- Email: [email protected]
Behaviour of plutonium in the phosphoric acid-trifluoroacetic acid system
J. Inorg. Nucl. Chem., 1964, Vol. 26, pp. 461 to 467. Pergamon Press Ltd. Printed in Northern Ireland
BEHAVIOUR OF PLUTONIUM IN THE PHOSPHORIC ACID-TRIFLUOROACETIC ACID SYSTEM* J. M. C L E V E L A N D ' ~ Heavy Element Chemistry, Chemical Research Operation, Hanford Laboratories, Richland, Washington
(Received 7 June 1963; in revisedJbrm 15 August 1963) Abstract--Plutonium (II1) and (IV) are efficiently extracted from concentrated trifluoroacetic acid solution into 85 per cent phosphoric acid. Extraction of plutonium (II1), followed by further contacting with the trifluoroacetic acid phase, results in rapid conversion of the plutonium to the tetravalent state, the trifluoroacetic acid acting as a catalyst for the air oxidation. The same phosphate complex results from the extraction-oxidation of plutonium(Ill)and the extraction of plutonium(IV), and it does not contain trifluoroacetic groups. Plutonium (1V) trifluoroacetic, Pu(CF3COO)4"xHzO was prepared as a precipitate by allowing a trifluoroacetic acid solution of plutonium (III) to air oxidize. TRIFLUOROACETICacid, a liquid at r o o m temperature, is chiefly k n o w n as a moderately strong acid (estimated dissociation c o n s t a n t ---- 1.8 (1)) that is miscible with water a n d a wide variety of organic liquids. N o t so well k n o w n is the fact that it is immiscible with c o n c e n t r a t e d phosphoric acid solutions, suggesting the possibility of using this system for solvent extraction studies. The system is u n i q u e in that phosphoric acid, rather t h a n a phosphate ester, is the extractant, both phases are moderately strong acids, a n d the system is almost (but n o t quite) n o n a q u e o u s . EXPERIMENTAL Trifluoroacetic acid manufactured by the Minnesota Mining and Manufacturing Company and Baker and Adamson's 85 per cent (15 molar) phosphoric acid were used without further purification. Solutions of plutonium (III) in trifluoroacetic and phosphoric acids were prepared by dissolving metal turnings in the appropriate volume of acid. In each case a few drops of water Was added to the acids to accelerate dissolution; the small amount of water added had a negligible effect on acid concentrations, however. With the exception of the four extractions with 71 per cent (12.5 molar) phosphoric acid, all the acid solutions used in the extraction experiments were concentrated [99+ per cent (13 molar) trifluoroacetic acid, 85 per cent (15 molar) phosphoric acid]. Dissolution in phosphoric acid is complete and reasonably rapid. Trifluoroacetic acid was found to dissolve plutonium slowly and to leave a residue of undissolved material (chiefly plutonium oxide). Before use, the solutions were decanted away from these insoluble residues. Since solutions of plutonium in trifluoroacetic acid become turbid on standing, they were used soon after preparation to minimize this effect, and there was no evidence that the turbidity had any harmful influence on the extraction. Plutonium concentrations in the acids were generally 0.01-O.02 molar. All extractions were performed by shaking equal volumes of the appropriate solutions for the specified time in 60 ml separatory funnels at room temperature (25-26°C). Since generally the volume of liquid did not exceed 25 ml, there was an air volume of at least 35 ml present in the separatory funnel during each extraction. In those extractions performed under oxygen-free conditions, the separator), funnel was flushed with argon before, during, and after addition of the solutions. All plutonium solution analyses were performed by direct mounting and alpha-energy analysis. All visible spectra were measured from 350 to 1400 m# on a Cary Model 14 recording spectrophotometer, using the infrared source over the entire range. Spectra of the solutions were measured * General Electric Company, Richland, Washington. Work performed under Contract No. AT(45-1)-1350 for the U.S. Atomic Energy Commission. 1" Present address: The Dow Chemical Co., Rocky Flats Division, P.O. Box 888, Golden, Colorado. ~1) G. C. HOOD, O. REDLICHand C. A. REILLY, J. Chem. Phys. 23, 2229 (1955). 461
462
J.M. CLmeI~LAm3
in one-centimeter quartz cells, except for that of the pink salt dissolved in phosphoric acid, which was determined in a five-centimetercell. The curves obtained were photographed to produce the spectra reported herein. RESULTS AND DISCUSSION The distribution ratio is defined as follows: Drr=
[Pu]n,ro, [PU]cFsCOOH "
Table 1 lists distribution ratios obtained for various extraction times and for two different initial phosphoric acid concentrations. The distribution ratio decreases rapidly with decre~ising phosphoric acid concentration; unfortunately this dependence could not be studied further since phosphoric acid solutions more dilute than about twelve molar are miscible with triflu0roacetic acid. TABLE I.--DISTRIBUTION RATIOS FOR THE EXTRACTION OF PLUTONIUM FROM TRIFLUOROACETIC ACID INTO PHOSPHORIC ACID
Distribution ratio Extraction time 5 see
5 sec 5 see 10 see 2 min 2min 2 min 4 rain 4 rain 4 rain 4 rain 4 m_in 4 min
15 M HaPO4
12"5 M HsPO4
73 119 105 77 104 114 98 104 126 126 116 152 137
21.5 27.4 11.0" 22.9
* Low value probably due to incomplete phase separation. The colour of the phosphoric acid phase varies considerably as a function of the extraction time---from blue after five seconds to pink after 4 min--suggesting that the plutonium is extracted into phosphoric acid in the trivalent state and subsequently oxidised to the tetravalent state. This is indeed the case, as is indicated in Figs. 1 and 2 by the similarity of the spectrum of the phosphoric acid phase after 5 see extraction to that of a phosphoric acid solution of plutonium (III), and by the similarity of the spectrum of the phosphoric acid phase after 4 min extraction to that of a solution of plutonium in phosphoric acid that had been allowed to stand exposed to the air long enough (20 days) to oxidise the plutonium (III) to plutonium (IV). Furthermore, since extraction equilibrium appears to be reached rapidly, it is concluded that the values for four min extraction represent the distribution ratio for plutonium (IV), while the values for shorter extraction times are ratios for mixtures containing varying amounts of plutonium (IlI). The values for 5 sec extraction are essentially
Behaviour of plutonium in the phosphoric acid-trifluoroacetic acid system
400
500
600
700
800
900
I000
11100 1200
15GO
463
HO0
WAVELENGTH, mF
1~o. l.--Absorption spectra of plutonium (III) in phosphoric acid (1) 0.02 M PuflII) in H,PO4 (2) Pu in HsPO, after 5 see extraction from CFsCOOH.
I
i
]
400
500
600
I
700
I
[
800 900 IOOO WAVELENGTH, m~
[0
II 0
'
1200
1300
r400
Fz~. l--Absorption spectra of plutonium (iV) in phosphoric acid (1) 0'02 M Pu(IV) in HsPO4 (2) Pu in HsPO~ after 4 rain extraction from CF,COOH. the distribution ratios for plutonium (III) with only a minor amount of plutonium (IV) present. The spectrum of plutonium (III) in trifluoroacetic acid is identical to that of the phosphoric acid solution, and is therefore not shown. Both spectra are similar to the spectrum reported by COHEN<2~for plutonium (III) in perchloric acid solution.
464
J . M . CLEVELAND
Further evidence that oxidation of plutonium to the tetravalent state is not a necessary part of the extraction was obtained by extractions performed under oxygenfree conditions, in which the plutonium was efficiently extracted and remained in the trivalent state, even after a 2-min extraction. The distribution ratio was somewhat lower under these conditions, however. Extractions performed in an argon atmosphere using both boiled and unboiled trifluoroacetic acid gave essentially the same results, indicating that the major part of the oxidation is caused by the oxygen present in the air space above the liquids, rather than by the dissolved oxygen initially present in the trifluoroacetic acid. The fact that the spectrum of plutonium (IV) in phosphoric acid is not dependent on whether the solution was prepared by metal dissolution or by extraction from trifluoroacetic acid indicates that the plutonium (IV) species after extraction-oxidation does not contain trifluoroacetate ligands. Thus the species present in phosphoric acid after a 4 rain extraction-oxidation is apparently simply the plutonium (IV) species formed in 85 per cent phosphoric acid. DENOTKINA and SCHEVCHENKOta) have studied phosphate complexes of plutonium (IV) by a solubility method, and have concluded that in six molar phosphoric acid solutions the predominant plutonium complex is Pu(HPO4)56-; whether this is also the predominant species in concentrated phosphoric acid is not known. The most striking feature of this system is the rapiditywithwhich plutonium (III) is oxidized to plutonium (IV) in phosphoric acid solutions when shaken with trifluoroacetic acid. Studies were made of the rate of oxidation of plutonium(III)in phosphoric acid under three different conditions. In one set of experiments, plutonium (IlI) was extracted from trifluoroacetic acid and spectra were run of the resulting phosphoric acid solutions after shaking times varying from 5 sec to 4 min. A semi-logarithmic plot of the rate of disappearance of the plutonium (III) peak at 590 m# as a function of time, shown in Fig. 3, indicates that under these conditions the reaction is first-order in plutonium (III) concentration. The half-time for the oxidation is approximately 55 sec. In another study, whose results are also shown in Fig. 3, the phosphoric acid solutions of plutonium (III) were prepared by metal dissolution, rather than by extraction from trifluoroacetic acid. These solutions were shaken with trifluoroacetic acid for periods up to 4 min and their spectra determined. This reaction is also first-order in plutonium (III) concentration, with a half-time of 63 secwnot significantly different from that of the first study. In still another series of experiments, solutions of plutonium (III) in phosphoric acid (prepared by metal dissolution) were shaken alone for varying time periods and their spectra determined. As shown in Fig. 3, the rate of oxidation is much slower; by extrapolation the half-time was estimated to be approximately 18 rain. The turbidity problem prevented a quantitative study of the rate of oxidation of plutonium (III) in trifluoroacetic acid alone, but qualitative observations indicated that the rate is slower than in the phosphoric acid-trifluoroacetic acid system. The ne.ed for phosphoric acid in order to attain rapid oxidation is probably due to its superior ability to stabilize plutonium (IV) by complex formation. t*~ D. COHEN, d. Inorg. Nucl. Chem. 18, 211 (1961). ta~ R. G. DENOTKINA and V. B. SHEVCHENKO, Russ. J. lnorg. Chem. (English Translation), 6, 756 (1961).
Behaviour of plutonium in the phosphoric acid-trifluoroacetic acid system
465
The data indicate that plutonium (III) is extracted into phosphoric acid from trifluoroacetic acid, and that with continued shaking the latter acts as a catalyst for the air oxidation of plutonium (III) to (IV), greatly increasing the rate. The solubility of oxygen in trifluoroacetic acid is high (0.2 rail per rail of acid at a partial pressure of 640 mm (4)) and it is likely that oxidation is accomplished by the dissolved oxygen, which is continually replenished from the air space above the liquids. Thus the oxidation reaction takes place between two liquid phases, rather than between a gas and a liquid phase.
K
u
~
l
E O
u Z q
"'i 0
O,OI,
I
60
I
L
~
120 180 240 TIME, SECONDS
I
300
360
FI~. 3.--Rate studies--oxidation of plutonium (ILl) (1) Pu(III) in HsPO4 shaken in the absence of CFaCOOH ¢t = 18 rain (2) Pu(III) in HsPO4 shaken with an equal volume of CFsCOOH ~'t = 63 see (3) Pu(III) in CFsCOOH extracted with an equal volume of HgPO, r½ -----55 see.
The species actually responsible for the oxidation is not known. It is possible that dissolved oxygeh forms peroxides which result in the formation of peroxytrifluoroacetic acid, which then acts as the oxidizing agent. Tdfluoroacetie acid readily forms the peroxyacid in the presence of hydrogen peroxide, (5) a fact which supports the above mechanism. On the other hand it was observed that the presence of a peroxide '~) G. S. FOJXOKAand G. H. CADY, J. Amer. Chem. Soc. 79, 2451 (1957). ,s) Trifluoroacetic Acid (Booklet) p. 13. Halocarbon Products Corporation, Hackensack, New Jersey (1963).
466
J.M. CI~WLAND
suppressor (0.01 molar ferrous ion) in the trifluoroacetic acid phase had no effect on the oxidation; thus the role of peroxides, if any, is not known. An interesting sidelight of the rate studies was the observation that the peak at about 1400 m/t, due to the presence of water, decreased in the phosphoric acid spectra with increasing extraction time, suggesting that trifluoroacetie acid is capable of extracting water from 85 per cent phosphoric acid. This observation was Confirmed by analysis of the phosphoric acid solutions by Karl Fischer titration. It was found that a 4 min extraction with an equal volume of trjfluoroacetic acid reduced the water content of the phosphoric acid from 14.5 per cent to 10.5 per cent.
i
400
500
600
700
I
I
800 900 I000 WAVELENGTH, rn/~
I
I100
I
1200
I
1300
1400
FIG. 4.--Absorption spectra of various plutonium (IV) species (1) Pink precipitate from CF3COOH--in fluorocarbon mull (2) Pu(IV) in CFaCOOH (3) Pink precipitate dissolvedin HsPO4. Trifluoroacetic acid solutions of plutonium (III) become turbid, the turbidity increasing with time. On standing five days, one such solution had turned salmon pink and had deposited an appreciable quantity of a pink precipitate. The spectra of the solution and the precipitate (the latter in a fluorocarbon mull) are shown in Fig. 4. Both are apparently plutonium (IV) spectra and are somewhat similar, though not identical. Differences between the spectrum of this solution and the spectrum of plutonium (IV) in perchloric acid, as reported by COHEN(2) lead to the conclusion that plutonium (IV) is complexed in trifluoroacetic acid, as would be expected. Coulometric analysis of the pink precipitate indicated that it was Pu(CFaCOO)a , with an unknown degree of hydration. [Pu found: 33.2 per cent. Calculated for Pu(CFaCOO)4:34.6 per cent.] Efforts to obtain X-ray data on the salt were unsuccessful, probably due to its microcrystallinity. If allowed to stand in moist air, the salt loses weight and becomes tan in colour, presumably due to hydrolysis to form Pu(OH)a and CFaCOOH , the latter being lost by evaporation.
Behaviour of plutoniumin the phosphoricacid-trifluoroaceticacid system
467
The salt is soluble in phosphoric acid, although the rate of solution is slow because of the viscosity of the acid. The spectrum of such a solution, shown in Fig. 4, is identical to those of plutonium (IV) in phosphoric acid solutions shown in Fig. 2, giving further confirmation to the conclusion that the salt is a compound of plutonium (IV). Plutonium (IV) was found to extract efficiently from trifluoroacetie acid into phosphoric acid, and the spectrum of the resulting solution was identical to that of the phosphoric acid solutions of plutonium (IV) shown in Fig. 2. Thus, although all of the previous runs involved the extraction of plutonium (III) followed by its oxidation, the extraction of plutonium (IV) appears to take place equally well.
BEHAVIOUR OF PLUTONIUM IN THE PHOSPHORIC ACID-TRIFLUOROACETIC ACID SYSTEM* J. M. C L E V E L A N D ' ~ Heavy Element Chemistry, Chemical Research Operation, Hanford Laboratories, Richland, Washington
(Received 7 June 1963; in revisedJbrm 15 August 1963) Abstract--Plutonium (II1) and (IV) are efficiently extracted from concentrated trifluoroacetic acid solution into 85 per cent phosphoric acid. Extraction of plutonium (II1), followed by further contacting with the trifluoroacetic acid phase, results in rapid conversion of the plutonium to the tetravalent state, the trifluoroacetic acid acting as a catalyst for the air oxidation. The same phosphate complex results from the extraction-oxidation of plutonium(Ill)and the extraction of plutonium(IV), and it does not contain trifluoroacetic groups. Plutonium (1V) trifluoroacetic, Pu(CF3COO)4"xHzO was prepared as a precipitate by allowing a trifluoroacetic acid solution of plutonium (III) to air oxidize. TRIFLUOROACETICacid, a liquid at r o o m temperature, is chiefly k n o w n as a moderately strong acid (estimated dissociation c o n s t a n t ---- 1.8 (1)) that is miscible with water a n d a wide variety of organic liquids. N o t so well k n o w n is the fact that it is immiscible with c o n c e n t r a t e d phosphoric acid solutions, suggesting the possibility of using this system for solvent extraction studies. The system is u n i q u e in that phosphoric acid, rather t h a n a phosphate ester, is the extractant, both phases are moderately strong acids, a n d the system is almost (but n o t quite) n o n a q u e o u s . EXPERIMENTAL Trifluoroacetic acid manufactured by the Minnesota Mining and Manufacturing Company and Baker and Adamson's 85 per cent (15 molar) phosphoric acid were used without further purification. Solutions of plutonium (III) in trifluoroacetic and phosphoric acids were prepared by dissolving metal turnings in the appropriate volume of acid. In each case a few drops of water Was added to the acids to accelerate dissolution; the small amount of water added had a negligible effect on acid concentrations, however. With the exception of the four extractions with 71 per cent (12.5 molar) phosphoric acid, all the acid solutions used in the extraction experiments were concentrated [99+ per cent (13 molar) trifluoroacetic acid, 85 per cent (15 molar) phosphoric acid]. Dissolution in phosphoric acid is complete and reasonably rapid. Trifluoroacetic acid was found to dissolve plutonium slowly and to leave a residue of undissolved material (chiefly plutonium oxide). Before use, the solutions were decanted away from these insoluble residues. Since solutions of plutonium in trifluoroacetic acid become turbid on standing, they were used soon after preparation to minimize this effect, and there was no evidence that the turbidity had any harmful influence on the extraction. Plutonium concentrations in the acids were generally 0.01-O.02 molar. All extractions were performed by shaking equal volumes of the appropriate solutions for the specified time in 60 ml separatory funnels at room temperature (25-26°C). Since generally the volume of liquid did not exceed 25 ml, there was an air volume of at least 35 ml present in the separatory funnel during each extraction. In those extractions performed under oxygen-free conditions, the separator), funnel was flushed with argon before, during, and after addition of the solutions. All plutonium solution analyses were performed by direct mounting and alpha-energy analysis. All visible spectra were measured from 350 to 1400 m# on a Cary Model 14 recording spectrophotometer, using the infrared source over the entire range. Spectra of the solutions were measured * General Electric Company, Richland, Washington. Work performed under Contract No. AT(45-1)-1350 for the U.S. Atomic Energy Commission. 1" Present address: The Dow Chemical Co., Rocky Flats Division, P.O. Box 888, Golden, Colorado. ~1) G. C. HOOD, O. REDLICHand C. A. REILLY, J. Chem. Phys. 23, 2229 (1955). 461
462
J.M. CLmeI~LAm3
in one-centimeter quartz cells, except for that of the pink salt dissolved in phosphoric acid, which was determined in a five-centimetercell. The curves obtained were photographed to produce the spectra reported herein. RESULTS AND DISCUSSION The distribution ratio is defined as follows: Drr=
[Pu]n,ro, [PU]cFsCOOH "
Table 1 lists distribution ratios obtained for various extraction times and for two different initial phosphoric acid concentrations. The distribution ratio decreases rapidly with decre~ising phosphoric acid concentration; unfortunately this dependence could not be studied further since phosphoric acid solutions more dilute than about twelve molar are miscible with triflu0roacetic acid. TABLE I.--DISTRIBUTION RATIOS FOR THE EXTRACTION OF PLUTONIUM FROM TRIFLUOROACETIC ACID INTO PHOSPHORIC ACID
Distribution ratio Extraction time 5 see
5 sec 5 see 10 see 2 min 2min 2 min 4 rain 4 rain 4 rain 4 rain 4 m_in 4 min
15 M HaPO4
12"5 M HsPO4
73 119 105 77 104 114 98 104 126 126 116 152 137
21.5 27.4 11.0" 22.9
* Low value probably due to incomplete phase separation. The colour of the phosphoric acid phase varies considerably as a function of the extraction time---from blue after five seconds to pink after 4 min--suggesting that the plutonium is extracted into phosphoric acid in the trivalent state and subsequently oxidised to the tetravalent state. This is indeed the case, as is indicated in Figs. 1 and 2 by the similarity of the spectrum of the phosphoric acid phase after 5 see extraction to that of a phosphoric acid solution of plutonium (III), and by the similarity of the spectrum of the phosphoric acid phase after 4 min extraction to that of a solution of plutonium in phosphoric acid that had been allowed to stand exposed to the air long enough (20 days) to oxidise the plutonium (III) to plutonium (IV). Furthermore, since extraction equilibrium appears to be reached rapidly, it is concluded that the values for four min extraction represent the distribution ratio for plutonium (IV), while the values for shorter extraction times are ratios for mixtures containing varying amounts of plutonium (IlI). The values for 5 sec extraction are essentially
Behaviour of plutonium in the phosphoric acid-trifluoroacetic acid system
400
500
600
700
800
900
I000
11100 1200
15GO
463
HO0
WAVELENGTH, mF
1~o. l.--Absorption spectra of plutonium (III) in phosphoric acid (1) 0.02 M PuflII) in H,PO4 (2) Pu in HsPO, after 5 see extraction from CFsCOOH.
I
i
]
400
500
600
I
700
I
[
800 900 IOOO WAVELENGTH, m~
[0
II 0
'
1200
1300
r400
Fz~. l--Absorption spectra of plutonium (iV) in phosphoric acid (1) 0'02 M Pu(IV) in HsPO4 (2) Pu in HsPO~ after 4 rain extraction from CF,COOH. the distribution ratios for plutonium (III) with only a minor amount of plutonium (IV) present. The spectrum of plutonium (III) in trifluoroacetic acid is identical to that of the phosphoric acid solution, and is therefore not shown. Both spectra are similar to the spectrum reported by COHEN<2~for plutonium (III) in perchloric acid solution.
464
J . M . CLEVELAND
Further evidence that oxidation of plutonium to the tetravalent state is not a necessary part of the extraction was obtained by extractions performed under oxygenfree conditions, in which the plutonium was efficiently extracted and remained in the trivalent state, even after a 2-min extraction. The distribution ratio was somewhat lower under these conditions, however. Extractions performed in an argon atmosphere using both boiled and unboiled trifluoroacetic acid gave essentially the same results, indicating that the major part of the oxidation is caused by the oxygen present in the air space above the liquids, rather than by the dissolved oxygen initially present in the trifluoroacetic acid. The fact that the spectrum of plutonium (IV) in phosphoric acid is not dependent on whether the solution was prepared by metal dissolution or by extraction from trifluoroacetic acid indicates that the plutonium (IV) species after extraction-oxidation does not contain trifluoroacetate ligands. Thus the species present in phosphoric acid after a 4 rain extraction-oxidation is apparently simply the plutonium (IV) species formed in 85 per cent phosphoric acid. DENOTKINA and SCHEVCHENKOta) have studied phosphate complexes of plutonium (IV) by a solubility method, and have concluded that in six molar phosphoric acid solutions the predominant plutonium complex is Pu(HPO4)56-; whether this is also the predominant species in concentrated phosphoric acid is not known. The most striking feature of this system is the rapiditywithwhich plutonium (III) is oxidized to plutonium (IV) in phosphoric acid solutions when shaken with trifluoroacetic acid. Studies were made of the rate of oxidation of plutonium(III)in phosphoric acid under three different conditions. In one set of experiments, plutonium (IlI) was extracted from trifluoroacetic acid and spectra were run of the resulting phosphoric acid solutions after shaking times varying from 5 sec to 4 min. A semi-logarithmic plot of the rate of disappearance of the plutonium (III) peak at 590 m# as a function of time, shown in Fig. 3, indicates that under these conditions the reaction is first-order in plutonium (III) concentration. The half-time for the oxidation is approximately 55 sec. In another study, whose results are also shown in Fig. 3, the phosphoric acid solutions of plutonium (III) were prepared by metal dissolution, rather than by extraction from trifluoroacetic acid. These solutions were shaken with trifluoroacetic acid for periods up to 4 min and their spectra determined. This reaction is also first-order in plutonium (III) concentration, with a half-time of 63 secwnot significantly different from that of the first study. In still another series of experiments, solutions of plutonium (III) in phosphoric acid (prepared by metal dissolution) were shaken alone for varying time periods and their spectra determined. As shown in Fig. 3, the rate of oxidation is much slower; by extrapolation the half-time was estimated to be approximately 18 rain. The turbidity problem prevented a quantitative study of the rate of oxidation of plutonium (III) in trifluoroacetic acid alone, but qualitative observations indicated that the rate is slower than in the phosphoric acid-trifluoroacetic acid system. The ne.ed for phosphoric acid in order to attain rapid oxidation is probably due to its superior ability to stabilize plutonium (IV) by complex formation. t*~ D. COHEN, d. Inorg. Nucl. Chem. 18, 211 (1961). ta~ R. G. DENOTKINA and V. B. SHEVCHENKO, Russ. J. lnorg. Chem. (English Translation), 6, 756 (1961).
Behaviour of plutonium in the phosphoric acid-trifluoroacetic acid system
465
The data indicate that plutonium (III) is extracted into phosphoric acid from trifluoroacetic acid, and that with continued shaking the latter acts as a catalyst for the air oxidation of plutonium (III) to (IV), greatly increasing the rate. The solubility of oxygen in trifluoroacetic acid is high (0.2 rail per rail of acid at a partial pressure of 640 mm (4)) and it is likely that oxidation is accomplished by the dissolved oxygen, which is continually replenished from the air space above the liquids. Thus the oxidation reaction takes place between two liquid phases, rather than between a gas and a liquid phase.
K
u
~
l
E O
u Z q
"'i 0
O,OI,
I
60
I
L
~
120 180 240 TIME, SECONDS
I
300
360
FI~. 3.--Rate studies--oxidation of plutonium (ILl) (1) Pu(III) in HsPO4 shaken in the absence of CFaCOOH ¢t = 18 rain (2) Pu(III) in HsPO4 shaken with an equal volume of CFsCOOH ~'t = 63 see (3) Pu(III) in CFsCOOH extracted with an equal volume of HgPO, r½ -----55 see.
The species actually responsible for the oxidation is not known. It is possible that dissolved oxygeh forms peroxides which result in the formation of peroxytrifluoroacetic acid, which then acts as the oxidizing agent. Tdfluoroacetie acid readily forms the peroxyacid in the presence of hydrogen peroxide, (5) a fact which supports the above mechanism. On the other hand it was observed that the presence of a peroxide '~) G. S. FOJXOKAand G. H. CADY, J. Amer. Chem. Soc. 79, 2451 (1957). ,s) Trifluoroacetic Acid (Booklet) p. 13. Halocarbon Products Corporation, Hackensack, New Jersey (1963).
466
J.M. CI~WLAND
suppressor (0.01 molar ferrous ion) in the trifluoroacetic acid phase had no effect on the oxidation; thus the role of peroxides, if any, is not known. An interesting sidelight of the rate studies was the observation that the peak at about 1400 m/t, due to the presence of water, decreased in the phosphoric acid spectra with increasing extraction time, suggesting that trifluoroacetie acid is capable of extracting water from 85 per cent phosphoric acid. This observation was Confirmed by analysis of the phosphoric acid solutions by Karl Fischer titration. It was found that a 4 min extraction with an equal volume of trjfluoroacetic acid reduced the water content of the phosphoric acid from 14.5 per cent to 10.5 per cent.
i
400
500
600
700
I
I
800 900 I000 WAVELENGTH, rn/~
I
I100
I
1200
I
1300
1400
FIG. 4.--Absorption spectra of various plutonium (IV) species (1) Pink precipitate from CF3COOH--in fluorocarbon mull (2) Pu(IV) in CFaCOOH (3) Pink precipitate dissolvedin HsPO4. Trifluoroacetic acid solutions of plutonium (III) become turbid, the turbidity increasing with time. On standing five days, one such solution had turned salmon pink and had deposited an appreciable quantity of a pink precipitate. The spectra of the solution and the precipitate (the latter in a fluorocarbon mull) are shown in Fig. 4. Both are apparently plutonium (IV) spectra and are somewhat similar, though not identical. Differences between the spectrum of this solution and the spectrum of plutonium (IV) in perchloric acid, as reported by COHEN(2) lead to the conclusion that plutonium (IV) is complexed in trifluoroacetic acid, as would be expected. Coulometric analysis of the pink precipitate indicated that it was Pu(CFaCOO)a , with an unknown degree of hydration. [Pu found: 33.2 per cent. Calculated for Pu(CFaCOO)4:34.6 per cent.] Efforts to obtain X-ray data on the salt were unsuccessful, probably due to its microcrystallinity. If allowed to stand in moist air, the salt loses weight and becomes tan in colour, presumably due to hydrolysis to form Pu(OH)a and CFaCOOH , the latter being lost by evaporation.
Behaviour of plutoniumin the phosphoricacid-trifluoroaceticacid system
467
The salt is soluble in phosphoric acid, although the rate of solution is slow because of the viscosity of the acid. The spectrum of such a solution, shown in Fig. 4, is identical to those of plutonium (IV) in phosphoric acid solutions shown in Fig. 2, giving further confirmation to the conclusion that the salt is a compound of plutonium (IV). Plutonium (IV) was found to extract efficiently from trifluoroacetie acid into phosphoric acid, and the spectrum of the resulting solution was identical to that of the phosphoric acid solutions of plutonium (IV) shown in Fig. 2. Thus, although all of the previous runs involved the extraction of plutonium (III) followed by its oxidation, the extraction of plutonium (IV) appears to take place equally well.