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Neptunium complex ion species extracted from nitric acid-perchloric acid mixtures by tributyl phosphate
J. Inorg. NucL Chem., 1~0, Vol: 16, pp. 138 to 141. PergamonPress Ltd. Printedin Northern Ireland
NEPTUNIUM FROM
COMPLEX NITRIC
MIXTURES
ION
SPECIES EXTRACTED
ACID-PERCHLORIC
BY TRIBUTYL
ACID
PHOSPHATE*
W. E. KEDER Hanford Laboratories Operation, General Electric Company, Richland, Washington (Received 22 December 1959; in revised form 2 February 1960)
Abstract--Neptunium(IV) and neptunium(Vl) were extracted into TBP-xylene from nitric acidperchloric acid mixtures and the absorption spectra of the neptunium were measured in both phases. The spectra of the neptunium in the TBP solulions were found to vary with the aqueous phase composition. It was concluded that more than one complex ion species extracts in each case and that three neptunium(VI) species are indicated.
RECENTLY the stability of nitrate complexes of thorium ~1~ and zirconium ~2~ and the chloride complexes of zirconiumt~, a~ have been measured by a tributyl phosphate (TBP) extraction procedure. This method ~1~depends on the extraction of the tetraco-ordinated species from an acidic-aqueous phase into a nonmiscible TBP phase. Its application demands that no other species extracts. In practice, extractions have been from solutions in which ionic strength was held constant with perchloric acid. Spectrophotometric measurements have been made which show that more than one neptunium(IV) and neptunium(VI) complex ion species extracts into TBP from nitric acid-perchloric acid mixtures. This observation raises considerable doubt about the applicability of TBP as an extraction agent for the determination of complex ion stabilities. EXPERIMENTAL Neptunium was obtained in nitric acid solution which was converted to perchloric acid solution by successive precipitations of neptunium(V) hydroxide. Neptunium(IV) was prepared by catalytic hydrogen reduction and neptunium(VI) by electrolysisJ 4~ A several-fold excess of semicarbazide was added to the neptunium(IV) to act as a holding reductant. Four molar HNO8 and HCIO~ solutions were prepared from Baker and Adamson reagent grade acids. One molar TBP was prepared from redistilled tributyl phosphate and Baker and Adamson reagent grade xylene. Before each neptunium solution was extracted, a portion Of TBP was brought to equilibrium with the acid by three successive contactings with equal volumes of acid. Acid mixtures were prepared at the desired nitrate concentrations, at total acid equal to 4.0 molar~ and neptunium was added to each mixture. The absorption spectrum was measured from 400 to 1300 mp with a Cary Model 14 recording spectrophotometer. Each solution was then contacted for 5 min with an equal volume of 1"0 M TBP and the spectrum of each phase was measured. The concentration of the organic phase was determined by radiometric assay. The neptunium concentrations of all aqueous phases, except 4.0 M HC104, were about 0.008 molar before contacting. * Work performed under Contract No. AT(45-1)-1350 for the U.S. Atomic Energy Commission. ~lj V. V. FOMINand E. P. MAIOROVA,Zh. Neorg. Khim. 1, 1703 (1956); AERE-LIB-TR-802 (1958). t2~A. S. SOLOVKIN,Zh. Neorg. Khim. 2, 611 (1957); AERE-LIB-TR-791 (1958). ~a~A. E. LEVITTand H. FREUrqO,J'. Amer. Chem. Soc. 78, 1545 (1956). tia~ D. COHEN and J. C. HINDMAN,J. Amer. Chem. Soc. 74, 4679, 4682 (1952); ~ W. C. WAGGENER,J. Phys. Chem. 62, 382 (1958). 138
Neptunium complex ion species extracted from nitric acid-perchloric acid mixtures RESULTS
AND
139
CONCLUSIONS
The spectra of neptunium(IV) in nitric-perchloric acid mixtures at constant total acid concentration equal to 4.0 M are shown in Fig. 1. These spectra change with solution composition because of nitrate complex formation. The spectra of neptunium(IV) in 1.0 M TBP are shown in Fig. 2. The neptunium in each solution represented in this figure was extracted from an aqueous solution shown in Fig. 1. The neptunium spectra in the TBP solutions are clearly a function of the composition of the aqueous phase from which the neptunium was extracted. It has been
--.-4.o~
H~
7O 6O
P,
~o
3O 2O IO
Fro. l.--Absorption spectra of neptunium(IV) in 4.0 M nitric acid-perchloric acid solutions.
shown that the absorption spectra of ions in solution are very nearly independent of the solvent environment in cases where the particular inner sphere complexes present are not altered. (5) The complex ion species extracted into the TBP must, therefore, change with aqueous solution composition. No part of these spectra appears to be due to higher oxidation states of neptunium. Neptunium(VI) has no significant absorption peaks in the wavelength region considered. The extractability of neptunium(V) is low and the spectra of the aqueous phases after extraction did not show appreciable amounts of this oxidation state. Since spectral measurements were made immediately following extraction, no neptunium(V) should have appeared in the organic phase. The absence of neptunium(V) absorptions in the TBP spectra is also shown by the following observation. A TBP solution of neptunium(IV) developed new peaks at 980 and 1003 m/~ after several days time, but its former spectrum returned when it was back extracted with the original acid solution. The 980 m/~ peak of neptunium(V) then appeared in the aqueous solution. It was expected that Np(NOa)4.2TBP would be the chief extracted species at all nitrate-perchlorate compositions containing sufficient nitrate to form the complex. (sJ j. BJERRUM,A. W. ADAMSONand O. BOSTRUP,Acta Chem. Scand. 10, 329 (1956).
140
W.E. KEDER 0.! M 0.SM I-OM - - - - - 4,0M
70
.....
.....
HNO3 HNO3 HN03 HN03
60
20
t
I0
700
800
Wovelenc~ FIG.
1000
900 mp
2.--Absorption spectra of solutions of neptunium(IV) in
!'0 M
TBP extracted from the
aqueous solutions in Fig. 1.
Apparently, this is not true. Since only uncharged species are extracted by the unionized TBP, one must conclude that at least one perchlorate-containing complex of neptunium(IV) is extracted. SOLOVKIN(~) has described a TBP extraction technique for determining stability constants and has measured the stability of four zirconium nitrate complexes. FOMIN and MAmROVAa) have employed the same method for determination of the stability of thorium nitrate complexes. In these studies 4.0 M perchloric-nitric acid mixtures were used. It was assumed in the treatment of the data that the tetranitrato complex is the only species extracted. The fact that other species extract in the case of neptunium(IV) raises some question about the validity of this assumption in the cases of zirconium and thorium. Similar measurements have been made with neptunium(VI). The results are 5(:
#
4C
3C
2C
IC
0 I000
I100
Wovelengrn,
1200
1300
m/z
FIG. 3.--Absorption spectra of neptunium(VI) in nitric acid-perchloric acid solutions. 1. 0.00 M HNO8 @ 4.0 M H C I O 4 5.
6. 7.
0.5 M H N O a at- 3.5 M H C I O 4 1.0 M H N O a -~ 3.0 M H C l O 4 4.0 M H N O a -p 0 ' 0 M H C I O 4
Neptunium complex ion speciesextracted from nitric acid-perchloric acid mixtures
141
LO0 I
90
II II I|
80 70 60
15
30
10
ilOO Wavele~glh,
12oo
13oo
m/~
Flo 4.--Absorption spectra of neptunium(VI) in 1.0 M TBP extracted from nitric acidperchloric acid solutions. 1. 0.00 M H N O s -I- 4.0 M HCIO 4 2. 0.05 M H N O s -k 3.95 M HCIO 4 3. 0 . 1 0 M H N O a d - 3 ` 9 M H C I O 4 4. 0 . 3 M H N O a-I- 3.7 M H C I O 4 5. 0 . S M H N O s-~ 3,5 M H C I O 4 6. 1 . 0 M H N O s-k 3.0 M H C I O 4 7. 4 . 0 M H N O a ~ 0 . 0 MHCIO4
shown in Figs. 3 and 4. The spectra at wavelengths shorter than those shown gave no evidence of other oxidation states. Neptunium(VI) is considerably more extractable from HCIO 4 than neptunium(IV); so, in this case, spectra were obtained over the entire range of acid mixtures from 4.0 M HNO 3 to 4.0 M HC10 4. The spectra of neptunium(VI) in TBP solutions, Fig. 4, indicate that three separate species extract from the acid solutions studied. Neptunium(VI) has been shown to extract from nitric acid as NpO2(NOa)f2TBP. t6~ A corresponding diperchlorato complex might be expected to be the species extracted from pure perchloric acid. Only one nitratoperchlorato complex is possible. This species seems a more reasonable postulate for the source of the intermediate spectrum that either a charged or hydrolysed TBP complex. Further work is in progress on the spectra of other actinide oxidation states in TBP solutions. t6) j. K. DAWSON, R. ELLIOTTand H. A. C. McKAY, AERE/C/R-1117 (1953).
NEPTUNIUM FROM
COMPLEX NITRIC
MIXTURES
ION
SPECIES EXTRACTED
ACID-PERCHLORIC
BY TRIBUTYL
ACID
PHOSPHATE*
W. E. KEDER Hanford Laboratories Operation, General Electric Company, Richland, Washington (Received 22 December 1959; in revised form 2 February 1960)
Abstract--Neptunium(IV) and neptunium(Vl) were extracted into TBP-xylene from nitric acidperchloric acid mixtures and the absorption spectra of the neptunium were measured in both phases. The spectra of the neptunium in the TBP solulions were found to vary with the aqueous phase composition. It was concluded that more than one complex ion species extracts in each case and that three neptunium(VI) species are indicated.
RECENTLY the stability of nitrate complexes of thorium ~1~ and zirconium ~2~ and the chloride complexes of zirconiumt~, a~ have been measured by a tributyl phosphate (TBP) extraction procedure. This method ~1~depends on the extraction of the tetraco-ordinated species from an acidic-aqueous phase into a nonmiscible TBP phase. Its application demands that no other species extracts. In practice, extractions have been from solutions in which ionic strength was held constant with perchloric acid. Spectrophotometric measurements have been made which show that more than one neptunium(IV) and neptunium(VI) complex ion species extracts into TBP from nitric acid-perchloric acid mixtures. This observation raises considerable doubt about the applicability of TBP as an extraction agent for the determination of complex ion stabilities. EXPERIMENTAL Neptunium was obtained in nitric acid solution which was converted to perchloric acid solution by successive precipitations of neptunium(V) hydroxide. Neptunium(IV) was prepared by catalytic hydrogen reduction and neptunium(VI) by electrolysisJ 4~ A several-fold excess of semicarbazide was added to the neptunium(IV) to act as a holding reductant. Four molar HNO8 and HCIO~ solutions were prepared from Baker and Adamson reagent grade acids. One molar TBP was prepared from redistilled tributyl phosphate and Baker and Adamson reagent grade xylene. Before each neptunium solution was extracted, a portion Of TBP was brought to equilibrium with the acid by three successive contactings with equal volumes of acid. Acid mixtures were prepared at the desired nitrate concentrations, at total acid equal to 4.0 molar~ and neptunium was added to each mixture. The absorption spectrum was measured from 400 to 1300 mp with a Cary Model 14 recording spectrophotometer. Each solution was then contacted for 5 min with an equal volume of 1"0 M TBP and the spectrum of each phase was measured. The concentration of the organic phase was determined by radiometric assay. The neptunium concentrations of all aqueous phases, except 4.0 M HC104, were about 0.008 molar before contacting. * Work performed under Contract No. AT(45-1)-1350 for the U.S. Atomic Energy Commission. ~lj V. V. FOMINand E. P. MAIOROVA,Zh. Neorg. Khim. 1, 1703 (1956); AERE-LIB-TR-802 (1958). t2~A. S. SOLOVKIN,Zh. Neorg. Khim. 2, 611 (1957); AERE-LIB-TR-791 (1958). ~a~A. E. LEVITTand H. FREUrqO,J'. Amer. Chem. Soc. 78, 1545 (1956). tia~ D. COHEN and J. C. HINDMAN,J. Amer. Chem. Soc. 74, 4679, 4682 (1952); ~ W. C. WAGGENER,J. Phys. Chem. 62, 382 (1958). 138
Neptunium complex ion species extracted from nitric acid-perchloric acid mixtures RESULTS
AND
139
CONCLUSIONS
The spectra of neptunium(IV) in nitric-perchloric acid mixtures at constant total acid concentration equal to 4.0 M are shown in Fig. 1. These spectra change with solution composition because of nitrate complex formation. The spectra of neptunium(IV) in 1.0 M TBP are shown in Fig. 2. The neptunium in each solution represented in this figure was extracted from an aqueous solution shown in Fig. 1. The neptunium spectra in the TBP solutions are clearly a function of the composition of the aqueous phase from which the neptunium was extracted. It has been
--.-4.o~
H~
7O 6O
P,
~o
3O 2O IO
Fro. l.--Absorption spectra of neptunium(IV) in 4.0 M nitric acid-perchloric acid solutions.
shown that the absorption spectra of ions in solution are very nearly independent of the solvent environment in cases where the particular inner sphere complexes present are not altered. (5) The complex ion species extracted into the TBP must, therefore, change with aqueous solution composition. No part of these spectra appears to be due to higher oxidation states of neptunium. Neptunium(VI) has no significant absorption peaks in the wavelength region considered. The extractability of neptunium(V) is low and the spectra of the aqueous phases after extraction did not show appreciable amounts of this oxidation state. Since spectral measurements were made immediately following extraction, no neptunium(V) should have appeared in the organic phase. The absence of neptunium(V) absorptions in the TBP spectra is also shown by the following observation. A TBP solution of neptunium(IV) developed new peaks at 980 and 1003 m/~ after several days time, but its former spectrum returned when it was back extracted with the original acid solution. The 980 m/~ peak of neptunium(V) then appeared in the aqueous solution. It was expected that Np(NOa)4.2TBP would be the chief extracted species at all nitrate-perchlorate compositions containing sufficient nitrate to form the complex. (sJ j. BJERRUM,A. W. ADAMSONand O. BOSTRUP,Acta Chem. Scand. 10, 329 (1956).
140
W.E. KEDER 0.! M 0.SM I-OM - - - - - 4,0M
70
.....
.....
HNO3 HNO3 HN03 HN03
60
20
t
I0
700
800
Wovelenc~ FIG.
1000
900 mp
2.--Absorption spectra of solutions of neptunium(IV) in
!'0 M
TBP extracted from the
aqueous solutions in Fig. 1.
Apparently, this is not true. Since only uncharged species are extracted by the unionized TBP, one must conclude that at least one perchlorate-containing complex of neptunium(IV) is extracted. SOLOVKIN(~) has described a TBP extraction technique for determining stability constants and has measured the stability of four zirconium nitrate complexes. FOMIN and MAmROVAa) have employed the same method for determination of the stability of thorium nitrate complexes. In these studies 4.0 M perchloric-nitric acid mixtures were used. It was assumed in the treatment of the data that the tetranitrato complex is the only species extracted. The fact that other species extract in the case of neptunium(IV) raises some question about the validity of this assumption in the cases of zirconium and thorium. Similar measurements have been made with neptunium(VI). The results are 5(:
#
4C
3C
2C
IC
0 I000
I100
Wovelengrn,
1200
1300
m/z
FIG. 3.--Absorption spectra of neptunium(VI) in nitric acid-perchloric acid solutions. 1. 0.00 M HNO8 @ 4.0 M H C I O 4 5.
6. 7.
0.5 M H N O a at- 3.5 M H C I O 4 1.0 M H N O a -~ 3.0 M H C l O 4 4.0 M H N O a -p 0 ' 0 M H C I O 4
Neptunium complex ion speciesextracted from nitric acid-perchloric acid mixtures
141
LO0 I
90
II II I|
80 70 60
15
30
10
ilOO Wavele~glh,
12oo
13oo
m/~
Flo 4.--Absorption spectra of neptunium(VI) in 1.0 M TBP extracted from nitric acidperchloric acid solutions. 1. 0.00 M H N O s -I- 4.0 M HCIO 4 2. 0.05 M H N O s -k 3.95 M HCIO 4 3. 0 . 1 0 M H N O a d - 3 ` 9 M H C I O 4 4. 0 . 3 M H N O a-I- 3.7 M H C I O 4 5. 0 . S M H N O s-~ 3,5 M H C I O 4 6. 1 . 0 M H N O s-k 3.0 M H C I O 4 7. 4 . 0 M H N O a ~ 0 . 0 MHCIO4
shown in Figs. 3 and 4. The spectra at wavelengths shorter than those shown gave no evidence of other oxidation states. Neptunium(VI) is considerably more extractable from HCIO 4 than neptunium(IV); so, in this case, spectra were obtained over the entire range of acid mixtures from 4.0 M HNO 3 to 4.0 M HC10 4. The spectra of neptunium(VI) in TBP solutions, Fig. 4, indicate that three separate species extract from the acid solutions studied. Neptunium(VI) has been shown to extract from nitric acid as NpO2(NOa)f2TBP. t6~ A corresponding diperchlorato complex might be expected to be the species extracted from pure perchloric acid. Only one nitratoperchlorato complex is possible. This species seems a more reasonable postulate for the source of the intermediate spectrum that either a charged or hydrolysed TBP complex. Further work is in progress on the spectra of other actinide oxidation states in TBP solutions. t6) j. K. DAWSON, R. ELLIOTTand H. A. C. McKAY, AERE/C/R-1117 (1953).