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Determination of the stability constants of thorium nitrate complexes with anion-exchange resins
.I. Inorg. Hucl. Chem., 1960. Vol. 13. pp. 112 to 118. Perpmon Preu Ltd. Printed in Northern Ireland
DETERMINATION OF THE STABILITY CONSTANTS OF THORIUM NITRATE COMPLEXES WITH ANION-EXCHANGE RESINS J. DANON Centro Brasileiro de Pesquisas Fisicas, Avenida Wenceslau Brfis 71, Rio de Janeiro, Brasil (Received 3 June 1959)
Abstract--The distribution of thorium between Dowex-1 nitrate resin and solution was measured by the aid of ~UThin the range,---1.55-8.44M lithium nitrate. The activity of nitrate in the resin was determined and a distribution curve corrected from invasion-effectsof the supporting electrolyte in the resin phase was constructed according to the Marcus and Coryell methods. The stability constants of the complex thorium species, considered to be formed from the uncharged complex Th (NOa)4 ~ Th(NOs)~_~ + iNOs were calculated by curve fitting methods for i = 1 to +2, and the following values were found: log ~+~ ~ + 0.22, log/30 = 0.00, log/~_~ = - 0-80 ~ 0.17, log/3_s -- -- 1.70 4- 0.22. THE determination o f the stability constants of the nitrato complexes of thorium has been investigated by solvent-extraction methods. Using a solution of thenoyltrittuoracetone in benzene as extracting agent with nitrate media of 0"5 ionic strength DAY and STOUGHTON(1) found the value 4.73 for the first nitrate complex of thorium. ZEngosi
Determination of the stability comtants of thorium nitrate complexes
113
constants which is particularly suitable for the species formed in the neighborhood of the neutral complex. DISCUSSION
AND RESULTS
The adsorption of thorium by Dowex-1, 8 ~ DVB, 50-100 mesh in the nitrate form was investigated with thorium-234 (/~, 7, 24.1 days period). This isotope was separated from large amounts of uranium in 8 M HCI solution with Dowex-1 columns/m Further purification was made by adsorbing ~ in a small Dowex-I column from 7 M HNOs solution; followed by ehition with water. All measurements were made in a scintillation counter. We shall assume the following mechanism~S,9:°~ for the adsorption of thorium from fithium nitrate media by the anion-exchange resin. The neutral complex Th(NOs)4 is transferred from the aqueous solution to the resin phase ThL 4 (sol.) ~ ThL4 (res.)
(1)
followed by the reaction with the resin ligands ThL 4 (res.) ~ 4RL ~ R4ThL s
(2)
where L stands for the nitrate iigand. The number of resin ligands was chosen in order to form the coordinatively saturated complex in the resin phase. As has been shown by KATZIN et al. ¢Iz~ the co-ordination number of Th a+ is in general eight. From (I) and (2) we can write for the thermodynamic equilibrium equation of the adsorption process K, = aR,ThL, a~.~l~, aRL-1 (3) or K r ~---[ R 4 T h L s ] .
?RIThL s . aTI~L" . a X L -I
(4)
where brackets represent concentrations and 7 the corresponding activitycoefficients. The distributioncoefficientD is given by D ---[R4ThLs] [Th'+ ] + [ThL s+] + [ThL• + ] + . . .
(5)
The sum in the denominator extends to all specieswhich can be formed in aqueous solution. From (4) and (5) we obtain l
O = K,. Y~,hr.,. ax~z'. [Th,+] a~h~, q- [ThLs+ ] a~-h~, -I- • • •
(6)
We shall now define the formation of a complex by relat~g it to the neutral complex ThL4~- ThL~_~ q- iL
(7)
where the index i gives the charge of the complex. The thermodynamic equilibrium constant for (7) is ff~ = a a ~ _ , , aL'. a~hlL,
(8)
~xl~K. A. Ks~os, G. E. Mooat and F. NtLSON, J. Amer. Chem. Soc. 711, 2692 (1956). ~xs~L. I. KATZIN,J. R. FeRaARO, W. W. WtTCDLANDTand R, L. M c B ~ , 1. Amer. Chem. Soc. 711,5139 (1956).
114
J. DANON
which gives [ThL~-i] = /~iyi-laL- ~.
(9)
aThL,
Introducing (9) into (6) we get D = K r . y~lThL, altL 4 . ( ~ , Yi-laL--t)--I
(I0)
We shall now make the following assumptions: (1) The activity of the nitrate ion in solution is given by aL -----[L]
4" ) • ~'(LiNO8
(11)
where [L] is the concentration of nitrate ion in the aqueous phase and 7~lso,) the corresponding mean ionic activity coefficient of LiNO3. (2) The activity of the nitrate ion in the resin phase is given by a~, -----aL. [L]R - t • [Li]R - t
(12)
where aL is the activity in the aqueous phase according to (1 I) and [L]R and [Li]R the concentrations of nitrate and lithium respectively in the resin phase. The reference state for the activity aRL is taken arbitrarily as ara~° for at. = 1. Relation (12) can be derived from Donnan equilibrium of the electrolyte between the aqueous solution and the resin phase. ~13~ (3) The activity coefficients 7'~,ThS, and 7t are independent of the value of as. The justifications for these assumptions are given in MARCUS and CORYFJ~Lpapers. Introducing the above assumptions in (10) we find log D = log (K,.
-1 s . a R S 40) -~?R4ThL
4(log
aRL --
log
a R L °) - -
log Z/~iyi-laL -i
(13)
We shall now make K,* = K,. 7£,ITh~. aRs '0 8,* =
-1
(14) (15)
According to the assumptions made K,* and fl/* are independent of as. We define a correction function F(a) as F(a) = log aRL -- log aRL°
(16)
Introducing (14), (15) and (16) into (13) we find log D = log Kr* q- 4F(a) -- log ~ fli*as -~
(17)
The difference log D O= log D -- 4F(a) gives the distribution coefficients corrected from the invasion effects of the supporting electrolyte in the resin phase: log Do = log D -- 4F(a) = log K,* -- log Y. ill*as -~
(18)
It is easily calculated that d log D O - = d log aL
[=
4--
a
(19)
where [ is the mean charge number of the complex species and a the mean number of ligands as defined by Bjerrum. (la~ K. A. K l t a u s and G. E. MOOR~, .LAmer. Chem. $oc. 75, 1457 0953).
Determination of the ~ability comtants of tlmrium nitrate complexes
115
The values of the distribution eoeffr.iznts as a function of the LilqOs molarity are given in Table 1. This data was obtained in equilibration experiments (24 hr) with the resin at room temperature (25 4- 3°C). The concentration of the LiNOs solutions was measured in a flame photometer. All LiNOs solutions were made ~07 M in TABLe 1 MLINO* I
1"55 2"00 2"39 2"53 2"89 3"40
log D
MLINOg
4"02 4"35 5"06 5"20 6"24 6"32 8"44
1"10" 1"45" 1 "72* 1"83
2"21 2"36
log D
2"81 2"91 3"21 3"35 3"69* 3"67* 4"33*
* Values with errors larger than -I-0-05 log units,
4
!
0
o
s
o
"~
.
.
.
.
~
IogAL FIG. l.--Adsorption of Th(IV) by Dow¢~x-1at varying LiNOs activities.
HNOs in order to avoid hydrolysis of thorium nitrate. However even at these acidities hydrolytical reactions appear to occur at LiNO s molarities lower than 2 M.t Fig. 1 shows the values of log D as a function of log aT.. The values of log % were calculated from mean activity coefficients of LiNOs taken from RoerssoN and STOKES.(14) Experiments with 0.1 g of glass wool per 10 ml of solution showed in thes~ conditions glass adsorption in amounts which are not negligible compared to the resin adsorption. ¢]4) R. A. ROmNSONand 11. H. STOKe, Electrolyte Solutions. Butterworths Scientific Publications, London (]955).
116
J. DANON
Fig. 2 shows the variation of the invasion function F(a) with log aL. This data was obtained with Dowex-1, X-8, 50-100 mesh in the nitrate form, from the same batch as was used in the equilibration experiments with thorium, according to the procedure described by KRAUS and NeX.SON.(xa) Details about this data will be published in a further paper. Fig. 3 shows the variation of the corrected distribution log D O as a function of log a,.
/
0 J, A o tl.
-0-5
IogA L
FZG. 2.--Invasion function F(a) vs. LiNO s activity.
In the range of activities investigated the values of ~= d log Do/d log aL go from +0"6 to --1.8 indicating the presence of the successive species ThLa+, ThLt o, ThLe- , ThL6*- in solution. Preliminary values of 8~,x*, 8o*, 8-1" and 8-=* constants were derived from the relation log 8*+x
--log aL
( i = i + ½)
With the values obtained for 8+1" and/~* we formed the equation in three parameters which was solved by curve fitting method of SILL~ :(~) 1
Do
8+a*
1
K. a'~ ~ +
.~
8K*
.a + ~
. aI
(20)
With the values of K,* and/3-2* obtained from (20) we solved the equation Kr* a 8+x* 1 8-1" D--~ "~_--~ - a~ . . . .8-,* . + ~ . a + ~ _ ~ .0= ¢11) L. G. SILL~r~, Acta Chem. $cand. 10, 186 (1956).
(21)
Determination of the stability constants of thorium nitrate complexes
117
By this procedure the following set of values were derived for the constants: log Kr~ 1"22; log ~+x*= ~0"22; log /~0----0"00; log ~ _ ~ * - - - 0 " 8 0 ± 0 " 1 7 ; log/~-I* ~ --1"70 :J: 0"22. With these values the solid curves shown in Fig. 1 and Fig. 3 were calculated from equations (17) and (18) respectively. The fitting of the experimental points is satisfactory. The data could also be fitted by a curve log De ~ log Kr* -- log(l ~- ~_l*a-~ /~_~*a~) which would mean that no Th(NOs)3+ species are present at the lower activity
I'C
0,I Q0 o
-O~
o
0~'
~
,.'.,
IogA L
FIG. 3.--Corrected distribution vs. LiNO s activity.
range investigated. However at low LiNOs activities the hydrolytical reactions of thorium nitrate tend to increase the values of the distribution coefficients. For this reason the value found for ~+1" may be considered as the upper limit of the association constant. The comparison between the values of stabilityconstants/~* (referredto the neutral complex) calculated from FOMIN and M~oRovKs data c8~ and those obtained from the anion-exchange data is shown in Table 2. In the above comparison we disregard differences in activity coefficients between the two set of data. The stability constants ~ * calculated from the anion-exchange data represent the product of true thermodynamic equilibrium constants by activity coefficients 7,~ whereas FOM~ and M~oRovA's data are concentration constants evaluated at constant ionic strength. The values obtained for/~-x* and/~-i* show that the formation of thorium nitrate negatively charged complexes occur at high concentrations of nitrate ion ( ~ 3 M). This is in agreement with the results of FOMn~and MxloRovxCS)which have shown that the extraction behaviour of thorium nitrate by TBP up to a concentration of 2.2 M
118
J. DANON
in nitrate ion was essentially not influenced by the formation of negatively charged complexes. No evidence was found in the range of activities investigated for the presence of complexes with more negative charge than --2, although the possibility of the existence of such species at higher activities of nitrate ion cannot be ruled out entirely on the basis of the anion-exchange data. TABLE 2 log 3÷1"
log/~o*
log 3-1"
log t~-.~*
FOMIN and MAIOrtOVA(,u = 2)
+0'26
0'00
--
--
Present work (variable LiNO3)
+0.22
0.00
--0.80
-1.70
The overall stability constant for the reaction Zr 4+ + 6NO8 ~- Zr(NO3)62has been investigated by solvent extraction studies with TBP (16)and its value was found to be fie = 0.02, at/z = 4. From FOMIN and MAIOROVA'Sdata and the results of the present study we found for the analogous reaction for Th 4+ the value fls = 0" 11. This shows an increasing tendency in the stability of the negatively charged nitrate complexes with increasing ionic radius in going from Zr 4+ to Th 4+. The same trend was observed with the first nitrate complexes of these elements. (17) Acknowledgements--We are indebted to Miss Z. CAILLAUXfor the lithium analysis. This work was supported by the Conselho Nacional de Pesquisas. (16) A, S. SOLOVKIN,Zh. Neorg. Kkim. 2, 611 (1957). (Iv) j. C. HINDMAN,lonic and Molecular Species of Plutonium in Solution, NNES, Plutonium Project Record, Vol. 14-A, p. 341. McGraw-Hill, New York (1954).
DETERMINATION OF THE STABILITY CONSTANTS OF THORIUM NITRATE COMPLEXES WITH ANION-EXCHANGE RESINS J. DANON Centro Brasileiro de Pesquisas Fisicas, Avenida Wenceslau Brfis 71, Rio de Janeiro, Brasil (Received 3 June 1959)
Abstract--The distribution of thorium between Dowex-1 nitrate resin and solution was measured by the aid of ~UThin the range,---1.55-8.44M lithium nitrate. The activity of nitrate in the resin was determined and a distribution curve corrected from invasion-effectsof the supporting electrolyte in the resin phase was constructed according to the Marcus and Coryell methods. The stability constants of the complex thorium species, considered to be formed from the uncharged complex Th (NOa)4 ~ Th(NOs)~_~ + iNOs were calculated by curve fitting methods for i = 1 to +2, and the following values were found: log ~+~ ~ + 0.22, log/30 = 0.00, log/~_~ = - 0-80 ~ 0.17, log/3_s -- -- 1.70 4- 0.22. THE determination o f the stability constants of the nitrato complexes of thorium has been investigated by solvent-extraction methods. Using a solution of thenoyltrittuoracetone in benzene as extracting agent with nitrate media of 0"5 ionic strength DAY and STOUGHTON(1) found the value 4.73 for the first nitrate complex of thorium. ZEngosi
Determination of the stability comtants of thorium nitrate complexes
113
constants which is particularly suitable for the species formed in the neighborhood of the neutral complex. DISCUSSION
AND RESULTS
The adsorption of thorium by Dowex-1, 8 ~ DVB, 50-100 mesh in the nitrate form was investigated with thorium-234 (/~, 7, 24.1 days period). This isotope was separated from large amounts of uranium in 8 M HCI solution with Dowex-1 columns/m Further purification was made by adsorbing ~ in a small Dowex-I column from 7 M HNOs solution; followed by ehition with water. All measurements were made in a scintillation counter. We shall assume the following mechanism~S,9:°~ for the adsorption of thorium from fithium nitrate media by the anion-exchange resin. The neutral complex Th(NOs)4 is transferred from the aqueous solution to the resin phase ThL 4 (sol.) ~ ThL4 (res.)
(1)
followed by the reaction with the resin ligands ThL 4 (res.) ~ 4RL ~ R4ThL s
(2)
where L stands for the nitrate iigand. The number of resin ligands was chosen in order to form the coordinatively saturated complex in the resin phase. As has been shown by KATZIN et al. ¢Iz~ the co-ordination number of Th a+ is in general eight. From (I) and (2) we can write for the thermodynamic equilibrium equation of the adsorption process K, = aR,ThL, a~.~l~, aRL-1 (3) or K r ~---[ R 4 T h L s ] .
?RIThL s . aTI~L" . a X L -I
(4)
where brackets represent concentrations and 7 the corresponding activitycoefficients. The distributioncoefficientD is given by D ---[R4ThLs] [Th'+ ] + [ThL s+] + [ThL• + ] + . . .
(5)
The sum in the denominator extends to all specieswhich can be formed in aqueous solution. From (4) and (5) we obtain l
O = K,. Y~,hr.,. ax~z'. [Th,+] a~h~, q- [ThLs+ ] a~-h~, -I- • • •
(6)
We shall now define the formation of a complex by relat~g it to the neutral complex ThL4~- ThL~_~ q- iL
(7)
where the index i gives the charge of the complex. The thermodynamic equilibrium constant for (7) is ff~ = a a ~ _ , , aL'. a~hlL,
(8)
~xl~K. A. Ks~os, G. E. Mooat and F. NtLSON, J. Amer. Chem. Soc. 711, 2692 (1956). ~xs~L. I. KATZIN,J. R. FeRaARO, W. W. WtTCDLANDTand R, L. M c B ~ , 1. Amer. Chem. Soc. 711,5139 (1956).
114
J. DANON
which gives [ThL~-i] = /~iyi-laL- ~.
(9)
aThL,
Introducing (9) into (6) we get D = K r . y~lThL, altL 4 . ( ~ , Yi-laL--t)--I
(I0)
We shall now make the following assumptions: (1) The activity of the nitrate ion in solution is given by aL -----[L]
4" ) • ~'(LiNO8
(11)
where [L] is the concentration of nitrate ion in the aqueous phase and 7~lso,) the corresponding mean ionic activity coefficient of LiNO3. (2) The activity of the nitrate ion in the resin phase is given by a~, -----aL. [L]R - t • [Li]R - t
(12)
where aL is the activity in the aqueous phase according to (1 I) and [L]R and [Li]R the concentrations of nitrate and lithium respectively in the resin phase. The reference state for the activity aRL is taken arbitrarily as ara~° for at. = 1. Relation (12) can be derived from Donnan equilibrium of the electrolyte between the aqueous solution and the resin phase. ~13~ (3) The activity coefficients 7'~,ThS, and 7t are independent of the value of as. The justifications for these assumptions are given in MARCUS and CORYFJ~Lpapers. Introducing the above assumptions in (10) we find log D = log (K,.
-1 s . a R S 40) -~?R4ThL
4(log
aRL --
log
a R L °) - -
log Z/~iyi-laL -i
(13)
We shall now make K,* = K,. 7£,ITh~. aRs '0 8,* =
-1
(14) (15)
According to the assumptions made K,* and fl/* are independent of as. We define a correction function F(a) as F(a) = log aRL -- log aRL°
(16)
Introducing (14), (15) and (16) into (13) we find log D = log Kr* q- 4F(a) -- log ~ fli*as -~
(17)
The difference log D O= log D -- 4F(a) gives the distribution coefficients corrected from the invasion effects of the supporting electrolyte in the resin phase: log Do = log D -- 4F(a) = log K,* -- log Y. ill*as -~
(18)
It is easily calculated that d log D O - = d log aL
[=
4--
a
(19)
where [ is the mean charge number of the complex species and a the mean number of ligands as defined by Bjerrum. (la~ K. A. K l t a u s and G. E. MOOR~, .LAmer. Chem. $oc. 75, 1457 0953).
Determination of the ~ability comtants of tlmrium nitrate complexes
115
The values of the distribution eoeffr.iznts as a function of the LilqOs molarity are given in Table 1. This data was obtained in equilibration experiments (24 hr) with the resin at room temperature (25 4- 3°C). The concentration of the LiNOs solutions was measured in a flame photometer. All LiNOs solutions were made ~07 M in TABLe 1 MLINO* I
1"55 2"00 2"39 2"53 2"89 3"40
log D
MLINOg
4"02 4"35 5"06 5"20 6"24 6"32 8"44
1"10" 1"45" 1 "72* 1"83
2"21 2"36
log D
2"81 2"91 3"21 3"35 3"69* 3"67* 4"33*
* Values with errors larger than -I-0-05 log units,
4
!
0
o
s
o
"~
.
.
.
.
~
IogAL FIG. l.--Adsorption of Th(IV) by Dow¢~x-1at varying LiNOs activities.
HNOs in order to avoid hydrolysis of thorium nitrate. However even at these acidities hydrolytical reactions appear to occur at LiNO s molarities lower than 2 M.t Fig. 1 shows the values of log D as a function of log aT.. The values of log % were calculated from mean activity coefficients of LiNOs taken from RoerssoN and STOKES.(14) Experiments with 0.1 g of glass wool per 10 ml of solution showed in thes~ conditions glass adsorption in amounts which are not negligible compared to the resin adsorption. ¢]4) R. A. ROmNSONand 11. H. STOKe, Electrolyte Solutions. Butterworths Scientific Publications, London (]955).
116
J. DANON
Fig. 2 shows the variation of the invasion function F(a) with log aL. This data was obtained with Dowex-1, X-8, 50-100 mesh in the nitrate form, from the same batch as was used in the equilibration experiments with thorium, according to the procedure described by KRAUS and NeX.SON.(xa) Details about this data will be published in a further paper. Fig. 3 shows the variation of the corrected distribution log D O as a function of log a,.
/
0 J, A o tl.
-0-5
IogA L
FZG. 2.--Invasion function F(a) vs. LiNO s activity.
In the range of activities investigated the values of ~= d log Do/d log aL go from +0"6 to --1.8 indicating the presence of the successive species ThLa+, ThLt o, ThLe- , ThL6*- in solution. Preliminary values of 8~,x*, 8o*, 8-1" and 8-=* constants were derived from the relation log 8*+x
--log aL
( i = i + ½)
With the values obtained for 8+1" and/~* we formed the equation in three parameters which was solved by curve fitting method of SILL~ :(~) 1
Do
8+a*
1
K. a'~ ~ +
.~
8K*
.a + ~
. aI
(20)
With the values of K,* and/3-2* obtained from (20) we solved the equation Kr* a 8+x* 1 8-1" D--~ "~_--~ - a~ . . . .8-,* . + ~ . a + ~ _ ~ .0= ¢11) L. G. SILL~r~, Acta Chem. $cand. 10, 186 (1956).
(21)
Determination of the stability constants of thorium nitrate complexes
117
By this procedure the following set of values were derived for the constants: log Kr~ 1"22; log ~+x*= ~0"22; log /~0----0"00; log ~ _ ~ * - - - 0 " 8 0 ± 0 " 1 7 ; log/~-I* ~ --1"70 :J: 0"22. With these values the solid curves shown in Fig. 1 and Fig. 3 were calculated from equations (17) and (18) respectively. The fitting of the experimental points is satisfactory. The data could also be fitted by a curve log De ~ log Kr* -- log(l ~- ~_l*a-~ /~_~*a~) which would mean that no Th(NOs)3+ species are present at the lower activity
I'C
0,I Q0 o
-O~
o
0~'
~
,.'.,
IogA L
FIG. 3.--Corrected distribution vs. LiNO s activity.
range investigated. However at low LiNOs activities the hydrolytical reactions of thorium nitrate tend to increase the values of the distribution coefficients. For this reason the value found for ~+1" may be considered as the upper limit of the association constant. The comparison between the values of stabilityconstants/~* (referredto the neutral complex) calculated from FOMIN and M~oRovKs data c8~ and those obtained from the anion-exchange data is shown in Table 2. In the above comparison we disregard differences in activity coefficients between the two set of data. The stability constants ~ * calculated from the anion-exchange data represent the product of true thermodynamic equilibrium constants by activity coefficients 7,~ whereas FOM~ and M~oRovA's data are concentration constants evaluated at constant ionic strength. The values obtained for/~-x* and/~-i* show that the formation of thorium nitrate negatively charged complexes occur at high concentrations of nitrate ion ( ~ 3 M). This is in agreement with the results of FOMn~and MxloRovxCS)which have shown that the extraction behaviour of thorium nitrate by TBP up to a concentration of 2.2 M
118
J. DANON
in nitrate ion was essentially not influenced by the formation of negatively charged complexes. No evidence was found in the range of activities investigated for the presence of complexes with more negative charge than --2, although the possibility of the existence of such species at higher activities of nitrate ion cannot be ruled out entirely on the basis of the anion-exchange data. TABLE 2 log 3÷1"
log/~o*
log 3-1"
log t~-.~*
FOMIN and MAIOrtOVA(,u = 2)
+0'26
0'00
--
--
Present work (variable LiNO3)
+0.22
0.00
--0.80
-1.70
The overall stability constant for the reaction Zr 4+ + 6NO8 ~- Zr(NO3)62has been investigated by solvent extraction studies with TBP (16)and its value was found to be fie = 0.02, at/z = 4. From FOMIN and MAIOROVA'Sdata and the results of the present study we found for the analogous reaction for Th 4+ the value fls = 0" 11. This shows an increasing tendency in the stability of the negatively charged nitrate complexes with increasing ionic radius in going from Zr 4+ to Th 4+. The same trend was observed with the first nitrate complexes of these elements. (17) Acknowledgements--We are indebted to Miss Z. CAILLAUXfor the lithium analysis. This work was supported by the Conselho Nacional de Pesquisas. (16) A, S. SOLOVKIN,Zh. Neorg. Kkim. 2, 611 (1957). (Iv) j. C. HINDMAN,lonic and Molecular Species of Plutonium in Solution, NNES, Plutonium Project Record, Vol. 14-A, p. 341. McGraw-Hill, New York (1954).