Tenth Annual Summer Symposinm-Nncleonics and Analytical Chemistry
The Analytical Chemistry of Plutonium CHARLES F. METZ The University o f California, 10s Alamos Scientific laboratory, 10s Alamos, N.
b This paper reviews the general chemistry of plutonium and applications of its analytical chemistry. Plutonium may b e separated from impurities b y precipitation, ion exchange, and solvent extraction. It may b e determined b y potentiometric titration, spectrophotometric, and radiochemical methods. The rare earth-like absorption spectra of its oxidation states enable spectrophotometric determination of th, 0 concentration o f the various oxidation states in solution. Caution must be exercised, because of the effects of pH, temperature, and nature of anions present on the molar extinction coefficients of the valence states. Spectrochemical methods may b e used to determine a large number o f metallic impurities, i f the line-rich spectrum of plutonium i s absent from the spectra of the impurities. This may b e done b y a carrier-distillation technique, using a carrier such as gallium sesquioxide, extraction of plutonium from impurities with an organic solvent, or (a less sensitive method) b y placing a sample containing not over 50 y o f plutonium on a copper electrode and exciting with an alternating current arc. PIutonium(II1) does not form a polarographic wave between 0.0 and -0.9 volt vs. the saturated calomel electrode; this permits polarographic determination of uranium, titanium, vanadium, and other elements in the presence of plutonium. Alpha-counting methods may be used to measure plutonium concentrations in the microgram per liter range and to determine plutonium-238 and plutonium-239 isotopic concentrations, if plutonium-240 and americium-241 are not present. initial discovery of the plutonium-238 isotope in 1940 by Seaborg, McMillan, Kennedy, and Wahl (46), and of the plutonium-239 isotope in 1941 by Kennedy, Seaborg, Segre, and Wahl (69), was followed by the discovery in 1941 (29) that the plutonium-239 isotope was fissionable with slow neutrons with the release of tremendous amounts of energy. Possible military applications shortly led to a study of this man-made element, which was magnificent in both magnitude and results. The initial HE
1748
ANALYTICAL CHEMISTRY
M.
isolation of fairly pure plutonium compounds in 1942 by Cunningham and Werner (17') made possible intensive studies of its chemical properties. It is impossible here to give proper recognition to all tvho so diligently and brilliantly searched out information that grew to impressive proportions within a few years. Not within the history of chemistry has so much information been obtained about a single element in so short a time. This becomes all the more astounding when one remembers that much of this work was done with microgram and even submicrogram quantities. This paper discusses chemical properties of importance in the analytical chemistry of the element. Much of the information pertaining to the general chemistry of plutonium was obtained from the National Yuclear Energy Series (44,46). There has been a gieat deal of discussion concerning the place of elements with atomic numbers 93 to 97 in the periodic table. In view of the supporting evidence revealed by studies of chemical properties, absorption spectra in aqueous solutions and crystals, and data of crystallographic structure, magnetic susceptibility, and spectroscopy, Seaborg (43) has suggested that the elements from actinium through californium form the first part of a new rare earth-like series, which might logically be named the actinide series and assigned a place in the periodic table. The peculiar properties of the analogous lanthanide series have been attributed to an incompleted shell of 4 j electrons, which is filled up as the series is ascended. In an analogous manner, the properties of the actinide elements are attributed to the presence of an incompleted 5f electron shell, which is filled up as the series is ascended. Table I shows electronic configurations for the actinide elements, suggested by Seaborg (4.9). One of the characteristic properties of the members of the lanthanide series is the sharp absorption bands, mostly in the visible region, a property characteristic of 4 j electrons. The actinide elements show a striking similarity in this property. Chemical Properties of Metal. Plu-
Table I. Suggested Electron Configurations for Gaseous Atoms of Actinide Elements
Atomic 89 90
Element Actinium Thorium
91
Protactinium
92
93
Uranium NeDtunium
94
Plutonium
95 96 97
lZmericium Curium Berkelium Californium
KO.
98
Electron Configuration 6d7s2 6d7s2 (or 5f6d7s2) 5f26d7se (or 5fGd27s2) 5f66d7sz
5P7s2 " (or 5f4Gd7s2)
5f67s2 (or 5f56d7s2) 5y\7-s-* -,- -- - I
5f76d7s2 577.~2 5flo7s2
toniuni is a chemically reactive metal (16, 91). ilt somewhat elevated temperature, atmospheric oxidation proceeds rapidly, especially a t moderate humidities. At high relative humidities, the metal corrodes rapidly a t room temperatures. Like uranium, the finely divided metal is pyrophoric. Plutonium dioxide is the oxide usually formed in these cases. The metal reacts with chlorine, bromine, and iodine a t moderate temperature, forming the corresponding trihalide, but fluorine apparently forms a protective coating on the metal. Ammonia reacts a t 1000" C. to form the mononitride, and carbon reacts a t 1300" C. to form the monocarbide. At somewhat higher temperatures, there is evidence of the formation of the tricarbide. Plutonium metal reacts with hydrogen a t room temperature to form the dihydride, or the trihydride, depending on the amount of hydrogen available. The metal, n-hose melting point is approximately 630" C., is readily soluble in hydrochloric, hydrobromic, hydriodic, 72% perchloric, 85% phosphoric, and concentrated trichloroacetic acids. Moderately concentrated sulfuric acid dissolves the metal slowly, but nitric acid is without action. Water reacts very slowly with the metal a t room temperatures. Alkali hydroxides do not attack the metal. METHODS
Precipitation.
OF SEPARATION
Among the insoluble
plutonium compounds important in the analysis of plutonium-containing materials are the peroxide, the tetraiodate, the tetrahydroxide, the dioxalate, the trifluoride, and the tetrafluoride. Table I1 shows the conditions and extent of their solubilities (21). In many cases advantage is taken of these low solubilities in removing plutonium from solution before proceeding with the analysis. For example, work a t Los Alamos has shown that plutonium may be precipitated as the oxalate prior to determination of boron (181, the iodate prior to determination of free acid in the solution (51), or as the peroxide prior to determination of titanium (8),indium (57), cadmium ( 5 ) ,bismuth (@,and several other metal ions. All these examples illustrate the philosophy that, because of health hazards and danger in handling, it is a good precautionary measure to remove the plutonium from solution before determining other elements. However, sufficient plutonium always remains in solution to require special and careful handling in the laboratory. Ion Exchange. Another interesting technique for separating plutonium from a significant number of metal ions is ion exchange. Work done a t Chalk River ( I ) , and unpublished work a t Los illamos, have shonn that plutonium(V1) may be absorbed on Don ex 1 anion resin from a concentrated nitric acid solution. Additional unpublished work at Los Alamos has shown that under similar conditions plutonium(I1') is absorbed and held more tightly than plutonium(V1). I n either case, the plutonium can be eluted with hydroxylamine and 1N nitric acid, the hydroxylamine acting as a reducing ageiit and dilute nitric acid acting as an eluting agent for the plutonium(II1). Experience a t Los Alamos indicates that recovery of the plutonium is not complete by as much as 0.1 to 0.2%. This means that subsequent assay for the plutonium is usually biased to this extent on the negative side. I n addition, removal of the organic material prior to potentiometric titration has been time-consuming. Additional unpublished work a t Los Alamos has indicated that plutonium(1V) can also be absorbed on Dowex 1 anion from concentrated hydrochloric acid containing 0.25M hydriodic acid, and thus separated from most metal ions. Iron is essentially the only metal that follows the plutonium. Solvent Extraction. Still another useful technique for separating plutonium from other metal ions is solvent extraction. Even though considerable information regarding the solubilities of plutonium in organic solvents was obtained early in the history of plutonium (52) and investi-
Table II. Solubilities of Slightly Soluble Plutonium Compounds Solubility, Compound Composition of Solution Mg. Pu/Liter
a
Equilibration period, G days.
gations have continued, this method of separation has found only limited use in the chemical analysis of plutoniumcontaining materials. Table I11 sh0n.s the behavior of plutonium toward a few organic solvents under one set of conditions. I n several cases, the solubilities are extremely favorable for the separation. Thenoyltrifluoroacetone (TTA) in benzene is an excellent solvent for plutonium(1V) in nitric acid, and diethyl ether is excellent for plutonium(V1). Organic solvents that will extract uranyl ion are also good solvents for plutonium(V1). I n general, however, the nitric acid concentration should be somewhat higher for plutonium(V1) than for uranium(V1) (0.5 to 2J1). OXIDATION STATES
Because the element exists in several states of oxidation (14), it is to be expected that its chemistry will be correspondingly complex. Plutonium can exist in the +2, 1-3, $4, +5, and +6 oxidation states. Although the electronic structure in the +2 state is not known, it is assumed that the higher states correspond to the radon conipleted shell structure, with the reniaining electrons in the 5f shell. The $2 State. The dipositive oxidation state is unimportant in analysis. It is exemplified by a n oxide and sulfide; apparently it cannot exist in aqueous solution. The $3 State. Many solid compounds contain plutonium(II1) : a n oxide, all the anhydrous halides, a hydroxide, perchlorate, sulfate, nitrate, etc. I n aqueous solution, plutonium(II1) exists as P u + + + ion. The +3 state may be formed by reduction of the +4 state with sulfur dioxide, hydroxylamine, mercurous ion in chloride solution, titanous ion, zinc amalgam, ferrous ion, stannous ion, and iodide ion. I n moderately acid solutions it is relatively stable with respect to air oxidation, being oxidized rather slo~dy. I n general, the chemical properties are similar to those of the rare earths of oxidation number +3. The +4 State. The tetrapositive state of oxidation is very stable and is obtained on ignition of the metal or most plutonium compounds in air. It may be formed by oxidation of the $ 3 state with oxidizing agents such as
Table 111. Behavior of Plutonium toward Some Organic Solvents (Conditions. 10M "*NOS, 1M Hx03) Pu(IT'), Pu(s'I), Solvent % % Diethyl Cellosolve 97 9G Ethyl butyl Cellosolve 66 73 8-8-Dibutoxvethvl ether 93 91 Methyl ethil keione, 15yGxylene 79 .. Methyl isobutyl ketone (Hexone) 82 .. Acetophenone 89 .. Cyclopentanone 91 .. hlethylcyclohexanone 82 .. Tributyl phosphate 99 97 &Quinolinol in chloroform 90 .. I
,
permanganate, dichromate, bromate, nitrite, concentrated nitric acid, OXYgen, and ceric ion. Plutonium tetrafluoride is the only solid anhydrous halide of the $4 state that has been definitely identified. All others appear to be thermodynamically unstable and decompose to the +3 halide and free halogen (11). Many hydrated +4 compounds exist: halides, sulfate, nitrate, oxalate, iodate, etc. Unhydrolyzed plutonium(1V) ion is believed to exist in aqueous solutions of 0.lM hydrogen ion or greater. It forms complexes with sulfate ion, nitrate ion, fluoride ion, oxalate ion, phosphate ion, and chloride ion in high acid concentrations. I n one respect, plutonium(IV) solutions are unstable, undergoing partial self-oxidation and reduction to plutonium(II1) and plutonium(V1). The extent of this disproportionation depends markedly on the acidity and complexing action of the negative ions present-a 2 x 1 O - a M solution of plutonium(1V) in 0.5M hydrochloric acid and a t 25' C. will disproportionate to the following extent
+
3 P u + ~ 2 H10 F? 2 Pu+++ PuO~*+ 4 H + 27.270 58.4% 13.6%
+
+
and require about 9 days to reach equilibrium (15). Absorption spectra evidence of this behavior (Figure l) (15) clearly shows the absorption peaks of the +3 and +4 valence states. The rate of disproportionation of plutonium(1V) varies directly as the second or third power of its concentration and inVOL. 29, NO. 12, DECEMBER 1957
.
1749
versely as the cube of the hydrogen ion concentration. The chemistry of plutonium(1V) is very similar to that of cerium(1T') and uranium(1V). The hydroxide shows no amphoteric properties and is very insoluble. The +5 State. The + 5 state of oxidation was the last one t o be discoyered, and for a time v a s thought t o be unstable. I n the p H range 2 t o 7 . it is fairly stable. Only a few compounds have been prepared in \I hich plutoniuni(V) is present. One contains potassium and hydroxide ions. The pentafluoride may also exist. I n aqueous solution, plutonium(V) is believed to exist as PuOz+. The + 5 state is not directly important in the analytical chemistry of the element. The +6 State. The hexavalent state of oxidation is well characterized by a number of plutonyl and plutonate salts, which resemble very closely t h e corresponding uranyl and uranate salts, I n aqueous solution, plutonium(V1) is believed to exist as PUOZ*+ ion. It may be formed from the $4 state by oxidizing agents such as hot permanganate, dichromate, bromate, hot dilute nitric acid, and hot perchloric acid. It may be reduced by sulfur dioxide, iodide, hydroxylamine, hydrogen peroxide, ferrocyanide, and by electrolysis. Unlike uranyl compounds, plutonyl compounds exhibit no fluorescence. OF SOLUTIONS Aqueous solutions of the various oxidation states of plutonium exhibit typical sharp-band rare earth-like absorption spectra, which make it possible to determine plutonium spectrophotometrically in each of its valence states. Figure 2 (15) shows the absorption spectrum of plutonium(III), (IV), and (V) in 0.531 hydrochloric acid and plutonium(V1) in 0.5-11 nitric acid. Thus an analytical absorption band is found at about 600 mp for plutonium(III), at 470 mp for plutonium(IV), at 569 mp for plutonium(V), and at 833 mp for plutonium(V1). RIuch of the early information about the behavior of plutonium in solutions toward oxidizing and reducing agents, and kinetics of reactions, including disproportionation. \vas obtained by observing the solutions spectrophotometrically (16). More recently, Allison (9)has reported that the plutonium(II1) band a t 602 mp can be used with a precision within 1% a t a concentration range of 2 to 4 mg. per ml. The plutonium(1V) peak at 476 mp may be used for concentrations as low as about 1 mg. per ml. and the plutonium(V1) peak a t 833 mp for concentrations as low as 0.5 mg. per ml., with l-cm. cells. However, the molal extinction coefficients are dependent on temperature, pH, and type of acid
!
230
,
i
l
l
1
1
'
1
-
w
I?
LL LL W
0
0
z
p 20 0
z
IX W -I
U _I
0
E 40 -
0 3000
j
~
4000
l
5000
~
,
6000 7000 8000 WAVE ,LENGTH (A,)
~
,
r
9000 40,000 44,000
Figure 1 , Absorption spectrum of equilibrium mixture obtained upon disproportionation of plutonium(1V) in 0.5M hydrochloric acid at about 24" c.
40 c
-1
ABSORPTION SPECTROSCOPY
1759
e
ANALYTICAL CHEMISTRY
L
w
50
0 40
e s
LL
Figure 2. Absorption spectra of pIutonium(III), (IV), and (V) in 0.5M hydrochloric acid and plutonium(V1) in 0.5M nitric acid
0
30 20 10
+
0
EI-
20
V
t: $
2
h r
10 3 50
i
40
10
400
500
600
700
800
900
1,000
1,200
WAVE LENGTH m p
ions, as shown in Figures 3 (16),4 (Wb), and 5 (62). Such behavior is due to complex formation between the plutonium ions and the anions of the acid. There is less complexing tendency with perchlorate ions than with other anions (68). Figure 6 (21) brings together the absorption spectra of plutonium(III), (IV), and (VI) in relatively uncomplesed states.
Additional evidence of the complexing tendencies of plutonium ions in various states of oxidation is found in the migration tendencies of the ions under various conditions of acidity (37). Table IV (37) shons such evidence for plutonium(II1) and (IV) ; Table V (37) shoms similar evidence for plutonium(VI). Although such evidence cannot lie taken as quantitative proof of the
l
extent of complex formation, it proves that coniplex formation does occur. Measurements of absorption spectra and electrical transference have led to generalizations regarding the tendencies of anions to form negatively charged plutonium conipex ions.
-Table IV.
70Plutonium Olidation State 111 111
so3, c1
Evidence has aisu been obtained for positively charged phosphate complexes of plutonium, as indicated by King (50),but they are not important
,0°
w
250
8
79 69 5 97
a
17 5
16 6JI H2S04 2JI HC1 1 O M HC1
17 17
131 HzSO4 11Tf HSO3 1oJr “ 0 3
I5
04
b
1 98 5 12
4
18 17
1I
a
84 0 05 99 8 c-
99 9.5 0.2 25
5
Migrated To anode 21 30 5
16
17 5 16
0 lA1f 0 531 HzSOr
11IV IT-
To cathode
Hours
0 5&!f
ITITITIT-
ClOI
Time,
Concentration 1M HC1 l O M HC1
111 I11
so,, CzH301, c?oc
Strong Moderate Weak or zero
Electrical Migration of Plutonium(lll) and (IV) Ions
99 1 5 98 8
Table V. Electrical Migration of PIutonium(V1) Ions
yo Plutonium Acid Migrated To To ConcenTime, tration Hours cathode anode 2M HC1 17 100 10M HC1 17 . 100 1M HnSOa 16 27 73 2.44 HNOj 17 99.1 0 9 14.6M HSOI 3.5 5 95 9MHClOc 16.5 99.9 0 1 0.1M HNOa
4 Figure 3.
Effect of temperature and hydrochloric acid concentration on main absorption peak of plutonium(V1)
L e f t . 0 2M HC1. 2.09 x 10-131 PU Cehter. 0.5M HCI, 1 24 X 10-4.
