Dec. 5 , 1954
THERMODYNAMICS O F NEPTUNIUM(IV) SULFATE COMPLEX IONS
ing such a solution with an equiconcentrated but nearly neutral solution of +5 arsenic, so t h a t the concentration of +5 arsenic is kept constant while t h a t of hydrochloric acid is varied, produces the effect shown by the succeeding curves of this figure. The waves are nearly completely suppressed even in 10.2 M acid (Fig. 5e), and absent altogether from 9 M acid (Fig. 5f). No reduction wave was found either in a hydrochloric acid-perchloric acid medium containing 10.7 M hydrogen ioii and 7.7 iM chloride ion, or in a hydrochloric acid-lithium chloride medium containing 8.2 M hydrogen ion and 11.2 M chloride ion. Evidently both hydrogen [CONTRIBUTION FROM
THE
5931
and chloride ions a t very high activity are essential for the conversion of the +5 arsenic into the reducible species, which must be a n ion in which some or all of the oxygen atoms of the arsenate ion have been replaced by chloride. Though the increase of i d / C with increasing concentration of +5 arsenic above about 0.4 m M is clearly disadvantageous in practical analytical work, no maximum suppressor has been found which eliminates this behavior without causing a severe distortion of the entire polarogram. XELVHAVEN,CONN.
CHEMISTRY DIVISION,ARGONNE NATIONAL LABORATORY]
Thermodynamics of the Neptunium(IV) Sulfate Complex Ions BY J. C. SULLIVAN AND J. C. HINDMAN RECEIVED JUNE 9, 1954 The extraction of neptunium( IV) into benzene with the chelating agent, thenoyltriiluoroacetone, has been used to demonstrate the existence of the +4 ion of neptunium in acid perchlorate solution. This $4 ion of neptunium has been shown to associate with sulfate in perchloric acid solution a t an ionic strength of 2.00 according to the reactions N ~ + ~ ( a q . )HS0,- = NpS04++ H-, NpS04*+ HSOp- = Sp(S04)z H+. The association constants for the stepwise reactions a t 25" are kl = 270 zt 27 and kz = 11.0 f 2.2. The partial molal heat changes for the reactions are,respectively, -1.17 i 0.16 and +3.64 f 1.02 kcal./rnole. The corresponding partial molal entropy changes are 7.2 f 0.6 and 18.7 5~ 3.6 e.u.
+
+
+
+-
Studies of the distribution of a metal cation between an aqueous and a benzene phase with the aid of the chelating agent, thenoyltrifluoroacetone, have proved extremely useful in determining both the charge on a metal ion in the absence of a complexing ligand and the association constants in complexing media.'S2 This method has been used in the present paper to definitely establish the existence of the Np+*(aq.) ion as the predominant species in perchloric acid solution. The measurements have been extended t o a study of the association behavior of Npf4 (as.) in sulfate media. In order to determine the factors affecting the association with this ligand, calculations of the heat and entropy changes involved in the association process have been made from measurements of the temperature dependence of the equilibrium constants. Experimental A. Materials.-The perchloric acid was G. Frederick Smith, reagent grade double-vacuum distilled standardized against SaOH. The sodium perchlorate was prepared from perchloric acid and sodium carbonate and twice recrystallized from distilled water. Solutions were prepared on a weight basis. The sulfuric acid was reagent grade awayed both for total sulfate and for acidity. The thenoyl trifluoracetone (abbreviated TTA, hereafter) was a product of the Dow Chemical Cs. The T T A solutions were made by weighing a given amount of the solid into a known volume of benzene. The S p ( I V ) was prepared by the method of Cohen and Hindman.3 B. Procedure.-The total ionic strength was kept constant a t 2.00 by use of sodium perchlorate and perchloric acid. For the sulfate studies the acidity was kept constant a t 2.00 N . Two ml. of aqueous and benzene solutions were pipetted into 10-ml. erlenmeyer flasks fitted with ground glass stoppers. The stoppers were waxed and the flasks were then covered with special rubber containers. The covered vessels were agitated in a constant temperature (1) R. E. Connickand W. H. MCVey,THISJOWRNAL,71,3182(1949). (2) R. E. Connick and W. H. Reas, ibid.. 73, 1171 (1951). (3) D. Cohen and J. C. Hindman, ibid., 74, 4679 (1952).
water-bath until equilibrium was reached. It was found that the same equilibrium was attained if a concentrated solution of the neptunium(1V) chelate in benzene was agitated with a perchloric acid solution as when a perchloric acid solution of neptunium(1V) was agitated with a benzene phase containing the chelating agent, thus demonstrating the reversibility of the system. Duplicate aliquots of each layer were mounted on Pt plates and radiochemically assayed using a methane proportional counter. These duplicate assays checked to within the statistical accuracy of the counting, f 0 . 5 % . Duplicate samples checked to within ztl%. The isotope of neptunium was the long lived a-emitter, NpZ37.
Results and Discussions Formula of the TTA Complex of Neptunium Extracted into Benzene.-As Connick and McVeyl have pointed out, the first step in the process of studying complex formation by the TTA method is the assignment of the formula of the metal ion complex extracted into the benzene layer and the determination of the degree of complex formation by the chelating species in the aqueous phase. A general equation for the reactions between Np(1V) and TTA may be written
-' + (P - j ) H B = N P ( O H ) ~ - - j ) H f + (i + 9 - 4)H20
( X P ( O H ) ~ B ~ ) +- ~j (4 i
p
Bp (1)
+
where HB is used t o represent undissociated TTA. I n the following equations, brackets are used to indicate concentrations of the enclosed species. [Np(OH)iBj+4-i-j] = complexed species in aqueous phase [Np(OH)c.pBp] = cornplexed species in benzene phase'
The extraction coefficient may be defined as
(4) The possible species in the benzene layer are subject to the requirement that only neutral molecules be present and limited by the maximum co6rdination number of the neptunium(1V) which is considered to be 8 for oxygen.
J. C.SULLIVAK AND J. C . HIKDMAN
5932
I t can be readily shown from equations 1 and 2 that if the Np(1V) is chelated with 4 molecules of TTh in the benzene phase (p = 4) and no chelate complex is formed in the dqueous layer ( J = O ) , then log E' = 1 log HB
YOI. 76
Graphical analysis showed t h a t a straight line with the slope of +4.0 fits the data up to an initial HB concentration of 0.1 AT. Therefore, j = 0 and p = 4 and equation 2 niay be rewritten in the form
(d
(5)
where E' is defined by the equation E' =
1
(4)
To compute E' from the measured extraction In the experiments in the following sections, the coefficient, E , it is necessary t o assume that the HI3 concentrations were adjusted so that the exactivity coefficients of all neptunium species in the traction was carried out in the region where the benzene layer are equal to 7 x N pwhich ~l is in turn fourth power dependence of the extraction coefassumed to be Y N ~ B=~ Y U B ~ .Y E B ~has been de- ficient on H B was obeyed. of the Neptunium(1V) in Perchlorate ~ termined by Reasja and found t o be equal to ~ H B . ~Formula equations I and 3 with the The dependence of E on HB concentration a t Media.-Combining equation for hydrolysis of Np-? various temperatures and 2.00 A f HC104 are given Sp -'(as) + ? H 2 0 Sp(OH).+4-a iH" (fi) in Table I. TABLE I and the previously developed equations6 we have THE DEPENDENCE O F THE EXTRACTION C O E F F I C I E h T E' for the average number of OH- groups, %,' bound O U THE COSCEVTR.4TIOV O F THENOYLTRIFLUOROACETOXE per neptunium(1V) ion
+
[ H - ; = 2 00,p = 2 00, Sp(I1.) s l X 10-4 11 itnltial aqueou5) 1 0 2 i 20 2 i L i 2' 3>,7 = 2' log E'
log [ H B ]
1 0 713
-0.936 -1.057 -1.285 -1.441 -1.481 -1.586 -1.678 -1.959
+
,387 539 271 402 845 188 38j
-1
-1 -1 -2
-3
1.5
log I '
log [ H B ]
log
r
log [ H B l
-~0,620 tl.010 --i).754 1.en;: - 7-G t ,712 - ,852 ,316 - ,032 1 ill:< - ,855 ,781 - 890 - ..?I7 -1,iI-K -. lJ1 - .YO0 f ,171 -1.072 -1.455 , 4 8 5 - ,970 -1.316 -1.812 -1.584 ,0022 -1.111 - ,Mfi -1 2 i 0 -2,239 - 1 . 6 6 8 -:3.41:3 -1.960 - 1 . 4 7 % -1.480 1.391
+
I
I
I .o
-
,t =
0.5
E" = E/[HB]'[-yHBI4
T h e data in Table I1 show t h a t Np(1V) exists as an unhydrolyzed tetrapositive ion from 2.00 U [ H + ] to 0.51 -11[H+]. The deviation of the extraction curve from linearity a t low acid suggests that hydrolysis of neptunium(1V) is occurring. A determination of the TTB dependence of the extraction coefficient a t lower acidity, Fig. 1, shows marked deviation from the fourth power dependence on T T A A concentration This suggests that appreciable complex formation between the T T X and the neptunium(1V) is occurring in the aqueous phase. T h e low value of log E' suggests formation of complexes of the type Kp,(OH),B,. TABLE I1 DCPETDE\CE O F THE EXTRACTION COEFrICIEYT, E " , o\ THE COSCE~TRATIOY OF HYDROGE\ lo\ = 2 00 Wp(IT )] = 1 x 10-4 11 (initial q u c o u i ) log I:"
I
w
(1 log [Hi']
where E" is defined by the equation
TIIE
_
- d log E"
10 2
* 0.20 log [ € I + ]
6.051 f j . 341
4 484 4 721 1 986 5 5,5l C i 176 i; 703 R 75iI 6 948 i 238
6.981 7 . 101 7.175 7 265
7 310 7 43.3 7 ,552 7 710
4 3
