934
T. W. NEWTONAND F. B. BAKER
Vol. 61
CHLORIDE COMPLEX IONS OF Pu(V1)I BY T. W. NEWTONAND F. B. BAKER University of California, Los Alamos Scientijk Laboratory, Los Alamos, NEWMexico Received February $2, i967
The effect of chloride ion on the absorption spectrum of Pu(V1) has been investigated in solutions with = 2. A t eight different wave lengths the absorption was determined as a function of (Cl-) from 0 to 1.8 M . Making the usual assamptions, it was found that the data a t any one wave length were consistent with the formation of a single complex ion with a constant association quotient and constant absorptivity. However, the apparent association quotients obtained in this way were not constant, but were a function of wave length. I n order to explain this behavior it was found necessary to assume that a higher complex is present, the absorptivities are dependent on (Cl-) at some of the wave lengths, or both. Experiments a t 50” indicated more complexing a t higher temperatures.
Introduction The existence of chloride complexes of Pu(V1) is suggested by several types of data. Ward and Welch2 have studied the effect of chloride ion on the equilibrium between Pu(II1) and a cationexchange resin. Their results, together with those of Rabideauwhohasreported values for the Pu(II1)Pu(V1) formal potential in 1M HC10d3and in 1M HCl,4 enable an association quotient to be estimated if the pertinent activity coefficients may be assumed constant. Ward and Welch report 1.1 for the apparent dissociation quotient for PuCl++ in 1 M HC1 and Rabideau found the Pu(II1)Pu(V1) potential to be 1.0 mv. more negative in 1 M HC1 than in 1 M HC104. These results lead to 0.7 for the apparent association quotient of PuOzCl+. For the analogous equilibrium for the formation of UOZCl+, Day and Powers5 found an apparent quotient, a t 25’ and at p = 2, of 0.88 by using a distribution method. As pointed out by these authors, and discussed by Young and Jonesle changes in activity coefficients rather than complex ion formation may be responsible for the observed changes in distribution coefficient ratios. This comment applies with equal force to ion-exchange resin equilibrium studies. The results of these two investigations give only the changes in the appropriate ratios of stoichiometric activity coefficients on going from perchlorah to chloride solutions. I n the first example the ratio is y G & H C l / ~ 4 ~ P u C and 1~ in the second i t is y 3 & ~ ~ 1 ~ 1 2 / y 4 f H The ~ ~ . existence of complex ions, let alone their equilibrium quotients, cannot be inferred from such data unless accurate predictions can be made of the expected changes in activity coefficient ratios in the absence of complexing.6J I n order to estimate the size of the error which might be introduced through neglect of activity coefficient changes, we have recalculated the association quotient for U02C1+ approximating the changes in the pertinent activity coefficient (1) This work was done under the auspices of the U. S. Atomic Energy Commission. (2) M. Ward and G. A. Welch, J . Inorg. NucE. Chem., 2 , 395 (1956). (3) S. W. Rabideau, J . A m . Chem. Soc., 7 8 , 2705 (1956), Table IV. (4) S. W. Rabideau and H. D. Cowan, ibid., 77, 6145 (1955). Table IV. ( 5 ) R. A. Day and R. M. Powers, ibid., 76, 3895 (1954). (6) T. F. Young and A. C . Jones, “Annual Review of Physical Chemistry,” Vol. 3, Annual Reviews, Inc., Stanford, California, 1952, p. 286. (7) 0. Redlich, Chem. Reus., 89, 333 (194G).
ratios. It was assumed that the ratio of the actjvity coefficient of UOa++ in NaCl t o its activity coefficient in NaC104 is the same as the analogous ratio for Ba++, an ion which presumably is not complexed by C1-. This Ba++ ion activity coefficient ratio was estimated from the activity coefficients of pure BaClz and Ba(C104)zby using Guggenheim’s treatment of mixed electrolytes.* It has been shown that ?&€IC1 is the same in NaCl and NaC104 solution^.^ This led to an apparent association quotient for U02C1+ only about half as large as before. I n spite of the approximations involved, this calculation suffices to show that where complexing is slight, large errors are to be expected if changes in qctivi’ty coefficients are neglected. Another approach to the problem of the existence of the complex ion Pu02Clf can be made by studying the effect of chloride ion on the absorption spectrum of Pu(V1). ConnickIo has given spectra which show that in the region from 8300 to 8400 A., the height of a side band increases as the chloride ion concentration is increased, and that the main band first increases then decreases. I n the present work, the spectra were reexamined using a spectrophotometer of much better resolution in the hopes of getting data which could be used in a quantitative description of the Pu(V1)-C1- system.
Experimental Part Reagents.-The Pu( VI) solutions were prepared by dissolving weighed amounts of pure plutonium metal in standardized concentrated HCIO,. The resulting solution was diluted and then oxidized by paasing ozone for several hours. The excess ozone was swept out with a stream of oxygen. The oxidized solution was then diluted to mark in a volumetric flask to give the desired concentration. All measurements were made within 10 hours of the oxidation to minimize the effects of self a-irradiation. No changes in absorbance of any of the experimental solutions were observed in this time. Solutions of HClO, were prepared by diluting the concentrated acid and were standardized by titration. The concentrated acid was purified by boiling a t atmospheric pressure and then again under reduced pressure. The HCl solutions were prepared from reagent grade concentrated material and were used without further purification. Sets of solutions for the determination of absorbance versus chloride concentration were prepared by pipetting varying (8) E. A. Guggenheim, “Thermodynamics,” Second Edition, North-Holland Publ. Co., Amsterdam, 1950, p. 315. Or see R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” Butterworth Scientific Publications, London, 1955, p. 425. (9) S. J. Bates and J. W. Urmston, J . Am. Cham. Soc., 65, 4068 (1933). (IO) R. E. Connick, sz al., paper 4.20, “The Transuranium Elements,” edited by G. T. Seaborg, J . J. Kat5 a n d W. M. Manning, National Nuclear Energy Series, Division IV, VoI. 14B, McGrawHill Book Co., Inc., New York, N. Y.,1949, pp. 559 and 586.
,
CHLORIDE COMPLEX IONS OF PLUTONIUM(VI)
July, 1957
935
TABLE I TESTOF BEER’SLAWIN Pu(V1) SOLUTIONS AT 25’ Wave length, Solvent
A.
7
8302
2 fi1 HC104
-
8304 2 M HCI-
r
0.08 Slit width, mm. 0.07 0.10 0.15 0.20 7.6 3.0 Bandhalf-width, A.a 2.7 3.8 5.7 554.7 546.6 541.3 532.3 316.7 E , M-1 cm.-l Mean dev.b 0.002 0.0001 0.0005 0.001 0.000 Max. dev.b 0.004 0.001 0.001 0.002 0.001 a Calculated from data supplied with the instrument. These sorbances and those calculated using the tabulated B values. known amounts of standard HC1 solution into each of a set of volumetric flasks, the required amounts of HC104 solution were added from a buret, and finally the same volume of a standard Pu(V1) solution was pipetted into each flask. All solutions were thermostated in the absorption cell before placing in the spectrophotometer. For the runs at 50”, a thermostated cell holder was used. Spectrophotometer.-All measurements were made with a Cary Recording Spectrophotometer, Model 14, No. 5. This instrument uses a double monochromator and is considerably better with regard to resolution and stray light than the instruments which were available in the previous work on the absorption spectrum of Pu(V1). Moore and Krausll reported molar absorptivities1*for the sharp Pu(V1) peak near 8310 A. which varied strongly with plutonium concentration and with slit width. The maximum value was 365 M - 1 cm.-1 and was obtained a t the narrowest slit by extrapolating to zero Pu( VI) concentration. Using the present instrument, adherence to Beer’s law was tested in solutions of HC104 and in solutions of HCl. I n both solutions the law was obeyed a t least as far as an absorbancell of 0.85. Absorbance data taken a t several slit widths on solutions with six different Pu( VI) concentrations were used to calculate molar absorptivities by least squares. Absorbances calculated from these absorptivities agreed with the experimental ones within the experimental error. These data and results are shown in Table I. The results show that the absorbance scale of the instrument is essentially linear, resolution of the peaks is satisfactory, and that any chloride complexes which might have formed are predominantly mononuclear. At the narrower slits the response of the instrument is quite noisy in this wave length region, so most of the absorbance measurements were made with a slit of 0.15 mm. The maximum molar absorptivity, 555 M-’ cm.-1, is much greater than the value 365 which was reported previously. This difference is very likely due to the greater resolution and smaller stray light in the present instrument.
Results and Discussion When the degree of association is small, it is important to define clearly what is meant by “association” or “complex ion formation.” It is sometimes defined in terms of departure from expected behavior, but for our purposes we find it better to consider association or complex formation meaningful only when there are clearly distinguishable species present in the solution. This definition is not so restrictive (nor, perhaps, so exact) as that proposed by Redlich.’ A change in the spectrum of a certain substance when another is added does not necessarily imply complex formation. l3 For example, the broadening of the Pu(V1) band a t about 8300 A. caused by substituting NOS- for Clod- is best described as a medium effect (see Fig. 1.) The width of the band shows that not all the Pu(V1) ions have the ( 1 1 ) G. E. Moore and K. A. Kraus, paper 4.22, “The Transuranium Elements,” edited by G. T. Seaborg, J. J. Kats and W. M. Manning, National Nuclear Energy Series, Division I V , Vol. 14B, McGraw-Hill Book Co., h a . , New York, N. Y., 1949, p. 608. (12) H. K. Hughes, Anal. Chem., Z4, 1349 (1952). (13) See M. Smith and M. C. R. Symons. J . Chem. Phas., 25, 1074 (1956).
-
0.10 0.15 3.8 5.7 315.2 311.0 0.002 0.002 0.004 0.004 deviations refer
8373 M HCl-----------
2-
0.20 0.08 0.10 0.15 0.20 7.6 3.0 3.8 5.7 7.6 303.9 127.8 126.7 126.5 126.0 0.004 0.0005 0.001 0.002 0.002 0.009 0.001 0.003 0.003 0.005 to the differences between observed ab-
same energies in either the ground or the excited states. Substituting NO3- for clod- increases the range of these energies. No direct evidence for complex formation is provided, though, since no way of distinguishing a new species is indicated. Similarly, the general increase in Pu(V1) absorption caused by chloride ion (see Fig. 2) below 5000 b. provides no direct evidence for chloride complexes. General spectral changes such as these can, however, supply indirect evidence for complex formation in those cases where only small concentrations of complexer are needed and the medium effect may be assumed small. I
I
I
600
400
$
k
2
300
a
0
v)
m U
[r
200
U
J
0
I 100
82
50
WAVE LENGTH (ANGSTROMS).
Fig. 1.-Spectra of Pu(V1) in 2 M NaC104 and in 2 AI NaNOs: ---, NaC104; - - - -, NaNOa.
The effect of chloride ion on the Pu(V1) spectrum above 5000 b. does, however, provide direct evidence for complex formation; new bands appear in a t least four regions, 5050, 6350, 8370 and 9870 A. These bands imply Pu(V1) ions with distinctly different sets of energies in either the ground or excited states. This is evidence for a distinguishable new species, which we consider necessary for proof of complex formation. Our results in the 8300 A. region (see Fig. 3) are in disagreement with those reported by C ~ n n i c k , ~ in that we found no increase in the main band with increasing chloride concentration. We believe
936
T.W. NEWTON AND F. B. BAKER
WAVE LENGTH (ANGSTROMS).
Fig. 2.-Spectra of Pu(V1) in 2 M HClOd and in 2 M HCI: -- , HC101; - - - -, HCI.
Vol. 61
only the first complex, PuO&1+, is responsible for the spectral changes, (b) i t has a constant association quotient, pl,and (c) it has a constant absorptivity, el. I n this circumstance r = p1 and s = €1. Since the data at any one wave length are in agreement with equation 1 they seem t o indicate that the three conditions are satisfied. However, at least one of them cannot be met since the parameter r, the apparent association quotient, is not the same at all the wave lengths. The problem was investigated further in an attempt to find which ones of the three conditions are not met. TABLE I1
b
APPARENTASSOCIATION QUOTIENTS,r, AND APPARENT ABSORPTIVITIES, 8 , AT VARIOUS WAVELENGTHS (25' and 2 M total acid except where noted) Wave length,
A.
2940 3850 4300 4578 5079 6320 8303 8373 8303" 8373" 8304b 8373b a 50'.
Mean dev.
T
IO
8
0.002 0.47f0.04 341 1072 .002 .50 f .02 12.5 300 .001 . 4 0 f .01 5.5 69.9 .001 . 5 O i .02 12.8 42.6 ,002 .70 f .15 7.8 13.4 .001 .80f .12 2.5 10.41 .001 .65 f .05 538 136 8.9 194 .003 .90 f .04 .001 .83 f .05 490 77 .005 1.32 9.9 188 .001 0 . 5 4 f .05 537 140 9.6 193 .003 .76 f .05 b Total acid 0.02 M ; total (Na+) = 1.98 M .
T o start, we will make the provisional assumption that only the first complex is formed in the concentration range studied; that is, condition (a) is satisfied. We may write z = fO€O j l € l (2) where the f's represent the fractions of the total plutonium in the original and in the first complex species, respectively, and the E'S refer to their molar absorptivities. Since f~ f1 = 1, equation 2 can be put into the form :- eo = f i ( 6 1 - eo) The fraction, fi sY.ill be an unspecified function of the chloride concentration, but will be the same for a given solution at all wave, lengths, so for any chloride concentration we can write, for example
+
WAVE LENGTH ( A N G S T R O M S ) .
Fig. 3.-Spectra of Pu(V1) in the 8300 A. region: --in 2 M HCIO,; - - - -, in 1 M HCIO, 1 M HCI; - in 2 M HCI.
+
-:
that the earlier results were in error probably due to insufficient resolution of the narrow band involved. At eight different wave lengths between 2940 and 8373 A. the average molar absorptivity was determined as a function of chloride concentration. Average molar absorptivity, 1, is defined by 1 = A / b c , where A is the absorbance, b is the path length, and c is the total concentration of Pu(V1). All the data for a.ny single wave length can be reproduced within the experimental error by the equation where EO is the absorptivity of the uncomplexed Pu(V1) and r and s are adjustable parameters. The best values found for r and s a t the various wave lengths are shown in Table 11. * The behavior described by equation 1 is required if the following three conditions are met: (a)
+
are constant as If the absorptivities, EO and they are often assumed to be, the ratio, R, on the left of equation 3 should be a constant. Values for R are given in the eighth and ninth columns of Table 111. It is seen that R is not constant, but increases by about 14% in the first example and decreases by about 25% in the second. Since equation 3 does not depend on condition (b), our results require either a second complex, absorptivities which change wit,h the medium, or both. Continuing under the assumption of only one complex, it is of interest t o determine the minimum
CHLORIDE COMPLEX IONS OF PLUTONIUM(VI)
July, 1957
937
TABLE I11 AVERAGEMOLARABSORPTIVITY versus CHLORIDE ION CONCENTRATION AT SEVERAL WAVELENGTHS Total acid, 2 M , temperature, 25", slit, 0.15 mm. 4300
0.000 ,302 .604 ,900
1,208 1.510 1.812
a
5.5 12.4 18.0 22.6 26.5 29.7 32.6
(z
-
8303 €
CO)
... 6.9 12.5 17.1 21.0 24.2 27.1
,
538 473 424 388 361 339 321
(fD
8373
- i) ...
RBiB
R%t
€0)
**.
8.9 47.9 74.2 92.8 105.8 116.0 123.3
65 114 150 177 199 21 7
-
(i
E
(1.65)" 1.67 1.75 1.79 1.83 1.86 1.90
39.0 65.3 83.9 06.9 107.1 114.4
(5.88)" 5.65 5.22 4.91 4.61 4.42 4.22
These values were obtained by extrapolation.
changes in absorptivities sufficient to account for the data. Before this can be done it is necessary to have a value for el a t a t least one wave length. Considering the data for 8303 8.we find that it is possible to set reasonable limits on the value of el. An upper limit of 210 M-I cm.-l was set by a conservative extrapolation of B versus l/(Cl-) and the lower limit is, of course, zero. As indicated in Table 11, if p1and el are assumed to have constant values in the range 2 M HClOd to 2 1cf HCl then el will be 136 M-' cm.-l. Or if it is assumed that the peak a t 8373 is symmetrical about the wave length of its maximum, el a t 8303 will be about 50 M-l em.-'. It was found that if the change in absorptivity with C1- concentration was assumed to be a linear function, E = €0 [ l f a(CI-)], the changes in the ratio, Ri$& could be reproduced satisfactorily. The smallest changes in the E'S a t these wave lengths will occur in the special circumstance where the two a values are the same but with the appropriate choice of sign. Limiting absorptivity functions found in this way for 4300 and 8373 b. are given in Table IV. The functions given for 8303 A. were determined from R:!!: values and the functions previously determined for 8373 A. Eight and one-half per cent. per M of C1- is required a t 4300 and 8373 A. in order to fit the data no matter which limit on €1 at 8303 is taken. These wave lengths gave the most divergent values of the apparent association quotient (Table 11); so smaller changes will suffice for the other wave lengths. For comparison, the effects of various other medium changes on the 8303 b. Pu(V1) peak are shown in Table V. If it could be shown that a change as large as 8.5% were impossible, the existence of a second complex would be indicated. The association quotient, pl, is given by the equation P I = [(EO - € l ) / ( Z - EI)I[(C1-)1-1- [(Cl-)]-l It is seen that it cannot be determined uniquely since it depends on the value of e l . We can, however, set limits on the values of p1 which are consistent with the data since el a t 8303 8.must lie between 0 and 210 M-I cm.-l. Taking the lower limit, calculated values for p, went from 0.44 in 0.3 d l C1- to 0.35 in 1.8 M C1- and were given by the equation log p1 = -0.340 - 0.066 (Cl-). Taking el = 136 gives p1 the constant yalue of 0.65 over the whole range of C1- concentra-
TABLE IV Wave, length,
A.
4300 8303 8373
Molar absorptivity functions For lower limit on a(8303) For upper limit on a(8303) I O = 5.5[1 O.O839(CI-)l fa = 5.5[1 O.O827(Cl-)] e1 = f31.0[1 O.O839(CI-)] €1 = 39.3[1 O.O827(Cl-)1 eo = 538[1 O.O154(C1-)1 ea 53811 O.O076(CI-)I 61 = 0 ei = 2lO[l O.O076(CI-) I eo = 8.911 f O.O839(Cl-)1 ea 8.9[1 -I- O.O827(CI-)] 61 = 335[1 O.O839(C1-)1 el = 207.7[1 O.O827(CI-) I
-
+
-
-
-
+ +
-
TABLE V hfOLAR
ABSORPTIVITIESO F P U ( V I ) I N VARIOUS SOLUTIONS Temperature, 25" and slit, 0.15 mm. Wave length a t niax. Solution
A.
Molar absorptivity, AI - 1 cm. - 1
0 0034 M HClOa 20 MHCIOa 4.0 MHClOi 20 M NaCIOa 40 MNaCI04 20 MNaNOa 40 A4NaNOI
8305 8302 8298 8304 8302 8304 &YO5
553 541 529 538 525 508 470
~
tions, as was mentioned earlier. The upper limit for el a t 8303 leads to PI values from 0.80 up to 1.07 in the range 0.3 to 1.8 19 C1-. These values are given by the equation log PI = -0.111 0.075(Cl-). Any of these expressions for p1 together with the appropriate values of eo and el will reproduce the exrerimental values of c a t 8303 with a maximum deviation of 2 d1-l ern.-'. These results illustrate the interesting fact that in general if the actual values of p1 change with increasing ligand concentration, its apparent value, calculated under the assumption that it is constant, will not even be in the range of the actual values. The limiting changes in with C1- concentration are not unreasonable since an approximate calculation, again using Guggenheim's treatment,B indicates that p1 should decrease somewhat more than what was actually found a t the lower limit, We do not attach much significance to the fact that a decrease was predicted since in solutions as concentrated as these it is unlikely that the treatment is accurate enough to predict even the sign of the effect with certainty. Turning now t o the very reasonable assumption that a second complex as well as the first is important we find that the problem becomes extremely complicated. However, it can be shown that the data at all the wave lengths can be explained without the need of assuming variations in the absorptivities
+
T. W. NEWTON AND F. B. BAKER
938
or association quotients. This can be done readily by an application of Kruh’s theorem. This theorem comes directly from some equations given by R. Kruh14 and may be stated as follows: If a set of average absorptivities, B versus (z), the free ligand concentration, can satisfactorily be explained under the assumption of a single mononuclear complex, MX, with an association quotient K : and an absorptivity E : , it can be explained equally well under the assumption of two complexes MX and MX2 with a whole family of equilibrium quotients and absorptivities, K 1 , K z , el and EZ. These constants are related to the ones which apply to the single complex by the following equations Ki = Kl’(1 =k d m ) / 2 R ,
€1
= s’(2R)/(1 f
dl
- 4 R ) and
€2
= el’
where R = K*/K1. K Z is defined in terms of the first complex, thus PI = K 1 and P 2 = K1Kz. Applying the theorem to the present problem, r values from Table I1 were taken as K ’ a t each of the wave lengths 4300, 8303 and 8373 A. and enough values of P1 and PZwere calculated to enable plots to be made of P1 versus Pz for values which fit the data a t each of the three wave lengths. These plots were essentially straight lines which did not cross a t the same point but did define a small triangle. The points a t the corners would, according to the theorem, fit the data from the two wave lengths represented and a point in the center should fit the data from all three wave lengths nearly as well. This center point P1 = 1.25 and PZ = 0.35 was tested as follows. Values for f1 and fz (fractions of the Pu(V1) i n the first and second complexes) were calculated for each of the experimental C1- concentrations. A least squares method was then used to evaluate best values for and €2 a t each pf the wave lengths. These values were used to calculate values for the absorb(14) R. Kruh, J . Am. Chem. Soc., 76, 4865 (1954). The derivation in this note is unnecessarily complicated: the required relations can be derived easily by equating the expressions for c in terms of only one complex and in terms of two
This equation is cross multiplied and since it must hold for all values of (D), the sums of the coefficients of each power of ( z ) must be zero. This leads to three simultaneous equations, the solutions to which are the desired relations.
Vol. 61
ances of each solution; these were found to agree with the experimental ones within the experimental error. As might be expected, other nearby PI& pairs were found which also fit the data within the experimental error. The results of some of these calculations are shown in Table VI. Values obtained in this way can be considered to be only approximate at best since possible changes in absorptivities and probable changes in activity coefficients have been ignored. TABLE VI EQUILIBRIUM QUOTIENTSAND ABSORPTIVITIES CONSISTENT WITH THE DATAAT 25’ 81 1.25 1.20 1.35 82 0.35 0.30 0.40 €1 (4300) 25.9 27.1 24.3 €2 (4300) 69.2 72.1 68.5 339 324 314 €1 (8303) 115 120 €2 (8303) 124 €1 (8373) 141 146.6 133 195 189 189 e2 (8373) 0.002 0.002 AV. (Aobsd - Aealod) 0.001 0.006 0.004 0.007 Max. (Aobad - Acalcd) 110 77 128 1o62(Aobsd - Acalod)’
I n conclusion, we believe that these experiments show definitely the existence of a t least one Pu(V1)C1- complex and give approximately the extent of complexing in solutions with an ionic strength of 2. They also show the importance of making measurements not only a t different wave lengths but on different absorption bands. If only the results obtained below 5000 A. had been available, i t would have been concluded that only one complex was important and that its absorptivities and association quotient were essentially independent of the C1- concentration. However, when the measurements on the different absorption bands are considered, it becomes evident that a second complex is important, C1- causes unusually large changes in some of the molar absorptivities, or both. Acknowledgments.-The authors gratefully acknowledge many helpful discussions with Prof. Henry Taube of the University of Chicago and with Prof. Harold Friedman of the University of Southern California. They also wish to acknowledge the many helpful discussions with, and the interest of, Dr. J. F. Lemons, under whose general direction this work was done.
I,
>
