- Email: [email protected]
Extraction of phosphoric acid with long-chain tertiary amines—I
',;;,~'
,u,,!
I he<
!~7~ '~o[ ~. pp ~'035-2040. Pergamon Press
Prinled in (Jreal Britain
EXTRACTION OF PHOSPHORIC ACID WITH LONG-CHAIN TERTIARY AMINES--1 EXTRACTION FROM AQUEOUS PHOSPHORIC
ACID SOLUTIONS
Y. MARCUS and L. E. ASHER Department of Inorganic & Analytical Chemistry, The Hebrex~ University, Jerusalem, Israel Receired l0 NoL:ember 1976; re{eit'ed jor publicatimt 3 March 1977)
Abstract--The extraction of phosphoric acid from its aqueous solutions with the long-chain tertiary amine Alamine 336 in toluene was studied at 25°C. Alamine 336 is intermediate in its extraction properties between trioctylamine and tridecylamine, but the spread in the distribution coefficientsbetween these two does not exceed 10%.Admixing20% by volume of dodecane with the toluene diluent produces a notable synergistic effect, but high concentrations of the aliphatic diluent lead to third phase formation. Some amine is extracted from the organic phase into dilute aqueous phosphoric acid solutions, affecting the titration of this acid with base. The distribution data can be quantitatively explained by the formation in the organic phase of two amine phosphate species with stoichiometries R3N:H~PO4 of l:l and 1:2. At high acid concentrations (3,4.2 M), excess acid over the 1:2 species is extracted. INTRODUCTION
The extracuon of mineral acids by long-chain tertiary amines in hydrocarbon diluents has received much attention, but while the monobasic acids have been studied very thoroughly, the tribasic phosphoric acid has not. Early workers[l,2] used the unsymmetrical methyldioctylamine in the hydrogen bonding diluent chloroform, but presented only very few data. A study by Matutano et al.[3] involving 0.087 M tridodecylamine in xylene at 18°C gave a distribution isotherm (Fig. 1) which shows a maximum, that is, at the given amine concentration a given phosphoric acid concentration in the organic phase is in equilibrium with two different aqueous phosphoric acid solutions. This is possible only for two organic phases which differ in some respect, and indeed, the water contents were found[3] to differ. However, subsequent work did not confirm the maximum in the isotherm, neither a study with 0.19 and 0.38 M trioctylamine in benzene (at an unspecified temperature)h4], nor a more detailed study by Sato[5] with 0.1 M trioctylamine in benzene at 20°C (Fig. 1). It is not the change of diluent among the aromatic hydrocarbons[6] which could produce the far-reaching changes in the behavior, and it was decided, therefore, to reinvestigate the system thoroughly, and try Io resolve this question. Phosphoric acid exists in aqueous solutions principally as the undissociated H~PO4 and as the ions H* and H:P(L . Extraction involving the anions HPO4: and PO~ ~ in the aqueous phase, as suggested by Sato[5] is therefore unlikely. This does not preclude in organic phases, containing excess amine and which are therefore basic, the formation of species such as (R3NH)2HPO~, where R3N signifies the tertiary amine, and a bar designates the organic phase. On the other hand, as Fig. I shows, excess acid is extracted over that stoichiometr!cally equivalent to the amine, and a species such as (R3NH)H2PO4, H3PO4 may be formed. A subsequent paper[71 will examine the question of the extraction of phosphoric acid from solutions at higher pH, where the ions H2PO~ and HPO42- predominate over
O3~. / ! O2~
'
I
//
~/.
,o%. '
:
/~
!
:Y
oot~ 0
2
4
6 CH3PO,~/M
8
!
!0
Fig. I. Distribution isotherms of phosphoric acid between 0.1 M solutions of tertiary amines in aromatic hydrocarbon diluenl:s and aqueous solutions. Circles: this work, toluene diluent, 25°C, O, trioctylamine; {), Alamine 336; 0, tris(decyl)amine; II, from Sato[5], trioctylamine in benzene diluent, 20°C: £> from Matutano[3]; tridodecylamine in xylene diluent, 18°C (the lower dashed curve is a second set of data); , , from Smirnov and Gorlov[4], trioctylamine is benzene diluent, room temperature. The ordinate is normalized to a common organic amine concentration of 0.1 M. the acid H,I~O4, but in this study the extraction from phosphoric acid solutions by the general reaction nH3PO4 + mR~N = (R3N),, (H~PO4),
(1)
will be considered. The bulk of the study involves the tertiary amine Alamine 336 in toluene as diluent (chosen for its relative nonvolatility, and freedom from isomeric impurities). Some experiments were also done with t.rioctylamine and tris(decyl)amine, to see the effect of the chain length, and with an aliphatic diluent, dodecane, to see its effects. All the extractions were done at 25°C.
2035
2036
Y. MARCUS and L.E. ASHER EXPERIMENTAL
Reagents. Two batches of Alamine 336 (trademark of General Mills, Inc.) representing a mixture of tertiary C8 and C~o amines were used without further purification. Batch I had an equivalent weight of 423-+-+2, while batch II had an equivalent weight of 401-+2gequiv -j. The variation of the equivalent weights of Alamine 336 among different batches indicates a lack of consistency with respect to composition. The average molecular weight of 392 reported by the manufacturer was determined by mass spectrometry and refers only to the tertiary amine fraction. The average amine number, i.e. the number of milligrams of potassium hydroxide equivalent to one gram of sample, corresponds [8] to an average equivalent weight of 418. The presence of varying amounts of non-basic impurities (which in effect raise the apparent equivalent weight) and secondary and primary amines (which in effect lower the apparent equivalent weight) would explain the batch dependence. Tri-n-octyl amine (Fluka practical) was distilled under vacuum. Tris(decyl)amine (Eastman practical), octylamine, dioctylamine and tri-hexylamine were used without further purification. Since the amine solutions slowly undergo photo-catalysed oxidation, all organic solutions were prepared fresh daily. Phosphoric acid (Frutarom analytical grade) was appropriately diluted with triply distilled water for the extractions. Titrations were performed using BDH calibrated solutions. Equipment. Extractions were performed using a shaker bath which agitated the solutions in a horizontal direction with a frequency of ca. 170 cycles/minute and a displacement of 3 cm. The temperature in the bath was kept at 25.0-+0.2°C. Titrations were performed with a Radiometer 11 automatic titrator, SBR2C titrigraph, and model 26 pH-meter. Extraction procedures. Except where indicated otherwise, extractions were performed by agitating 10.0ml each of solvent and aqueous phase in stoppered 50 ml volumetric flasks in the shaker bath. No significant difference was observed for duplicate runs as a function of time, as agitation time was varied from 20 min to 2 hr. After agitation, the phases were allowed to separate overnight while thermostated. When this was not sufficient to effect complete separation, centrifugation was employed. The possible effect of such centrifugation was investigated, using a system which separated under normal conditions, and no significant difference was observed. However, for systems at very low phosphoric acid concentrations, the appearance of cloudiness in the organic phase even after centrifugation prevented the accurate determination of distribution coefficients under these conditions. Analytical techniques. Several methods were employed for the determination of phosphoric acid after extraction. (1) An aliquot from the organic phase was added to 70% ethanol and titrated with NaOH while recording the titration curve. For extractions where Cn3po4< CR3N (C signifying"total concentration), only one endpoint is observed, corresponding to titration of two equivalents of acid, respectively amine salt. When Crt:,o4 > CR3N, two endpoints are observed, the first corresponding to one equivalent of the excess extracted phosphoric acid, and the second endpoint corresponding to the second equivalent of the excess acid plus two equivalents of amine salt. The titration phase under these conditions is generally not homogeneous and appropriate standards were prepared to check both the accuracy of the method as well as the endpoint behaviour. (2) Some organic phases from extractions were analysed for total phosphate using a modified phosphomolybdate method employing acetone[9]. This method was particularly valuable for analysing solutions with very low phosphate content. (3) The aqueous phases were analysed by titration with sodium hydroxide either potentiometrically or using a mixed indicator consisting of three parts phenolphthalein and one part a-naphtholphthalein. As will be discussed below, there was often a significant discrepancy between the first and second endpoints of the potentiometric titrations. In these cases, the second endpoint was taken for determination of phosphate concentrations. In general, two titrations were performed for every sample, the results being in agreement to better than -+1%. A further check on the analytical method is provided by the sum of total phos-
phoric acid in both phases after extraction, the agreement usually being better than 1% and always better than 2%, except when significant volume changes took place as a result of the extractions. Volume changes were determined by repeated manual inversion in the course of several hours of thermostated samples in a modified burette which allowed both ease of equilibration as well as precision in volume reading to -+1%. Several methods were tried for the determination of amines extracted into the aqueous solutions[10-13]. No analysis was found to be completely satisfactory from the standpoint of reproducibility, and only one gave reproducible calibration curves even to within a reasonable margin. This method[13] was used to determine the extracted amine, and its reliability is represented by the standard deviations of + 8%. The equivalent weight of amines was determined by two titration techniques. The first was by potentiometric titration with HCI in i-propanol solution, while the second method was by potentiometric titration using perchloric acid in acetic acid solution [14]. RESULTS
Extraction of H3PO4 by toluene alone The extraction of H3PO4 by pure toluene over the range CH3Po4= 0 . 1 - 3.7 M yielded in all cases organic phases whose upper limit for CR3Poa was less than that reported by Ricci, Sebastiani and Variali[6], CH~po4= 0.0013 M after extraction from 0.1 M HaPO4. It should be noted, however, that their determination for extraction of phosphoric acid into benzene is also very high with respect to that reported by Sato[5]: CHaPO4= 0.0014M after extraction from 0.1MH3PO416] compared to C'H3Po4= 0.0006 M after extraction from 4 M phosphoric acid[5].
Extraction of H3PO4 by Alamine 336 as a function of diluent The extraction of H3PO4 by Alamine 336 in the aromatic diluent toluene was compared to its extraction by the aliphatic diluent, dodecane. These latter extractions were complicated by the appearance of a third phase after equilibration, the amount of this third phase increasing with increasing concentrations of acid in the aqueous phase. Extractions performed with kerosene diluent indicated that there were conditions under which no third phase was formed; under these conditions, however, H3PO4 extraction was insignificant (at 0.1 M Alamine 336, D < 0.02). Significant extraction of H3PO4, without the appearance of a third phase, was observed in various mixtures of dodecane with toluene, and the results of such extractions are listed in Table 1. Under these conditions third phase formation is apparently limited by increasing aromatic content and lower total H3PO4 concentration. For example, in 80% dodecane v/v third phase formation is observed only beyond 0.585 M H3PO4, while in 70% dodecane no third phase was observed even at 1.028 H3PO4, and in 60% dodecane 1.48 M H3PO4 was necessary to bring about third phase formation. Figure 2 consists of plots of the distribution ratio D vs diluent composition at constant aqueous H3PO4 concentrations. It shows the interesting synergistic extraction of the 20% diluent mixture, with respect to extraction by the amine in either of the pure diluents.
Extraction of H3PO4 by tertiary amines in toluene Most of the extractions of H3PO4 were performed with Alamine 336, at the three initial amine concentrations 0.1,
2nI"
Extraction of phosphoric acid with long-chain tertiary amines--1
Table 2. Distribution of phosphoric acid between ter'tiar} amines in toluene and aqueous solutions.
Table I. Distribution of phosphoric acid between 0. l M Alamine 336 in dodecane-toluene mixtures and aqueous solutions. Dodecane (9~)
(=rex
CH,,O~
M
M
40
60
7(1
80
M
Trioctylamine 0.154 0.201 (I.642 (I. 159
D
(1.195 0.684
0.241 0.175
0.201 0.572 0.676 0.697 1.029
0.14(1 0.140
C'H,P );
M
M
(0.400)
0.074 0.082 O.IOR
2.71 2% 261
1]'
0.207 0,688 1.48'
ill 16 0.167
(1.218
0.030
0.358 0.558 0.6,99 (L814 1.028
0.078 0. 115 (/. I 17 0.088 0.(134
0,585+
--
1.69
(I.0g 1
0.121
2.43
354
0.050
0.149
2l7 !.K~ l~,t I.!9 U.'¢X (}.4~) ~.4~
I}.}94
0.178
Alamine 336 0.065 0.155 0.158 0.253
0.100
0.143 O, I 17
0.306 O.M6
0.651 (1.760 [,350 1.69 1.70
2.14 2.8O
3.55 4.22 5.55 7,13 0.037 0. 046 0.054 OO60
+Third phase formal(on. 0.400
/J~
(~R~N
Clt~pO4 0.100
20
D
0.224 O.U,9 0.438
0. 110 (i.186 (3.180 (I.217 0.2IV 0.152 0.151 0.136
1~15 (!.t~22 0{138
0.700
- ~,1
0144
4.17
0.092
l{)17
(l~,h
0.080 0.077
I 42
u
2 {)9
IlaS
0.068 O.057
e,<
Tris(dec,, Ilaminc 0 169 U (186 0.317 (! i75 0.6q4 II [38 1.70 0.07" 3.% OO46
0.048 0.O45
0. I00
0.040
O.038 1.70 2.11 2.40 2.62
0 iiX7
<62
- J_
2. These data for Alamine 336 are plotted as 1)1(',,,, (where ('k~N represents the total amine concentration} vs Cu3,,~a/(~R3~ in Fig. 3. It is seen that at all three amine concentrations a maximum in D/C~,~, is reached, for Cm,'o4 approximately equal to one half the total amine
O :2 ~'/
x\
O i
'.\
,o F i
f
i Yo v/v dodecane
Fig. 2. Distribution ratios of phosphoric acid between 0.1 M
Alamine 336 in mixtures of dodecane and toluene, and aqueous phosphoric acid solutions at constant equilibrium concentrations of 0.69 ± 0.01 M I@1 and (1.21± 0.02 M (©). The points for '90~'" dodecane are for kerosene, rather than for a dodecane-toluene mixture. 0.4 and 0.7 M. Whereas at 0.1 M amine, volume changes of either phase were insignificant, definite volume chang e occurred when extraction was performed at the higher amine concentrations (the organic phase expanded at the expense of the aqueous phase). The volume change was as much as 10% for some extractions at 0.7 M amine. Since as a rule the concentrations of H3P(L in both phases were determined, there is no effect on the reported distribution coefficients; nevertheless, the total amine concentration in the organic phase obviously decreased with the increase in volume. The distribution coefficients (D = (2.~pm/CH~PO ~ = ratio of total molarity of H~P()4 in the organic phase to that in the aqueous phase) for H3PO4 extraction as a function of the nature of the amine, the total amine concenlration and the aqueous H~P(L concentration are listed in Table
U
! /
CR~N
I
I
¢F
i/2 z,i
~'! ,
I
JI l
01M
•//7
~ %.
,,, i
"\
[,
i 1 [
"\
I
0
....... l 05
10
i5
C HspQ / C R 3N
Fig. 3. Normalized distribution ratios of phosphoric acid bc~ tween 0.1 M (O), 0.4 M (ID) and 0.7 M (O) Alamine 336 in toluene and aqueous solutions vs normalized concentrations of phos phoric acid in the organic phase. Normalization is by division into the total amine concentratkm (~RN
2038
Y. MARCUS and L. E. ASHER
concentration. However, in the corresponding isotherms CH3Po4 vs CH3r,o4 (Fig. 1), no maximum is obtained, and the course of the data is in good agreement with those of Sato[5] for trioctylamine in benzene, and in disagreement with those of Matutano[3] for tridodecylamine in xylene. The data for trioctylamine and tris(decyl)amine are seen (Fig. 1) to follow closely those for Alamine 336, the data for trioctylamine being somewhat higher, those for tris(decyl)amine somewhat lower, the spread not exceeding 10% Clq3PO4, Distribution results for the two batches of Alamine 336 are the same within the experimental error of -+2% in D. Data obtained at given, constant, equilibrium aqueous acidities and different Alamine 336 concentrations in toluene are shown in Table 3. in
Table 3. Distribution of phosphoric acid between solutions of Alamine 336 in toluene and aqueous solutions at constant acidities, 0.247_+0.002M.
DISCUSSION
Theoretical model for extraction Most of the data in this study were obtained with Alamine 336 in toluene--those with the other amines show the same trends--so that only the former will be discussed in detail. Consider the following two extraction equilibria
H3PO4 + R3N~RaNH.HzPO4 : K~
(2)
2H3PO4 + R3N ~R3NH.HzP O4.H3PO4 : K_~.
(3)
The stoichiometries of the extracted species do not necessarily imply structures. In the aqueous phase, the dissociation equilibrium H3P O4~.-~-H÷ + H2PO4- ; K.
(4)
occurs. In the above equations, K1, Ks and Ka are the thermodynamic equilibrium constants. The distribution ratio is D CH3POJCH3PO4 =
(~R~N: Dob~: D~.~lf~:
0.101 0.213 0.25
0.200 0.642 0.64
0.301 1.09 1.06
0.400 1.54 1.49
0.599 2.45 2.37
0.700 2.93 2.81
?Calculated with eqn (10) with the parameters K I =70CR3 N and K~ = 0.25.
Extraction of Alamine 336 by aqueous phosphoric acid solutions The previously mentioned endpoint discrepancy during analysis of many aqueous phase samples prompted further investigation into the back-extraction of amine into aqueous phosphoric acid. It was found that extraction of Alamine 336 from 0.1 M toluene solutions into aqueous phosphoric acid increased as the acid concentration decreased in the range 0.3-0.03 M: CR3N= 4.8X10-4+2.1X10 5X C.3Po 4 - ' - 1. These values are significantly higher than the solubility of the amine in water[15], 0.125 x 10-4 M, so that the back extraction into the phosphoric acid solutions must reach a maximum at C.3Po4<0.03 M. The back-extraction values were not sufficient to explain the endpoint discrepancy completely, and it is concluded that lower chain-size and lower order amines were back-extracted, which did not show up in the analyses. The endpoint discrepancy manifested itself positively at low acid and high amine concentrations, and under these conditions formation of a white precipitate during NaOH titration of the aqueous phase was noted. This precipitate, whose odour was not characteristic of Alamine 336, redissolved upon addition of excess acid, and it thus seems likely that the precipitate is due to a basic impurity in the Alamine 336, most likely an amine (or amines) whose acid salt is relatively soluble in water and whose distribution coefficient favours considerable extraction into the aqueous phase. Addition to aqueous alcoholic solutions of primary (octyl), secondary (dioctyl) and tertiary (trihexyl) amines, indeed produced similar endpoint discrepancies, as observed, supporting the impurity hypothesis for equilibrations with Alamine 336 solutions. Pretreatment of the amine with acid before extraction experiments greatly reduced the endpoint discrepancy, although it was not completely eliminated.
= ([R3NH.H2P04] + 2[R3NH'H2PO4'H3P04]) /([H2P04 ] + [H3P04])
(5)
where square brackets denote molar concentrations. Those in the aqueous phase are related to the activity of undissociated phosphoric acid a. as [H2PO4 ] +
Ka~/2a.t/2y+_-t+ a.y. '
[H3PO4] =
(6)
= a.(K,Y2a,,-~:2y+ ~+ y° ') = a.X
where y_+and y. are the molar activity coefficients of the dissociated (ionic) and undissociated phosphoric acid, and X, the expression in parentheses, is a function of the aqueous phase only. The activity a. and activity coefficients were obtained from the work of Elmore et al.[16], employing pK. =2.148 at 298K of Bates[17]. The concentration of phosphoric acid in the organic phase species can be expressed as [R3NH.H2PO4] + 2[R3NH.H2PO4.H~PO4] -
-
I
-
-
I
= K~yNYNp[R3N]a,, + 2K2yNYNp~[R3N]a,,
= K~(1 + 2K;a.)[R3N]a.
2
(7)
where K; = KlyN)NP, - - - K"- = K2K~ . YNPYNP2, ... ' and y are molar activity coefficients of the species R3N (YN), R3NH.H:PO4(.fNp) and RsNH.H2PO4"H3PO4(37Npz). The concentration of free amine in the organic phase [R3N] is obtained from the difference between the total amine concentration CR3S and that bound to phosphoric acid [R3N] = 6"R3N- ([RaNH.H2PO4] + [R3NH.H2PO4.H3PO4]) =
CR3N/(I+ Kla.(1 + K~a.))
(8)
where the last equation is obtained with the aid of eqn (7) with obvious modification. From eqns (5), (6), (7) and (8) the distribution ratio is D = K'ffl + 2K2a.)CR3N/X(1 ' ' ' + K,a.(l + Kza.)).
(9)
A normalized quantity D' is convenient for further cap
Extraction of phosphoric acid with long-chainterliary amines--1
2117')
culatiuns: i
D' = DXa,IO<. = (1 + 2K" a.)/(1 + K" a,, + llK'~a.).
(10) Provided that K'~ and K~ are independent of a, and of (=~,~, D' is a unique function of the aqueous phase concentration of phosphoric acid, which determines a,. Its limits are zero at very small, and two at very large, values of a,. It turns out (Fig. 4) that D' is not unique, but depends on Ca3~' In view of the strong dependence of D / C ~ on C ~ exhibited in Fig. 3, it may be assumed that the additional equilibrium H~PO, + 2R~N~(R3NH):HPO~: K,
(11)
should be invoked in order to explain this dependence, This follows from the fact that at low phosphoric acid concentrations D/CR3~ is roughly proportional to CR~,, i,e. D is roughly proportional to C~N. However, it is not the total amine concentration that is relevant, but the activity of amine not bound to phosphoric acid, [R3N]YN. In fact, equilibrium (lll, which would add a term KI[R~N] to the numerator of eqn (10) and a term 2K~IR~N] to the denominator, where K~ = K~K, '_%5~Npf,~;e, would cause deviations of the cab culated D' opposite to those observed, assuming K',, K~ and K~ to be constants. For any choice of K', and K; that agrees with the data and for positive values of K'3, not enough amine is bound in the species__(_RxNH)2HPO4, reducing the free amine concentration [R3N], in order to compensate for the additional extraction caused by it. With the alternative assumption that equilibrium (11) is disregarded, but instead the activity coefficient ratio f~/~]~., is set to be proportional to C~3,, the total solute concentration in the organic phase, the data can be fitted well by eqn. (10). The quasi-constant K',=K~v]s/,]yp becomes K I = K,k. C~y = K'(CR~s. Substitution of this in eqn (/0) leads to an expression with two constants K'~' and K~ and the parameter C~3~. The equation can be recast into the form I
+ 1/D')a,dR3N = ILK",+ [(1 -21D')a,,2CR3,IK" (12)
and solved by least squares to give K'/=70_+7 M-: and K5 0.25-+0.05 M = for the distribution data D'(a,) at the three (~R~Nvalues of 0.1, 0.4 and 0.7 M. (The three lowest points at 0.1 M were excluded from the correlation). The agreement of the calculated D' vs a, with the data (Fig. 4 and Table 3) is within an average dex iation of _*3.0% which is considered quite good, in view of the average error of a single distribution point D of :=2%, which is somewhat magnified by the normalization to D'. This agreement does not imply that the activity coefficients of the amine phosphate salts, YNp and YNp,, are independent of concentrations in the organic phase, and confirm the proportionality of .VNand CR~Nfor the partly neutralized, i.e. mixed free amine-amine salt solutions, making K'( and K; constant. A closer look shows that the constancy of K~ implies only the weaker condition that the ratio .%H%P: is independent of concentrations[19], rather than the stronger one that they are individually independent. Also, amines in aromatic diluents behave not far from ideality[18], while their salts are quite non ideal, so that the condition ,VN/Y'NP kCR3N may perhaps indicate the inverse proportionality of YNP and CR~N. A =
08p/:~' 0 4 ~! ;'// ¢,'
i
00~0
~
04
0~8
12
6
20
o~.M Fig. 4. Normalized distribution ratios of phosphoric :reid D = DXa,/(Tu~N vs phosphoric acid activities (a,,) in the aqueom, phase, for 0.1 M (©), 0.4M (I[7))and 0.7M (0) Alamine 336 in toluene. Curves calculated from eqn (10) with K I K'~(:,.:,, 70(~a3N and K: = 0.25. strong decrease in the activity coefficients of amine salts with increasing concentrations is generally observed in binary systems. It is surmised to be true also in wet, partially neutralized systems, such as the present one. It is remarkable that the agreement of eqn (10) or (12) with the data extends to quite high aqueous phosphoric acid activities (a, - 10, corresponding to Cu,po~ - 4,2 M, beyond the range shown in Fig. 4) i.e. as long as Cmp,,~ 2CR3N, SO that the equilibria (2) and (3) suffice. At very low phosphoric acid concentrations, however, there are small systematic deviations (see Table 31, which may perhaps be partly explained by the analytical difficulties and incomplete phase separation described in the experimental section. The evidence is against the formation of a species with the stoichiometry (R3NH)2HP(L in the organic phase under the present conditions. If formed, it would have caused deviations in the sense opposite to that observable in Fig. 4, as discussed above. That such a species would pass into the aqueous phase, in agreement with the direction of the deviations, is unlikely, in viev, of the largely organic constitution. However, the loss of amine (shorter chain or lower order fractions of the Alamine 336) at low phosphoric acid concentrations noted in the results section, is also in the right direction, and should contribute to the deviations. The conclusion from this study is that the general extraction eqn (1) expresses the extraction data for Alamine 336 concentrations in toluene up tv 07 M and aqueous phosphoric acid concentrations up to 4.2 M with m = 1 and n = 1 and 2. There is no evidence that from acid solutions, even when CaR3N> 2C.,~,,,~ species with m > 1 are formed in the organic phase, while only at very high acidities (C.,poa>4.2M) are species with rz >2 apparently formed.
Comparison with previous studies Of the published publications on extraction of phosphoric acid with tertiary amines[t-6], only those of Matutano et al.[3] and of Sato [5] are sufficiently detailed to permit close comparison. As Fig. 1 shows, the maximum in the distribution isotherm suggested b} Matutano et al.[3] could not be confirmed, while the general trend suggested by Sato[5] was indeed confirmed, taking into account the differences in the alkyt chain length of the amine and the solvent. The reliability of the data of
2(140
Y. MARCUS and L. E. ASHER
Matutano et a/.[3] is to be questioned also because the detailed report contains a second set of distribution points (giving the lower dashed curve in Fig. 1), which is inconsistent with their own published data. The data shown in Table 3 for 0.247 M phosphoric acid correspond to a slope of 1.4 for a plot of log D vs log CR,N (in the range 0.1 < CR~N<0.7), in good agreement with a slope of 1,45 found by Sato[5] for 0.2 M acid and 0.01 < CR~N< 0.1. Our data for higher acidities also correspond to a lower slope, as found by Sato. However, the interpretation proposed by Sato, in terms of the species (R3NH)3PO4, (R3NHhHPO4 and (R3NH)H,,PO4 is not supported by the evidence. Only the latter species occurs in the organic phase at low acidities, while at high acidities, the additional species (R3NH)H2PO4.H3PO4 must be invoked, to explain C~3Po4 > C~3N, a fact found by Sato but not considered by him. His infrared spectral data are not inconsistent with our interpretation either. The effect of the diluent on the extraction is of interest. Ricci et a/.[61 found for 0.41 M trioctylamine in toluene and 0.0373 M aqueous phosphoric acid (at equilibrium) D = 1.68, in good agreement with the value of D = 1.70 found for 0.037 M phosphoric acid and 0.400 M Alamine 336 in this study. According to Ricci[6], benzene is a better diluent (and o-xylene a worse one): for 0.42 M trioctylamine in benzene at CH3po4= 0.0257, D = 2.89, in reasonable agreement with the datum of Smirnov[4] for 0.38M trioctylamine in benzene at CH3po4= 0.0246, D = 3.20. The same trend arises from Sato's data at low acidities, shown in Fig. 1. However, at high acidities Sato's data for benzene diluent are lower than ours for toluene diluent, while Smirnov's data are not really comparable, being obtained at higher (~R3~. What is remarkable is the finding by Ricci[6] that pentane and ligroin are acceptable diluents for trioctylamine in phosphoric acid extraction, yielding results comparable to xylene and benzene respectively. The acidity employed (CH3Po4<0.05 M) is however well below the threshold for third phase formation, a phenomenon observed in our study as well as in others[3,4]. For Alamine 336 in toluene, no third phase formation was observed for 0.1 M amine up to 7.2 M acid, for 0.7 M amine up to 2.1 M acid, but the conditions for its formation at higher concentrations were not specifically sought. Smirnov[4] observed third phase formation above 7.6 M aqueous acidity for both 0.19 and 0.38 M trioctylamine in benzene, while Matutano[3] did not observe third phase formation with 0.087 tridocecylamine in xylene at high acidities, up
to ca. 8 M. On the other hand, at low acidities (~<0.5 M) he did observe some third phase formation, which corresponds to our difficulties with phase separations and observations of cloudiness at low acidities too, see experimental section. No information on mixtures of aromatic and aliphatic diluents, such as the dodecane-toluene mixtures used in the present study, is available in the published literature for comparison, nor is the solubilization and extraction of long-chain amines from organic diluents into aqueous phosphoric acid mentioned elsewhere. Acknowledgement--The authors acknowledge the support of Israel Chemicals, Ltd. for this research. One of us (L.E.A.) is indebted to the Lady Davis Fellowship Trust for a post doctoral fellowship. The data on dodecane-toluene mixtures were provided by Mrs. S. Druckman. REFERENCES
1. E. L. Smith and J. E. Page, J, Soc. Chem. Ind. (London) 67, 48 (1948). 2. F. L. Moore, Anal. Chem. 29, 1660 (1957). 3. L. Matutano, French Report CEA-R2760 (1965); H. Brusset, R. Hamelin and L. Matutano, Compt. Rend. 261, 1979 (1965). 4. V. S. Smirnov and E. E. Gorlov, Russ. J. lnorg. Chem. 12, 428 (1967). 5. T. Sato, J. Appl. Chem. 15, 10 (t965). 6. P. F. Ricci, E. Sebastiani and G. Variali, Ann. Chim. (Rome) 60, 116 (1970). 7. Y. Marcus, L. E. Asher and Ch. Barak, to be published. 8. P. Pattison, General Mills, Inc., personal communication (1975). 9. D. N. Barnbart and A. R. Wreath, Anal. Chem. 27, 440 (1955). 10. H. M. N. H. Irving and J. J. Markham, Anal. Chim. Acta 39, 7 (1967). 11. H. M. N. H. Irving and D. Damodaran, Analyst 90, 180 (1965). 12. A. Makurgee and P. Makurgee, J. Appl. Chem. 12, 127 (1%2). 13. Y. M. Florence and Y. J. Farrar, Anal. Chim. Acta 63, 255 (1973). 14. J. S. Fritz, Acid-Base Titrations in Non-Aqueous Solvents, Allyn and Bacon. (1973). 15. "Alamine 336", prospectus for LIX Chemical Division o[ General Mills, Inc., Illinois. 16. K. L. Elmore, J. D. Hatfield, R. L. Dunn and A. D. Jones, J. Phys. Chem. 69. 3520 (1965). 17. R. G. Bates, J. Res. Nat. Bur. Stand. 47, 127 (195l). 18. A. S. Kertes and G. Markovitz, J. Phys. Chem. 72, 4202 (1968). 19. S. Lindenbaum, J. Phys. Chem. 66, 1382 (1962).
,u,,!
I he<
!~7~ '~o[ ~. pp ~'035-2040. Pergamon Press
Prinled in (Jreal Britain
EXTRACTION OF PHOSPHORIC ACID WITH LONG-CHAIN TERTIARY AMINES--1 EXTRACTION FROM AQUEOUS PHOSPHORIC
ACID SOLUTIONS
Y. MARCUS and L. E. ASHER Department of Inorganic & Analytical Chemistry, The Hebrex~ University, Jerusalem, Israel Receired l0 NoL:ember 1976; re{eit'ed jor publicatimt 3 March 1977)
Abstract--The extraction of phosphoric acid from its aqueous solutions with the long-chain tertiary amine Alamine 336 in toluene was studied at 25°C. Alamine 336 is intermediate in its extraction properties between trioctylamine and tridecylamine, but the spread in the distribution coefficientsbetween these two does not exceed 10%.Admixing20% by volume of dodecane with the toluene diluent produces a notable synergistic effect, but high concentrations of the aliphatic diluent lead to third phase formation. Some amine is extracted from the organic phase into dilute aqueous phosphoric acid solutions, affecting the titration of this acid with base. The distribution data can be quantitatively explained by the formation in the organic phase of two amine phosphate species with stoichiometries R3N:H~PO4 of l:l and 1:2. At high acid concentrations (3,4.2 M), excess acid over the 1:2 species is extracted. INTRODUCTION
The extracuon of mineral acids by long-chain tertiary amines in hydrocarbon diluents has received much attention, but while the monobasic acids have been studied very thoroughly, the tribasic phosphoric acid has not. Early workers[l,2] used the unsymmetrical methyldioctylamine in the hydrogen bonding diluent chloroform, but presented only very few data. A study by Matutano et al.[3] involving 0.087 M tridodecylamine in xylene at 18°C gave a distribution isotherm (Fig. 1) which shows a maximum, that is, at the given amine concentration a given phosphoric acid concentration in the organic phase is in equilibrium with two different aqueous phosphoric acid solutions. This is possible only for two organic phases which differ in some respect, and indeed, the water contents were found[3] to differ. However, subsequent work did not confirm the maximum in the isotherm, neither a study with 0.19 and 0.38 M trioctylamine in benzene (at an unspecified temperature)h4], nor a more detailed study by Sato[5] with 0.1 M trioctylamine in benzene at 20°C (Fig. 1). It is not the change of diluent among the aromatic hydrocarbons[6] which could produce the far-reaching changes in the behavior, and it was decided, therefore, to reinvestigate the system thoroughly, and try Io resolve this question. Phosphoric acid exists in aqueous solutions principally as the undissociated H~PO4 and as the ions H* and H:P(L . Extraction involving the anions HPO4: and PO~ ~ in the aqueous phase, as suggested by Sato[5] is therefore unlikely. This does not preclude in organic phases, containing excess amine and which are therefore basic, the formation of species such as (R3NH)2HPO~, where R3N signifies the tertiary amine, and a bar designates the organic phase. On the other hand, as Fig. I shows, excess acid is extracted over that stoichiometr!cally equivalent to the amine, and a species such as (R3NH)H2PO4, H3PO4 may be formed. A subsequent paper[71 will examine the question of the extraction of phosphoric acid from solutions at higher pH, where the ions H2PO~ and HPO42- predominate over
O3~. / ! O2~
'
I
//
~/.
,o%. '
:
/~
!
:Y
oot~ 0
2
4
6 CH3PO,~/M
8
!
!0
Fig. I. Distribution isotherms of phosphoric acid between 0.1 M solutions of tertiary amines in aromatic hydrocarbon diluenl:s and aqueous solutions. Circles: this work, toluene diluent, 25°C, O, trioctylamine; {), Alamine 336; 0, tris(decyl)amine; II, from Sato[5], trioctylamine in benzene diluent, 20°C: £> from Matutano[3]; tridodecylamine in xylene diluent, 18°C (the lower dashed curve is a second set of data); , , from Smirnov and Gorlov[4], trioctylamine is benzene diluent, room temperature. The ordinate is normalized to a common organic amine concentration of 0.1 M. the acid H,I~O4, but in this study the extraction from phosphoric acid solutions by the general reaction nH3PO4 + mR~N = (R3N),, (H~PO4),
(1)
will be considered. The bulk of the study involves the tertiary amine Alamine 336 in toluene as diluent (chosen for its relative nonvolatility, and freedom from isomeric impurities). Some experiments were also done with t.rioctylamine and tris(decyl)amine, to see the effect of the chain length, and with an aliphatic diluent, dodecane, to see its effects. All the extractions were done at 25°C.
2035
2036
Y. MARCUS and L.E. ASHER EXPERIMENTAL
Reagents. Two batches of Alamine 336 (trademark of General Mills, Inc.) representing a mixture of tertiary C8 and C~o amines were used without further purification. Batch I had an equivalent weight of 423-+-+2, while batch II had an equivalent weight of 401-+2gequiv -j. The variation of the equivalent weights of Alamine 336 among different batches indicates a lack of consistency with respect to composition. The average molecular weight of 392 reported by the manufacturer was determined by mass spectrometry and refers only to the tertiary amine fraction. The average amine number, i.e. the number of milligrams of potassium hydroxide equivalent to one gram of sample, corresponds [8] to an average equivalent weight of 418. The presence of varying amounts of non-basic impurities (which in effect raise the apparent equivalent weight) and secondary and primary amines (which in effect lower the apparent equivalent weight) would explain the batch dependence. Tri-n-octyl amine (Fluka practical) was distilled under vacuum. Tris(decyl)amine (Eastman practical), octylamine, dioctylamine and tri-hexylamine were used without further purification. Since the amine solutions slowly undergo photo-catalysed oxidation, all organic solutions were prepared fresh daily. Phosphoric acid (Frutarom analytical grade) was appropriately diluted with triply distilled water for the extractions. Titrations were performed using BDH calibrated solutions. Equipment. Extractions were performed using a shaker bath which agitated the solutions in a horizontal direction with a frequency of ca. 170 cycles/minute and a displacement of 3 cm. The temperature in the bath was kept at 25.0-+0.2°C. Titrations were performed with a Radiometer 11 automatic titrator, SBR2C titrigraph, and model 26 pH-meter. Extraction procedures. Except where indicated otherwise, extractions were performed by agitating 10.0ml each of solvent and aqueous phase in stoppered 50 ml volumetric flasks in the shaker bath. No significant difference was observed for duplicate runs as a function of time, as agitation time was varied from 20 min to 2 hr. After agitation, the phases were allowed to separate overnight while thermostated. When this was not sufficient to effect complete separation, centrifugation was employed. The possible effect of such centrifugation was investigated, using a system which separated under normal conditions, and no significant difference was observed. However, for systems at very low phosphoric acid concentrations, the appearance of cloudiness in the organic phase even after centrifugation prevented the accurate determination of distribution coefficients under these conditions. Analytical techniques. Several methods were employed for the determination of phosphoric acid after extraction. (1) An aliquot from the organic phase was added to 70% ethanol and titrated with NaOH while recording the titration curve. For extractions where Cn3po4< CR3N (C signifying"total concentration), only one endpoint is observed, corresponding to titration of two equivalents of acid, respectively amine salt. When Crt:,o4 > CR3N, two endpoints are observed, the first corresponding to one equivalent of the excess extracted phosphoric acid, and the second endpoint corresponding to the second equivalent of the excess acid plus two equivalents of amine salt. The titration phase under these conditions is generally not homogeneous and appropriate standards were prepared to check both the accuracy of the method as well as the endpoint behaviour. (2) Some organic phases from extractions were analysed for total phosphate using a modified phosphomolybdate method employing acetone[9]. This method was particularly valuable for analysing solutions with very low phosphate content. (3) The aqueous phases were analysed by titration with sodium hydroxide either potentiometrically or using a mixed indicator consisting of three parts phenolphthalein and one part a-naphtholphthalein. As will be discussed below, there was often a significant discrepancy between the first and second endpoints of the potentiometric titrations. In these cases, the second endpoint was taken for determination of phosphate concentrations. In general, two titrations were performed for every sample, the results being in agreement to better than -+1%. A further check on the analytical method is provided by the sum of total phos-
phoric acid in both phases after extraction, the agreement usually being better than 1% and always better than 2%, except when significant volume changes took place as a result of the extractions. Volume changes were determined by repeated manual inversion in the course of several hours of thermostated samples in a modified burette which allowed both ease of equilibration as well as precision in volume reading to -+1%. Several methods were tried for the determination of amines extracted into the aqueous solutions[10-13]. No analysis was found to be completely satisfactory from the standpoint of reproducibility, and only one gave reproducible calibration curves even to within a reasonable margin. This method[13] was used to determine the extracted amine, and its reliability is represented by the standard deviations of + 8%. The equivalent weight of amines was determined by two titration techniques. The first was by potentiometric titration with HCI in i-propanol solution, while the second method was by potentiometric titration using perchloric acid in acetic acid solution [14]. RESULTS
Extraction of H3PO4 by toluene alone The extraction of H3PO4 by pure toluene over the range CH3Po4= 0 . 1 - 3.7 M yielded in all cases organic phases whose upper limit for CR3Poa was less than that reported by Ricci, Sebastiani and Variali[6], CH~po4= 0.0013 M after extraction from 0.1 M HaPO4. It should be noted, however, that their determination for extraction of phosphoric acid into benzene is also very high with respect to that reported by Sato[5]: CHaPO4= 0.0014M after extraction from 0.1MH3PO416] compared to C'H3Po4= 0.0006 M after extraction from 4 M phosphoric acid[5].
Extraction of H3PO4 by Alamine 336 as a function of diluent The extraction of H3PO4 by Alamine 336 in the aromatic diluent toluene was compared to its extraction by the aliphatic diluent, dodecane. These latter extractions were complicated by the appearance of a third phase after equilibration, the amount of this third phase increasing with increasing concentrations of acid in the aqueous phase. Extractions performed with kerosene diluent indicated that there were conditions under which no third phase was formed; under these conditions, however, H3PO4 extraction was insignificant (at 0.1 M Alamine 336, D < 0.02). Significant extraction of H3PO4, without the appearance of a third phase, was observed in various mixtures of dodecane with toluene, and the results of such extractions are listed in Table 1. Under these conditions third phase formation is apparently limited by increasing aromatic content and lower total H3PO4 concentration. For example, in 80% dodecane v/v third phase formation is observed only beyond 0.585 M H3PO4, while in 70% dodecane no third phase was observed even at 1.028 H3PO4, and in 60% dodecane 1.48 M H3PO4 was necessary to bring about third phase formation. Figure 2 consists of plots of the distribution ratio D vs diluent composition at constant aqueous H3PO4 concentrations. It shows the interesting synergistic extraction of the 20% diluent mixture, with respect to extraction by the amine in either of the pure diluents.
Extraction of H3PO4 by tertiary amines in toluene Most of the extractions of H3PO4 were performed with Alamine 336, at the three initial amine concentrations 0.1,
2nI"
Extraction of phosphoric acid with long-chain tertiary amines--1
Table 2. Distribution of phosphoric acid between ter'tiar} amines in toluene and aqueous solutions.
Table I. Distribution of phosphoric acid between 0. l M Alamine 336 in dodecane-toluene mixtures and aqueous solutions. Dodecane (9~)
(=rex
CH,,O~
M
M
40
60
7(1
80
M
Trioctylamine 0.154 0.201 (I.642 (I. 159
D
(1.195 0.684
0.241 0.175
0.201 0.572 0.676 0.697 1.029
0.14(1 0.140
C'H,P );
M
M
(0.400)
0.074 0.082 O.IOR
2.71 2% 261
1]'
0.207 0,688 1.48'
ill 16 0.167
(1.218
0.030
0.358 0.558 0.6,99 (L814 1.028
0.078 0. 115 (/. I 17 0.088 0.(134
0,585+
--
1.69
(I.0g 1
0.121
2.43
354
0.050
0.149
2l7 !.K~ l~,t I.!9 U.'¢X (}.4~) ~.4~
I}.}94
0.178
Alamine 336 0.065 0.155 0.158 0.253
0.100
0.143 O, I 17
0.306 O.M6
0.651 (1.760 [,350 1.69 1.70
2.14 2.8O
3.55 4.22 5.55 7,13 0.037 0. 046 0.054 OO60
+Third phase formal(on. 0.400
/J~
(~R~N
Clt~pO4 0.100
20
D
0.224 O.U,9 0.438
0. 110 (i.186 (3.180 (I.217 0.2IV 0.152 0.151 0.136
1~15 (!.t~22 0{138
0.700
- ~,1
0144
4.17
0.092
l{)17
(l~,h
0.080 0.077
I 42
u
2 {)9
IlaS
0.068 O.057
e,<
Tris(dec,, Ilaminc 0 169 U (186 0.317 (! i75 0.6q4 II [38 1.70 0.07" 3.% OO46
0.048 0.O45
0. I00
0.040
O.038 1.70 2.11 2.40 2.62
0 iiX7
<62
- J_
2. These data for Alamine 336 are plotted as 1)1(',,,, (where ('k~N represents the total amine concentration} vs Cu3,,~a/(~R3~ in Fig. 3. It is seen that at all three amine concentrations a maximum in D/C~,~, is reached, for Cm,'o4 approximately equal to one half the total amine
O :2 ~'/
x\
O i
'.\
,o F i
f
i Yo v/v dodecane
Fig. 2. Distribution ratios of phosphoric acid between 0.1 M
Alamine 336 in mixtures of dodecane and toluene, and aqueous phosphoric acid solutions at constant equilibrium concentrations of 0.69 ± 0.01 M I@1 and (1.21± 0.02 M (©). The points for '90~'" dodecane are for kerosene, rather than for a dodecane-toluene mixture. 0.4 and 0.7 M. Whereas at 0.1 M amine, volume changes of either phase were insignificant, definite volume chang e occurred when extraction was performed at the higher amine concentrations (the organic phase expanded at the expense of the aqueous phase). The volume change was as much as 10% for some extractions at 0.7 M amine. Since as a rule the concentrations of H3P(L in both phases were determined, there is no effect on the reported distribution coefficients; nevertheless, the total amine concentration in the organic phase obviously decreased with the increase in volume. The distribution coefficients (D = (2.~pm/CH~PO ~ = ratio of total molarity of H~P()4 in the organic phase to that in the aqueous phase) for H3PO4 extraction as a function of the nature of the amine, the total amine concenlration and the aqueous H~P(L concentration are listed in Table
U
! /
CR~N
I
I
¢F
i/2 z,i
~'! ,
I
JI l
01M
•//7
~ %.
,,, i
"\
[,
i 1 [
"\
I
0
....... l 05
10
i5
C HspQ / C R 3N
Fig. 3. Normalized distribution ratios of phosphoric acid bc~ tween 0.1 M (O), 0.4 M (ID) and 0.7 M (O) Alamine 336 in toluene and aqueous solutions vs normalized concentrations of phos phoric acid in the organic phase. Normalization is by division into the total amine concentratkm (~RN
2038
Y. MARCUS and L. E. ASHER
concentration. However, in the corresponding isotherms CH3Po4 vs CH3r,o4 (Fig. 1), no maximum is obtained, and the course of the data is in good agreement with those of Sato[5] for trioctylamine in benzene, and in disagreement with those of Matutano[3] for tridodecylamine in xylene. The data for trioctylamine and tris(decyl)amine are seen (Fig. 1) to follow closely those for Alamine 336, the data for trioctylamine being somewhat higher, those for tris(decyl)amine somewhat lower, the spread not exceeding 10% Clq3PO4, Distribution results for the two batches of Alamine 336 are the same within the experimental error of -+2% in D. Data obtained at given, constant, equilibrium aqueous acidities and different Alamine 336 concentrations in toluene are shown in Table 3. in
Table 3. Distribution of phosphoric acid between solutions of Alamine 336 in toluene and aqueous solutions at constant acidities, 0.247_+0.002M.
DISCUSSION
Theoretical model for extraction Most of the data in this study were obtained with Alamine 336 in toluene--those with the other amines show the same trends--so that only the former will be discussed in detail. Consider the following two extraction equilibria
H3PO4 + R3N~RaNH.HzPO4 : K~
(2)
2H3PO4 + R3N ~R3NH.HzP O4.H3PO4 : K_~.
(3)
The stoichiometries of the extracted species do not necessarily imply structures. In the aqueous phase, the dissociation equilibrium H3P O4~.-~-H÷ + H2PO4- ; K.
(4)
occurs. In the above equations, K1, Ks and Ka are the thermodynamic equilibrium constants. The distribution ratio is D CH3POJCH3PO4 =
(~R~N: Dob~: D~.~lf~:
0.101 0.213 0.25
0.200 0.642 0.64
0.301 1.09 1.06
0.400 1.54 1.49
0.599 2.45 2.37
0.700 2.93 2.81
?Calculated with eqn (10) with the parameters K I =70CR3 N and K~ = 0.25.
Extraction of Alamine 336 by aqueous phosphoric acid solutions The previously mentioned endpoint discrepancy during analysis of many aqueous phase samples prompted further investigation into the back-extraction of amine into aqueous phosphoric acid. It was found that extraction of Alamine 336 from 0.1 M toluene solutions into aqueous phosphoric acid increased as the acid concentration decreased in the range 0.3-0.03 M: CR3N= 4.8X10-4+2.1X10 5X C.3Po 4 - ' - 1. These values are significantly higher than the solubility of the amine in water[15], 0.125 x 10-4 M, so that the back extraction into the phosphoric acid solutions must reach a maximum at C.3Po4<0.03 M. The back-extraction values were not sufficient to explain the endpoint discrepancy completely, and it is concluded that lower chain-size and lower order amines were back-extracted, which did not show up in the analyses. The endpoint discrepancy manifested itself positively at low acid and high amine concentrations, and under these conditions formation of a white precipitate during NaOH titration of the aqueous phase was noted. This precipitate, whose odour was not characteristic of Alamine 336, redissolved upon addition of excess acid, and it thus seems likely that the precipitate is due to a basic impurity in the Alamine 336, most likely an amine (or amines) whose acid salt is relatively soluble in water and whose distribution coefficient favours considerable extraction into the aqueous phase. Addition to aqueous alcoholic solutions of primary (octyl), secondary (dioctyl) and tertiary (trihexyl) amines, indeed produced similar endpoint discrepancies, as observed, supporting the impurity hypothesis for equilibrations with Alamine 336 solutions. Pretreatment of the amine with acid before extraction experiments greatly reduced the endpoint discrepancy, although it was not completely eliminated.
= ([R3NH.H2P04] + 2[R3NH'H2PO4'H3P04]) /([H2P04 ] + [H3P04])
(5)
where square brackets denote molar concentrations. Those in the aqueous phase are related to the activity of undissociated phosphoric acid a. as [H2PO4 ] +
Ka~/2a.t/2y+_-t+ a.y. '
[H3PO4] =
(6)
= a.(K,Y2a,,-~:2y+ ~+ y° ') = a.X
where y_+and y. are the molar activity coefficients of the dissociated (ionic) and undissociated phosphoric acid, and X, the expression in parentheses, is a function of the aqueous phase only. The activity a. and activity coefficients were obtained from the work of Elmore et al.[16], employing pK. =2.148 at 298K of Bates[17]. The concentration of phosphoric acid in the organic phase species can be expressed as [R3NH.H2PO4] + 2[R3NH.H2PO4.H~PO4] -
-
I
-
-
I
= K~yNYNp[R3N]a,, + 2K2yNYNp~[R3N]a,,
= K~(1 + 2K;a.)[R3N]a.
2
(7)
where K; = KlyN)NP, - - - K"- = K2K~ . YNPYNP2, ... ' and y are molar activity coefficients of the species R3N (YN), R3NH.H:PO4(.fNp) and RsNH.H2PO4"H3PO4(37Npz). The concentration of free amine in the organic phase [R3N] is obtained from the difference between the total amine concentration CR3S and that bound to phosphoric acid [R3N] = 6"R3N- ([RaNH.H2PO4] + [R3NH.H2PO4.H3PO4]) =
CR3N/(I+ Kla.(1 + K~a.))
(8)
where the last equation is obtained with the aid of eqn (7) with obvious modification. From eqns (5), (6), (7) and (8) the distribution ratio is D = K'ffl + 2K2a.)CR3N/X(1 ' ' ' + K,a.(l + Kza.)).
(9)
A normalized quantity D' is convenient for further cap
Extraction of phosphoric acid with long-chainterliary amines--1
2117')
culatiuns: i
D' = DXa,IO<. = (1 + 2K" a.)/(1 + K" a,, + llK'~a.).
(10) Provided that K'~ and K~ are independent of a, and of (=~,~, D' is a unique function of the aqueous phase concentration of phosphoric acid, which determines a,. Its limits are zero at very small, and two at very large, values of a,. It turns out (Fig. 4) that D' is not unique, but depends on Ca3~' In view of the strong dependence of D / C ~ on C ~ exhibited in Fig. 3, it may be assumed that the additional equilibrium H~PO, + 2R~N~(R3NH):HPO~: K,
(11)
should be invoked in order to explain this dependence, This follows from the fact that at low phosphoric acid concentrations D/CR3~ is roughly proportional to CR~,, i,e. D is roughly proportional to C~N. However, it is not the total amine concentration that is relevant, but the activity of amine not bound to phosphoric acid, [R3N]YN. In fact, equilibrium (lll, which would add a term KI[R~N] to the numerator of eqn (10) and a term 2K~IR~N] to the denominator, where K~ = K~K, '_%5~Npf,~;e, would cause deviations of the cab culated D' opposite to those observed, assuming K',, K~ and K~ to be constants. For any choice of K', and K; that agrees with the data and for positive values of K'3, not enough amine is bound in the species__(_RxNH)2HPO4, reducing the free amine concentration [R3N], in order to compensate for the additional extraction caused by it. With the alternative assumption that equilibrium (11) is disregarded, but instead the activity coefficient ratio f~/~]~., is set to be proportional to C~3,, the total solute concentration in the organic phase, the data can be fitted well by eqn. (10). The quasi-constant K',=K~v]s/,]yp becomes K I = K,k. C~y = K'(CR~s. Substitution of this in eqn (/0) leads to an expression with two constants K'~' and K~ and the parameter C~3~. The equation can be recast into the form I
+ 1/D')a,dR3N = ILK",+ [(1 -21D')a,,2CR3,IK" (12)
and solved by least squares to give K'/=70_+7 M-: and K5 0.25-+0.05 M = for the distribution data D'(a,) at the three (~R~Nvalues of 0.1, 0.4 and 0.7 M. (The three lowest points at 0.1 M were excluded from the correlation). The agreement of the calculated D' vs a, with the data (Fig. 4 and Table 3) is within an average dex iation of _*3.0% which is considered quite good, in view of the average error of a single distribution point D of :=2%, which is somewhat magnified by the normalization to D'. This agreement does not imply that the activity coefficients of the amine phosphate salts, YNp and YNp,, are independent of concentrations in the organic phase, and confirm the proportionality of .VNand CR~Nfor the partly neutralized, i.e. mixed free amine-amine salt solutions, making K'( and K; constant. A closer look shows that the constancy of K~ implies only the weaker condition that the ratio .%H%P: is independent of concentrations[19], rather than the stronger one that they are individually independent. Also, amines in aromatic diluents behave not far from ideality[18], while their salts are quite non ideal, so that the condition ,VN/Y'NP kCR3N may perhaps indicate the inverse proportionality of YNP and CR~N. A =
08p/:~' 0 4 ~! ;'// ¢,'
i
00~0
~
04
0~8
12
6
20
o~.M Fig. 4. Normalized distribution ratios of phosphoric :reid D = DXa,/(Tu~N vs phosphoric acid activities (a,,) in the aqueom, phase, for 0.1 M (©), 0.4M (I[7))and 0.7M (0) Alamine 336 in toluene. Curves calculated from eqn (10) with K I K'~(:,.:,, 70(~a3N and K: = 0.25. strong decrease in the activity coefficients of amine salts with increasing concentrations is generally observed in binary systems. It is surmised to be true also in wet, partially neutralized systems, such as the present one. It is remarkable that the agreement of eqn (10) or (12) with the data extends to quite high aqueous phosphoric acid activities (a, - 10, corresponding to Cu,po~ - 4,2 M, beyond the range shown in Fig. 4) i.e. as long as Cmp,,~ 2CR3N, SO that the equilibria (2) and (3) suffice. At very low phosphoric acid concentrations, however, there are small systematic deviations (see Table 31, which may perhaps be partly explained by the analytical difficulties and incomplete phase separation described in the experimental section. The evidence is against the formation of a species with the stoichiometry (R3NH)2HP(L in the organic phase under the present conditions. If formed, it would have caused deviations in the sense opposite to that observable in Fig. 4, as discussed above. That such a species would pass into the aqueous phase, in agreement with the direction of the deviations, is unlikely, in viev, of the largely organic constitution. However, the loss of amine (shorter chain or lower order fractions of the Alamine 336) at low phosphoric acid concentrations noted in the results section, is also in the right direction, and should contribute to the deviations. The conclusion from this study is that the general extraction eqn (1) expresses the extraction data for Alamine 336 concentrations in toluene up tv 07 M and aqueous phosphoric acid concentrations up to 4.2 M with m = 1 and n = 1 and 2. There is no evidence that from acid solutions, even when CaR3N> 2C.,~,,,~ species with m > 1 are formed in the organic phase, while only at very high acidities (C.,poa>4.2M) are species with rz >2 apparently formed.
Comparison with previous studies Of the published publications on extraction of phosphoric acid with tertiary amines[t-6], only those of Matutano et al.[3] and of Sato [5] are sufficiently detailed to permit close comparison. As Fig. 1 shows, the maximum in the distribution isotherm suggested b} Matutano et al.[3] could not be confirmed, while the general trend suggested by Sato[5] was indeed confirmed, taking into account the differences in the alkyt chain length of the amine and the solvent. The reliability of the data of
2(140
Y. MARCUS and L. E. ASHER
Matutano et a/.[3] is to be questioned also because the detailed report contains a second set of distribution points (giving the lower dashed curve in Fig. 1), which is inconsistent with their own published data. The data shown in Table 3 for 0.247 M phosphoric acid correspond to a slope of 1.4 for a plot of log D vs log CR,N (in the range 0.1 < CR~N<0.7), in good agreement with a slope of 1,45 found by Sato[5] for 0.2 M acid and 0.01 < CR~N< 0.1. Our data for higher acidities also correspond to a lower slope, as found by Sato. However, the interpretation proposed by Sato, in terms of the species (R3NH)3PO4, (R3NHhHPO4 and (R3NH)H,,PO4 is not supported by the evidence. Only the latter species occurs in the organic phase at low acidities, while at high acidities, the additional species (R3NH)H2PO4.H3PO4 must be invoked, to explain C~3Po4 > C~3N, a fact found by Sato but not considered by him. His infrared spectral data are not inconsistent with our interpretation either. The effect of the diluent on the extraction is of interest. Ricci et a/.[61 found for 0.41 M trioctylamine in toluene and 0.0373 M aqueous phosphoric acid (at equilibrium) D = 1.68, in good agreement with the value of D = 1.70 found for 0.037 M phosphoric acid and 0.400 M Alamine 336 in this study. According to Ricci[6], benzene is a better diluent (and o-xylene a worse one): for 0.42 M trioctylamine in benzene at CH3po4= 0.0257, D = 2.89, in reasonable agreement with the datum of Smirnov[4] for 0.38M trioctylamine in benzene at CH3po4= 0.0246, D = 3.20. The same trend arises from Sato's data at low acidities, shown in Fig. 1. However, at high acidities Sato's data for benzene diluent are lower than ours for toluene diluent, while Smirnov's data are not really comparable, being obtained at higher (~R3~. What is remarkable is the finding by Ricci[6] that pentane and ligroin are acceptable diluents for trioctylamine in phosphoric acid extraction, yielding results comparable to xylene and benzene respectively. The acidity employed (CH3Po4<0.05 M) is however well below the threshold for third phase formation, a phenomenon observed in our study as well as in others[3,4]. For Alamine 336 in toluene, no third phase formation was observed for 0.1 M amine up to 7.2 M acid, for 0.7 M amine up to 2.1 M acid, but the conditions for its formation at higher concentrations were not specifically sought. Smirnov[4] observed third phase formation above 7.6 M aqueous acidity for both 0.19 and 0.38 M trioctylamine in benzene, while Matutano[3] did not observe third phase formation with 0.087 tridocecylamine in xylene at high acidities, up
to ca. 8 M. On the other hand, at low acidities (~<0.5 M) he did observe some third phase formation, which corresponds to our difficulties with phase separations and observations of cloudiness at low acidities too, see experimental section. No information on mixtures of aromatic and aliphatic diluents, such as the dodecane-toluene mixtures used in the present study, is available in the published literature for comparison, nor is the solubilization and extraction of long-chain amines from organic diluents into aqueous phosphoric acid mentioned elsewhere. Acknowledgement--The authors acknowledge the support of Israel Chemicals, Ltd. for this research. One of us (L.E.A.) is indebted to the Lady Davis Fellowship Trust for a post doctoral fellowship. The data on dodecane-toluene mixtures were provided by Mrs. S. Druckman. REFERENCES
1. E. L. Smith and J. E. Page, J, Soc. Chem. Ind. (London) 67, 48 (1948). 2. F. L. Moore, Anal. Chem. 29, 1660 (1957). 3. L. Matutano, French Report CEA-R2760 (1965); H. Brusset, R. Hamelin and L. Matutano, Compt. Rend. 261, 1979 (1965). 4. V. S. Smirnov and E. E. Gorlov, Russ. J. lnorg. Chem. 12, 428 (1967). 5. T. Sato, J. Appl. Chem. 15, 10 (t965). 6. P. F. Ricci, E. Sebastiani and G. Variali, Ann. Chim. (Rome) 60, 116 (1970). 7. Y. Marcus, L. E. Asher and Ch. Barak, to be published. 8. P. Pattison, General Mills, Inc., personal communication (1975). 9. D. N. Barnbart and A. R. Wreath, Anal. Chem. 27, 440 (1955). 10. H. M. N. H. Irving and J. J. Markham, Anal. Chim. Acta 39, 7 (1967). 11. H. M. N. H. Irving and D. Damodaran, Analyst 90, 180 (1965). 12. A. Makurgee and P. Makurgee, J. Appl. Chem. 12, 127 (1%2). 13. Y. M. Florence and Y. J. Farrar, Anal. Chim. Acta 63, 255 (1973). 14. J. S. Fritz, Acid-Base Titrations in Non-Aqueous Solvents, Allyn and Bacon. (1973). 15. "Alamine 336", prospectus for LIX Chemical Division o[ General Mills, Inc., Illinois. 16. K. L. Elmore, J. D. Hatfield, R. L. Dunn and A. D. Jones, J. Phys. Chem. 69. 3520 (1965). 17. R. G. Bates, J. Res. Nat. Bur. Stand. 47, 127 (195l). 18. A. S. Kertes and G. Markovitz, J. Phys. Chem. 72, 4202 (1968). 19. S. Lindenbaum, J. Phys. Chem. 66, 1382 (1962).