THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in
U. S. Patent Office) (Copyright, 1952, by the American Chemical Society)
Founded by Wilder D. Bancroft VOLUME56
JANUARY 17, 1958
NUMBER 1
CHEMISTRY OF THORIUM I N AQUEOUS SOLUTIONS. 11. CHLORIDE COMPLEXING AS A FUNCTION OF IONIC STRENGTH1 BY W. C. WAGGENER AND R.W.STOUGHTON Chemistry Divi&m, Oak Ridge Natimtal Laboratory, Oak Ridge, Tennessee Received August SO, 1061
Chloride complexing of aqueous thorium has been studied aa a function of ionic strength over the range 0.5-6.0 using the benzene-TTA solvent extraction method. Chloride data u to 4 M are ex lained in terms of successive complexing, and constants are estimated for formation of ThCl+a, ThCl*+a,&hCls+ and The&. Aqueous TTA-thorium species have been investigated by a new method, viz., by measuring the partition of TTA, under suitable conditions, between a benzene phase and an aqueous phase, as a function of both thorium and chloride concentration. A sin le complex, monothenoyltrifluoroT.p - ThT+a H+) at p = 2.00 acetonothorium ion (Le., ThT+'), has been found, and a tentative value for kT (Th+4 is 6.6 & 5%. There is no evidence for a double complex involving both TTA and chloride under our conditions.
+ fl
b
+
Introduction where K T , kh, ki, kT and kncl stand for equilibrium In the first paper of this series, Day and Stough- constants with the substances in equations (l), ton2 studied the interaction of aqueous thorium ( 2 4 , (2b), (3) and (a), respectively, expressed in with a number of monobasic acids a t a single ionic moles per liter, H T stands for TTA, T stands for strength and acidity of 0.5. The present work was the chelate radical, and the subscript B indicates initiated to obtain additional chloride data re- benzene phase. Under conditions where all aqueous thorium is in quired for thorium hydrolysis studies being conducted a t different ionic strengthlaas well as t,oex- the form of Th+*,equation (1)expresses the extracploit the advantages of the benzene-TTA extrac- tion of thorium into benzene-TTA, and the distribution ratio of thorium in the two phases is extion method employed. This solvent extraction method involves measure- pressed by ment of the change in the distribution ratio of radioactive thorium (Th-234) between a benzene phase containing the chelating agent, thenoylt~~uoroacetone (TTA), and an aqueous phase containing varyIf the ThT+saqueous complex, equation (3) were ing concentration of anions&., chloride-and per- taken into account, both the central part and the chlorate ion at constant ionic strength and acidity. right-hand side of equation ( 5 ) would contain the The following reactions involving aqueous tho- factor (1 ~T[HT]/[H+]). rium ion may occur Thorium hydrolysis, which is best represented by equations ( 2 4 and (2b), is negligible under condiTh+' + ~ H T B = T h T a + 4H+ KT (1) tions of acidity and thorium concentration emTh+' + Ha0 = T h o + + + 2H+ k h (2a) 2Thf' + Ha0 = ThrO+d + 2H+ k{ (2b) ployed.' If the relatively small amount of complexTh+' + H T = ThT+' + H + kT (3) ing represented by equation (3) be neglected then Th+' + nC1- = ThCl,+('-") kncl (4) when chloride ion is present in the aqueous phase the ratio of the total thorium in the aqueous phase (1) This document is based on work performed for the Atomic to that in the benzene phase is given by Energy Commission at the Oak Ridge National Laboratory. (2) R. A. Day and R. W . Stoughton. J . Am. Chem. Soc.. 19, 5662
+
+ . - + [ThCI:('-")] n = [Th+'] + [ThCI+'] + [ThCln+'] [ThTd B
(1950). (3) K. A. Kraus and R. W. Holmberg, U. S. Atomic Energy Coinn i k i o n Declassified Report, AYCD-2919 (1950).
(6)
1
W. C. WAGGENER AND R. W. STOUGHTON
2
from which may be derived the equation expressing the distribution ratio as a function of chloride ion concentration. R
Ro(1
+ kcl[C1-] + ~ Z C I [ C ~+- ] ' + knc~[Cl-]") * *
(7)
The constants kc^, k m , . ., kncl are the respective stability constants for possible chlorothorium species ThCl+*, ThCh+2, . . ., ThCl$'-"), and are written as indicated in equation (4). The stability constant for t'he monochlorothorium ion may be obtained a t low chloride where the contribution of higher species is negligible, from the slope of the curve obtained by plotting R versus [GI-], divided by Ro. The next higher species then may be estimated by plotting log { R / R o - (1 k c l [Cl-1) ] versus log [Cl-1, inserting the value for k c l initially determined. The limiting slope at low chloride gives the chloride power dependence of the species, and its ordinate intercept a t log [Cl-] = 0 numerically equals the constant. Stability constants for successive species may be estimated in turn by similar log-log plots. However, for the intermediate chlorothorium species which have small and only slightly different stabilities it is necessary to adjust the valuea of the constants by trial and error. These constants could also be obtained by a least squares method. However, the method outlined here is easier to perform and, we believe, just as accurate. The complexing of TTA by thorium in the aqueous phase according to equation (3) is significant, though relatively small (i.e., under 10%). Zebroski4 by the spectrophotometric method, measured k~ as a function of ionic strength in the range p = 0.11-2.11. Recently6 he has reported that his former values are probably too high by a factor of nearly two. During our present studies there arose a question of possible double complexes involving both TTA and chloride with thorium. If such complexing were appreciable our above interpretation in terms of lCnCl constants would be incorrect. By measuring the distribution of TTA, under suitable conditions, between a benzene phase and an aqueous phase as a function of thorium and of chloride, equation (3) has been verified as representing all Th+eTTA aqueous complexing of detectable magnitude, and a value for k~ was obtained in close agreement with Zebroski's later work. For purposes of disproving double complexing, it was assumed that the following reactions involving TTA and thorium might be taking place in the aqueous phase
+
+
+
Th+4 H T = ThT+3 H + k~ (3) Th+' HT C1- = ThClT+' H + ~ C I T (8)
+
+
+
Under conditions where the hydrolysis of thorium is negligible and its extraction into benzene also negligible, the partition of TTA between aqueous and benzene phases may then be written D==
+
[HT]A4- [ T ~ T + ~ A [ThC1TCa]r
W"B
(9)
(4) E. L. Zebroski, University of California Radiation Laboratory clasaified thesis, BC-63, 1947. (5) E. L. Zebroski and H. W. Alter, private communication.
Vol. 56
from which may be derived D
Do (1
+
[Th+4] kT
[H+l
+
kClT
where Do stands for the aqueous-over-benzene TTA distribution in absence of thorium and chloride in the aqueous phase at a given ionic strength. The , be evaluated in the absence of constant, k ~ may chloride from the slope of the line obtained by plotting D versus [Th+*I/[H+]. Then knowing k ~ the , value of ~ C I Tcould be obtained at constant [Th+4]/ [H+] from the slope of the curve obtained by plotting D versus [Th+4][C1-]/[H+], should the third term in the brackets of equation (10) be of significant magnitude. Experimental Aqueous Chlorothorium Studies.--Reagent grade chenucals were used throughout. Sodium perchlorate used to maintain ionic strength was repared in 8 M solution by neutralizing standardized HA04 with NaOH pellets. A visible amount of hydrated Fe(OH)s, formed upon standing 2-3 days a t a pH of 4 was filtered off after first boiling to eliminate carbonate, and diluting to volume. Analyses for sulfate and phosphate ions were reported as less than 0.001 M and and O.OOO1 M, respectively.e The TTA waa obtained from M. Calvin of the University of California. It waa repurified by vacuum distillation, and stored in darkness over PZO;. Crystals of the nearly colorleas product melted at 42.8-43.2". The aqueous solutions for a given experiment at constant ionic strength and acidity were prepared m a grou by pipetting measured volumes of components into 25-mf volumetric flasks and diluting to the mark. The benzene-TTA stock solution as prepared contained TTA 2.5% in excess of the nominal molarity in order to accommodate a decrease in the total concentration of chelating agent due to a ueous ph? solubility. Natural thorium, 10- or 10-6 and thonum tracer were introduced from 0.01 M HC1O4 solutions (previously equilibrated with benzene-TTA) by equilibrating overnight. Equilibrations were performed in 15-ml. glsss-stoppered centrifuge tubes which were rotated end-over-end a t 45 r.p.m. in a thermostated water-bath with temperature controlled at 25 f 0.1". Duplicate 4-ml. volumes of 11 or 15 stock solutions of a ueous phase were e uilibrated 1.5-3.5 hours with equal vJumes of benzene-T%A containing Th232 and tracer Th-234. Both phases were sampled as soon as practicable after centrifuging. Aliquots from the benzene phases were evaporated directly on 25-mm. glass plates for beta counting. Aliquots from the aqueous phases which contained salts were delivered to 2-ml. volumetric flasks, diluted, then equilibrated with0.5 d.of 0.5 M benzene-TTA. After centrifuging, the organic phases were transferred, i n toto, to counting plates. The samples were counted with a G M counter through a 20.7 mg./cm.z aluminum absorber, whereby only the 2.32 mev. beta of UXZ(1.1 minute Pa234 daughter of Th-234) waa counted, thereby making the count quite insensitive to small amounts of extraneous material. Material balances averaged 97-101% with a mean deviation of 1-2%. Aqueous TTA-Thorium Studies.-A stock solution of 0.2 M Th(ClO& in 0.057 M HC104 was prepared and its extraction into benzene-TTA waa compared with that. of carrier-free tracer Th-234 in 0.1 M HClO, in the followng m n n e r . Thorium nitrate was heated to dryness several times with HClOa. The solid w m taken up with enough 0.01 M HCIOI to give a solution 0.234 M in thorium m determined by precipitating the oxalate and igniting.to ThOz. An aliquot of this solution was &luted tenfo!d with dilute HClO4 and tracer added. The resulting solution which W&S approximately 0.38 M in acid and 0.0234 M in thorium was e uilibrated with an equal volume of 0.25 M.benzene-TTA. d e aqueous-over-benzene distribution ratios determned from duplicate gravimetric analyses of each phase were 0.430and 0.432. The corresponding ratio determined from
AI,
(6) These analyses were performed through the courtesy of P. A. Thomason, Oak Ridge National Laboratory Analytical Division.
~ H L O R I D E(hMPLEXINC: Ut' THORIUM AS A
Jan., 1952
the average beta count of four plates prepared from each phase was 0.430. In this experiment duplicate 4-mI. volunics of 11 aqueous stock solutions s t p = 2.0, [H+] = 0.04 A / , [Cl-] = 00.56 itf and [Th] = 0-0.10 fli were equilibrated 6 hourh with equal volunic: of 1.0 X 1 0 - 4 ilf. and 3.0 X lo-' Jf benzene-TTh. The concentrations of TTA in the benzene phases were mcasurcd spcctrophotonlctrically a t 320, 325 and 330 mp after 5- and %fold dilutions, respcctivcly. Th(* concentrations of TTA extracted into the aqueous phases were determined by equilibrating 3 - i d . aliquots, aciditicd to 0.5 M in HClO, overnight with 3 nil. o l fresh benzene. Then, the absorption of back extracted TTA was measured directly. The acidity of cach aqueous phase after equilibration was measurcd with a glass electrode-vibrating rccd t4ectrometcr which was standardized against 0.050 ill HClOd in 1.95 Jf NaClO,.'
3
FUNCTION OF IONlC STRENGTHS
0
07
06 Results and Discussion 05 b o i 0 2 0 3 0.4 [0C d5. o s The results obtained for the interaction of aqueous thorium and chloride ions are prescntccl Fig. X-Ihstrilmtion of thoriuiii in ttir graphically in Figs. 1-6. Tablc 1 gives a summary systeni: equililuation 1.5 a t 25 f 0.1': I
I
I
I
I
I
I 07
I
on
I 09
I Io
nc u(v)us I)cIizciie
6,
e\pPrinioitrtl the cquatioii lCh;:&,= 0.601 I 1 1.53[CI-]I.Aqueous phase ( p = 1.0): [Cl-] = 0.00-1.00 M ; [Clod-] = 1.OO-O.OOdl; ~ X I L + ] 0.8&0.00 '11; [H+] = 0.20df. Benzene phase: [HT] = 0.10; [T1i234] = tracer level; [TIP21 = 10-6M; (Th initially in benzene phase). points;
-, curve of
+
E
01 0
I
I
I
I
I
005
010
015
010
025
[C4
I 030
I 0.33
I
1
040
OW
OW
Fig. 1,-Distribution of thorium in the aqueous benzene system: equilibration 3.0 hr. at 25 f 0.1 : 0, experimental points, first count of plates; 0 ,experimental points, second count of plates; , curve of equation R t k . 0.368 [ 1 2.24[C1-1). .4queous phase(p = 0.5): [Cl-1 = O . W . 5 0 U ; [ClO,-] = 0.50-0.00 M ; [Na+] = 0.300.00 M; [€I+] = 0.20M . Benzene phase: [HTl= 0.10 ill; rrh*3'] = tracer level; [Th232] = 10-5M; (Th initially in benzene phase).
+
-
-
I
1
I 05
10
rc1-I
1.5
I
20
I
25
Fig. 4.--Distrihtion of thorium i i i the :tclric.ous Iwiizoiie y.st.cin: cqui1il)ration 2.0 hr. nt 25 5 0.1'; 0, n~-prrii~irxitrl points; __ , curve of tlic equntion ltb::z, = 1.00 I 1 1.21[c1-1 0 . 1 0 1 ~ 1 - I P 0.20[c1-13;. A ~ U C O U S ~ ~ ~ = W ( ~ 2.0): ICl-I = 0.00-2.00 JI; IClU4-] = 2.00-0.00 ill: [Sail = 1.80 M; [H'] = 0.20 .If. JJcneene phase: [IIT! = 0.10 ;If; [Th23'] = tracer Ievcl; IT112321 = 10-6.V; (Th initially in benzene).
+
Fig. 2.-Distributiori of thorium in the a ueous benzene experiequilibration 3.5 hr. at 25 f 0.1 'B : 0, mental points, first count of data; 0 ,experimental points, second count of data; -I curve of the equation mnl. = 0.508 f 1 1.78[CI-I 1- Aqueous Phase @ = 0.7): IC1-I E 0.00-0.50 M ; [CIOd-] = 0.70-0.20 ; [Na+] = 0.500.m M ; IH+I = 0.20 M. Benzene IHTJ ,=.0.10 A!,. (Th*'4] = tracer level; [Thz8Z]= . I f ; (Th initially in t)enzene .--- -phase). __
d
(7) Jleasureinents were the rourtesp of R. I V . IIoll,ll,ery, Oak I{idge Sational I.aboratory Chemistry Division.
+
+
of constants. These equilibrium coilstants are concentration (not activity) constants and, as such, contain the activity coefficients of the various speties involved as factors. These coefficients are assumed to remain essentially constant for a given experiment at constant, ionic strength and changing ~ chloride ratio. This :issumption appcni-s to 1 ) reaat ionic strengths in thc region of ion. chloride. Henve, our values for kc.1 should not be
W. C. WAGGENER ,WD R. W. STOUGHTON
Vol. 56
0
I
"
1
I
1
l
4
e PI. Fig. 5.-Distribution of thorium in the aqueous benzene system: equilibration 3.0 hr. at 25 f 0.1 : 0, ex erimental points; , curve of equation RC.= 0.280 [I+ 1.701Cl-I O.14[C1-l2 O.lOIC1-la 0.018[C1-]4]. Aqueous p h m ( p = 4.0): [Cl-] = 0.004.00 M ; IC104-] = 4 . 0 0 . 0 0 M ; INS+] = 3.68 M ; [H+] = 0.32 M. 13cnzene p l m e : [HT] = 0.25M; [Th**'] = tracer level; [Th*a*]= 10-+M; (Th initially in benzene phase).
+
-
+
+
Fig. 6.-Distribution of thorium in the ueom benzene system: equilibration 2.0 br. at 25 f O.lo;%, experimental pointa; -,curve of e uation RZA = 0.396II Z.lO[Cl-J 0.55[C1-11 O.35[81-JaI. Aqueous phase ( p = 6.0): [cl-] = 0.00-2.00 M; [Clod-] = 6.00-4.00 M; [Na+] = 5.68 M; [H+] = 0.32 M. Benzene phase: [HT] = 0.25M; [Thrs4]= tracer level; [Th*a*] = lO-'M; (Th imtially in benzene phase).
+
+
+
.
for the U(1V) hydrolysis agreed quite well with experiment in the region up to about p = 2.0. The value for kcl at p = 0.5 obtained in the present studies is significantly higher than the value of appreciably in error due to this assumption. The 1.76 obtained by Day and Stoughton.z In this , and krcl, however, are connection the distribution ratio, %, obtained by higher constants, ~ C I k3c1 cxpcctctl to be more affected by change in mcdium, Day and Stoughton was 0.63 compared to the present value of 0.37. Ten per cent. of the discrepancy since thcy are important only a t higher chloride. is due to our allowance for the solubility of TTA in TABLEI the aqueous phase (see Experimental). Another ~ ~ ~ Q U I I ~ I R R CONSTANTS IUM FOR CHLOROTIIORIUM COMPLEXESdifference w&s the purifying of the TTA for the AS A FUNCTION OF IONIC STRENGTH AT 25' present work. Values of Ro of up to unity have Concentration constants of formation for been obtained with old TTA samples whereas all chlorothorium species values using the repurified material were of the orc [ I i ' ] [ I l T ] [Thl ~ C I ~GI" krCia krcla 0 . 5 0.20 0.10 10-6 2.24 der of 0.37. Day and Stoughton did not purify 0 . 5 .50 .2.i 10-6 3.25 their TTA and may have had some water-soluble . I O 10-5 1.78 0.7 .30 complexing agent impurity prasent. Such a possi1.0 .N .in 1.53 bility would explain their lower k c l and would 2.0 .20 . I O I O + 1 . 3 1 0 . 1 (o.osn) 0 . 3 (2.0) 4 o % . ;. 10-3 1.70 0 . 1 4 ( 0 . 0 8 ~ o) . i o ( 0 . 7 1 ) o . n i 8 ( 0 . 1 8 ) mean that all their constants are too low. G.0 .32 .45 1 0 - 6 2 . 1 0 0 . 5 5 ( 0 . 2 6 ) 0.33(0.64) Our results a t ionic strength 4.0 are in substantial \':~lucsin pnrcntlicscs arc const:tnts of formation by addiagreement with those of Zebroski, Alter and Heutioil of a siiiglc cliloridc ion to the preceding complex. mann.s They have obtained the values 1.30,0.125, 0.037and 0.014 for k c l , ~ C I ~, J C Iand kra, respecThe concentration constant, kc1 (for Th+' C1- = ThCl+a) decreases from 2.24 a t p = 0.5 to a tively. It is noteworthy that our results for the formaminimum of 1.21 a t p = 2.0, then increases to 2.10 tion of successively higher chlorothorium complexes at p = 6.0. An approximate value for the corre- do not follow even qualitatively the Bjerrum thesponding activity constant kE1 ( p = zero), has been ory based upon statistical and electrostatic effects. m obtained assuming the Debye-Huckel equation It was thought at first that the data up to 4.0 M holds up to an ionic strength of 2. A plot of log chloride could be represented satisfactorily by the kcl versus p'/a/(l 2.465 ~ ' 1 1 )gave a value of 1281 = 24 on extrapolation t.0 zero ionic strength. assumption of two chlorospecies, the mono- and the (9) E. L. Zebroski. II. W. Alter and F. K. Heuniann. private CONKraus and Nelson* have found that a similar plot niunicalion. 10-6
+
+
(8) IC. A. Eraua and
P. Nelson, J . Am. Clicnz. Soc., 74, 3901 (19;O).
(IO) N. Bjerruin, Erusb. czakt. Nalurw.. 6, 125 (1926).
.
BINUCLEAR PEROXO COMPLEX COMPOUNDS OF COBALT
Jan., 1952
trichlorothorium ions. However, with additional ’ 5 data it has become apparent that while the monoand trichloro ions are the predominant chlorothorium species at all concentrations up to 4 M chloride the di- and tetrachlorothorium species are present in measurable concentration. Table I1 illustrates the relative importance, percentagewise, of the thorium species in 1, 2 and 4 M chloride solution at an ionic strength of four.
z
’
o
5
r’.,.... 1
t
SLOPL
o.zo
0.0732
TABLE I1 REI~ATIVE PERCENTAGE OF AQUEOUS THORIIJY SPECIES IN CHLORIDE SOLUTIONSAT AN IONICSTRENGTH OF 4.0 A N D 26” Species
Th+4 ThCl +a ThClr+I ThCl,+ ThClt
--Chloride
concentration-
1M
33.8 57.5 4.7 3.4 0.6
2M
4M
16.5 56.2 9.3 13.2 4.8
4.8 32.3 10.6 30.4 21.9
Thenoyltrifluoroacetonothorium Ion.-The results of studies at an ionic strength of two are presented in Fig. 7. Data obtained in perchlorate solution by varying the concentration of thorium are indicated by the open circles. The dark circles represent additional points calculated from data at constant total thorium with varying chloride by correcting for the chlorothorium species present (see Fig. 4 and Table I). These two sets of data agree quite well and show no evidence for the existence of either complexes containing both chloride and TTA radical or complexes containing more than one TTA radical per thorium. The best straight line through these data gives a value of k~ bet,ween 6.4 and 6.8. In interpreting these data, the distribution ratio at zero thorium concentration (0.0114) was weighted higher than the other values, since it was an average of a larger number of experimental results.
Fig. 7.-Distribution of thenoyltrifluoroacetone (HT) in the aqueous benzene system: 0, [Th] = 0.00-0.10 M ; 0 ,[Th+4] [Chloro-Th] = 0.10 M; [Cl] = 0.20-0.96 M; Aqueousphase(p = 2.0): [Th+r] = 0.004.10 M ; [Na+l = 0.00-1.96 M ; [H+] = 0.04 M ; [CIO-] = 0.04;-2.00 M; [Cl-] = O.O(M.96 M. Benzene phase (initially) [HT] = 1.0 X lo-‘ and 3.0 X 10-4M.
+
The effect of this complex is to make all the constants in equation (7) contain the factor 1/(1 ~T[HT]/[H+]). Hence the true values for any of the constants are obtained by multiplying the values given in this paper by (1 ~T[HT]/[H+I). This factor is 1.08 at ionic strength 2.0 at equilibrium at [H+] = 0.32 M and [HTIB = 0.25 M . As a state of equilibrium with respect to the complex ThT+S is not attained in much less than 25 hours due to the slowness of the TTA phase distribution, and since we shook the tubes for only 1.5 to 3.5 hours, t,he correction under the current experiments is much smaller, i.e., nearer to unity. At other ionic strengths we believe the correction t.0 be not much, if any, greater than at ionic strength 2.0. Hence we have not made the correction in the current work.
+
+
TITE BINUCLEAR PI;‘,R,OXO COMPOUNDS OF TRI- AND TETRAPOSITIVF, COBALT BY LAWRENCE R. THOMPSON A N D W. K. WILMARTII Department of Chemistry, University of Southern California, Los Angeles, California Received Aitgu8t 50, 1061
A repetition of the preparative work of A. Werner and others confirmed the existence of a series of green binuclear peroso compounds containing a cobalt atom in the tetrapositive oxidation state. A second series of red com ounds which Werner, using the conce t of secondary valence forces formulated as isomers proved to be the one electron refuction products of the green series a n f contained only tripositive cobalt. Analytically, the red compounds a peared to be isomeric because, while they are only weakly basic in solution, they normally precipitate as acid salts and tgus differ from the green series by a single proton. The reported ‘‘isomericconversions” are in reality all oxidation-reduction reactions and include the bromidecatalyzed disproportionation of the red series and reduction of the green series by hydroxide ion.
Introduction The present research is the beginning of a systematic attempt to clarify the general chemical nature of a remarkable series of binuclear complex compounds containing cobalt in the tetraposit,ive valence state. This valence appears to be confined to polynuclear complexes in which the metal ions are linked through a peroxo bridge in the following fashion
[(A)60-on-c0(
4+
A)&]
The subscript five indicates that in addition t,o the peroxo bridge each cobalt atom is also bonded in the usual fashion to five other groups, some of which may be bridging groups between the cobalt atoms. A may be one or more of the conventional complexing groups; the charge on the complex ion will of course depend upon which groups are present. It should be possible, in principle, to reduce each of these ions to the symmetrical unit
