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The distribution of barium cations between aqueous and nitrobenzene phases in the presence of dicarbolide anions
J. inorg, nucL Chem. Vol. 42, pp. 1481-1486 Pergamon Press Ltd., 1960.
Printed in Great Britain
THE DISTRIBUTION OF BARIUM CATIONS BETWEEN AQUEOUS AND NITROBENZENE PHASES IN THE PRESENCE OF DICARBOLIDE ANIONS I. PODZIMEK,? M. KYR~ and J. RA1S Nuclear Research Institute, 250 68 l~eL Czechoslovakia
(First received 9 July 1979; received/or publication 17 January 1980) Abstract--The extraction of Ba2+from HNO3 solutionsinto nitrobenzenecontainingH+ [~r-(3)-I,2-BgC2H,]2Co was studied. The data obtained obey the exchange reaction Ba2. + 2Ho+,8= Baor=2++ 2H+ with"log KB~2. = 0.67 to 0.94 for CaNo3-< 1 mole/I. The hydration of Ba2+ ion in the nitrobenzene phase was determined experimentally.The average hydration number is h~2÷= 11.5- 1. The individual extraction constants for the alkali and alkaline earth cations correlate well with their respective hydration numbers. INTRODUCTION The solvent extraction of alkali metals into nitrobenzene in the presence of the voluminous hydrophobic anion [Ir-(3)1,2-B9C2Ht j]2Co- (denoted as B-) was first described [1] in 1976. Later the usefulness of this system was Claimed for the isolation of ~37Csfrom radioactive wastes originating in the reprocessing of spent nuclear fuel[2]. This suggestion was based also on the high radiation stability of the dicarbolide [3]. The potential usefulness of this reagent for radiochemical analysis was also demonstrated (t37Cs isolation[4] and the radioanalytical determination of strontiur~ using ~Sr [5]). No systematic data have been published on the distribution of barium in the above system, although nonradioactive barium occurs in fission product solutions and may accompany the 9°Sr in the separation.
Accordingly, the solvent extraction step Ba
2+
+
~
2+
+
+ 2Hore~ Bao,= + 2H (equilibrium constant KBa/2H) (1)
log De,'
T,///J
4
0
i///
\
EXPERIMENTAL The experimental technique is essentially identical to that used previously[l]. The reagent was used as a nitrobenzene solution of the corresponding acid (HB). The water content in some extracts has been determined(-+5%) by Karl Fischer method usingthe dead stop indication of the end point, For the determination of the concentration of hydrogen ions in the presence of Ba, both in the aqueous and in the organic phase, automatic thermometric titration by pyridine[6] has been used (±0.5%). For the determination of the distribution ratios (20°C)of Ba (DB.) labelling with t33Ba(NO3)2 has been utilized, the 3, radioactivity being measured with a well-type NaI(Ti) detector connected to conventionalcountingequipment.The radiochemical purity of ~3Ba has been checked 7-spectrometrically.
-2
t
0
-2
log CON03
~
-I
0 log CH08
Fig. I. Dependence of Ba distribution c ~No~initial concentration
of HNO3in the aqueous phase; c ~tn--initiatconcentration of HB in the organic phase; Curve 1-c~a=0.305M; Broken linestheoretical slopes -2 and +2. 2--0.203M; 3--4).105M,4--0.046M, 5--C~No3--0.1M; 6--0.2M; 7--0.3M; 8---0.5M; 9--I.0M; 10-2.0M; 11--3.0M. Tracer amounts of Ba.
RESULTS AND DISCUSSION The dependence of Dea values for tracer amounts of Ba upon the concentration of HNO3 in the aqueous phase and upon the concentration of the reagent in the organic phase is shown in Fig. 1. It can be seen that log DBa values depend linearly upon the initial acidity of the aqueous phase c ~No3 (at constant initial re~gent concentration in the organic phase (c~e)), the slope being ~ - 2 . For constant C~No3 values in the aqueous phase log Dsa values increase linearly with c~ the slope being ~ + 2.
?The paper represents a part of the Thesis (Candidate of Sciences) of I.P.
N
-I
IogDBa
0 0.0 o
t -5
I -3
-~
~c o
Fig. 2. Influence of Ba concentration on its distribution.Curve I--0.3M HB; 2---0.2M HB; 3---0.1M HB; 4---0.04HB; 5--03M HNO3; c~--initial concentration of Ba(NO3h (molll) in the aqueous phase.
1481
1482
I. PODZIMEK et al.
can be assumed as the main equilibrium controlling the distribution of Ba. This is analogous to the reaction Cs + + H2,g~Cso+,s + H investigated previously [3] using the same anion and solvent. In order to corroborate the validity of eqn (1) two criteria were used: (i) testing the essential independence of the equilibrium constant of reaction (1) upon c ~INO3,C~a and c ~, and (ii) testing the equivalence of the exchange of two H + ions for one Ba2+. Using the first criterion the parameters of the solvent extraction experiments were systematically varied within the following ranges:
The value of Ka~2a obtained can be compared to other exchange constants in water-nitrobenzene systems owing to the fact, that this type of constant does not depend on the type of anion present so that from the values of constants of the type KMelMeand KMe'/Me, the value of KMe/Me"c a n easily be deduced. Generally, an algebraic combination of the log K values is feasible. (KMc/M¢'is the constant for exchanging the cation Me for Me'.) The following relationships can be used: log
Kaa/2. =
log Kca/2Li +
log Ku./c,
+ 2 log Kcs/H + 2 log KLi/Cs; HNO30.1-3M; c~a 0.04-0.5M; c~t~oml0-6-10-1M. For the sake of brevity only selected data are presented. Typical examples of the effect of HNOa and HB concentrations are given in Fig. l, those of Ba(NO3)2 concentration in Fig. 2. The values of Ka~/2Hfor tracer concentrations of Ba are calculated according to the formula
and log Kaa/2. = log KSr/2H + log Kua/sr.
The values of Kaa/2H calculated by this indirect method together with K values necessary for the calculation are summarized in Table 1. It can be seen that the value found experimentally is in a good accord with previous data (allowing for the various ion strength values used). Ke~/2~= [Ba2+]ora'[H+]2/[Ba2+].[H+]2rs Another way of verifying the mechanism expressed by o 2 o2 eqn (1) is to check the equivalence of the amounts of Ba2÷ "~ DB,'(c m~o3)/c .a. (3) and H + transferred between the two phases. The results of The use of eqn (3) implies the following assumptions: (i) All such experiments are summarized in Table 2. Analysis of the data given in Table 2 shows that on barium in the system is in the form of free Ba2÷ cations (see average one extracted Ba2+ ion replaces ~ 2.16 H + ions in equilibrium (1)); (ii) no HNO3 extraction occurs (this has been checked); (iii) The transfer of HB into the aqueous the organic phase and transfers ~ 1.87H+ ions into the phase is negligible (this can be expected from the value of aqueous phase. The average of the two (2.015) is very close the constant KH+B-= [H+]o,g[B-]o,J[H+][B-] ~ 103"2, to the theoretical value 2.000. The hydration of Ba 2÷ ions in the nitrobenzene phase found previously[l]; (iv) full dissociation of HB in the seems to be of interest since a correlation of hydration and organic phase (independent evidence given in paper[l]. selectivity of extraction in this type of system has been For higher Ba concentrations eqn (3) has to be modified established [10, 13]. (for examples of experimental results see Fig, 2). Under Published data are discrepant, the hydration number 4.0 this condition the following relationships are used (for was found in a paper[10] using relatively low concentla = t~orB): trations of barium dipicrylaminate, whereas in a recent investigation a much higher value (10.5) has been [H+]or, = c ~u - 2[BaZ+]o~,; reported[14] for a similar system. [H+] = c ~No3+ 2[BaZ+]o~; In this work data on the hydration of both H ÷ and Ba2÷ ions have been obtained so that the total change in water [Bae+]or,,= c ~atNo3~Daa/(1 + Da,). concentration in the organic phase due to reaction (1) might Consequently, knowing the three initial concentrations and be ascertained. Figure 3 shows the increase in the total concentration of the distribution of Ba, the Kaa/2. value can be found. water in the organic phase as a function of the initial An analysis of the Ka~2H values obtained yielded the concentration of HB (no Ba was present in the system) and following conclusions. Most log Ka,a2, values obtained for aqueous acidity ~ 1 M HNO3 fall between the values of the concentration of HNO3 in the aqueous phase. It can be seen that the concentration of water is nearly 0,67 and 0.94 which seems to be a rather narrow interval for linearly dependent upon the concentration of HB but the the wide concentration ranges used. Analysis of the bulk of slope increases slightly with higher HB concentrations. the data showed no significant dependence of Ka~/ea This form of [H20~or, vs [ion]or, dependence has been values upon c~, nor on c~n concentrations. The Kn~/z, found recently by Skarda et al.[15] for cations and by values diminish slowly with increase in HNO3 concenDiamond et al.[16] for the hydration of halide anions in tration in the aqueous phase. This relation could be nitrobenzene. If the hydration number is defined conrepresented for the above concentration range: ventionally as log Kaa/z, = 0.92-0.23c ~NO3.
h.÷ = ([H20]o~- [H20]o~,~,)/[H÷]o**,
Similar changes with aqueous acidity were found previously[3, 7] in the case of Kc,m in a similar extraction system. The most probable explanation of this phenomenon is through the change in activity coefficients in the aqueous phase with the ionic strength, as in the case of Kc,/,[3]. On the whole, the variations in the values of Ke~zn are sufficiently small that the system can be assumed to obey eqn (1) reasonably well.
where [H20]ors.o is the value of [H20]ors in the absence of any reagent in nitrobenzene, then values of hm within the range 5.2-7.6 are found. For one c ~No3concentration the h values, of course, increase with higher c ~B concentrations. The dependence of the [H20]org value upon the total concentration of Ba in the system is given in Fig. 4. The right-hand end of the curves corresponds to extracts practically saturated with Ba2+.
1483
The distribution of barium cations Table 1. Ion exchange solvent extraction constants nitrobenzene-water, ~ 20~C, data for lowest ionic strength available tTl~ of oeastsat
(KW,/W,,)
sa£~n l~'es~at
4-, ~
1o~
.y.t----
r@fox'@~o@
~W,/W,"
~L~veot / 4 g a t ~
K /2 , ,
a],
~r/2X
d~LoxrboXX4e
O. 70
9
~i/ca
dX~±oryZ--'~-to
o. ~
1o
F~Sr
d£p£ory~te
O. 27
10
KC~/I A
d£p£oryJJm/aat@
It. 0
11, ].2
10, ~
l~01T~,,- 01~,ala~
~ ~
7, z,,
K
dioarboZ./.de
(oOwJm,r£so~)
various
O. 5~t
various
O, 97
e,~/2e
8
0.92-0.23 w
tl~Ls work oltZod. £rcm
osJ.ed, t r ~ s
xsr/~a o w = omm 3 ( x ~ t ± a x omen. o t ~ o 3)
Table 2. Verification of the reaction Ba2+ + 2 H ~ ~ B a ~ + 2H+. Values indicate changes in concentrations (mol/1) of H + in the respective phase, A pertains to experimentally found values, theor.-retates to concentrations calculated from the above exchange reaction knowin8 the distribution of Ba. c ° denotes initial concentrations of the respective components
O•o--• 3
o,3M Imo 3 o
o~--~
o.3M aa
o.xM ~
o , o ~ at
o.31/ aa o . ~
~
0,3Z4 zm
theor. ~ aq - £ or6
0.171 0.171 o.16o
0.107 0.103 o.lop
0.069 O.063 0.055
0.1~1 0.1~2 o.125
0.075 O.073 o.o61
theor. ~ aq - ~org
O.116 O.105 0.IIi
O.O71 0.079 0.082
O.O~1 O.O47 0.065
O.IO8
O.2H HI]
0.~18
O.O/t7 0.029 0.063
@,114
ths~. mq
-Aorg
0.057 0.051
o.o66
0.032 0.023
0.02~ 0.020
0.040 0.034
0.017 0.015
theor. ~ aq - Aorg
0.021 0.015 0.016
-
-
O.O/tM HB
o.o3~
The main, rather spectacular, conclusion drawn from the Figure is the very weak dependence of the [I-120]o~ value upon the [l'I+]/[Ba2+] ratio in the extract. There is an + 2+ indication, however, that replacing 2H ~ ions by one Ba.~ leads to the water content of the extract growin8 to a certain extent. We would therefore expect the value of he.2+ to be slightly higher than double the value of h.+, JINCVol.42,No. [0--G
0 . . ~ Imo 3
0.026
0.O42
0.025
o.SM O.046 0.040 o.o43
0.010 0.015
O.O18
0.0135 0.013
Using an analosous relation to that esed for kw, the definition of he.'+ can be given as hn.2÷ = [Ba2÷]or~t([H20]o~ --'[H20]ors.o-hlsl~[I-I+]o~). This expression can be jugif~d only if both [H20].~ and h~+ values are independent of the [Ba2+]o~ concentration.
1484
I. PODZIMEK et aL
[H20]ot 4
3
2-
0
0.2
0.4
COHB
Fig. 3. Coextractionof water into nitrobenzene with HB.
[HzO]*~L
-0
~
~
_
_,
same charge. It was found that the higher the difference between hM~ and hM¢' the higher the value log KM~/M¢'. The present results, however, pertain to the exchange of a univalent and bivalent cation (H+-Ba2+). A simple analogy to the above method of Correlation for ions of different charges is hardly possible. Nevertheless a somewhat different approach to the above tentative correlation appears to be feasible. Since the h values pertain to one cation only, solvent extraction constants relating to single ions must be taken for correlation. The concept of these "individual extraction constants" and methods of their calculation have been described previously [18]. In the present case, the basis for the calculation is the relationship log KBa/2H = log K a y - 2 log log KH corresponding to the equation
K
_ [Ba2+]o,,./[H+]o,s~ z
(4)
which is a rearranged form of eqn (3). Knowing that l o g K a = - 5 . 7 (see [18]), inserting log KBa/2x = 0.9 we obtain log Kaa = 0.9-I 1.4 = 1.5 -10.5-+0.1. Figure 5 shows the correlation of h and log Km,,t~ values for several metals. The KB,, hn and hBa ,.o, 0 ¢ 3 values used stem from this work, other values were found in the literature [14, 18-20]. If for any metal two h values 0 0 4 ~ have been reported both points are shown in the Figure. It 0.5 k ~ 5 can be seen that the present results corroborate the I I I I I hypothesis that hydration of cations in the polar organic -5 -3 -I log 6- 0 phase in systems of this type is closely related to the Fig. 4. Influenceof Ba concentration on the coextractionof H~O selectivity of extraction. Highly hydrated cations are less into nitrobenzene; Curve 1--0.3M HB; 2----0.2MHB; 3--0.15M prone to enter the nitrobenzene phase. A comparison of the HB; 4--4).1MHB; 5--0.04M HB; open symbols0.3M HNO3;full values ha, with hydration numbers for aqueous solutions is symbols 0.SM HNO3. summarized in Table 3. From Table 3, it follows that the h.a2+ value in nitrobenzene lies well within the range of It is evident that since the difference between [H20]org and values for aqueous solutions, but the majority of aqueous ha÷'[H+]org is small for small Ba2+/H+ ratios in the extract hydration number values are lower than hn.~+. the calculation of ha,2÷ depends largely on the right hand This calls for comment in relation to the findings of branches of the curves in Fig. 4. Some authors [17] make a other authors on hydration of ions in polar organic correction for the influence of the growing content of solvents. The fact that the hn.2+ value is higher than the hydration water by assuming that the physical solubility of coordination number of barium in aqueous solutions [8] water relates only to that part of the organic phase indicates that the hypothesis that 8 = n + ha, suggested corresponding to the volume fraction of nitrobenzene in by Shivrin [32] for ion exchange extraction systems canthe extract. In this work no similar correction was applied because even for high concentrations of hydrated ions it becomes insignificant, the physical solubility of water in nitrobenzene (~0.17 M) being rather small compared to cs the total H20 concentration in the extract (up to 4 M). For these reasons, the physical meaning and reliability of "cx ha,2÷ values obtained is limited. The fact that the two -5' • oH curves denoted as I in Fig. 4 and also those denoted as 2 in O(D • Li the same Figure run almost parallel to each other seems to No ~5~x" indicate that hBa2+values depend on the aqueous acidity in a very similar way as those for h.÷. On the other hand the % accuracy of data obtained is not high enough to permit the -I0 " calculation of the ha, e÷ dependence upon the composition of the organic phase. Therefore it was considered reasonj ®'1~, ~co I able to calculate an average hB~2+value corresponding to 5 a n aqueous acidity 0.3 HNO3 only. The value found (hBF+ = 11.5 + 1) was in reasonable agreement with the Fig. 5. Correlationof individual cation extraction constants from value 10.5 reported for other hydrophobic anions and Ba2÷ water into nitrobenzenewith their hydrationnumbersin nitrobenzene. Full symbols--resultsfrom this work. If for one metal two by recent workers [14]. different data are available in the literature, one of the symbolsis An overall correlation between selectivity and crossed. For most cations values of K and h relatingto # --~0are difference in hydration of cations (Me, Me') in the nitro- used. For Ba the average values are shown since no significant benzenephase was observed[10, 13] for cations of the dependence of K upon # could be found.
Y
•R•b
25
10o7
28
25ol
22ol
15.0
value)
-
9,5
-
25
-
-
8
11
25
8
0.3
0,25M
005M
O,I-I.OM
IM
O.I-I,SM
-
~OM
001-I.8M
o£H20 seine. eolli~ative l~op®rtiee 0£ H~O-solns. eleo~rolytic ion tz~ua~er
aotivity ooef£o oolli~tive p~operties
pz'oportioe
souud v e l o o i t T ! oom3~reesibility eleot~olTtio trLne£er of ion 8otivity ooefT. eleotx'01ytio tTansfero£ ion dieloctrio
activity ooefTo ~ e t i c p~operties aotivit7 eoef£. d.tffusion meastrvemente
0.TM 25
0£
method o£ determination
oonono o£ electrolyte
3
(%)
5.7 707
temperature
hydration
number
Table 3. Examples of hydration numbers of Ba2+ in aqueous solutions
0
Br'~
Ci'i
CI'j
5
31
30
3O
29
28 NaC1 I-I
27
29
26
01-~
0
7
1~+t
23,25 25
29
Cl-j
0
22
21
raforonoo
Cs+j 0 Be2+ t .
ssumedhydz~tlonnutaber
referenoe ion~ad its
f3
c
o"
o
=
1486
I. PODZIMEK et al.
not be extended to the present case [8]. Here n denotes the number of the organic solvent molecules in the first solvation shell of the cation in the organic phase, n~--the number of H20 molecules in the first solvation shell (for the present case no = hBa~+) and 8 is the usual coordination number of the cation Ba 2+. This difference stems probably from the circumstance that Shivfin only considered solvents more basic than water and able to replace water in the hydration shells. Nitrobenzene as a solvent of insignificant basicity evidently shows quite different behaviour. Several other ideas explaining relatively high hydration numbers of ions in nitrobenzene have been published. Gere [17] found that the hydration number of bis(l,0-phenanthroline)Cu(I) + in nitrobenzene is 14+4 and offers the following explanation. The hydration is due to a certain number of primary watermetal ion bonds accompanied by several secondary water-water polymer bonds leading away from the primary site and limited by the extent of shielding afforded by the coordinated ligand atoms. Kenjo and Diamond[16] found the following average hydration numbers in nitrobenzene for CI-, Br- and I-, resp.: 3.3, 1.8, 1.0. The values increased with higher concentration of the anions in the nitrobenzene phase similarly to our results in Fig. 3. The authors believe that this effect has its origin in the ability of the anions to fit into and even promote a hydrogen-bonded network with H20. Tarui[33] studied the hydration of perchlorate anion accompanied by tris(1,10-phenanthroline)Fe(II) z+ in nitrobenzene and came to the conclusion that hydration in nitrobenzene may be higher than than in water. The author ascribes this phenomenon to a different mechanism of hydration in the two media. Owing to the fact that the distances between H20 molecules in nitrobenzene are much larger than those in aqueous solution, the hydration in nitrobenzene depends primarily upon the total energy of the interaction of cation with water molecules [33]. These ideas might also be relevant to the relatively high values of hB,2+ in nitrobenzene found in this work.
REFERENCES
1. J. Rais, P. Seluck~' and M. KyrL 7. Inorg. Nucl. Chem. 38, 1376 (1976). 2. M. KyrL L. Kadlecov~i,J. Rais, P. S¢luck~,,J. Tepid', B. Ya. Galkin, R. I. Ljubcev, S. I. Rovnyj, V. N. Romanovskij, N. S. Tichonov and D. N. Shishkin, Issledovania v oblasti pererabotki oblachonnovo topliva, Vol. 1., Proceedings of the IV. Sympoziam, held 28 March 1977 in Karlovy Vary, Cze-
choslovakia, p. 246, Paper KV 77/T5., Czechoslovak Atomic Energy Commission, Prague (1977). 3. F. Mac~l~ek, U Mfitel and M. KyrL Radiochem. Radioanal. Lett. 35, 247 (1978). 4. V. Koprda and V. ~asn,'tr, 9th Radiochemical Conference, Piegtany, Czechlslovakia, 1i September, p. 47; in Abstract of Papers (1978), Czechoslovak Chemical Society. 5. M. ~tefek, M. Kyr/~ and J. Rais, Zh. Anal. Khim. 31, 1364 (1976). 6. M. Z;imek, Coll. Czech. Chem. Commun. 4l, 3754 (1976); Czechosl. Patent appls. PV 2573-77, PV 2919-77. 7. J. Rais and M. KyrL Coll. Czech. Chem. Comman. 34, 949 (1969). 8. M. KyrL V. Michalukov~i,J. Bene~ and L. Kadlecov~i, Coll. Czech. Chem. Commun. 34, 1709(1969). 9. P. Vafiura, J. Rais, P. Seluck~, and M. KyrL Coll. Czech. Chem. Commun. 44, 157 (1979). 10. M. Pivofikov~iand M. KyrL 7. Inorg. Nucl. Chem. 31, 175 (1969). 1!. J. Rais, M. Kyr~ and M. Pivofikov:i,7. lnorg. Nacl. Chem. 30, 611 (1968). 12. I. Rais and M. Kyd in Solvent Extraction Chemistry Proceedings of a Conference, p. 595. North Holland, Amsterdam (196~). 13. J. Rais, M. Benegovfi-Pacltovfi,P. Seluck~' and M. Myrg, 7. lnorg. Nucl. Chem. 35, 633 (1973). 14. T. Iwachido, M. Minami,A. Sadakave and K. Toei, Chem. Left. (Tokyo) 1511, No. 12 (1977); fiNIS 9, RN 391015, p. 4664 (1978)). 15. V. ~karda, L Rais and M. Kyrs, 7. Inorg. Nacl. Chem. 41, 1443 (1979). 16. T. Kenyo and R. M. Diamond, 7. lnorg. Nucl. Chem. 36, 183 (1974). 17. D. R. Gere, DissertationAbstracts 26, 54 (1965). 18. J. Rais, Coll.Czech. Chem. Comm. 36, 3253 (1971). 19. M. Kawasaki, K. Toei and T. lwachido, Chem. Lett.Japan 417
(1972). 20. S. Motamizu,K. Tosi and T. Iwachido, Ball. Chem. Soc. Japan 42, 1006 (1969). 21. E. Glueckauf, Trans. Faraday Soc. 51, 12350959). 22. T. J. Swift and W. G. Sayre, J. Chem. Phys. 44, 3567 (1966). 23. K. Tamura and T. Sasaki, Bull. Chem. Soc. Japan 36, 975 (1963). 24. E. N. Gapon, Z. Anorg. AUg. Chem. 168, 125 (1927). 25. J. Padova, Bull.Res. Counc. Tsr.Sect.A Chem. 10, 63 (1961). 26. L. H. Collet, C.r.Acad. Sci.239, 266 (1954). 27. J. Baborovsk~, and O. Viktorin,Coll.Czech. Chem. Commun. 5, 518 (1933). 28. J. B. Hasted, D. M. Ritson and C. H. Collie,7.Chem. Phys. 16, I
(1948). 29. R. H. Stokes and R. A. Robinson, 7. Am. Chem. Soc. 70, 1870 (1948). 30. E Burion and E. Rouger, C.r. Acad. Sci. 196, llll (1933). 31. R. Haase, Z. Elektrochem. 62, 279 (1958). 32. G. N. Shivrinand B. N. Laskorin Dokl. Akad. Nauk SSSR 194, 877 (1970). 33. T. Tarui, J. Inorg. Nacl. Chem. 37, 1213 (1975).
Printed in Great Britain
THE DISTRIBUTION OF BARIUM CATIONS BETWEEN AQUEOUS AND NITROBENZENE PHASES IN THE PRESENCE OF DICARBOLIDE ANIONS I. PODZIMEK,? M. KYR~ and J. RA1S Nuclear Research Institute, 250 68 l~eL Czechoslovakia
(First received 9 July 1979; received/or publication 17 January 1980) Abstract--The extraction of Ba2+from HNO3 solutionsinto nitrobenzenecontainingH+ [~r-(3)-I,2-BgC2H,]2Co was studied. The data obtained obey the exchange reaction Ba2. + 2Ho+,8= Baor=2++ 2H+ with"log KB~2. = 0.67 to 0.94 for CaNo3-< 1 mole/I. The hydration of Ba2+ ion in the nitrobenzene phase was determined experimentally.The average hydration number is h~2÷= 11.5- 1. The individual extraction constants for the alkali and alkaline earth cations correlate well with their respective hydration numbers. INTRODUCTION The solvent extraction of alkali metals into nitrobenzene in the presence of the voluminous hydrophobic anion [Ir-(3)1,2-B9C2Ht j]2Co- (denoted as B-) was first described [1] in 1976. Later the usefulness of this system was Claimed for the isolation of ~37Csfrom radioactive wastes originating in the reprocessing of spent nuclear fuel[2]. This suggestion was based also on the high radiation stability of the dicarbolide [3]. The potential usefulness of this reagent for radiochemical analysis was also demonstrated (t37Cs isolation[4] and the radioanalytical determination of strontiur~ using ~Sr [5]). No systematic data have been published on the distribution of barium in the above system, although nonradioactive barium occurs in fission product solutions and may accompany the 9°Sr in the separation.
Accordingly, the solvent extraction step Ba
2+
+
~
2+
+
+ 2Hore~ Bao,= + 2H (equilibrium constant KBa/2H) (1)
log De,'
T,///J
4
0
i///
\
EXPERIMENTAL The experimental technique is essentially identical to that used previously[l]. The reagent was used as a nitrobenzene solution of the corresponding acid (HB). The water content in some extracts has been determined(-+5%) by Karl Fischer method usingthe dead stop indication of the end point, For the determination of the concentration of hydrogen ions in the presence of Ba, both in the aqueous and in the organic phase, automatic thermometric titration by pyridine[6] has been used (±0.5%). For the determination of the distribution ratios (20°C)of Ba (DB.) labelling with t33Ba(NO3)2 has been utilized, the 3, radioactivity being measured with a well-type NaI(Ti) detector connected to conventionalcountingequipment.The radiochemical purity of ~3Ba has been checked 7-spectrometrically.
-2
t
0
-2
log CON03
~
-I
0 log CH08
Fig. I. Dependence of Ba distribution c ~No~initial concentration
of HNO3in the aqueous phase; c ~tn--initiatconcentration of HB in the organic phase; Curve 1-c~a=0.305M; Broken linestheoretical slopes -2 and +2. 2--0.203M; 3--4).105M,4--0.046M, 5--C~No3--0.1M; 6--0.2M; 7--0.3M; 8---0.5M; 9--I.0M; 10-2.0M; 11--3.0M. Tracer amounts of Ba.
RESULTS AND DISCUSSION The dependence of Dea values for tracer amounts of Ba upon the concentration of HNO3 in the aqueous phase and upon the concentration of the reagent in the organic phase is shown in Fig. 1. It can be seen that log DBa values depend linearly upon the initial acidity of the aqueous phase c ~No3 (at constant initial re~gent concentration in the organic phase (c~e)), the slope being ~ - 2 . For constant C~No3 values in the aqueous phase log Dsa values increase linearly with c~ the slope being ~ + 2.
?The paper represents a part of the Thesis (Candidate of Sciences) of I.P.
N
-I
IogDBa
0 0.0 o
t -5
I -3
-~
~c o
Fig. 2. Influence of Ba concentration on its distribution.Curve I--0.3M HB; 2---0.2M HB; 3---0.1M HB; 4---0.04HB; 5--03M HNO3; c~--initial concentration of Ba(NO3h (molll) in the aqueous phase.
1481
1482
I. PODZIMEK et al.
can be assumed as the main equilibrium controlling the distribution of Ba. This is analogous to the reaction Cs + + H2,g~Cso+,s + H investigated previously [3] using the same anion and solvent. In order to corroborate the validity of eqn (1) two criteria were used: (i) testing the essential independence of the equilibrium constant of reaction (1) upon c ~INO3,C~a and c ~, and (ii) testing the equivalence of the exchange of two H + ions for one Ba2+. Using the first criterion the parameters of the solvent extraction experiments were systematically varied within the following ranges:
The value of Ka~2a obtained can be compared to other exchange constants in water-nitrobenzene systems owing to the fact, that this type of constant does not depend on the type of anion present so that from the values of constants of the type KMelMeand KMe'/Me, the value of KMe/Me"c a n easily be deduced. Generally, an algebraic combination of the log K values is feasible. (KMc/M¢'is the constant for exchanging the cation Me for Me'.) The following relationships can be used: log
Kaa/2. =
log Kca/2Li +
log Ku./c,
+ 2 log Kcs/H + 2 log KLi/Cs; HNO30.1-3M; c~a 0.04-0.5M; c~t~oml0-6-10-1M. For the sake of brevity only selected data are presented. Typical examples of the effect of HNOa and HB concentrations are given in Fig. l, those of Ba(NO3)2 concentration in Fig. 2. The values of Ka~/2Hfor tracer concentrations of Ba are calculated according to the formula
and log Kaa/2. = log KSr/2H + log Kua/sr.
The values of Kaa/2H calculated by this indirect method together with K values necessary for the calculation are summarized in Table 1. It can be seen that the value found experimentally is in a good accord with previous data (allowing for the various ion strength values used). Ke~/2~= [Ba2+]ora'[H+]2/[Ba2+].[H+]2rs Another way of verifying the mechanism expressed by o 2 o2 eqn (1) is to check the equivalence of the amounts of Ba2÷ "~ DB,'(c m~o3)/c .a. (3) and H + transferred between the two phases. The results of The use of eqn (3) implies the following assumptions: (i) All such experiments are summarized in Table 2. Analysis of the data given in Table 2 shows that on barium in the system is in the form of free Ba2÷ cations (see average one extracted Ba2+ ion replaces ~ 2.16 H + ions in equilibrium (1)); (ii) no HNO3 extraction occurs (this has been checked); (iii) The transfer of HB into the aqueous the organic phase and transfers ~ 1.87H+ ions into the phase is negligible (this can be expected from the value of aqueous phase. The average of the two (2.015) is very close the constant KH+B-= [H+]o,g[B-]o,J[H+][B-] ~ 103"2, to the theoretical value 2.000. The hydration of Ba 2÷ ions in the nitrobenzene phase found previously[l]; (iv) full dissociation of HB in the seems to be of interest since a correlation of hydration and organic phase (independent evidence given in paper[l]. selectivity of extraction in this type of system has been For higher Ba concentrations eqn (3) has to be modified established [10, 13]. (for examples of experimental results see Fig, 2). Under Published data are discrepant, the hydration number 4.0 this condition the following relationships are used (for was found in a paper[10] using relatively low concentla = t~orB): trations of barium dipicrylaminate, whereas in a recent investigation a much higher value (10.5) has been [H+]or, = c ~u - 2[BaZ+]o~,; reported[14] for a similar system. [H+] = c ~No3+ 2[BaZ+]o~; In this work data on the hydration of both H ÷ and Ba2÷ ions have been obtained so that the total change in water [Bae+]or,,= c ~atNo3~Daa/(1 + Da,). concentration in the organic phase due to reaction (1) might Consequently, knowing the three initial concentrations and be ascertained. Figure 3 shows the increase in the total concentration of the distribution of Ba, the Kaa/2. value can be found. water in the organic phase as a function of the initial An analysis of the Ka~2H values obtained yielded the concentration of HB (no Ba was present in the system) and following conclusions. Most log Ka,a2, values obtained for aqueous acidity ~ 1 M HNO3 fall between the values of the concentration of HNO3 in the aqueous phase. It can be seen that the concentration of water is nearly 0,67 and 0.94 which seems to be a rather narrow interval for linearly dependent upon the concentration of HB but the the wide concentration ranges used. Analysis of the bulk of slope increases slightly with higher HB concentrations. the data showed no significant dependence of Ka~/ea This form of [H20~or, vs [ion]or, dependence has been values upon c~, nor on c~n concentrations. The Kn~/z, found recently by Skarda et al.[15] for cations and by values diminish slowly with increase in HNO3 concenDiamond et al.[16] for the hydration of halide anions in tration in the aqueous phase. This relation could be nitrobenzene. If the hydration number is defined conrepresented for the above concentration range: ventionally as log Kaa/z, = 0.92-0.23c ~NO3.
h.÷ = ([H20]o~- [H20]o~,~,)/[H÷]o**,
Similar changes with aqueous acidity were found previously[3, 7] in the case of Kc,m in a similar extraction system. The most probable explanation of this phenomenon is through the change in activity coefficients in the aqueous phase with the ionic strength, as in the case of Kc,/,[3]. On the whole, the variations in the values of Ke~zn are sufficiently small that the system can be assumed to obey eqn (1) reasonably well.
where [H20]ors.o is the value of [H20]ors in the absence of any reagent in nitrobenzene, then values of hm within the range 5.2-7.6 are found. For one c ~No3concentration the h values, of course, increase with higher c ~B concentrations. The dependence of the [H20]org value upon the total concentration of Ba in the system is given in Fig. 4. The right-hand end of the curves corresponds to extracts practically saturated with Ba2+.
1483
The distribution of barium cations Table 1. Ion exchange solvent extraction constants nitrobenzene-water, ~ 20~C, data for lowest ionic strength available tTl~ of oeastsat
(KW,/W,,)
sa£~n l~'es~at
4-, ~
1o~
.y.t----
r@fox'@~o@
~W,/W,"
~L~veot / 4 g a t ~
K /2 , ,
a],
~r/2X
d~LoxrboXX4e
O. 70
9
~i/ca
dX~±oryZ--'~-to
o. ~
1o
F~Sr
d£p£ory~te
O. 27
10
KC~/I A
d£p£oryJJm/aat@
It. 0
11, ].2
10, ~
l~01T~,,- 01~,ala~
~ ~
7, z,,
K
dioarboZ./.de
(oOwJm,r£so~)
various
O. 5~t
various
O, 97
e,~/2e
8
0.92-0.23 w
tl~Ls work oltZod. £rcm
osJ.ed, t r ~ s
xsr/~a o w = omm 3 ( x ~ t ± a x omen. o t ~ o 3)
Table 2. Verification of the reaction Ba2+ + 2 H ~ ~ B a ~ + 2H+. Values indicate changes in concentrations (mol/1) of H + in the respective phase, A pertains to experimentally found values, theor.-retates to concentrations calculated from the above exchange reaction knowin8 the distribution of Ba. c ° denotes initial concentrations of the respective components
O•o--• 3
o,3M Imo 3 o
o~--~
o.3M aa
o.xM ~
o , o ~ at
o.31/ aa o . ~
~
0,3Z4 zm
theor. ~ aq - £ or6
0.171 0.171 o.16o
0.107 0.103 o.lop
0.069 O.063 0.055
0.1~1 0.1~2 o.125
0.075 O.073 o.o61
theor. ~ aq - ~org
O.116 O.105 0.IIi
O.O71 0.079 0.082
O.O~1 O.O47 0.065
O.IO8
O.2H HI]
0.~18
O.O/t7 0.029 0.063
@,114
ths~. mq
-Aorg
0.057 0.051
o.o66
0.032 0.023
0.02~ 0.020
0.040 0.034
0.017 0.015
theor. ~ aq - Aorg
0.021 0.015 0.016
-
-
O.O/tM HB
o.o3~
The main, rather spectacular, conclusion drawn from the Figure is the very weak dependence of the [I-120]o~ value upon the [l'I+]/[Ba2+] ratio in the extract. There is an + 2+ indication, however, that replacing 2H ~ ions by one Ba.~ leads to the water content of the extract growin8 to a certain extent. We would therefore expect the value of he.2+ to be slightly higher than double the value of h.+, JINCVol.42,No. [0--G
0 . . ~ Imo 3
0.026
0.O42
0.025
o.SM O.046 0.040 o.o43
0.010 0.015
O.O18
0.0135 0.013
Using an analosous relation to that esed for kw, the definition of he.'+ can be given as hn.2÷ = [Ba2÷]or~t([H20]o~ --'[H20]ors.o-hlsl~[I-I+]o~). This expression can be jugif~d only if both [H20].~ and h~+ values are independent of the [Ba2+]o~ concentration.
1484
I. PODZIMEK et aL
[H20]ot 4
3
2-
0
0.2
0.4
COHB
Fig. 3. Coextractionof water into nitrobenzene with HB.
[HzO]*~L
-0
~
~
_
_,
same charge. It was found that the higher the difference between hM~ and hM¢' the higher the value log KM~/M¢'. The present results, however, pertain to the exchange of a univalent and bivalent cation (H+-Ba2+). A simple analogy to the above method of Correlation for ions of different charges is hardly possible. Nevertheless a somewhat different approach to the above tentative correlation appears to be feasible. Since the h values pertain to one cation only, solvent extraction constants relating to single ions must be taken for correlation. The concept of these "individual extraction constants" and methods of their calculation have been described previously [18]. In the present case, the basis for the calculation is the relationship log KBa/2H = log K a y - 2 log log KH corresponding to the equation
K
_ [Ba2+]o,,./[H+]o,s~ z
(4)
which is a rearranged form of eqn (3). Knowing that l o g K a = - 5 . 7 (see [18]), inserting log KBa/2x = 0.9 we obtain log Kaa = 0.9-I 1.4 = 1.5 -10.5-+0.1. Figure 5 shows the correlation of h and log Km,,t~ values for several metals. The KB,, hn and hBa ,.o, 0 ¢ 3 values used stem from this work, other values were found in the literature [14, 18-20]. If for any metal two h values 0 0 4 ~ have been reported both points are shown in the Figure. It 0.5 k ~ 5 can be seen that the present results corroborate the I I I I I hypothesis that hydration of cations in the polar organic -5 -3 -I log 6- 0 phase in systems of this type is closely related to the Fig. 4. Influenceof Ba concentration on the coextractionof H~O selectivity of extraction. Highly hydrated cations are less into nitrobenzene; Curve 1--0.3M HB; 2----0.2MHB; 3--0.15M prone to enter the nitrobenzene phase. A comparison of the HB; 4--4).1MHB; 5--0.04M HB; open symbols0.3M HNO3;full values ha, with hydration numbers for aqueous solutions is symbols 0.SM HNO3. summarized in Table 3. From Table 3, it follows that the h.a2+ value in nitrobenzene lies well within the range of It is evident that since the difference between [H20]org and values for aqueous solutions, but the majority of aqueous ha÷'[H+]org is small for small Ba2+/H+ ratios in the extract hydration number values are lower than hn.~+. the calculation of ha,2÷ depends largely on the right hand This calls for comment in relation to the findings of branches of the curves in Fig. 4. Some authors [17] make a other authors on hydration of ions in polar organic correction for the influence of the growing content of solvents. The fact that the hn.2+ value is higher than the hydration water by assuming that the physical solubility of coordination number of barium in aqueous solutions [8] water relates only to that part of the organic phase indicates that the hypothesis that 8 = n + ha, suggested corresponding to the volume fraction of nitrobenzene in by Shivrin [32] for ion exchange extraction systems canthe extract. In this work no similar correction was applied because even for high concentrations of hydrated ions it becomes insignificant, the physical solubility of water in nitrobenzene (~0.17 M) being rather small compared to cs the total H20 concentration in the extract (up to 4 M). For these reasons, the physical meaning and reliability of "cx ha,2÷ values obtained is limited. The fact that the two -5' • oH curves denoted as I in Fig. 4 and also those denoted as 2 in O(D • Li the same Figure run almost parallel to each other seems to No ~5~x" indicate that hBa2+values depend on the aqueous acidity in a very similar way as those for h.÷. On the other hand the % accuracy of data obtained is not high enough to permit the -I0 " calculation of the ha, e÷ dependence upon the composition of the organic phase. Therefore it was considered reasonj ®'1~, ~co I able to calculate an average hB~2+value corresponding to 5 a n aqueous acidity 0.3 HNO3 only. The value found (hBF+ = 11.5 + 1) was in reasonable agreement with the Fig. 5. Correlationof individual cation extraction constants from value 10.5 reported for other hydrophobic anions and Ba2÷ water into nitrobenzenewith their hydrationnumbersin nitrobenzene. Full symbols--resultsfrom this work. If for one metal two by recent workers [14]. different data are available in the literature, one of the symbolsis An overall correlation between selectivity and crossed. For most cations values of K and h relatingto # --~0are difference in hydration of cations (Me, Me') in the nitro- used. For Ba the average values are shown since no significant benzenephase was observed[10, 13] for cations of the dependence of K upon # could be found.
Y
•R•b
25
10o7
28
25ol
22ol
15.0
value)
-
9,5
-
25
-
-
8
11
25
8
0.3
0,25M
005M
O,I-I.OM
IM
O.I-I,SM
-
~OM
001-I.8M
o£H20 seine. eolli~ative l~op®rtiee 0£ H~O-solns. eleo~rolytic ion tz~ua~er
aotivity ooef£o oolli~tive p~operties
pz'oportioe
souud v e l o o i t T ! oom3~reesibility eleot~olTtio trLne£er of ion 8otivity ooefT. eleotx'01ytio tTansfero£ ion dieloctrio
activity ooefTo ~ e t i c p~operties aotivit7 eoef£. d.tffusion meastrvemente
0.TM 25
0£
method o£ determination
oonono o£ electrolyte
3
(%)
5.7 707
temperature
hydration
number
Table 3. Examples of hydration numbers of Ba2+ in aqueous solutions
0
Br'~
Ci'i
CI'j
5
31
30
3O
29
28 NaC1 I-I
27
29
26
01-~
0
7
1~+t
23,25 25
29
Cl-j
0
22
21
raforonoo
Cs+j 0 Be2+ t .
ssumedhydz~tlonnutaber
referenoe ion~ad its
f3
c
o"
o
=
1486
I. PODZIMEK et al.
not be extended to the present case [8]. Here n denotes the number of the organic solvent molecules in the first solvation shell of the cation in the organic phase, n~--the number of H20 molecules in the first solvation shell (for the present case no = hBa~+) and 8 is the usual coordination number of the cation Ba 2+. This difference stems probably from the circumstance that Shivfin only considered solvents more basic than water and able to replace water in the hydration shells. Nitrobenzene as a solvent of insignificant basicity evidently shows quite different behaviour. Several other ideas explaining relatively high hydration numbers of ions in nitrobenzene have been published. Gere [17] found that the hydration number of bis(l,0-phenanthroline)Cu(I) + in nitrobenzene is 14+4 and offers the following explanation. The hydration is due to a certain number of primary watermetal ion bonds accompanied by several secondary water-water polymer bonds leading away from the primary site and limited by the extent of shielding afforded by the coordinated ligand atoms. Kenjo and Diamond[16] found the following average hydration numbers in nitrobenzene for CI-, Br- and I-, resp.: 3.3, 1.8, 1.0. The values increased with higher concentration of the anions in the nitrobenzene phase similarly to our results in Fig. 3. The authors believe that this effect has its origin in the ability of the anions to fit into and even promote a hydrogen-bonded network with H20. Tarui[33] studied the hydration of perchlorate anion accompanied by tris(1,10-phenanthroline)Fe(II) z+ in nitrobenzene and came to the conclusion that hydration in nitrobenzene may be higher than than in water. The author ascribes this phenomenon to a different mechanism of hydration in the two media. Owing to the fact that the distances between H20 molecules in nitrobenzene are much larger than those in aqueous solution, the hydration in nitrobenzene depends primarily upon the total energy of the interaction of cation with water molecules [33]. These ideas might also be relevant to the relatively high values of hB,2+ in nitrobenzene found in this work.
REFERENCES
1. J. Rais, P. Seluck~' and M. KyrL 7. Inorg. Nucl. Chem. 38, 1376 (1976). 2. M. KyrL L. Kadlecov~i,J. Rais, P. S¢luck~,,J. Tepid', B. Ya. Galkin, R. I. Ljubcev, S. I. Rovnyj, V. N. Romanovskij, N. S. Tichonov and D. N. Shishkin, Issledovania v oblasti pererabotki oblachonnovo topliva, Vol. 1., Proceedings of the IV. Sympoziam, held 28 March 1977 in Karlovy Vary, Cze-
choslovakia, p. 246, Paper KV 77/T5., Czechoslovak Atomic Energy Commission, Prague (1977). 3. F. Mac~l~ek, U Mfitel and M. KyrL Radiochem. Radioanal. Lett. 35, 247 (1978). 4. V. Koprda and V. ~asn,'tr, 9th Radiochemical Conference, Piegtany, Czechlslovakia, 1i September, p. 47; in Abstract of Papers (1978), Czechoslovak Chemical Society. 5. M. ~tefek, M. Kyr/~ and J. Rais, Zh. Anal. Khim. 31, 1364 (1976). 6. M. Z;imek, Coll. Czech. Chem. Commun. 4l, 3754 (1976); Czechosl. Patent appls. PV 2573-77, PV 2919-77. 7. J. Rais and M. KyrL Coll. Czech. Chem. Comman. 34, 949 (1969). 8. M. KyrL V. Michalukov~i,J. Bene~ and L. Kadlecov~i, Coll. Czech. Chem. Commun. 34, 1709(1969). 9. P. Vafiura, J. Rais, P. Seluck~, and M. KyrL Coll. Czech. Chem. Commun. 44, 157 (1979). 10. M. Pivofikov~iand M. KyrL 7. Inorg. Nucl. Chem. 31, 175 (1969). 1!. J. Rais, M. Kyr~ and M. Pivofikov:i,7. lnorg. Nacl. Chem. 30, 611 (1968). 12. I. Rais and M. Kyd in Solvent Extraction Chemistry Proceedings of a Conference, p. 595. North Holland, Amsterdam (196~). 13. J. Rais, M. Benegovfi-Pacltovfi,P. Seluck~' and M. Myrg, 7. lnorg. Nucl. Chem. 35, 633 (1973). 14. T. Iwachido, M. Minami,A. Sadakave and K. Toei, Chem. Left. (Tokyo) 1511, No. 12 (1977); fiNIS 9, RN 391015, p. 4664 (1978)). 15. V. ~karda, L Rais and M. Kyrs, 7. Inorg. Nacl. Chem. 41, 1443 (1979). 16. T. Kenyo and R. M. Diamond, 7. lnorg. Nucl. Chem. 36, 183 (1974). 17. D. R. Gere, DissertationAbstracts 26, 54 (1965). 18. J. Rais, Coll.Czech. Chem. Comm. 36, 3253 (1971). 19. M. Kawasaki, K. Toei and T. lwachido, Chem. Lett.Japan 417
(1972). 20. S. Motamizu,K. Tosi and T. Iwachido, Ball. Chem. Soc. Japan 42, 1006 (1969). 21. E. Glueckauf, Trans. Faraday Soc. 51, 12350959). 22. T. J. Swift and W. G. Sayre, J. Chem. Phys. 44, 3567 (1966). 23. K. Tamura and T. Sasaki, Bull. Chem. Soc. Japan 36, 975 (1963). 24. E. N. Gapon, Z. Anorg. AUg. Chem. 168, 125 (1927). 25. J. Padova, Bull.Res. Counc. Tsr.Sect.A Chem. 10, 63 (1961). 26. L. H. Collet, C.r.Acad. Sci.239, 266 (1954). 27. J. Baborovsk~, and O. Viktorin,Coll.Czech. Chem. Commun. 5, 518 (1933). 28. J. B. Hasted, D. M. Ritson and C. H. Collie,7.Chem. Phys. 16, I
(1948). 29. R. H. Stokes and R. A. Robinson, 7. Am. Chem. Soc. 70, 1870 (1948). 30. E Burion and E. Rouger, C.r. Acad. Sci. 196, llll (1933). 31. R. Haase, Z. Elektrochem. 62, 279 (1958). 32. G. N. Shivrinand B. N. Laskorin Dokl. Akad. Nauk SSSR 194, 877 (1970). 33. T. Tarui, J. Inorg. Nacl. Chem. 37, 1213 (1975).