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Equilibria between tri-n-octylamine and some mineral acids
J. Inorg. Nucl, Chem., 1961, VoL 23, pp. 323 to 332. Pergamon Press Ltd. Printed in Northern Ireland
EQUILIBRIA BETWEEN TRI-N-OCTYLAMINE A N D SOME MINERAL ACIDS U. BERTOCC[ and G. ROLANDI Laboratorio di Chimica del Centro di lspra, Varese, Italy (Received 5 June 1961 ; in recised f o r m 6 July 1961)
Abstract--Tri-n-octylamine dissolved in xylene was equilibrated with aqueous solutions of nitric, hydrochloric, hydrofluoric and sulphuric acids, and the acid and water contents were determined in the organic phase. The experiments were performed at several concentrations of amine and of acids. The results show that the acid concentration in the organic phase at equilibrium is generally higher than that necessary to convert all the amine into the corresponding salt. Third phase formation was only found for certain equilibrations with sulphuric acid. The solubility of water in the organic phase was found to be small. THE extraction o f inorganic acids by long chain aliphatic amines dissolved in an organic diluent has been described in several recent papers, following the w o r k o f SMITH a n d PaGz (1), a n d MooRe (2) has briefly described the possibility o f using the process for the e x t r a c t i o n o f m a n y different inorganic and organic acids. WILSON (3), in studies o f the extraction o f Pu from nitric acid solutions, gives values l b r the extraction o f H N O a into t r i - n - o c t y l a m i n e (TOA). BERTOCCI(4), and CARSWELL a n d LAWRENCE(5), give the equilibrium curves for nitric acid and b o t h tri-isononylamine a n d t r i - i s o - o c t y l a m i n e up to 10 M aqueous concentration, showing that the a m i n e solution extracts nitric acid in excess over that required for f o r m a t i o n o f the amine nitrate. The extraction o f sulphuric acid has been studied by several workers. ALI.EN¢6) published d a t a on equilibria between H~SO 4 a n d b o t h T O A a n d d i - n - d e c y l a m i n e ( D D A ) , p a r t i c u l a r l y as regards the ratio a m i n e sulphate/bisulphate as a function o f aqueous HeSO 4 concentration. McDOWELL a n d BAES(7) r e p o r t e d similar w o r k for D D A a n d H2SO 4. BOIRIE(8) published d a t a on the equilibrium between T O A a n d H2SO 4 up to 2 M aqueous acid and investigated the conditions for converting the a m i n e completely into bisulphate. BJZOT and TREMILLON(9) studied the extraction o f HCI into T O A at m o d e r a t e a q u e o u s acid concentrations, and interpreted the results in terms o f f o r m a t i o n o f the h y d r o c h l o r i d e in the organic phase. Finally, JENKINS a n d WAIN (1°) d e m o n s t r a t e d the conditions for third phase formation in equilibrations between acids a n d various amines. ~JJ E. L. SMITH anti J. E. PAGE, J. Soc. Chem. hulnstr)' ( L o n d o n ) , 67, 48, (1949). ('-'~ 1-'. L. MOORE, dnalyt. Chem. 29, 1660 (1957); O R N L - 1 3 1 4 (1952). /a, A. S. W~LSON, Proceedings of the Second International Col~'rence on the Peacefal Uaes of dtomi~ Ener~l,,
Geneva 1958. p. 544, United Nations (1958) (4~ U. BERrOCC~,AERE/R 2933 (1959). ~r,~ D. J. CARSWEt,t, and J. J. LAWRENCE,J. InorG Nucl. Chem. II, 69 (1959). 16~K. A. ALLEN,J. Phys. Chem. 60, 239, 943 (1956). (71 W. J. McDoWELLand C. F. BAES,J. Phys. Chem. 62, 777 (1958). (s~ C. BOmlE, Btdl. Soc. Chim. France 8-9, 980, 1088 (1958). I~ J. BIZOT, B. TREMILLON, Bidl. Soe. Chim. France 1, 122 (1959).
{~0}1. L. JENKINSand A. G. WMN, AERE/M-537 (1959). 323
324
U. BERTOCCI and G. ROLANDI
F r o m a n e x a m i n a t i o n o f the literature, however, it seems that i n f o r m a t i o n o n the extraction o f hydrochloric a n d sulphuric acids at high concentrations, as well as data c o n c e r n i n g hydrofluoric acid, are lacking. It was therefore decided to carry o u t a systematic investigation o f the equilibria between H N O a , HC1, H F a n d H2SO 4 u p to c o n c e n t r a t i o n s of 10 M a n d a tertiary amine, tri-n-octylamine, diluted with xylene. The solubility o f water in the organic phase was also studied. EXPERIMENTAL
Chemicals All experiments were carried out using TOA purchased from the firm FLUKA, with a nominal titre of 99.5 per cent. Titration of weighed samples with HCIO4 in glacial acetic acid indicated a molecular weight of 353 4- 2 compared with a theoretical value of 353. Solutions were prepared with C.P. xylene, a mixture of the three isomers. Before titration, the solutions were washed twice with deionized water in order to remove traces of low molecular weight amines. The aqueous acidic solutions were prepared from C.P. chemicals.
Methods of analysis The concentration of TOA in the solutions in xylene was determined by titration with HCIO4 in glacial acetic acid, with crystal violet as indicator. The accuracy is 4-0.0005 M for concentrations less than 0"05 M and 4-0'002 M for the other ones. The molarity of the aqueous acids is given with an accuracy of 4-0' 1 M for concentrations higher than 1 M, and +0.02 M for the other ones. Two methods were used to determine the acid content of the organic phase: (1) titration of the organic phase, diluted in acetone, with aqueous NaOH; (2) potentiometric titration of the organic phase, diluted in benzene/methanol, with potassium methoxide. The first method was used mainly for TOA solutions equilibrated with HF, the second for the others. Several titrations were carried out by both methods; the results were in good agreement, and also agreed with a third method, viz. back-extraction of the acid with excess alkali, and titration of the residual alkalinity in the aqueous phase. Potentiometric titrations were carried out using a glass electrode and S.C.E. reference electrode, both immersed in the organic solution. The glass electrode reached equilibrium rapidly and gave constant and reproducible values. The accuracy ranged from 4-0.001 M for 0"01 M TOA solutions, to 4-0'02 M for 0"5 M TOA solutions. The water content of the organic phase was determined by the Karl Fischer titration, using the standard dead-stop technique. The accuracy ranges from 4-0'005 M for the lowest TOA concentration to 4-0.02 M for the highest one.
Equilibrations TOA solutions in xylene (0.01, 0.02, 0'05, 0.1, 0.2 and 0.5 M) were prepared, and aqueous solutions of each acid from 0.5 to 10 M. Each TOA solution was equilibrated with aqueous solutions of the four acids at various concentrations, with an organic/aqueous ratio of 2:1. A mixing time of 5 min is sufficient to reach.equilibrium in nitric, hydrochloric and hydrofluoric acids. But for equilibrations with sulphuric acid a mixing time of 15 rain was chosen. After mixing, the phases were separated by centrifuging. Settling is fairly rapid when the pH of the aqueous phase is less than unity over the range of TOA concentrations used in this work. EXPERIMENTAL RESULTS (1) Stability of the amine A series of experiments was carried o u t to check the stability of the T O A solutions i n xylene towards chemical attack b y the acids. F o r this purpose, samples o f a 0.2 M
Equilibria between tri-n-octylamine and some mineral acids
325
T O A solution were contacted with each of the acids studied at concentrations of 2 M and 8 M. After 15 days' standing, the organic phase was washed with N a O H and the T O A titre checked; the results showed that the molarity of the amine was unchanged within the limits of experimental error.
(2)
Titrations in non-aqueous media
Fig. 1 shows titration curves for HNO3, HC1 and H2SO 4. The curves for HCI and H N O a are of the same shape, with two inflections due to the fact that both solutions - 500
- 400
- 500
O
200
>
tm
-
t
7 Tr;nt .1 i N ~ methox,de q,],% ant SC el~ctrode~ 5 c c cf 0 95 M TC,A ea,~i~ated v,,rh 5 q M H SC':
•
42C
+
3:c
?f F : I 9 7 M
TSA
10o
C
IC0
i /
2o0 sos ÷
/~..** :~--~/ [
j,/÷
¢ ~ ÷
-
[ Z
4 ~1.
Flo.
b
6
;
8
9
,2
,2
Of O.~.N K metho×ide benzene/methanol
l.--Potentiometric
titrations
'3
4
Ib
!6
in
of acids in TOA.
contain more acid than that corresponding to formation of the amine salt. The first inflexion corresponds to neutralization of excess acid and the second to neutralization of acid combined with the amine; the difference between the titres gives the molarity of the amine itself. The titration curve for the TOA solution equilibrated with H.~SO4 is more complex. showing three inflexion points ; in this case also the organic phase contains excess sulphuric acid above that corresponding to the stoichiometric T O A acid sulphate. By comparison with the other titration curves, it may be shown that the first inttexion corresponds to neutralization of excess H2SO4, and the second and third to neutralization of combined acid in two stages via the amine bisulphate.
326
U. BERTOCCIand G. ROLANDI
(3) Equilibrations The experimental results for the four acids are given in Tables 2, 3, 4 a n d 5, which show for each equilibration the m o l a r i t y of T O A , of acid, a n d of water in the organic phase, as well as the m o l a r i t y o f the acid in the aqueous phase. TABLE 1 . - - E X T R A C T I O N OF
(HNO3)aqtm (HNO3)orgcm (HzO)orgC.~l)
0"001 <0"001 0"006
4"4 0'003 --
HNOs 7"9 0"008 0"008
AND WATER INTO PURE XYLENE
8"7 0"013 --
8"7 0"022 0"015
9"7 0'032 --
10"3 0'043 --
TABLE 2.--EQUILIBRATIONOF TOA SOLUTIONSWITH AQUEOUSHNO3
(TOA)org (M)
(HNO3)aq (M)
(HNOz)org (M)
(HNO3)org (TOA) org
0'010 0"010 0'010 0"010 0"021 0"021 0"021 0"021 0"021 0"047 0-047 0-047 0.100 0.100 0-100 0.100 0.197 0.197 0.197 0.197 0-197 0.197 0-50 0"50 0.50 0.50 0.50 0.50
0'01 2"35 6'6 8'9 0"01 1'2 2-3 7-0 8-85 0'01 1'85 6'9 0'01 1"9 6"4 9"0 0-01 9-85 2"0
0'010 0'014 0'022 0"027 0"021 0'025 0'031 0.050 0'057 0'050 0"065 0'110 0-10 0.14 0"22 0"265 0"195 0'23 0'28 0"39 0"42 0'51 0'50 0.58 0'67 1'00 1-24 1"32
1'0 1-4 2-2 2"7 1"0 1"2 1'5 2'4 2'7 1.0 1-35 2.35 1'0 1"4 2.2 2-65 1-0 1-15 1-4 2-0 2-1 2-6 1-0 1.15 1-35 2.0 2.5 2.65
5"25 6"1 8"8 0"01 0'75 1"7 5"4 8'3 9"0
(H20)org (M)
(HNO3)org
0.007 0"01 0"01 0"01 0"01 0'02 0"02 0"03 0"04 0"05 0"04
4'4 5'0 5'7 5-0 6-5 5"5 5-0 4"7 5"5 5.3 4'9
0'06
4-7
0'08 0"09 0"08
5"2 5"6 6'2
0"10 0"15 0"20
6"7 6"7 6"2
(H20)org
* Water content not reported; the experimental values are of the same order of magnitude as the experimental error. As pure xylene extracts some H N O 3 a n d water, the values reported in the tables are corrected by subtracting the a m o u n t s given in Table 1, t a k i n g into a c c o u n t the m o l a r ratio o f xylene in the organic phase. N o corrections are applied for the extraction o f the other acids into xylene, since the a m o u n t s are negligible. T h e a m o u n t of water extracted by pure xylene contacted with HC1, H F a n d H2SO 4 is i n d e p e n d e n t o f the
327
Equilibria between tri-n-octylamine and some mineral acids TABLE 3.--EQUILIBRATION OF TOA SOLUTIONS WITH HCI (TOA)org (M)
(HCl)aq
0-0095 0.0095 0.0095 0.0095 0.0095 0-020 0.020 0.020 0.049 0.049 0.049 0.049 0.049 0,100 0'100 0.100 0.100 0.100 0.100 0.197 0.197 0-197 0.197
1-8 3.55 5-4 7.1 8.75
0.197
0.50 0.50 0.50 0.50 0.50 0.50
(M)
1.75
5.0 8-5 1.75
3.6 4.75 6.95 8.4 0.4 1-7 4.3 5.25 7.0 8.25 1.6 3.6 5.2 6'8 8.15 0.2 1.5 4.l 5.1 6.75 8.0
(HCl)org (M) 0"0095 0"0095 0"012 0"014 0-016 0"020 0-024 0'034 0"049 0"050 0"056 0'070 0"08 0-10 0"10 0-11 0.125 0-15 0'17 0"20 0"20 0-24 0"29 0"33 0"50 0.51 0.55 0.63 0-72 0.83
(HC1)org
(H~O)org
(TOA)or~ 1"0 1"0 1-25 1 "45
1'7 1'0 1'2 1.7 1"0 1"0 1"15 1 "45 1 "65 1'0 1'0 1"1 1 "25 1-5 l'7 1"0 1 "05 1 "25 1 '45 1'65
(HCI)org
(M)
(H20)org
0.015 0-015 0-01 0.015 0"0l 0-02 0"015 0'015 0"04 0"04 0-04 0-04
0.6 0-6 1.2 0.9 1.6 1.0 1.6 23 1.2 12 1.4 1.7
0'095 0"10 0"10 0"10 0"10 0"17 0.18 0-19 0"19 0'18
1.0 0.9 1.2 1.5 1.7 1.2 I.I 1.3 1.5 1.8
0.53 0.55 0.58 0.58
1.0 1.0 1.1 I-2
1"0 1 "0
1-I 1 "25 1 "45 1 '65
TABLE 4,--EQUILIBRATION OF TOA SOLUTIONS WITH HF
(TOA)org (M)
(HF)aa
0"010 0"010 0"010 0"010 0'010 0.100 0.100 0.100 0.100 0-100 0"51 0-51 0.51 0.51 0"51
0"2 0'95 4"0 6"l 9"75 0'2 0"85 3"9 5"58 9'6 0"1 0"5 3.5 5.5 8.9
(M)
(H F)org
(H F)org
(M)
(TOA)org
0"007 0"021 0'027 0"029 0'033 0"06 0"21 0"28 0"30 0"33 0'25 1"0 1.4 1.5 1.7
0"7 2"I 2"7 2"9 3"3 0"6 2'1 2"8 3"0 3"3 0"5 1 "95 2"75 2"95 3"3
i t !
(H20)org (M)
(HF)o~g (H20)o,.g
~0 ~0 0.005
6.6
0.035 0.03 0.035
6"0 9"3 8"6
0.08 0.06
12 23
0.06
28
328
U. BBRTOCCIand G. ROLANDI
aqueous acid concentration, and the corrections applied were 0.012, 0.013 and 0.015 respectively. F r o m the values reported, the ratios (acid)org/(TOA)org and (acid)org/(H20)org were calculated, and for each acid a curve was drawn showing the ratio (acid)org / (TOA)org versus the aqueous acid molarity (Fig. 2, 3, 4 a n d 5). TABLE 5.--EQUILIBRATION OF T O A SOLUTIONS WITH H~SO4
(TOA)org
(H2SO4)~q
(H2SO4)org
(H~SO4)org
(M)
(M)
(M)
(TOA)org
0'010 0-010 0.010 0-010 0.010 0.010 0.010 0.010 0.019 0.019 0.019 0.019 0.019 0.049 0"049 0-049 0.049 0"049 0"100 0.100 0"100 0"100 0'195 0'195 0-195 0-195 0.50 0-50 0-50 0-50 0.50
0"2 0"5 0'95 1"9 3.75 6"0 8-4 10.0 0.5 0.95 3.65 7.5 8.5 0.5
0'007 0"008 0"009 0"0095 0"010 0"011 0"014 0"015 0"015 0"018 0.019 0"025 0'027 0.040 0"045 0'050 0"055 0"075 0.07 0.095 0.10 0"145 0'16 0'19 0'22 0"24 0.30 0'45 0"50 0"57 0'63
0'7 0'8 0"9 0'95 1'0 1"1 1'4 1"5 0'8 0"95 1.0 1.3 1.4 0.8 1'0 1"0 1"1
1"85
-
3"7 6"0 I0.0 0.15 0'9 3.6 9'0 0.45 1.8 5"9 6"85 0.05 0-75 3-5 5-85 6-7
(H20)org (M)
(H2SO4)org
(H20)org
m
0.007
1.1
0"008 0.007 0.006
1.2 1.4 1.8
0'011 0.012 0.011
1.4 + 1.6 2.3
0"03 0"025
1.3 1.8
0'025
2.2
0-055 0'05 0'06
1.7 2.0 2.4
0"13 0"13
1.5 1-8 1-8
0-32 0.32 0.33
1-6 1.8 1-9
1.55
0.7 0-95 1"0 1"45 0"8 0'95 1 '15 1 "25 0'6 0'9 1"0 1.15 1.25
0.12
!
On equilibrating T O A solutions with sulphuric acid, two organic phases are formed at certain T O A and aqueous HzSO~ concentrations. In the presence o f a third phase, the system becomes univariant at constant temperature and pressure, and for each aqueous H2SO 4 concentration, the composition o f the two organic solutions is fixed. The width o f the region within which T O A solutions in equilibrium with aqueous H2SO 4 p r o d u c e two phases can be deduced approximately f r o m the compositions o f the conjugate solutions shown in Table 6, which also gives some results showing h o w the region o f third phase formation varies with the temperature. As the separation o f the two conjugate phases is rather slow, particularly when their compositions differ
Equilibria between tri-n-octylamine and some mineral acids
329
..f¢'* ° 2c,
A. ( : , ~ 3 &
~
~
~'~" II
d k5
•
TOA 0-5M
~
T0A
-~
TOA 0 . 1 N
0"197 M
TOA O'047 N e,
b0
I
I
E
2
3
4
5
•
TOA 0.021M
(:
TOA 0'01 M
I
I
1
I
6
7
8
9
I0
M HNO3a q. . . . . Flo. z.--t~atm
[HNO~]0 ~ vs. a q u e o u s H N O 3 c o n c e n t r a t i o n
at e q u i l i b r i u m .
2.0
1.5
c) < O
•*
c~O,,, L • ' " g ~
o
1.0
e-&
,
c} x
0"5
I
-5M
•
'
'
e
TOA 0"197M
41,
TOA
,5.
TOA 0 . 0 4 9 M
•
TOA 0"1
o
T0A 0'0095 M
[
0-020 M
M
P
M HCL aq. Fro. 3.--Ratio
TABLE
~[ H C l ] ° vs. a q u e o u s H C I c o n c e n t r a t i o n
at equilibrium.
6 . - - C O M P O S I T I O N OF CONJUGATE ORGANIC PHASES FORMED ON EQUILIBRATION
or TOA SOLUTIONSWITH H2SO4 Light phase Temp
(M)
"C)
20
9.0
i 27
Heavy phase
(I-I2SO,)~q
J
45 I
(TOA)org
(H~SOl)org
(M)
(M)
(TOA)org (H2OS4)org
0.11
0.16
1.45
100
0.035
0.055
1.55
9"0
0-20
10.0
0.10
--
(TOA)or.
056 of_6
((H2SO~)org (TOA)org (M) ~ g
o.s, 1.1
i--;4; 1.50
--
- 0 2 ...... _C
_
:
330
U. BERTOCCI and G. ROLANDI
2.0
1.5
p...-.--&-
o o I-
I-0
o
6(]3 £
0.5
i
4
5 M
e
TOA
0.01 M
•
TOA
0,019 M
t
TOA
0.1 M
,~
TOA
0.195 M
•
TOA
0.049
o
TOA
0.50
6
7
M M
8
9
H 2 S 0 4 0 q.
FIG. e.--t~att o [H2S04]o ~ vs. aqueous HzSO4 concentration at equilibrium. .
.
.
.
5.5
3.0
2.5
/-
0 k-
2.0
I
1.5
•
TOA OOI M TOA 0 1 M
@
TOA 0 " 5 M
1.0
0"5 4 ,
0
I
i
I
2
4
5 M
FIG.
5.--Ratio
[HF]o [T-"~'~o
Vs.
7
8
9
I0
HFoq
aqueous HF concentration at equilibrium.
Equilibria between tri-n-octylamine and some mineral acids
331
only slightly, the absence of third phase formation, even after three days' standing, cannot be taken as proof of the thermodynamic stability of the system. Table 7 gives the results of some specific gravity determinations of the organic phases after equilibration with H2SO4; the measurements were carried out at 25 ~ I°C. T~Bt.E 7,--DENSITY OF ORGANIC PHASES EQUILIBRATED WITH H,,SO4
(TOA)org (M)
(HaSO4)org (M)
D 25 (g/cmu)
0.50 0.50 0.50 0.50 0.195 0.195 0.195 0.195 0.100 0.100 0.100 0.100 0.11 0.56 0-035 0.76
0 0.45 0.50 0.57 0 0.19 0.22 0.24 0 0.07 o.[0 0.145 0.16 0.81 0.055 [.15
0-855 0.880 0.884 0.887 0.860 0.875 0.878 0.880 0"865 0-869 0-872 0"975 0-869 0-835 0.865 0,.873
DISCUSSION The results of the equilibrations show that all the mineral acids investigated tend to dissolve in the organic phase formed by the amine salt dissolved in xylene. This tendency is the greatest for nitric acid, the ratio (HNO3)orJ(YOA)org being more than one at relatively low aqueous HNO3 concentrations. This ratio increases linearly with the aqueous H N O 3 concentration, and is independent of the organic amine concentration. It is therefore impossible to formulate a particular complex between amine and acid having a definite stoichiometric ratio in the organic phase. The ratio of acid to amine is similar for different amines having analogous formulae, as inferred by comparison with other published results. The constancy of the ratio between the acid and amine concentrations in the organic phase at constant aqueous acid concentration is a general feature for all the acids studied, but the quantitative behaviour of the different acids varies, e.g. HC1 begins to dissolve in the amine hydrochloride only at aqueous concentrations higher than 3"5 M. The curve for H2SO 4 is more complex: below aqueous concentrations about 1 M, the organic phase contains both sulphate and bisulphate in various proportions, whereas above that value only bisulphate is present. The curve is also valid above the point where two organic phases appear, since in each of them the ratio is the same. In the case of hydrofluoric acid the curve differs from those for the other acids, the solubility of the acid in the organic phase is higher than for the other acids, probably because of the tendency of H F to polymerize, and rises very rapidly up to aqueous concentrations about ] M, and then more slowly. In equilibrium with a 0-2 M aqueous
332
U. BERTOCCIand G. ROLANDI
solution the amine is not fully converted into salt, since H F is a weaker acid than the others. With xylene as diluent, third phase formation occurs only with sulphuric acid; even in this case the extent of the zone in which certain TOA solutions cannot exist as a single phase is rather small, and it practically vanishes on raising the temperature to a moderate extent. This confirms the superiority of aromatic diluents over the aliphatic ones as far as third phase formation is concerned. It is difficult to draw quantitative deductions from the water content of the organic phases, because of the small amounts extracted, but from the values of the ratio of acid to water in the organic phase the following conclusions may be drawn. For H N O 3, a definite amount of water is associated with the acid, about one mole of water for every five or six moles of HNOa; for HC1, it appears that each mole of amine hydrochloride is associated with one mole of water, while the excess HC1 is anhydrous. In the case of H~SOa, it seems that one mole of water is associated with two moles of amine bisulphate, the excess H2SO 4 being anhydrous. For HF, the only conclusion which can be drawn is that water is only slightly soluble in the organic phase.
EQUILIBRIA BETWEEN TRI-N-OCTYLAMINE A N D SOME MINERAL ACIDS U. BERTOCC[ and G. ROLANDI Laboratorio di Chimica del Centro di lspra, Varese, Italy (Received 5 June 1961 ; in recised f o r m 6 July 1961)
Abstract--Tri-n-octylamine dissolved in xylene was equilibrated with aqueous solutions of nitric, hydrochloric, hydrofluoric and sulphuric acids, and the acid and water contents were determined in the organic phase. The experiments were performed at several concentrations of amine and of acids. The results show that the acid concentration in the organic phase at equilibrium is generally higher than that necessary to convert all the amine into the corresponding salt. Third phase formation was only found for certain equilibrations with sulphuric acid. The solubility of water in the organic phase was found to be small. THE extraction o f inorganic acids by long chain aliphatic amines dissolved in an organic diluent has been described in several recent papers, following the w o r k o f SMITH a n d PaGz (1), a n d MooRe (2) has briefly described the possibility o f using the process for the e x t r a c t i o n o f m a n y different inorganic and organic acids. WILSON (3), in studies o f the extraction o f Pu from nitric acid solutions, gives values l b r the extraction o f H N O a into t r i - n - o c t y l a m i n e (TOA). BERTOCCI(4), and CARSWELL a n d LAWRENCE(5), give the equilibrium curves for nitric acid and b o t h tri-isononylamine a n d t r i - i s o - o c t y l a m i n e up to 10 M aqueous concentration, showing that the a m i n e solution extracts nitric acid in excess over that required for f o r m a t i o n o f the amine nitrate. The extraction o f sulphuric acid has been studied by several workers. ALI.EN¢6) published d a t a on equilibria between H~SO 4 a n d b o t h T O A a n d d i - n - d e c y l a m i n e ( D D A ) , p a r t i c u l a r l y as regards the ratio a m i n e sulphate/bisulphate as a function o f aqueous HeSO 4 concentration. McDOWELL a n d BAES(7) r e p o r t e d similar w o r k for D D A a n d H2SO 4. BOIRIE(8) published d a t a on the equilibrium between T O A a n d H2SO 4 up to 2 M aqueous acid and investigated the conditions for converting the a m i n e completely into bisulphate. BJZOT and TREMILLON(9) studied the extraction o f HCI into T O A at m o d e r a t e a q u e o u s acid concentrations, and interpreted the results in terms o f f o r m a t i o n o f the h y d r o c h l o r i d e in the organic phase. Finally, JENKINS a n d WAIN (1°) d e m o n s t r a t e d the conditions for third phase formation in equilibrations between acids a n d various amines. ~JJ E. L. SMITH anti J. E. PAGE, J. Soc. Chem. hulnstr)' ( L o n d o n ) , 67, 48, (1949). ('-'~ 1-'. L. MOORE, dnalyt. Chem. 29, 1660 (1957); O R N L - 1 3 1 4 (1952). /a, A. S. W~LSON, Proceedings of the Second International Col~'rence on the Peacefal Uaes of dtomi~ Ener~l,,
Geneva 1958. p. 544, United Nations (1958) (4~ U. BERrOCC~,AERE/R 2933 (1959). ~r,~ D. J. CARSWEt,t, and J. J. LAWRENCE,J. InorG Nucl. Chem. II, 69 (1959). 16~K. A. ALLEN,J. Phys. Chem. 60, 239, 943 (1956). (71 W. J. McDoWELLand C. F. BAES,J. Phys. Chem. 62, 777 (1958). (s~ C. BOmlE, Btdl. Soc. Chim. France 8-9, 980, 1088 (1958). I~ J. BIZOT, B. TREMILLON, Bidl. Soe. Chim. France 1, 122 (1959).
{~0}1. L. JENKINSand A. G. WMN, AERE/M-537 (1959). 323
324
U. BERTOCCI and G. ROLANDI
F r o m a n e x a m i n a t i o n o f the literature, however, it seems that i n f o r m a t i o n o n the extraction o f hydrochloric a n d sulphuric acids at high concentrations, as well as data c o n c e r n i n g hydrofluoric acid, are lacking. It was therefore decided to carry o u t a systematic investigation o f the equilibria between H N O a , HC1, H F a n d H2SO 4 u p to c o n c e n t r a t i o n s of 10 M a n d a tertiary amine, tri-n-octylamine, diluted with xylene. The solubility o f water in the organic phase was also studied. EXPERIMENTAL
Chemicals All experiments were carried out using TOA purchased from the firm FLUKA, with a nominal titre of 99.5 per cent. Titration of weighed samples with HCIO4 in glacial acetic acid indicated a molecular weight of 353 4- 2 compared with a theoretical value of 353. Solutions were prepared with C.P. xylene, a mixture of the three isomers. Before titration, the solutions were washed twice with deionized water in order to remove traces of low molecular weight amines. The aqueous acidic solutions were prepared from C.P. chemicals.
Methods of analysis The concentration of TOA in the solutions in xylene was determined by titration with HCIO4 in glacial acetic acid, with crystal violet as indicator. The accuracy is 4-0.0005 M for concentrations less than 0"05 M and 4-0'002 M for the other ones. The molarity of the aqueous acids is given with an accuracy of 4-0' 1 M for concentrations higher than 1 M, and +0.02 M for the other ones. Two methods were used to determine the acid content of the organic phase: (1) titration of the organic phase, diluted in acetone, with aqueous NaOH; (2) potentiometric titration of the organic phase, diluted in benzene/methanol, with potassium methoxide. The first method was used mainly for TOA solutions equilibrated with HF, the second for the others. Several titrations were carried out by both methods; the results were in good agreement, and also agreed with a third method, viz. back-extraction of the acid with excess alkali, and titration of the residual alkalinity in the aqueous phase. Potentiometric titrations were carried out using a glass electrode and S.C.E. reference electrode, both immersed in the organic solution. The glass electrode reached equilibrium rapidly and gave constant and reproducible values. The accuracy ranged from 4-0.001 M for 0"01 M TOA solutions, to 4-0'02 M for 0"5 M TOA solutions. The water content of the organic phase was determined by the Karl Fischer titration, using the standard dead-stop technique. The accuracy ranges from 4-0'005 M for the lowest TOA concentration to 4-0.02 M for the highest one.
Equilibrations TOA solutions in xylene (0.01, 0.02, 0'05, 0.1, 0.2 and 0.5 M) were prepared, and aqueous solutions of each acid from 0.5 to 10 M. Each TOA solution was equilibrated with aqueous solutions of the four acids at various concentrations, with an organic/aqueous ratio of 2:1. A mixing time of 5 min is sufficient to reach.equilibrium in nitric, hydrochloric and hydrofluoric acids. But for equilibrations with sulphuric acid a mixing time of 15 rain was chosen. After mixing, the phases were separated by centrifuging. Settling is fairly rapid when the pH of the aqueous phase is less than unity over the range of TOA concentrations used in this work. EXPERIMENTAL RESULTS (1) Stability of the amine A series of experiments was carried o u t to check the stability of the T O A solutions i n xylene towards chemical attack b y the acids. F o r this purpose, samples o f a 0.2 M
Equilibria between tri-n-octylamine and some mineral acids
325
T O A solution were contacted with each of the acids studied at concentrations of 2 M and 8 M. After 15 days' standing, the organic phase was washed with N a O H and the T O A titre checked; the results showed that the molarity of the amine was unchanged within the limits of experimental error.
(2)
Titrations in non-aqueous media
Fig. 1 shows titration curves for HNO3, HC1 and H2SO 4. The curves for HCI and H N O a are of the same shape, with two inflections due to the fact that both solutions - 500
- 400
- 500
O
200
>
tm
-
t
7 Tr;nt .1 i N ~ methox,de q,],% ant SC el~ctrode~ 5 c c cf 0 95 M TC,A ea,~i~ated v,,rh 5 q M H SC':
•
42C
+
3:c
?f F : I 9 7 M
TSA
10o
C
IC0
i /
2o0 sos ÷
/~..** :~--~/ [
j,/÷
¢ ~ ÷
-
[ Z
4 ~1.
Flo.
b
6
;
8
9
,2
,2
Of O.~.N K metho×ide benzene/methanol
l.--Potentiometric
titrations
'3
4
Ib
!6
in
of acids in TOA.
contain more acid than that corresponding to formation of the amine salt. The first inflexion corresponds to neutralization of excess acid and the second to neutralization of acid combined with the amine; the difference between the titres gives the molarity of the amine itself. The titration curve for the TOA solution equilibrated with H.~SO4 is more complex. showing three inflexion points ; in this case also the organic phase contains excess sulphuric acid above that corresponding to the stoichiometric T O A acid sulphate. By comparison with the other titration curves, it may be shown that the first inttexion corresponds to neutralization of excess H2SO4, and the second and third to neutralization of combined acid in two stages via the amine bisulphate.
326
U. BERTOCCIand G. ROLANDI
(3) Equilibrations The experimental results for the four acids are given in Tables 2, 3, 4 a n d 5, which show for each equilibration the m o l a r i t y of T O A , of acid, a n d of water in the organic phase, as well as the m o l a r i t y o f the acid in the aqueous phase. TABLE 1 . - - E X T R A C T I O N OF
(HNO3)aqtm (HNO3)orgcm (HzO)orgC.~l)
0"001 <0"001 0"006
4"4 0'003 --
HNOs 7"9 0"008 0"008
AND WATER INTO PURE XYLENE
8"7 0"013 --
8"7 0"022 0"015
9"7 0'032 --
10"3 0'043 --
TABLE 2.--EQUILIBRATIONOF TOA SOLUTIONSWITH AQUEOUSHNO3
(TOA)org (M)
(HNO3)aq (M)
(HNOz)org (M)
(HNO3)org (TOA) org
0'010 0"010 0'010 0"010 0"021 0"021 0"021 0"021 0"021 0"047 0-047 0-047 0.100 0.100 0-100 0.100 0.197 0.197 0.197 0.197 0-197 0.197 0-50 0"50 0.50 0.50 0.50 0.50
0'01 2"35 6'6 8'9 0"01 1'2 2-3 7-0 8-85 0'01 1'85 6'9 0'01 1"9 6"4 9"0 0-01 9-85 2"0
0'010 0'014 0'022 0"027 0"021 0'025 0'031 0.050 0'057 0'050 0"065 0'110 0-10 0.14 0"22 0"265 0"195 0'23 0'28 0"39 0"42 0'51 0'50 0.58 0'67 1'00 1-24 1"32
1'0 1-4 2-2 2"7 1"0 1"2 1'5 2'4 2'7 1.0 1-35 2.35 1'0 1"4 2.2 2-65 1-0 1-15 1-4 2-0 2-1 2-6 1-0 1.15 1-35 2.0 2.5 2.65
5"25 6"1 8"8 0"01 0'75 1"7 5"4 8'3 9"0
(H20)org (M)
(HNO3)org
0.007 0"01 0"01 0"01 0"01 0'02 0"02 0"03 0"04 0"05 0"04
4'4 5'0 5'7 5-0 6-5 5"5 5-0 4"7 5"5 5.3 4'9
0'06
4-7
0'08 0"09 0"08
5"2 5"6 6'2
0"10 0"15 0"20
6"7 6"7 6"2
(H20)org
* Water content not reported; the experimental values are of the same order of magnitude as the experimental error. As pure xylene extracts some H N O 3 a n d water, the values reported in the tables are corrected by subtracting the a m o u n t s given in Table 1, t a k i n g into a c c o u n t the m o l a r ratio o f xylene in the organic phase. N o corrections are applied for the extraction o f the other acids into xylene, since the a m o u n t s are negligible. T h e a m o u n t of water extracted by pure xylene contacted with HC1, H F a n d H2SO 4 is i n d e p e n d e n t o f the
327
Equilibria between tri-n-octylamine and some mineral acids TABLE 3.--EQUILIBRATION OF TOA SOLUTIONS WITH HCI (TOA)org (M)
(HCl)aq
0-0095 0.0095 0.0095 0.0095 0.0095 0-020 0.020 0.020 0.049 0.049 0.049 0.049 0.049 0,100 0'100 0.100 0.100 0.100 0.100 0.197 0.197 0-197 0.197
1-8 3.55 5-4 7.1 8.75
0.197
0.50 0.50 0.50 0.50 0.50 0.50
(M)
1.75
5.0 8-5 1.75
3.6 4.75 6.95 8.4 0.4 1-7 4.3 5.25 7.0 8.25 1.6 3.6 5.2 6'8 8.15 0.2 1.5 4.l 5.1 6.75 8.0
(HCl)org (M) 0"0095 0"0095 0"012 0"014 0-016 0"020 0-024 0'034 0"049 0"050 0"056 0'070 0"08 0-10 0"10 0-11 0.125 0-15 0'17 0"20 0"20 0-24 0"29 0"33 0"50 0.51 0.55 0.63 0-72 0.83
(HC1)org
(H~O)org
(TOA)or~ 1"0 1"0 1-25 1 "45
1'7 1'0 1'2 1.7 1"0 1"0 1"15 1 "45 1 "65 1'0 1'0 1"1 1 "25 1-5 l'7 1"0 1 "05 1 "25 1 '45 1'65
(HCI)org
(M)
(H20)org
0.015 0-015 0-01 0.015 0"0l 0-02 0"015 0'015 0"04 0"04 0-04 0-04
0.6 0-6 1.2 0.9 1.6 1.0 1.6 23 1.2 12 1.4 1.7
0'095 0"10 0"10 0"10 0"10 0"17 0.18 0-19 0"19 0'18
1.0 0.9 1.2 1.5 1.7 1.2 I.I 1.3 1.5 1.8
0.53 0.55 0.58 0.58
1.0 1.0 1.1 I-2
1"0 1 "0
1-I 1 "25 1 "45 1 '65
TABLE 4,--EQUILIBRATION OF TOA SOLUTIONS WITH HF
(TOA)org (M)
(HF)aa
0"010 0"010 0"010 0"010 0'010 0.100 0.100 0.100 0.100 0-100 0"51 0-51 0.51 0.51 0"51
0"2 0'95 4"0 6"l 9"75 0'2 0"85 3"9 5"58 9'6 0"1 0"5 3.5 5.5 8.9
(M)
(H F)org
(H F)org
(M)
(TOA)org
0"007 0"021 0'027 0"029 0'033 0"06 0"21 0"28 0"30 0"33 0'25 1"0 1.4 1.5 1.7
0"7 2"I 2"7 2"9 3"3 0"6 2'1 2"8 3"0 3"3 0"5 1 "95 2"75 2"95 3"3
i t !
(H20)org (M)
(HF)o~g (H20)o,.g
~0 ~0 0.005
6.6
0.035 0.03 0.035
6"0 9"3 8"6
0.08 0.06
12 23
0.06
28
328
U. BBRTOCCIand G. ROLANDI
aqueous acid concentration, and the corrections applied were 0.012, 0.013 and 0.015 respectively. F r o m the values reported, the ratios (acid)org/(TOA)org and (acid)org/(H20)org were calculated, and for each acid a curve was drawn showing the ratio (acid)org / (TOA)org versus the aqueous acid molarity (Fig. 2, 3, 4 a n d 5). TABLE 5.--EQUILIBRATION OF T O A SOLUTIONS WITH H~SO4
(TOA)org
(H2SO4)~q
(H2SO4)org
(H~SO4)org
(M)
(M)
(M)
(TOA)org
0'010 0-010 0.010 0-010 0.010 0.010 0.010 0.010 0.019 0.019 0.019 0.019 0.019 0.049 0"049 0-049 0.049 0"049 0"100 0.100 0"100 0"100 0'195 0'195 0-195 0-195 0.50 0-50 0-50 0-50 0.50
0"2 0"5 0'95 1"9 3.75 6"0 8-4 10.0 0.5 0.95 3.65 7.5 8.5 0.5
0'007 0"008 0"009 0"0095 0"010 0"011 0"014 0"015 0"015 0"018 0.019 0"025 0'027 0.040 0"045 0'050 0"055 0"075 0.07 0.095 0.10 0"145 0'16 0'19 0'22 0"24 0.30 0'45 0"50 0"57 0'63
0'7 0'8 0"9 0'95 1'0 1"1 1'4 1"5 0'8 0"95 1.0 1.3 1.4 0.8 1'0 1"0 1"1
1"85
-
3"7 6"0 I0.0 0.15 0'9 3.6 9'0 0.45 1.8 5"9 6"85 0.05 0-75 3-5 5-85 6-7
(H20)org (M)
(H2SO4)org
(H20)org
m
0.007
1.1
0"008 0.007 0.006
1.2 1.4 1.8
0'011 0.012 0.011
1.4 + 1.6 2.3
0"03 0"025
1.3 1.8
0'025
2.2
0-055 0'05 0'06
1.7 2.0 2.4
0"13 0"13
1.5 1-8 1-8
0-32 0.32 0.33
1-6 1.8 1-9
1.55
0.7 0-95 1"0 1"45 0"8 0'95 1 '15 1 "25 0'6 0'9 1"0 1.15 1.25
0.12
!
On equilibrating T O A solutions with sulphuric acid, two organic phases are formed at certain T O A and aqueous HzSO~ concentrations. In the presence o f a third phase, the system becomes univariant at constant temperature and pressure, and for each aqueous H2SO 4 concentration, the composition o f the two organic solutions is fixed. The width o f the region within which T O A solutions in equilibrium with aqueous H2SO 4 p r o d u c e two phases can be deduced approximately f r o m the compositions o f the conjugate solutions shown in Table 6, which also gives some results showing h o w the region o f third phase formation varies with the temperature. As the separation o f the two conjugate phases is rather slow, particularly when their compositions differ
Equilibria between tri-n-octylamine and some mineral acids
329
..f¢'* ° 2c,
A. ( : , ~ 3 &
~
~
~'~" II
d k5
•
TOA 0-5M
~
T0A
-~
TOA 0 . 1 N
0"197 M
TOA O'047 N e,
b0
I
I
E
2
3
4
5
•
TOA 0.021M
(:
TOA 0'01 M
I
I
1
I
6
7
8
9
I0
M HNO3a q. . . . . Flo. z.--t~atm
[HNO~]0 ~ vs. a q u e o u s H N O 3 c o n c e n t r a t i o n
at e q u i l i b r i u m .
2.0
1.5
c) < O
•*
c~O,,, L • ' " g ~
o
1.0
e-&
,
c} x
0"5
I
-5M
•
'
'
e
TOA 0"197M
41,
TOA
,5.
TOA 0 . 0 4 9 M
•
TOA 0"1
o
T0A 0'0095 M
[
0-020 M
M
P
M HCL aq. Fro. 3.--Ratio
TABLE
~[ H C l ] ° vs. a q u e o u s H C I c o n c e n t r a t i o n
at equilibrium.
6 . - - C O M P O S I T I O N OF CONJUGATE ORGANIC PHASES FORMED ON EQUILIBRATION
or TOA SOLUTIONSWITH H2SO4 Light phase Temp
(M)
"C)
20
9.0
i 27
Heavy phase
(I-I2SO,)~q
J
45 I
(TOA)org
(H~SOl)org
(M)
(M)
(TOA)org (H2OS4)org
0.11
0.16
1.45
100
0.035
0.055
1.55
9"0
0-20
10.0
0.10
--
(TOA)or.
056 of_6
((H2SO~)org (TOA)org (M) ~ g
o.s, 1.1
i--;4; 1.50
--
- 0 2 ...... _C
_
:
330
U. BERTOCCI and G. ROLANDI
2.0
1.5
p...-.--&-
o o I-
I-0
o
6(]3 £
0.5
i
4
5 M
e
TOA
0.01 M
•
TOA
0,019 M
t
TOA
0.1 M
,~
TOA
0.195 M
•
TOA
0.049
o
TOA
0.50
6
7
M M
8
9
H 2 S 0 4 0 q.
FIG. e.--t~att o [H2S04]o ~ vs. aqueous HzSO4 concentration at equilibrium. .
.
.
.
5.5
3.0
2.5
/-
0 k-
2.0
I
1.5
•
TOA OOI M TOA 0 1 M
@
TOA 0 " 5 M
1.0
0"5 4 ,
0
I
i
I
2
4
5 M
FIG.
5.--Ratio
[HF]o [T-"~'~o
Vs.
7
8
9
I0
HFoq
aqueous HF concentration at equilibrium.
Equilibria between tri-n-octylamine and some mineral acids
331
only slightly, the absence of third phase formation, even after three days' standing, cannot be taken as proof of the thermodynamic stability of the system. Table 7 gives the results of some specific gravity determinations of the organic phases after equilibration with H2SO4; the measurements were carried out at 25 ~ I°C. T~Bt.E 7,--DENSITY OF ORGANIC PHASES EQUILIBRATED WITH H,,SO4
(TOA)org (M)
(HaSO4)org (M)
D 25 (g/cmu)
0.50 0.50 0.50 0.50 0.195 0.195 0.195 0.195 0.100 0.100 0.100 0.100 0.11 0.56 0-035 0.76
0 0.45 0.50 0.57 0 0.19 0.22 0.24 0 0.07 o.[0 0.145 0.16 0.81 0.055 [.15
0-855 0.880 0.884 0.887 0.860 0.875 0.878 0.880 0"865 0-869 0-872 0"975 0-869 0-835 0.865 0,.873
DISCUSSION The results of the equilibrations show that all the mineral acids investigated tend to dissolve in the organic phase formed by the amine salt dissolved in xylene. This tendency is the greatest for nitric acid, the ratio (HNO3)orJ(YOA)org being more than one at relatively low aqueous HNO3 concentrations. This ratio increases linearly with the aqueous H N O 3 concentration, and is independent of the organic amine concentration. It is therefore impossible to formulate a particular complex between amine and acid having a definite stoichiometric ratio in the organic phase. The ratio of acid to amine is similar for different amines having analogous formulae, as inferred by comparison with other published results. The constancy of the ratio between the acid and amine concentrations in the organic phase at constant aqueous acid concentration is a general feature for all the acids studied, but the quantitative behaviour of the different acids varies, e.g. HC1 begins to dissolve in the amine hydrochloride only at aqueous concentrations higher than 3"5 M. The curve for H2SO 4 is more complex: below aqueous concentrations about 1 M, the organic phase contains both sulphate and bisulphate in various proportions, whereas above that value only bisulphate is present. The curve is also valid above the point where two organic phases appear, since in each of them the ratio is the same. In the case of hydrofluoric acid the curve differs from those for the other acids, the solubility of the acid in the organic phase is higher than for the other acids, probably because of the tendency of H F to polymerize, and rises very rapidly up to aqueous concentrations about ] M, and then more slowly. In equilibrium with a 0-2 M aqueous
332
U. BERTOCCIand G. ROLANDI
solution the amine is not fully converted into salt, since H F is a weaker acid than the others. With xylene as diluent, third phase formation occurs only with sulphuric acid; even in this case the extent of the zone in which certain TOA solutions cannot exist as a single phase is rather small, and it practically vanishes on raising the temperature to a moderate extent. This confirms the superiority of aromatic diluents over the aliphatic ones as far as third phase formation is concerned. It is difficult to draw quantitative deductions from the water content of the organic phases, because of the small amounts extracted, but from the values of the ratio of acid to water in the organic phase the following conclusions may be drawn. For H N O 3, a definite amount of water is associated with the acid, about one mole of water for every five or six moles of HNOa; for HC1, it appears that each mole of amine hydrochloride is associated with one mole of water, while the excess HC1 is anhydrous. In the case of H~SOa, it seems that one mole of water is associated with two moles of amine bisulphate, the excess H2SO 4 being anhydrous. For HF, the only conclusion which can be drawn is that water is only slightly soluble in the organic phase.