0.9767 0.9780 0"9815 0.9864 0"9875 1.006 1 "029 l "068 !'110 1 "151 1-193 1 '220 1 "342 1 "360
0.0000 0"0375 0"106 0" 198 0"214 0"583 1 "02 l "84 2"71 3 "62 4"53 5 "09 7"71 8"10
0.0000 0.0055 0"0150 0 "0257 0-0255 0"0654 0".105 0"218 0.312 0"388 0"463 0"500 0"654* 0"670
0.499 0"492 0-502 0 "538 0"582 0-574 0"576 0"415 0"346 0"312 0"267 0"252 0'175 • 0"165
0.501 0.502 0"483 0 "436 0"393 0"361 0"320 0"367 0"342 0"299 0"270 0"248 0"175 • 0:165
0.0000 0-0924 0"126 0" 157 0-161 0"228 0"261 0"283 0"295 0"318 0'351 0"374 0"485 0"500
........
I
* Graphical interpolation.
I
0"4
.......
I
........
]
........
I
t o SOLUTIONCOUNTING • TITRATION
/ /
~a 0.3 j
0"2 0-t _
,
• • _/
IO-3 IO-2 IO-~ 4 10 MOLARITY OF PHOSPHORIC ACID IN EQUILIBRIUMAQUEOUS PHASE
FIG. i.--The relationship between distribution c.o-efficient and the equilibrium aqueous phase H3PO4 concentration. the aqueous phase concentration. Each section also mirrors the water extraction behaviour (columns 7 and 8, Table 1 and Fig. 2). At first E ° increased linearly from 0-01 to 0.1 over the range 10-3-0-5 Ma, with the T B P - H 2 0 ratio remaining constant at one in the organic phase. In the second section E ° increased more rapidly with co-extraction o f water and H3PO 4. The m a x i m u m water to TBP ratio o f 2 in the organic phase occurred at 4 M,. E ° then began to level off as the addition o f more
418
C.E. HIoOINs and W. H. BALDWIN
phosphoric acid salted water out of the organic phase rapidly. In the last section E ,° rose rapidly from 0.3 at ~ 10 M a to 0.5 at 16 Ma. Through this range the T B P - H 2 0 ratio was that of the monohydrate regardless of the accompanying phosphoric acid concentration. When the phosphoric acid concentrations in each phase were converted to mole fractions, columns 3 and 6, Table 1, it was noted that generally the mole fraction of phosphoric acid in the organic phase was nearly the same as that in the corresponding aqueous phase.
I-"
IV '7I! t.6
/
o 4.5 t.4
o" '
~ t.2
.
~ t.t
.
t.0 0-9
0
1 2 3 MOLE RATIO) H3PO4 )o TBP
4
FIG. 2.--Water behaviour in the organic phase. The amount of TBP going into the aqueous phase was not appreciable over the range 0-15 M a H3PO4 (see Table 2). Phosphoric acid dissolved less TBP than water alone until the concentration exceeded 15 M a H3PO 4. Anhydrous H3PO4 dissolved TBP in all proportions at 25 ° C. TBP IN PHOSPHORIC ACID Equilibrium concentrations
TABLE 2 . - - S O L U B I L I T Y OF
SOLUTIONS AT 2'5 ° C
H3PO4 in queous
TBP in aqueous
(M)
(mg/l.)
0"00 0.12 0.29 0.58 1.14 12.4
414" 388* 370* 336* 290* " 110÷ 14.8 390f TBP miscible with anhydrous H3PO( * Determined using TBPJ2P. t Determined by weight recovery.
Interactions between tributyl phosphate, phosphoric acid and water
419
The water behaviour in acid systems extracted by TBP is peculiar to the acid itself. Phosphoric acid carried water into the organic phase to the extent of about five molecules per molecule of acid (initial slope, Fig. 2). Water co-extracted with the halogen acids in ratios varying from 3:1 to 4~1 ~1~. From evidence obtained by an organic volume change technique,( TM Tuck and DIAMOND(12) have recently postulated the presence of a trihydrated hydronium ion in the TBP phase after contact with aqueous HBr, HCI, and HCIO4. Small differences were found, however, and from these and other results~l, 2) the amount of water initially co-extracted by each acid appears to decrease in the order H3PO4> HI > HBr > HCI > HCIO4 > H2SO4 > HNO3. Nitric acid is a special case since it forces water from the TBP phase until the 1: 1 ratio of TBP and HNO3 is reached.el3, 1) As additional nitric acid goes into the organic phase the water content begins to increase.~13) The TBP appears to have approached its original monohydrated state when the organic phase is about 8 M HNO3. The organic phase acid concentration at which water is squeezed out by additional acid also varies with the acid. From the water-phosphoric acid curve (Fig. 2) the maximum appears to have occurred when the organic phase has the approximate composition 3 TBP.H3PO4.6 H20. If the organic solutions resulting from the extraction of perchloric and sulphuric acids by H~FORD and McKAy~2) have densities similar to those reported here for the organic solutions containing phosphoric acid and water then recalculating their data on a molal basis results in maxima at compositions of approximately 2 TBP'HCIO4"4 H20 and TBP.2 H2SO4"2 H20. The former does agree with the HCIO4 extraction data of SX~KI~tSgI and GWOZDZ.~14) Maxima for the halides(l) occur at roughly TBP.O.7HX.3 H20. The aforementioned minimum in nitric acids' water curve, for which the densities of the equilibrium organic solutions are known up to about 5 M a HNO3,~ls~ occurs wbe~ the composition of the organic phase is close to TBP'HNO3"~ H20. The exact ratio of HNO3 to TBP at the minimum water content is actually 1.2.c13) In previous work~l) most of the univalent electrolytes tested approached a 1~1 TBP--electrolyte compQsition in the organic phase, nitric acid alone exceeding this ratio. Here the polyvalent phosphoric acid to TBP ratio was ~ 1 at 9 M H3PO4 in the equilibrium aqueous phase, rose to 2~1 at 13.6 M,, and continued to increase rapidly. This indicates polymerization in the organic phase at the higher acidities. Other evidence will be given in a later section. The maximum ratio obtained here was four, at an equilibrium aqueous phase H3PO4 concentration of 16 M. Polymerization increased and the distribution ratio rose rapidly since the limiting condition in the aqueous phase was being approached, that is, anhydrous phosphoric acid which was miscible with TBP. The extraction of nitric acid by TBP up to as much as 2½ HNO3 molecules per TBP has been previously noted by ALCOCKet al.(13) TUCK(15)has postulated addition of the extra nitric acid molecules to the P-O-C oxygen atoms. KORPAKand DI'.'PTULA,{16) OH)D. G. TUCK,J. Chem. Soc. 3202 (1957). ~12~D. G. TUCKand R. M. DL~MOND,J. Phys. Chem. 65, 193 (1961). (~-~ K. ALCOCK,S. S. GRIMLEY,T. V. HEALY,J. KENNEDYand H. A. C. McKAY, Trans. Faraday Soc. 52, 39 (1956). ~4~ S. SW.KmaSKXand R. GwozDz, Nukleonika 5, 205 (1960). ~) D. G. Tucg, J. Chem. Soc. 2783 (1958). ([6) W. KORPAKand C. D~FrUL^, Nucleonika S, 63 (1960).
420
C.E. HIGOINSand W. H. BALDWIN
in presenting evidence for the formation of the complexes TBP'HNO3, TBP'2HNO3, TBP'3HNO3, and possibly TBP'4HNO3, have made the same suggestion. Such a bonding arrangement would appear to be consistent with the observed viscosities of the organic solutions. As shown by H~FORO and McKAY,~2) the viscosity increased very little, in fact beyond 1 Mo HNO3 decreasing until at 5 Mo it was the same as the viscosity of monohydrated TBP itself. The extraction of phosphoric acid by TBP is a somewhat different case in that the TBP is contacting an inorganic relative which also has a phosphoryl group. Its dative bonded P ~ O provides the opportunity for hydrogen bonding by an additional phosphoric acid molecule: at the higher acidities this results in several phosphoric acid molecules being associated with each TBP molecule. Furthermore, cross linkages must be present since the structure of liquid phosphori'c acids is known to consis[ of a complicated hydrogen bridge system. ~17) The water curve, Fig. 2, though, does lead one to suspect that b y t h e time the 1:I ratio of TBP to H3PO4 has been attained the phosphoric acid is connected to the P --, O of the TBP by a hydronium ion. The acid extraction data in Table 1 have been recalculated on a molal basis and plotted in Fig. 3. Two straight lines result, the one of steeper slope lying between I and 7 molal in the aqueous phase having the equation: mo = 0.17+0.295m a where mo and m= are moles HaPO4 per kg of TBP in the organic phase and moles HaPO4 per kg H20 in the aqueous phase, respectively. The other, above 7 m,, is linear out to nearly 100 molal and bears the relationship: mo=0.75+0.157m a I ,,
~2
I
I
I
I
,
,oO*/oTBP
=
2*/, TBP IN CYCLOHEXANE
= 2% TBP IN CARBON TETRA-
I
I
I / o
/
-
i,o _z
0
10 20 30 40 50 60 70 80 90 MOLES H3PO4 PER Kq. WATER IN AOUEOUS PHASE
1OO
FIG. 3.--Tbe distribution of H3PO4 between TBP and water.
The difference in slopes in Fig. 3 suggests that perhaps differences in extraction mechanism exist in the two regions covered. The intersection of the lines occurs at 1"8 molal HaPO4 in the organic phase, representing a mole ratio of H2PO4 to TBP of 0-5. From Fig. 2 it can be seen that this point is essentially the boundary between (17) J. R. VAN WAZER, Phosphorus and Its Compounds; Chemistry, Voi. 1, p. 489. Interscience, Ne w Y o r k (1958).
Interactions between tributyl phosphate, phosphoric acid and water
421
the region where the H3PO 4 is highly hydrated and that in which only the one water molecule per TBP is tolerated. The steeper sloped line covers the region where the multihydrated H3PO 4 is extracted. The lesser sloped line, which covers the bulk of the plot, applies to the region in which compound formation between TBP and the phosphoric acid is probable. In phosphoric acid, then, we have a specimen apparently exhibiting both the Class I and Class II properties previously mentioned/l) i.e., behaviour strongly affected by hydration (up to 1.2 m in the organic phase), and specific TBP-acid complex formation (above 3-4 m), respectively. T h e stipulation in the latter case here is that a molecule of water is involved regardless of the number of phosphoric acid molecules in the polymer, presumably as a hydronium io~ bridge between the TBP and the first phosphoric acid molecule in the chain. Additional evidence of two different extraction mechanisms was found by extracting at different temperatures solutions representative of each region in Fig. 3. When stock solutions of 2.92 M and 15 M H3PO4 were extracted with TBP at 2, 25 and 55°C the distribution co-efficient for the sample in the multi-hydrated H3PO4 region varied considerably, being 0.33, 0"23 and 0-14, respectively. In the region 2 case, on the other hand, the distribution co-efficient was totally unaffected, remaining at 0.37 over the temperature span tested. The bonds in the concentrated region are thus found to be stronger than those in the dilute range. Water analysis divulged the information that the region 2 sample likewise was unaltered from the 1:1 TBP-H20 ratio at the temperatures of the test, but the water content of the region I sample was affected greatly by temperature. At 55° the ratio was one. At 2° the water to TBP ratio was 1"30, considerably less than the ratio obtained when an equivalent amount of H3PO4 was extracted at 25° C, and not greatly different than the 1.2 H20's/TBP found for TBP saturated with water at 0° C. The water curve at the low temperature is at least displaced, while at 55° it appears to have flattened. The effect of a change in diluents on the extraction of H3PO 4 by TBP was obtained using 12 M stock H3PO4 and 2 per cent (by volume) solutions of TBP in carbon tetrachloride, hexane, cyclohexane, dibutyl ether and methyl ethyl ketone. The equilibrium organic phase acid concentrations were 0.003 M, 0.034 M and 0.036 M when the equilibrations were made with TBP in carbon tetrachloride, hexane, and cyclohexane, respectively--while the diluents alone extracted no acid. TBP in dibutyl ether extracted the most and was 0.46 M, but dibutyl ether alone accounted for over half of this value, being 0.25 M in H3PO4 when no TBP was present. Methyl ethyl ketone was miscible' with the' 12 M H3PO4. The oxygen-containing solvents such as ethanol,(ls) butanol, ethyl acetate and acetone, (19) oxygenated cycloparaffins(20) and ketone~, esters and ethers~21) have all been used as solvents in the commercial production and purification of phosphoric acid. The distribution of phosphoric acid between water and ether has also been reported.( 22. 23) A detailed survey of solvents and extraction processes for phosphoric acid has been published by DOUMAS.(24) (ts) p. j. Fox, J. Industr. Engng. Chem. 6, 828 (1914). (19) j. H. GRAVELL,U.S. Pat. 1499611 (1924). Chem. Abstr. 18, 2790 (1924). (2o) C. A. VAWA,U.S. Pat. 1968544 (1934). Chem. Abstr. 28, 5937 (1934). (2t) C. H. KELLER,U.S. Pat. 1981145 (1934). Chem. Abstr. 29, 559 (1935). (22) M. BACHELET,E. CliEYLANand J. I.~ Bp.IS, J. Chim. Phys. 44, 302 (1947). (23) B. HEL~mCtl and U. BAUMA~, Ber. Dtseh. Chem. Ges. 85, 461 (1952). (24) B. C. DOUMAS,Purification of Phosphoric Acid by Solvent Extraction, p.p. 56--64. University Microfilms Ann Arbor, Mich. (1961).
422
C.E. HIOGtNSand W. H. BALDWIN
Pronounced differences were found in the abilities of pure and diluted TBP to extract H3PO4. With a 2 per cent solution of TBP in cyclohexane E~° increased from 2 × 10- 6 at 4 M , to 5 × 10- 3 at 15 M a (Table 3). This is more than a two thousandfold increase wliereas with pure TBP E ° rose only from 0.26 to 0"37 over this same concentration range. In Fig. 3 is shown the extraction behaviour of TBP alone and diluted with cyclohexane and carbon tetrachloride. An aqueous phase molality of 16 is necessary before the 2 per cent TBP in cyclohexane begins to extract the HaPO 4 appreciably, while 27 m is required for the same concentration of TBP in CC14. The difference in pure and diluted TBP's extracting ability must be a result of the concentration change, since in a diluent the TBP molecules are much fewer and farther apart, combined with a decrease in polarity in the organic phase. The poorer extraction obtained when TBP is dissolved in carbon tetraehloride instead of cyclohexane is probably attributable to the effect of the diluent in altering the polarity of the phosphoryl group. TABLE 3.--EXTRACTION OF H j P O 4 - - 3 2 P BY 2 ~o TBP IN CYCLOHEXANE, t = 25 ° c
H3PO4 concentration (M) Aqueous phase 4.02
6.00 7.62 8'-02 9.20 10.0 I 1.3 12.5 13-1 14.9
Organic phase 8 × 10 -6 8"4 x 10 -5 3.60 x 10 -4 6.53 x 10 -4 3.59 × 10 -3 9.57 × 10 -3 2.64 x 10 -2 4.34 × 10 -2 5.05 x 10 -2 7.28 × 10 -2
E° 2 x 10-0 1"4 x 10-5 4.7 x 10 -5 8"14 × 10-5 3.90 x 10 -4 9.57 x 10 -4 2.34 x 10-3 3.47 × 10-3 3.85 × 10 -3 4.89 × 10-3
From Fig. 3 it is evident that the H3PO4:TBP ratio did not exceed 1 in the dilute TBP solutions. With cyclohexane as diluent the H3PO4 concentration levelled off at 3.64 mole per kg TBP, very close to the 1:1 complex. The extraction of TBP from a 2 per cent solution in cyclohexane by phosphoric acid solutions was negligible over the range shown in Fig. 3, although anhydrous H3PO 4 removed all the TBP from the diluents. The usual method of determining the species extracted is to obtain the slope of a log-log plot of E ° vs. TBP concentration, keeping the aqueous phase at a constant composition.(2s) Normally the plot holds good up to 5 or 10 volume per cent TBP before deviations occur. However, from Fig. 4, in which values of E ° resulting from the extraction of 13.1 M H3PO4--32p stock solution by TBP in cyclohexane are plotted over the range 0-2-100 per cent TBP, it appears that the complex here extracted depends on the amount of TBP present. The slope over the range 0.2-0-4 per cent TBP is 3.4, from 0.8 to 4 per cent it is 1.6 to 1"4, and from 9 to 100 per cent it is 1.04. The organic phase was saturated with one H3PO 4 molecule per TBP at 9.27 ~o TBP, increasing somewhat above I:i at 20 and 100~o TBP, the resultant aqueous phase concentrations being 13.0, 12.9 and 11.9 M, respectively. The other aqueous acidities remained at 13.1 M. (25) T. V. HEALYand H. A C. McKAv, Rec. Tray. Chim. 75, 730 (1956).
Interactions between tributyl phosphate, phosphoric acid and water
423
The solvation numbers for a multitude of metal-TBP complexes have been established(2e) and good evidence of 1 : 1 species of HNO3( 2s, Is), HCI, etcC2) has also been presented. The charge distribution on the cation is uniform in these instances, however, whereas phosphoric acid has three hydrogen atoms, all ionizable to different degrees. '
tC~t -
402
io -4
'
'
'''"1
~
~
'~''"1
'
'
' '"
f/
_
I t0 TBP CONCENTRATION, %
~00
Fio. 4.--Variation in distribution co-eff~ient as a function of TBP concentration in cyclohexane. Initial aqueous phase H3PO4 concentration was 13.1 M. The extraction of a higher concentration solution of H3POa-32P (15 M) as a function of TBP concentration in cyclohexane (Table 4) led to the formation of a third phase, between the aqueous and cyclohexane phases, when the TBP concentration was 4-35+ per cent. The TBP to H3PO4 ratio in the third phase over this range was 1:2, the same as in 100% TBP after contact with 15 M H3PO4. Upon raising the temperature the third phase disappeared at 40° C. Molecular weight determinations were made on anhydrous phosphoric acid-TBP mixtures of varying mole proportions in the hopes that some clue as to a preferred for example Reference 25; D. F. l~pv~,tD, J. P. FAres, P. R. GRAYand G. W. MASON, J. Phys. Chem. 57, 294 (1953); D. So,altOn.L, K. ALCOCK,J. M. FLETCHr~,E. HESIFORD and H. A. C. McKAY, J. Inorg. Nucl. Chem. 4, 304 (1957); H. A. C. McKAY and T. V. HEALY,Progress in Nuclear Energy, Series III, Process Chemistry, (Edited by F. R. BRucI~,J. M. Fl.grcnmt and H. H. H ~ N ) , Vol. 2, pp. 551-552, P~gamon Press, New York, London, Paris, Los Angeles (1958).
(2~) See
C. E. HIGG~NSand W. H. BALDWIN
424
species might be obtained. The mixtures weren't sufficiently stable in boiling cyclohexane or soluble enough" when the H3PO4 to TBP ratio was greater than one; however, when mixtures of ratios 1.5-3 were extracted with cyclohexane at temperatures of 10--55° C the material recovered from the cyclohexane phase had the composition of roughly one HaPO4 molecule per TBP. TABLE 4 . - - E X T R A C T I O N OF H 3 P ~ ) 4 - - 3 2 p FROM 15 M
H3PO4--a2P AT
VARIOUS
TBP CONCENTRATIONS IN CYCLOHEXANE t = 2 5 ° C
EquilibriumH3PO4 concentrations (M)
TBP conch, (voi. ~o)
Third phase volume (~o of total)
Organic phase
Third phase
Aqueous phase
E°
0 0.5 1.0 1.5 2-0 2.5 4.0 9.3 20.0 35.0 50.0 100
0 0 0 0 0 9 2.5* 8.1" 21" 42* 0 0
0 0.0068 0.0285 0.0514 0.0780 0-0864 0.0935 0.118 0.155 0.260 2.84 5.12
0 0 0 0 0 ~ -3.32 3.12 2.69 0 0
15.0 14.9 14.9 14.9 14.9 14.9 14.9 14.8 14.8 14.5 14.4 13.7
0 0.00046 0.00191 0.00345 0.00523 0.00580 0.00628 t 0.00797 t 0.0105 t 0.0179 t 0.197 0.374
* Third phase miscible with organic phase at 40° C. Moles H3PO4/I. in lighter org. phase -- moles HaPO4/I. in aq. phase. Ether was then used as the solvent for the molecular weight determinations by boiling point elevation. TBP, used as standard, behaved ideally over the range tested (0.2-1.5 g solute per 30 ml ether) while increasing association manifested itself as the mole per cent of phosphoric acid inca'eased. Molecular weights at the upper limit of 1.5 g of mixture per 30 ml ether increased from the 266 of the TBP to 330, 360, 420, 480, 510 and 570 for solutions of H3PO4 in TBP of 25, 33.3, 50, 66.7, 75 and 100 mole per cent H3PO4, respectively. Similar behaviour previously was noted by KOSOLAr~rr and POW~LL~27) when determining molecular weights of organophosphonic acids cryoscopically in naphthalene. They found the molecular weight tO exceed that of a hexamer and to still be rising at the higher concentrations. Dibasic organophosphoric acids polymerized in l i k e manner in n a p h t h a l e n e 3 2 S ) O u r ebullioscopically determined values, which varied directly with the concentration over the range studied, all fell within the molecular weight region of 265-300 when extrapolated to infinite dilution. It thus appeared that the phosphoric acid behaved as a trimer at infinite dilution in ether, and no specific complex.between TBP and anhydrous phosphoric acid was detected by the ebullioscopic method o f molecular weight determination. ~27)G. M. KOSOLAI'OrFand J. S. POWELL,J. Chem. Soc. 3535 (1950). ~2s) D. F. PEPPARD,J. R. FERRARO and G. W. MASON, J. Inorg. NucL, Chem. 7, 231 (1958).
Interactions between tributyl phosphate, phosphoric acid and water
425
The refractivity of solutions comprised of two substances in varying mole proportions in a diluent~29) and of mixtures of the pure substances themselves t30~ has been used to demonstrate hydrogen bonding and to indicate the resultant species. In Table 5 are listed the refractive indices of anhydrous mixtures of HaPO4 and TBP. The refractive index increased linearly from 1.4225 (1Sure TBP) to 1.4314 at 66"7~o H3PO4, then curved rapidly up to the value of 1-4508 at 1 0 0 ~ H3PO4. The maximum deviation between the observed refractive indices and those calculated from either the Gladstone and DaleOi) or Lorenz and Lorentz(31) refractivities occurred at 66.7 mole per cent H3PO 4 (Table 5). The values obtained from the empirical Gladstone and Dale mixture rule are listed because they were closer to the observed refractivities. As noted by HOLMES,(32) the maximum deviation between observed and calculated refractive indices corresponded to the greatest change in volume on mixing, in this case as shown by the difference in calculated and observed densities (Table 5, column 4). Calculating the refractivity differences by other methods, however, caused the maximum deviation in (n obs.-n calc.) to occur at different mole fractions of H3PO4. TABLE 5.--DENSITY AND REFRACTIVE INDEX OF ANHYDROUS H3PO4-TBP SOLUTIONS AT 25 ~ c i
Density (g/ml) Mole H 3PO4 0 25 "0 33 "3 50 "0 66 "7 75 '0 80 "0 84 "3 90.0 91 "4 95 "2 100.0
Observed 0"9722 •031 •057 •126 •233 •306 '367 I "424 1-540 1'680 1 "868
n~
Calc.*
Diff.
Observed
Calc. ~r
Diff.
I "026 1 "050 I -I 16 1-220 I "299 I "361
0"005 0-007 0"010 0"013
I "4225 1"4258 1 '4270 I '4287 1.4314 I "4337
I '4263 1"4277 I "4307 1-4347 I "4350
-0"0005 --0"0007 -0"0020 --0"0033 --0"0013
I '539
0"001
1-4361 1-4406
0-0010 -0-0002
I "681
--0'00 I
0'007
0.006 I "4371 1.4404 1.4415 I "4448 I "4508
I "4445
0'0003
* Calculated assuming no volume change. From the Gladstone and Dale mixture rule, i.~t~. The results of simple viscometric measurements on the anhydrous H3PO4-TBP mixtures have been plotted in Fig. 5. The viscosity increased slowly up to 33 ~ H3PO4, then rapidly increased to the maximum of 630 cpoise at 7 5 ~ H3PO 4, almost thrice t29~ F. M. ARSHID, C. H. GLEES, E. CI McLuRE, A. OGILVlE a n d T. J. ROSE, J. Chem. Sot'.
67 (1955). ~a0~N. A. PUSCHINand P. G. MATAVULJ, Z. Phys. Chem. A 158, 290 (1932); N. A. PUSHIN, P. MATAVULJ,I. 1. RIKOVSK!and M. NENADOVIC,Bull. Soc. Chim. Belgrade, 11, No. 3/4, 72(1940-46), Chem. Abstr. 42, 2167 (1948); N. A. PUSHIN,P. MATAVUUand I. I. RIKOVSKI, Bull. Soc. Chim. Belgrade, 13, 45 (1948), Chem. Abstr. 46, 4298 (1952). t3~ j. R. PARTINGTON, An Advanced Treatise on Physical Chemistry; Physico-Chemical Optics, Vol. 6, p. 72. Longmans, Green, London, New York, Toronto (1953). ~32~j. HOLMES,J. Chem. Soc. 103, 2165 (1913); 107, 1471 (1915).
426
C.E. HIOol~s and W. H. BAt.DWXN
the viscosity of liquid 100~o H3PO4 itself. The values obtained can be considered only approximate due to the type of apparatus employed and the use of a single standard in calculating viscosities of vastly different magnitude. Nonetheless, the results for TBP, dibutyl phosphoric acid, and H3PO4--3"7, 70 and 226 cpoise, respectively--are not unreasonably different than the reported results of 3.422, <33} 71 (at 20° C), <2s}and 200 cpoise (by graphical interpolation).<34~ 700 '
'
'
'
I
'
'
'
600
u
500
400
>.I-0 300 0
2O0
t00
0 TBP
J
I
0-5
I
I H3PO4
MOLE FRACTION
FIo. 5.--The viscosity of anhydrous TBP-H3PO4 solutions. Monobutyl- and dibutyl phosphates, the partial esters of phosphoric acid, were tested and compared with the dry TBP-H3PO4 solutions of 66.7 and 33.3 % H3PO4 respectively, said solutions having the same ratio of butoxy and hydroxy groups per phosphoryl group as do the corresponding partial esters. Monobutyl phosphate, C4HgOP(O)(OH)2, has a density at 25°C of 1.226 g/ml and a refractive index of 1-4309 compared with 1.240 g/ml and ng 1.4314 for the TBP-2H3PO4 solution. Dibutyl phosphate, (C4H90)2P(O)OH, at the same temperature has a density of 1.061 g/ml and n D of 1-4268 compared with 1.057 g/ml and n~ of 1-4270 for the H3PO4"2TBP solution. The viscosity found for dibutyl phosphate, 70 cpoise, was about twice that of its corresponding mixture, but monobutyl phosphate's viscosity, ~3~)D. P. EVANSand W. J. JONES,J. Chem. Soc. 985 (1932). {34~j. R. VANWAZeR,Phosphorusandits Compounds; Chemistry, Vol. 1, p, 485. Interscience, New York (1958).
Interactions between tributyl phosphate, phosphoric acid and water
427
500 cpoise, was close to that of the TBP.2H3PO4 solution, 538 cpoise. Dibutyl phosphoric acid's viscosity can be attributed to the fact that it dimedzes,¢~, 28, 35~ thus exhibiting more structure than the 2TBP'H3PO4 solution. Monobutyl phosphoric acid, on the other hand, is a dibasic acid such as has been shown to readily polymerize.t27,28~ The TBP.2H3PO 4 solution may be polymerized in such a fashion that its structure resembles that of monobutyl phosphoric acid. Associated liquids have greater viscosities because of their hydrogen bonding.~36> The most association in the system reported here occurred at one TBP per three phosphoric acid molecules (Fig. 5). Although the ratio at which a maximum is found does not necessarily agree with the ratio of a known complex in the system investigated, as shown by ADDISONand SMITH,t37~the degree of deviation of the viscosity curve is nevertheless an indication of the amount of association. FEI~RAROand pEpp^RDtaS~ have shown by the method of continuous variation, cryoscopically with benzene and by infra-red with cyclohexane, that TBP and mono-(2 ethylhexyl) phosphoric acid (H2MEHP) form associated complexes having the ratio (TBP)2/(H2MEHP)6 in benzene and (TBP)4/(H2MEHP)12 in cyclohexane. While we cannot tell the number of units of each substance involved in the undiluted solutions of TBP and H3PO4, the basic ratio of one TBP to three acid units is likewise indicated by the viscosity curve. It may be of interest to note that in X-ray diffraction structure studies on anhydrous phosphoric acid four formula weights were found per unit cell of crystals.t39 The TBP'3H3PO4 ratio (four units total) giving the maximum viscosity may be a fortuitous circumstance. From the density and viscosity results it is evident, though, that the major interactions between TBP and anhydrous H3PO4 do occur at 67-75 mole per cent H3PO 4. t35~B. J. THAMER,J. Phys. Chem. 64, 694 (1960). t36) G. C. PIMI~Wn~Land A. L. MCCLELLAN,The Hydrogen Bond, pp. 61-63. Freeman, San Francisco and London (1960). t37~C. C. ADDISONand B. C..SMrrn, J. Chem. Soc. 1783 (1960). 13s~j. R. FERRAROand D. F. PEPPARD,J. Phys. Chem. 65, 539 (1961). ~39~j. R. VANWAZER,Phosphorus andlts Compounds; Chemistry, Vol. 1, p. 486. Interscience, New York (1958).