technetium tetrabutylammonium pertechnetates solubility

1. Journal of R adioanalytical and Nuclear Chemistry, Articles VoL 121, No. 2 (1988) 515-521
ION ASSOCIATION IN TETRAALKYLAMMONIUM
PERTECHNETATE SOLUTIONS
K. E. GERMAN, S. V. KRJUCHKOV,L. L BELYAEVA,V. I. SP1TSYN
Institute of PhysiCal Chemistry, USSR Academy of Sciences,
Moscow (USSR)
(Received November 19, 1987)
The solubility of tetraalkylammonium perteehnetates in aqueous deetrolyte solutions
has been studied. It does not practically depend on ionic strength of the solution when
specific interaction of own ions and ions of the dissolved electrolyte (e.g. LiNO~ up to 5
tool/l) does not occur. A remarkable dependence of Bu~NTeO~ solubility on acidity of the
solution has been found. The influence of ion association on the solubility is discussed.
Introduction
Some properties of substances under consideration make us to take into account
ion association. Such are solubility and its variation while a solution composition is
varied. There are some ions in solutions taking part in complexation equilibria (such
as-association with protons or c6-ions). It influences the solubility values enabling
us to get information on the corresponding equilibrium constants.
Experimental and discussion
We studied the influence of solution acidity and tetrabutyl-ammonium concentration on the solubility of Bu4NTcO4. Solubility was determined by ~-counting of
99Tc. Solubility values obtained are shown in Table I.
Experimental data show that solubility of tetra-n-butyl-ammonium pertechnetate
does not practically depend on the ionic strength of the solution in the absence of
specific interaction of own ions and ions of the dissolved electrolyte (e.g. LiNO3) up
to 5 mol/1. Suffice it to note the practical identity of Bu4NTcO4 solubility in constant ionic strength solutions consisting of LiNO3 and HNO3 and solubility in pure
nitric acid solutions of corresponding concentration.
Solubility measurements for Bu4NTcO4 in nitric acid and in mixtures of HNO3
and IANO3 showed the unexpected remarkable dependence of solubility values on
acidity of the solution.
Elsevier Sequoia S. A., Lausanne
A kad~miat Kiad6, Budapest

2. K. E. GERMAN et al.: ION ASSOCIATION
Table 1
Solubility of Bu4NTeO4 in aqueous
electrolyte solutions at 20 ~
Electrolyte
concentration,
moll1
H~O
_
LiNOs
0.25
0.50
1.00
2.00
5.00
HNO~
0.25
0.50
L00
2.00
3.00
4.00
5.00
Bu4NOH
0.0042
0.010
0.097
0.210
LiNO~ + HNO3
4.50 0.50
4.00 1.00
3.00 2.00
2.00 "3.00
1.00 4.00
Solubility,
mmol/l
4.220.2
4.2 • 0.2
4.3 +-0.2
4.3 • 0.2
4.2 • 0.2
4.3 • 0.2
5.2•
6.220.3
8.1 • 0.3
12.7 • 0.4
15.9 • 0.5
25.9 • 0.8
34.0 • 1.5
4.2 • 0.2
1.3 20.1
0.39 • 0.03
0.24 • 0.02
6.3 •
8.3 •
12.0x0.5
16.0 • 0.5
25.8 • 0.8
To o u r m i n d , o n e o f t h e e x p l a n a t i o n m a y be t h e l o w e r i n g o f H T c O 4 dissociat i o n d u e t o t h e following equilibria:
K1
Bu4NTc04 ~
B u r N § -v TcO~-
K1 = SP,
(1)
K2 = K - 1
diss"
(2)
K2
H+ + TcO~
516
HTcO4

3. ICE. GERMANet al.: ION ASSOCIATION
In such a case it is possible to linearize solubility as follows:
2
2
Cs/Cs, ~ -
1 = K2 "
(A)
aHNO~
C$
- solubility of Bu4 NTc04, mol/1,
C2
= SP -- true product of solubilities of Bu4NTc04,
K2
where
- effective association constant of HTc04,
St 0
aHN03 -
activity of nitric acid, mol/1 (2), anNO, < 3.
The value of 4.1 was computed from this equation for the constant 1(2. Such a
calculation does not take into account the possibility of association of pertechnetate
ion with tetra-n-butyl ammonium, which follows from Table 1, where reduction of
concentration [TeO~ in solutions with different [Bu4N§ is lower than expected
(calculated from the solubility product).
K3
Bu4 Naq + TcO~'aq
+
r
Bu4 NTc04 aq
(3)
A simultaneous consideration of Eqs (1)-(3) leads us to the equation:
-Ks
/
* SP = 1+K2
9
aHNOs
(B)
It is possible to determine Ka by means of extrapolation ufing solubility values of
Bu4NTc04 at large concentration of Bu4NOH (Table 1).
Ca = [Bu4NTcO4]aq "
I
1t
1 + Ka[Bu4 N+]
(c)
Rearranging Eq. (C) we obtain
SP
Cs -
C s , o = [ B u 4 N +]
(D)
517

4. K. E. GERMAN et al.: ION ASSOCIATION
So far as graphical extrapolation in terms of Eq. (D) leads to the value of Cs,o =
= (0.2 • 0.02) X 10-3tool/I, the value K3, calculated using equation D is K3 = 15 • 3
l/mol.
It should be noted that the values measured for RANTcO4 (0.13 tool/1 for R = CH3,
and 0.012 tool/1 for R = C2Hs and 0.0042 mol/l for R = C4H9) are significantly
larger than expected for the salts of univalent cations because of the monotonic reduction of solubility among the alkali metal pertechnetates (Fig. 1).
A
,I-
:
r olLi" ~
/ rt'b
N
Oata r/Z
bNSu~.
~-3
Fig. 1. Solubility of pertechnetates
The latter is at leasI partly connected with the influence of association of [RAN]
[TcO4] in aqueous solutions. The usage of the expression for concentration of TcO,~
according to Eqs. ( I ) and (3) shows that the concentration of the associated salt in
the solution can be introduced as a product of two constants:
Cs,o = [Bu4NTcO4]a q = SP 9 K3
(4)
where from the value of the true solubility product, corrected for association of acid
in the mixture will be introduced as SP = (1.3 -+ 0.5) 9 10 -s mol 2 9 1-2 .
The values obtained enabled us to calculate the correct value of the association constant of the ion TcO~ with protons (K2) using Eq. (B).
As a result, we obtained the value K2 = 4.0 -+ 0.9 mol-1 being almost identical to
the value determined from Eq. (A).
The values of the association constant show that the association of TcOi ions with
Bu4N + and protons should be taken into account in evaluation of the solubility of
518

5. K. E. GERMAN et al.: ION ASSOCIATION
Table 2
Assodation constants of pertechnetate - ion with proton
Determination method
log K
Ionic force
References
Solubility of salt
Potentiometry
Calculation frcm liquid
extraction
Anion e~change
Liquid extraction
Raman spectroscopy
Kinetics
0.60
0.30
0-3
1-4
New data
4
0. 20
0.033
-0.3
-8.0
-8.0
1-4
5
5
0-6
6
7
8
9
-
Bu4NTcO4. Papers devoted to investigations of ion exchange, potentiometry, liquid
extraction, equilibrium work on solutions containing TcO~ ions pointed to the great
importance of taking into account association of pertechnetate ions with protons.
Association constant obtained in this work is compared with literature data in Table 2.
Table 2 shows that methods such as Raman spectroscopy and kinetics lead to pKa
values that conform to PAULING'sI 0 and RICCI'sI 1 hypothesis and indicate complete HTcO4 dissociation in aqueous solutions, while works based on study of real
equilibria in the solution point to a significant decrease of activity of TcO~ ions in
the acid solutions.
The hypothesis introduced in Reference 6 is evidently true. It indicates the presence of a weak outer-sphere (coritact ion pair) complex as well as a strong innersphere one (associate HTcO4). The Observation of this or that complex depends on
the method applied,
i
X
V"
Z
*
Fig. 2. Structure of Bu4NTcO4 ! 2
519

6. K. E. G E R M A N et al.: ION ASSOCIATION
A
O c~
@ c2
0 c3
@c4
|
N~
|
N2
Oo
Fig. 3. Structure of Me, NTcO4 ~~
The problem of the thermodynamic stability o f such weak outer-sphere complexes
is still under discussion. The stability of such associates is evidently connected with
the results of X-ray single crystal analysis showing pseudo-JAHN-TELLER distortion
of the d~
TcOg It was found that TcO4 ion in the structure of tetrabutylammonium pertechnetate has C3v. symmetry (as a result o f shortening of three
T c - O bonds to 1.58 A 12 (Fig. 2). Almost the same structure (Fig. 3) but with one
shorter bond was noted in the course of work with tetramethylammonium. 13 If it
were possible to show that the formation of ion pairs in the mixture is accompanied
by analogous changes, we would have one more argument for enhanced thermodynamic stability o f such associates.
References
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prirneneniya, Vol. 9. VINITI, Moscow, 1984, p. 119.
520

7. IC E. GERMAN et al.: ION ASSOCIATION
4. C. L. RUFS, R. F. HIRSCH, R. A. PACER, Nature, 199 (1963) 66.
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11. J. E. RICCI, J. Amer. Chem. Soc., 70 (1948) 109.
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Set Kristallografii I., 293 (1987) 101.
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521