- Email: [email protected]
The stability constants of polyvalent cations and aromatic arsonic acids—II Complexes of the uranyl ion with arsonic acid
J. inorg,nucl.Chem.,1969,Vol. 31, pp. 2845to 2851. PergamonPress. Printedin Great Britain
THE STABILITY CATIONS AND
CONSTANTS OF POLYVALENT AROMATIC ARSONIC ACIDS-II
COMPLEXES OF THE U R A N Y L ION WITH ARSONIC ACID M. (D h - E I D H I N and S. (3 C I N N E I D E Chemistry Department, University College, Galway, Ireland
(First received 17 July 1968: in revised form 22 November 1968) Abstract - Stability constants have been measured for the complexes of the uranyl ion with 2-hydroxybenzene arsonic acid and 2-4 dihydroxy benzene arsonic acid in aqueous media at 25 _+0. I°C. Possible structures are postulated for the complexes. Intramolecular hydrogen bonding, which prevents intermolecular association of arsonyl (---As = 0) oxygens with uranyl ions in other molecules of the complexes, may be responsible for their relatively high solubilities compared to those of uranyl complexes with benzene arsonic acid and 4-hydroxybenzene arsonic acid, in which intermolecul~r association via arsonyl oxygen atoms is possible.
INTRODUCTION A PREVIOUS publication[l] presented thermodynamic dissociation constants for a number of aromatic arsonic acids. Evidence has also been presented[l, 2] for complex formation between these acids and polyvalent cations, including the uranyl ion. Of these acids, only those with hydroxyl groups ortho to the arsonic acid groups, viz. 2-hydroxybenzene arsonic acid (2-HBAA) and 2,4-dihydroxybenzene arsonic acid (2-4 D H B A A ) , give uranyl complexes of sufficient solubility to permit a determination of their compositions and stability constants in aqueous media by spectrophotometric and potentiometric methods. This investigation is concerned with these determinations. EXPERIMENTAL Materials and method The apparatus, arsonic acid reagents, and experimental procedure were the same as previously described [1]. The concentration of uranyl ion in a stock solution of uranyl nitrate was estimated from the U:~O8 formed on evaporation to dryness of aliquots of the solutions and ignition of the residues to constant weight. Potentiometric data were obtained for solutions (50 ml) which were 1.00 × 10-3 M in uranyl nitrate and which contained various integral mole ratios of arsonic acid reagents to uranium. Spectra were measured in "suprasil" cells of path length 1 era.
RESULTS
AND
DISCUSSION
The titration curves in Fig. 1 are those for solutions of uranyl ion and 2-HBAA at/x-* 0. The inflexion in curve 2 at the point a = 2 corresponds to the consumption of two moles of base (KOH) per mole of uranyl ion present. This corresponds to the reaction UO22++ H3X ~ U O 2 ( H X ) + 2 H + (1) 1. M. (3 h-Eidhin and S. (3 Cinn~ide, J. inorg, nucl. Chem. 30, 3209 (1968). 2. R. V. Davies, J. Kennedy (S. (3 Cinn~ide), R. W. Mcllroy, R. Spence and K. M. Hill, Natare. Lond. 203, ~ 110 (1964). :!845
2846
M. (3 h - E I D H I N and S. (3 C 1 N N E I D E
I
2
Io
4
3: Q.
0
I I
I
2
I
I
~
4
I
5
I
6
0
Fig. I. Plots of pH vs. mole equivalents (a) of base consumed in titration of solutions of: (1) 10-3M2-HBAA(H3X); (2) 10-3MH3X+I03MUO2~+; (3) 2XI0-aMH3X+10 -3MUO~2+; (4) 3X10-3MH3X + 10-aMUO22+.
where H3X represents the tribasic acid 2-HBAA and HX 2- the ligand (L). It is evident from curve 2 that the uranium is all in the form U O d H X ) at pH 4.2. The constancy of K1 values, calculated for various points on curves 2 and 3 (see below) is supporting evidence for Equation 1, and shows that this mechanism of complex formation is the only one operative at pH ~ 4-0 even when H3X is in stoichiometric excess of the uranyl ion. Curve 1, Fig. 2, shows the spectrum of a solution at pH 3.72 which is 10 -3 M in both total uranyl ion and 2-HBAA. The spectrum is characterized by two broad bands at 370 and 450 m~. Similar spectra not included in Fig. 2, were obtained for other solutions at pH 3.0-4.0 containing mole ratios of 2-HBAA to uranyl ion of 2 and 3. This also indicates that the same complex is formed between 2-HBAA and the uranyl ion at all pH values between 3.0 and 4.0. The formula UO2(HX) inferred from potentiometric data (Equation 1) is consistent with the maxima displayed at 0.5 in continuous variation curves at different wavelengths for solutions conforming mixtures of the reagents at pH 4-0 (Fig. 3a). Although the formation of the complex UOz(HX) is complete at pH 4.2 for systems with a ratio of ligand to uranyl ion of 1 : 1, an increase in pH from 4.2 to 11.0 does not result in precipitation, except when extraneous salts such as potass-
Some stability constants - 11
2847
1.5
a - !-0
-o
c)
0.5
I 320
|
I 360
I
t 400
I
I 440
I
[ 480
I
/ .520
I
Wavelength,m/J. Fig. 2. Absorption spectra of solution mixtures of 2-HBAA (H3X) and uranyl nitrate at various pHs; (1) 10-3MH3X+ 10-3MUO22+ at pH 3.72: (2) as in (I) but at pH 5.81: (3) 2× 10 :~MH:,X+ 10-aMUO22+ at pH 5.13; (4) as in (3) but at pH 5.90; (5) 3× 10:1 MH3X+ 10 3MUO._,~+at pH 5.99.
ium nitrate are present. The absence of a precipitate at high pH indicates that the iigand remains associated with the uranyl ion, although hydrolysis or deprotonation of the hydrated complex at pH :~ 5.0 is evident from the additional alkali (a > 2) consumed (Curve 2, Fig. 1). The disappearance of the two bands characteristic of UO.,(HX) in the spectrum of the ! : 1 system at pH5.8, (Curve 2. Fig. 2) is also consistent with a change of species. Spectra of solutions containing 2 - H B A A and uranyl ion in ratios 2 : 1 and 3 : 1 and at pH 5-6 are, however, characlerized by two broad bands with maxima at 350 and 450 i ~ (Curves 3, 4 and 5, Fig. 2). This fact, together with the appreciably higher D values for these solutions compared to those of the 1 : 1 systems, indicates the formation of a complex other than UO2(HX) or its hydrolytic products. N o further increase in D is observed when the ratios of 2-H BAA to uranyl ion are increased beyond - 5.0 in solutions at pH 6.0-6-5. This implies that under such conditions the formation of the complex is complete. Except for the slightly higher D values resulting from the higher concentrations of complex species, the
2848
M. 0 h-E1DH1N and S. 0 CINNI~IDE
^ 386 m/x
0.3
4.
C0'2 "o
8 0
0.1
S Z I 0.2
\ I
1
I 0"5
I
I
I 0.8
I 0'5
I I 0-6 0-7 0.8
I 0'9
; a
b
Fig. 3. Continuous variation curves at different wavelengths for system UO.2~+2-HBAA (H3X)at (a) pH 4.0; (b) pH 6.5. spectra of these solutions are the same as that of a solution with a 3 : 1 ratio at
pH 5-99 (Curve 5, Fig. 2). Broad maxima at 0.67 in continuous variation measurements on solutions at pH 6.5 containing 2- H B A A and uranyl ion (Fig. 3b), indicate incomplete formation of a complex with a ligand to uranium ratio of 2: 1. Curves 3 and 4 (Fig. l) may be shown (see below) to be consistent with the formation of such a complex, with four equivalents of hydrogen ion being liberated per mole of complex formed in accordance with the equation: UO22++ 2H3X ~ [UO2(HX)212-+ 4H +.
(2)
It is also evident from spectrophotometric data that UOz(HX) is the only complex present in solutions of 2-HBAA and uranyl ion at pH < 4.2 even when the ratio of the former to the latter reagent is 6 : 1. Hence the respective stepwise stability constants K, and Kz for the complexes UO2(HX) and [UO2(HX)2] 2- do not overlap, and can be calculated from the equations of Irving and Rosotti[3] viz. K1=~/(1-h)[L]
3. H. Irvingand H. Rosotti,J.
chem. Soc.
and
K2=(~-l)/(2--~)[L].
3397 (1953).
Some stability constants - ! I
2849
Here L represents the ligand HX 2- and ~, the formation factor, varies from 0 to 1.0 in the former equation and from 1.0 to 2.0 in the latter. Values of [H +] which are required for the calculations of stoichiometric constants in media with ~ = 0. I(KNO3) were obtained by dividing the hydrogen ion activities (deduced from pH) by 0-785-the activity coefficient of the ion in these medial4]. Constant values of Kl were obtained for ratios of 2-HBAA to uranyl ion of 1,2 and 3. At pH ~ 4-8 precipitation of a uranium species occurred in media of/z ~ 0.05 (KNO3) containing uranyl ion and arsonic acid reagent. This ruled out the possibility of obtaining K2 values for the complexes in media of constant/x, and accordingly the values were obtained only for media containing no extraneous salts (/x ~ 0). In addition, and because of hydrolysis, values of K2 for the 2 : 1 system increased slightly with increasing pH, but constant values were obtained for the 3 : 1 and 4 : I systems. A similar series of experiments demonstrated that the complexes UO2(H2Y) and [UO2(HzY)2]"- are formed between the uranyl ion and the tetrabasic acid 2,4-DHBAA, (H4Y). The stepwise stability constants for these complexes were obtained using the same procedure as that used for the complexes with 2-HBAA. Only slight differences exist between the corresponding spectra of the 1 : I and 2 : 1 complexes with both reagents. The logarithms of the acid association constants, determined by the method outlined in [ 1], togethe, with logarithms of K, and K~ for the complexes, are given in Table 1. Table 1. Stability constants of complexes of arsonic acids and uranyl ion at 25 + 0. I°C Log K Acid 2-HBAA(HaX)
2-4DHBAA(H4Y)
Reaction H + + + H 2 X - <> H3X H + + H X 2 X H2X UO22++ H X 2- ~ UO.,(hx) H X 2 + U O ~ ( H X ) ~ [UO~(HX)z] H + + H 3 Y - <> H4Y H + + H 2 Y z- <> HaYUO22++ H~Y ~- ~ UO~(H2Y) H2Y'-'-+ UO2(H2Y) ~ [UO2(HzY)._,]2
/z = 0.1 3.83±0.02 7.50±0-02 8.64+_0.02 -4.01 ±0.02 7.56_+0.02 8.76_+_0.03 --
jx --~ 0 4-02+-0.02 7.75-+0.02 8.75 _+0.02 5.11 _+0.05 4.19_+0.02 7.82+_0.02 8.83 +0.02 5"29-+0"02
It has already been demonstrated [5], that saturation of a condensation polymer of formaldehyde, resorcinol and 2,4-DHBAA with uranyl ion occurs with a uranium to arsenic ratio of unity. This implies the formation of a complex with a ratio of the arsonate ligand to uranyl ion of 1 : 1 .in the polymer phase. Resins of lower loading were not investigated with the object of determining the composition of the adsorbed complex. However, it appears from the present investigations with monomeric models of the functional groups in the polymer, that when the latter, at pH > 4-8, is ~ 50% saturated with uranyl ion, the composition of the 4. C. W. Davies. Ion Association, p. 41. Butterworths, London (1962). 5. E. S. Lane and J. L. Willans, Private communication quoted in Ref. 2 (1964).
2850
M. 0 h - E I D H I N and S. 6 CINNI~IDE
complex in the polymer phase is likely to consist of two functional groups associated with each uranyl ion. It was also argued [2] that the structure of the 1 : 1 uranyl complex with 2,4D H B A A could be either A or B (Fig. 4), although the evidence presented tended to favour structure A. O~H
f--~ ~,/o. b HO~/~'-AS~o/U02 A
~ 9-u~ /~o . ~ HO
AS~o - • B
Fig. 4. Possible structures for complexes of uranyl ion with 2-4DHBAA.
Additional evidence arising from the present investigation also supports structure A for the 1 : 1 complex. For, should structure B be correct, it is reasonable to assume that the hydrogen, which would be associated in acid solutions with the negatively-charged oxygen in such a complex, would be at least as labile as the most labile hydrogen in the free acid. This would, however, result in an inflexion at a = 3 in the titration of the 1 : 1 system, instead of at a = 2 as observed, consistent with the unionized species, A*. Four-membered ring structures are, of course, quite common in complexes of the uranyl ion e.g. [UO2(CO3)3] 4-, [ U O 2 ( O A c ) 3 ] 1 - , [ U O 2 ( N O 3 ) 3 ] l - and [ U O 2 ( N O z ) 2 ] [ O P ( O E ) 3 1 2 , [6, 7]. Structure C is proposed for the anionic 2:1 complex with 2,4-DHBAA.* Except for the absence of the hydroxyl groups para to the complexing arsonate groups, structures analogous to A and C are also postulated for the uranyl complexes with 2-HBAA.* It is of interest that, while the solubilities of complexes of the uranul ion with 2-HBAA and 2,4-DHBAA are not less than 10-2moles/l., the solubilities of uranyl complexes of benzene arsonic acid and 4-hydroxy benzene arsonic acid (4-HBAA) are not greater than 10 -5 moles/1. While this may not be unexpected with benzene arsonic acid, one might anticipate a similar order of solubility for the uranyl complexes with the two isomers, 2-HBAA and 4-HBAA. A likely explanation for the relatively low solubility of the complex with 4-HBAA is that the uranyl ion is associated with the arsonate group as in structures A and C (Fig. 4), but in contrast with these structures the arsonyl (= As = 0) oxygen is free to associate with uranyl ions in other molecules of the complex. Such association would result in polymeric species which, coupled with the additional displacement of water of coordination, would tend to give a species which is much less soluble than the hydrated monomeric complexes (structures A and C and their analogues with 2-HBAA), whose arsonyl oxygens would be unable to *Note added in proof. The evidence does not, however, preclude structures with one or two ligands associated with the uranyl ion as in Fig. 4B, but in which the o-phenolic hydrogens are not displaced. Such alternative structures would not affect the values for the stability constants (Table 1). 6. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, p. 1085. Interscience, London (1966). 7. J. Kennedy, Chem. Ind. 950 (1958).
Some stability c o n s t a n t s - I I
2851
associate with uranyl ions in other complex molecules because of intramolecular hydrogen bonding in a 6-membered ring. Acknowledgements-The support of the United Kingdom Atomic Energy Authority and that of the !fish Agricultural Research Institute is gratefully acknowledged.
THE STABILITY CATIONS AND
CONSTANTS OF POLYVALENT AROMATIC ARSONIC ACIDS-II
COMPLEXES OF THE U R A N Y L ION WITH ARSONIC ACID M. (D h - E I D H I N and S. (3 C I N N E I D E Chemistry Department, University College, Galway, Ireland
(First received 17 July 1968: in revised form 22 November 1968) Abstract - Stability constants have been measured for the complexes of the uranyl ion with 2-hydroxybenzene arsonic acid and 2-4 dihydroxy benzene arsonic acid in aqueous media at 25 _+0. I°C. Possible structures are postulated for the complexes. Intramolecular hydrogen bonding, which prevents intermolecular association of arsonyl (---As = 0) oxygens with uranyl ions in other molecules of the complexes, may be responsible for their relatively high solubilities compared to those of uranyl complexes with benzene arsonic acid and 4-hydroxybenzene arsonic acid, in which intermolecul~r association via arsonyl oxygen atoms is possible.
INTRODUCTION A PREVIOUS publication[l] presented thermodynamic dissociation constants for a number of aromatic arsonic acids. Evidence has also been presented[l, 2] for complex formation between these acids and polyvalent cations, including the uranyl ion. Of these acids, only those with hydroxyl groups ortho to the arsonic acid groups, viz. 2-hydroxybenzene arsonic acid (2-HBAA) and 2,4-dihydroxybenzene arsonic acid (2-4 D H B A A ) , give uranyl complexes of sufficient solubility to permit a determination of their compositions and stability constants in aqueous media by spectrophotometric and potentiometric methods. This investigation is concerned with these determinations. EXPERIMENTAL Materials and method The apparatus, arsonic acid reagents, and experimental procedure were the same as previously described [1]. The concentration of uranyl ion in a stock solution of uranyl nitrate was estimated from the U:~O8 formed on evaporation to dryness of aliquots of the solutions and ignition of the residues to constant weight. Potentiometric data were obtained for solutions (50 ml) which were 1.00 × 10-3 M in uranyl nitrate and which contained various integral mole ratios of arsonic acid reagents to uranium. Spectra were measured in "suprasil" cells of path length 1 era.
RESULTS
AND
DISCUSSION
The titration curves in Fig. 1 are those for solutions of uranyl ion and 2-HBAA at/x-* 0. The inflexion in curve 2 at the point a = 2 corresponds to the consumption of two moles of base (KOH) per mole of uranyl ion present. This corresponds to the reaction UO22++ H3X ~ U O 2 ( H X ) + 2 H + (1) 1. M. (3 h-Eidhin and S. (3 Cinn~ide, J. inorg, nucl. Chem. 30, 3209 (1968). 2. R. V. Davies, J. Kennedy (S. (3 Cinn~ide), R. W. Mcllroy, R. Spence and K. M. Hill, Natare. Lond. 203, ~ 110 (1964). :!845
2846
M. (3 h - E I D H I N and S. (3 C 1 N N E I D E
I
2
Io
4
3: Q.
0
I I
I
2
I
I
~
4
I
5
I
6
0
Fig. I. Plots of pH vs. mole equivalents (a) of base consumed in titration of solutions of: (1) 10-3M2-HBAA(H3X); (2) 10-3MH3X+I03MUO2~+; (3) 2XI0-aMH3X+10 -3MUO~2+; (4) 3X10-3MH3X + 10-aMUO22+.
where H3X represents the tribasic acid 2-HBAA and HX 2- the ligand (L). It is evident from curve 2 that the uranium is all in the form U O d H X ) at pH 4.2. The constancy of K1 values, calculated for various points on curves 2 and 3 (see below) is supporting evidence for Equation 1, and shows that this mechanism of complex formation is the only one operative at pH ~ 4-0 even when H3X is in stoichiometric excess of the uranyl ion. Curve 1, Fig. 2, shows the spectrum of a solution at pH 3.72 which is 10 -3 M in both total uranyl ion and 2-HBAA. The spectrum is characterized by two broad bands at 370 and 450 m~. Similar spectra not included in Fig. 2, were obtained for other solutions at pH 3.0-4.0 containing mole ratios of 2-HBAA to uranyl ion of 2 and 3. This also indicates that the same complex is formed between 2-HBAA and the uranyl ion at all pH values between 3.0 and 4.0. The formula UO2(HX) inferred from potentiometric data (Equation 1) is consistent with the maxima displayed at 0.5 in continuous variation curves at different wavelengths for solutions conforming mixtures of the reagents at pH 4-0 (Fig. 3a). Although the formation of the complex UOz(HX) is complete at pH 4.2 for systems with a ratio of ligand to uranyl ion of 1 : 1, an increase in pH from 4.2 to 11.0 does not result in precipitation, except when extraneous salts such as potass-
Some stability constants - 11
2847
1.5
a - !-0
-o
c)
0.5
I 320
|
I 360
I
t 400
I
I 440
I
[ 480
I
/ .520
I
Wavelength,m/J. Fig. 2. Absorption spectra of solution mixtures of 2-HBAA (H3X) and uranyl nitrate at various pHs; (1) 10-3MH3X+ 10-3MUO22+ at pH 3.72: (2) as in (I) but at pH 5.81: (3) 2× 10 :~MH:,X+ 10-aMUO22+ at pH 5.13; (4) as in (3) but at pH 5.90; (5) 3× 10:1 MH3X+ 10 3MUO._,~+at pH 5.99.
ium nitrate are present. The absence of a precipitate at high pH indicates that the iigand remains associated with the uranyl ion, although hydrolysis or deprotonation of the hydrated complex at pH :~ 5.0 is evident from the additional alkali (a > 2) consumed (Curve 2, Fig. 1). The disappearance of the two bands characteristic of UO.,(HX) in the spectrum of the ! : 1 system at pH5.8, (Curve 2. Fig. 2) is also consistent with a change of species. Spectra of solutions containing 2 - H B A A and uranyl ion in ratios 2 : 1 and 3 : 1 and at pH 5-6 are, however, characlerized by two broad bands with maxima at 350 and 450 i ~ (Curves 3, 4 and 5, Fig. 2). This fact, together with the appreciably higher D values for these solutions compared to those of the 1 : 1 systems, indicates the formation of a complex other than UO2(HX) or its hydrolytic products. N o further increase in D is observed when the ratios of 2-H BAA to uranyl ion are increased beyond - 5.0 in solutions at pH 6.0-6-5. This implies that under such conditions the formation of the complex is complete. Except for the slightly higher D values resulting from the higher concentrations of complex species, the
2848
M. 0 h-E1DH1N and S. 0 CINNI~IDE
^ 386 m/x
0.3
4.
C0'2 "o
8 0
0.1
S Z I 0.2
\ I
1
I 0"5
I
I
I 0.8
I 0'5
I I 0-6 0-7 0.8
I 0'9
; a
b
Fig. 3. Continuous variation curves at different wavelengths for system UO.2~+2-HBAA (H3X)at (a) pH 4.0; (b) pH 6.5. spectra of these solutions are the same as that of a solution with a 3 : 1 ratio at
pH 5-99 (Curve 5, Fig. 2). Broad maxima at 0.67 in continuous variation measurements on solutions at pH 6.5 containing 2- H B A A and uranyl ion (Fig. 3b), indicate incomplete formation of a complex with a ligand to uranium ratio of 2: 1. Curves 3 and 4 (Fig. l) may be shown (see below) to be consistent with the formation of such a complex, with four equivalents of hydrogen ion being liberated per mole of complex formed in accordance with the equation: UO22++ 2H3X ~ [UO2(HX)212-+ 4H +.
(2)
It is also evident from spectrophotometric data that UOz(HX) is the only complex present in solutions of 2-HBAA and uranyl ion at pH < 4.2 even when the ratio of the former to the latter reagent is 6 : 1. Hence the respective stepwise stability constants K, and Kz for the complexes UO2(HX) and [UO2(HX)2] 2- do not overlap, and can be calculated from the equations of Irving and Rosotti[3] viz. K1=~/(1-h)[L]
3. H. Irvingand H. Rosotti,J.
chem. Soc.
and
K2=(~-l)/(2--~)[L].
3397 (1953).
Some stability constants - ! I
2849
Here L represents the ligand HX 2- and ~, the formation factor, varies from 0 to 1.0 in the former equation and from 1.0 to 2.0 in the latter. Values of [H +] which are required for the calculations of stoichiometric constants in media with ~ = 0. I(KNO3) were obtained by dividing the hydrogen ion activities (deduced from pH) by 0-785-the activity coefficient of the ion in these medial4]. Constant values of Kl were obtained for ratios of 2-HBAA to uranyl ion of 1,2 and 3. At pH ~ 4-8 precipitation of a uranium species occurred in media of/z ~ 0.05 (KNO3) containing uranyl ion and arsonic acid reagent. This ruled out the possibility of obtaining K2 values for the complexes in media of constant/x, and accordingly the values were obtained only for media containing no extraneous salts (/x ~ 0). In addition, and because of hydrolysis, values of K2 for the 2 : 1 system increased slightly with increasing pH, but constant values were obtained for the 3 : 1 and 4 : I systems. A similar series of experiments demonstrated that the complexes UO2(H2Y) and [UO2(HzY)2]"- are formed between the uranyl ion and the tetrabasic acid 2,4-DHBAA, (H4Y). The stepwise stability constants for these complexes were obtained using the same procedure as that used for the complexes with 2-HBAA. Only slight differences exist between the corresponding spectra of the 1 : I and 2 : 1 complexes with both reagents. The logarithms of the acid association constants, determined by the method outlined in [ 1], togethe, with logarithms of K, and K~ for the complexes, are given in Table 1. Table 1. Stability constants of complexes of arsonic acids and uranyl ion at 25 + 0. I°C Log K Acid 2-HBAA(HaX)
2-4DHBAA(H4Y)
Reaction H + + + H 2 X - <> H3X H + + H X 2 X H2X UO22++ H X 2- ~ UO.,(hx) H X 2 + U O ~ ( H X ) ~ [UO~(HX)z] H + + H 3 Y - <> H4Y H + + H 2 Y z- <> HaYUO22++ H~Y ~- ~ UO~(H2Y) H2Y'-'-+ UO2(H2Y) ~ [UO2(HzY)._,]2
/z = 0.1 3.83±0.02 7.50±0-02 8.64+_0.02 -4.01 ±0.02 7.56_+0.02 8.76_+_0.03 --
jx --~ 0 4-02+-0.02 7.75-+0.02 8.75 _+0.02 5.11 _+0.05 4.19_+0.02 7.82+_0.02 8.83 +0.02 5"29-+0"02
It has already been demonstrated [5], that saturation of a condensation polymer of formaldehyde, resorcinol and 2,4-DHBAA with uranyl ion occurs with a uranium to arsenic ratio of unity. This implies the formation of a complex with a ratio of the arsonate ligand to uranyl ion of 1 : 1 .in the polymer phase. Resins of lower loading were not investigated with the object of determining the composition of the adsorbed complex. However, it appears from the present investigations with monomeric models of the functional groups in the polymer, that when the latter, at pH > 4-8, is ~ 50% saturated with uranyl ion, the composition of the 4. C. W. Davies. Ion Association, p. 41. Butterworths, London (1962). 5. E. S. Lane and J. L. Willans, Private communication quoted in Ref. 2 (1964).
2850
M. 0 h - E I D H I N and S. 6 CINNI~IDE
complex in the polymer phase is likely to consist of two functional groups associated with each uranyl ion. It was also argued [2] that the structure of the 1 : 1 uranyl complex with 2,4D H B A A could be either A or B (Fig. 4), although the evidence presented tended to favour structure A. O~H
f--~ ~,/o. b HO~/~'-AS~o/U02 A
~ 9-u~ /~o . ~ HO
AS~o - • B
Fig. 4. Possible structures for complexes of uranyl ion with 2-4DHBAA.
Additional evidence arising from the present investigation also supports structure A for the 1 : 1 complex. For, should structure B be correct, it is reasonable to assume that the hydrogen, which would be associated in acid solutions with the negatively-charged oxygen in such a complex, would be at least as labile as the most labile hydrogen in the free acid. This would, however, result in an inflexion at a = 3 in the titration of the 1 : 1 system, instead of at a = 2 as observed, consistent with the unionized species, A*. Four-membered ring structures are, of course, quite common in complexes of the uranyl ion e.g. [UO2(CO3)3] 4-, [ U O 2 ( O A c ) 3 ] 1 - , [ U O 2 ( N O 3 ) 3 ] l - and [ U O 2 ( N O z ) 2 ] [ O P ( O E ) 3 1 2 , [6, 7]. Structure C is proposed for the anionic 2:1 complex with 2,4-DHBAA.* Except for the absence of the hydroxyl groups para to the complexing arsonate groups, structures analogous to A and C are also postulated for the uranyl complexes with 2-HBAA.* It is of interest that, while the solubilities of complexes of the uranul ion with 2-HBAA and 2,4-DHBAA are not less than 10-2moles/l., the solubilities of uranyl complexes of benzene arsonic acid and 4-hydroxy benzene arsonic acid (4-HBAA) are not greater than 10 -5 moles/1. While this may not be unexpected with benzene arsonic acid, one might anticipate a similar order of solubility for the uranyl complexes with the two isomers, 2-HBAA and 4-HBAA. A likely explanation for the relatively low solubility of the complex with 4-HBAA is that the uranyl ion is associated with the arsonate group as in structures A and C (Fig. 4), but in contrast with these structures the arsonyl (= As = 0) oxygen is free to associate with uranyl ions in other molecules of the complex. Such association would result in polymeric species which, coupled with the additional displacement of water of coordination, would tend to give a species which is much less soluble than the hydrated monomeric complexes (structures A and C and their analogues with 2-HBAA), whose arsonyl oxygens would be unable to *Note added in proof. The evidence does not, however, preclude structures with one or two ligands associated with the uranyl ion as in Fig. 4B, but in which the o-phenolic hydrogens are not displaced. Such alternative structures would not affect the values for the stability constants (Table 1). 6. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, p. 1085. Interscience, London (1966). 7. J. Kennedy, Chem. Ind. 950 (1958).
Some stability c o n s t a n t s - I I
2851
associate with uranyl ions in other complex molecules because of intramolecular hydrogen bonding in a 6-membered ring. Acknowledgements-The support of the United Kingdom Atomic Energy Authority and that of the !fish Agricultural Research Institute is gratefully acknowledged.