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UNITED STATES ATOMIC ENERGY COMMISSION
THE FORMAL POTENTIAL OF THE Am(V)-Am(VI)
R. A. Penneman
L. B. Asprey
[TID Issuance Date]
Los Alamos Scientific Laboratory
L Technical Information Division, ORE, Oak Ridge, Tennessee
U. -'S ,.' ., ..- -- i
Manuscript for a 15 minute talk at the Chicago
meeting of the American Chemical Society,
Tuesday, September 5, 1950, at 10:40 A. M.
Reproduced direct from copy
as submitted to this office.
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AEC, Oak Ridge, Tenn., 10-31-50-67S.A24341
THE FORMAL POTENTIAL OF TlE Am(V)-Am(VI) COUPLE
R. A. Penneman and L. B. Asprey
The formal potential of the Am(V)--m(VI) couple was determined by
three methods. The first was direct measurement of the potential in a
solution prepared from; pure Am(VI) after partial reduction to Am(V). The
second method involved direct measurement in a solution containing Am(Vl)
and Am(V) derived from the disproportionation of pure Am(V). The third was
an indirect value calculated from the partial oxidation of Am(V) to Am(Vl)
with ceric perchlorate.
Americium (VI) was prepared by oxidation of Am(III) in perchloric acid
with ammonium pero.jdisulfate. Sulfate ion was removed by the addition of
barium perchlorate. Americium (V) was prepared by hypochlorite oxidation
of Am(IIl) in carbonate solution according to the method of 'Vemer and
Perian. The precipitate of Am(V) was dissolved in 1S perchloric acid and
diluted to 1M after disproportionation had yielded sufficient Am(VI) to
permit measurement s.
The first series of measurements were made with Am(V)-Am(VI) in 0.3M
HC10 solution(preparea by the peroxydisulfate method). A shiny platinum
electrode was used in the americium solution and the potential was measured
versus the saturated calamel electrode. Contact with the reference electrode
*This document is based on work performed at the Los Alamos Scientific
Laboratory of the University of California under AEC Contract W-7405-O-g-36.
was made by means of an agar-potassium chloride and an agar-sodium perchlor-
ate bridge. The potential was measured with a Rubicon Type B potentiameter.
The cell was tested with cerium prior to the americium measurement.
The magnitude of the junction potentials was +0.037 volts at 0.081 cerium,
and +0.015 volts at 0.002 V cerium. Since the americium concentration was
0.007 U, a value of +0.015 volts was applied as an approximate correction
to the americium potentials. The uncertainty in the americium potential in
this measurement is of the order of 0.01 volts. Concentrations of Am(V)
and Am(VI) were determined with the Beckman DU spectrophotoamter. Results
appear in Slide 1.
Slide 1 Potential of the Am(V)-Am(VI) Couple in 0.3M IC104 va. Sat. Calamel
If a, o (sat. calomel) 0.099 log Am( + E
A0.059 log ,(V E. (measured)* ZR(formal) Potential Xr foril
A(Vr (volta) (volts) (volts) (volts)
2.19 +0.020 -1.407 -1.633 e.015 -1.618
1.93 +0.017 -1.403 -1.632 +.015 -1.617
0.338 -0.028 -1.357 -1.631 +.015 -1.616
0.167 -0.046 -1.332 -1.62t *.015 -1.609
0.122 -0.054 -1.324 -1.624 ..015 -1.609
*Measured against saturated calomel electrode taken as -0.246 volts.
The second series of measurements was made on a similar solution of
Am(VI) using cerous-ceric perchlorate in 1M HC104 as the reference electrode.
In this case a two-compartnenx, cell separated by an alundum diffusion disk
was used. The shiny platinum electrodes were cleaned in aqua regia and
boiled in distilled water before each series of measurements. The electrodes
AEEC U-93 3
intohed each other within 0.1 millivolts when usej in the reference solution.
Both compartments were filled with the reference solution, the electrodes
inserted and the potential determined. One compartment was drained ana
rinsed several times with lU HC10I, and the electrodes cleaned. The
nericium solution was then added and the measurements iiade. From ~m~lytical
values of the ceric-cerou- ratio, the reference, cell wai cCiLculated to have
a value of -1.695 volts. Results appear in Slide 2.
Slide 2 Am(V)-Am(VI) Potential in 1M HC104 vs. Ce(III)-Ce(IV)
(moles x 103) (moles x 103) (VI)/(V) G.U59 log(Vl)/(V) 4eas. Ef
1.72 7.45 4.33 0.0375 0.052 -1.605
2.48 7.09 2.86 0.0269 0.062 -1.606
3.48 5.61 1.61 0.0124 0.084 -1.599
3.96 4.93 1.24 0.0055 0.090 -1.600
5.68 3.58 0.631 -0.0118 0.108 -1.599
6.35 2.27 0.358 -0.0263 0.124 -1.597
*The potential of the cerium solution was -1.695 volts.
The potential measurements made on an Am(V)-Am(VI) solution derived
from disproportionation of Am'V) are presented in Slide 3.
Slide 3 Am(V)-Am(VI) Potential in IN HC104 vs. Hg2', Hg"*
(moles x 103) (moles x l03) (VI),(V) 0.059 log(V1)/(V) seas. Sf
0.82 2.84 3.46 0.0319 -0.711 -1.600
1.67 1.89 1.13 0.0032 -0.680 -1.598
2.13 1.31 0.615 -0.0125 -0.662 -1.596
The value of the reference cell was -0.921 volts calculated from r o = -0.907,
using analytically determined values of Hg') and Hgl*. This cell gave a
value agreeing within one millivolt of the calculated potential when
masured vs. the cwic-cerous perchlorate reference cell.
Immediately after taking the last reading, solid ammonium sulfate was
added to make the solution 1M in selfate. A shift In the measure potential
fro -0.662 to -0,620 occurred, a decrease of 0.042 volts, indicating oamplex
ion formation. Magusson, Hindama, and LaChapelle found that the
neptunium (V)/(VI) couple is shifted +0.05 volts in IM H!S04 from Its value
in perchloric acid. The same optical density was observed for the Am(V)
and Am(VI) peaks as before sulfate addition. However, the Am(VI) peak was
shifted 4 millimicrons towards the red.
A check on the value of the potential was made using ceric perchlorate
to partially oxidize Am(V) to Am(VI). From the amount of Am(VI) forced,
the ratio of Ce(IV)/(III) was calculated, assuming that one Ce(IV) was re-
quired for each Am(V) oxidized. Some post precipitation of barium sulfate
made spectrophotometric measurements subject to large background errors.
The results shown on Slide 4 are included merely to dhow that values for the
potential are in essential agreement.
Slide 4 The Potential of the Am(V)-Am(VI) Couple in 0.3y HC104 Using a
Standard Solution of Ce(IV)-Ce(III)
(Values after Oxidation)
Ce(IV E CeIII/IV AmI) A(V)-Am(VI)
CaIII) (volts) Am(V (volts)
0.138 -1.643 0.776 -1.65
0.168 -1.648 0.520 -1.66
No direct measurement of the Ce(IV), Ce(IlI) ratio was made after the
solution had reached equilibrium. Since the solution lad stood overrni ,
it is likely that reduction of Ce(IV) had occurred due to products of alpha
radiation on water. This may explain the discrepancy between this value and
the values obtained by direct measurement against three different reference
From the series of direct measurements, the value of the Am(V)-Am(VI)
couple may be taken as -1.600 A 0.005 volts in 1M HC1Q4. Similar masure-
ments on U, Np, and Pu ahw that the (V)-(VI) couple is independent of H*
ion ooncentration ir the range C.1M to 1k. The completing effect of sulfate
on americyl ion ib about the same as for neptunyl ion.
Taking the value of the Am(III)-Am(VI) potential reported earlier, and
the value of the Am(Ill)-Am(IV) potential reported by Cunningham and co-
workers, it is possible to construct a potential scheme for the americium
Slide 5 Oxidation Potentials of Americium in Acid
Am(III) -2.3 A (IV) -1.8 Ai(V) -1.60 Am(VI)
END OF DOCUMENT
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