UNITED STATES ATOMIC ENERGY COMMISSION 
 
 AECU-936 
 
 THE FORMAL POTENTIAL OF THE Am(V)-Am(VI) 
 COUPLE 
 
 By 
 
 R. A. Penneman 
 
 L. B. Asprey 
 
 [TID Issuance Date] 
 
 Los Alamos Scientific Laboratory 
 
 L 
 
 Technical Information Division, ORE, Oak Ridgo, Tonnosso* 
 
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. 
 
 PRINTED IN USA 
 PRICE 5 CENTS 
 
 AEC, Oak Ridge, Tenn., 1 0-31-50— 675-A24348 
 
THE FOftlAL POTTITIAI OP THR Am(V)-Am(VI) COUPLE* 
 R. A. Penneman and L. B. Aaprey 
 
 The formal potential of the Am(.V)-nm(VI) couple was determined by 
 three methods. The f ir3t was direct measurement of the potential in a 
 solution prepared froi. 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 froin the disproportionation of pure Am(V). The third was 
 an indirect value calculated from the partial oxidation of Am(V) to Am(Vl) 
 with eerie perchlorate. 
 
 Americiuro (VI) was prepared by oxidation of Am(III) in perchloric acid 
 with ammonium peroxydisuJ f ate. Sulfate ion was removed by the addition of 
 barium perchlorate. Americiura (V) was prepared by hypochlorite oxidation 
 of Am(IIl) in carbonate solution according to the method of Vemer and 
 Perlman. The precipitate of Am(V) was dissolved in 8M perchloric acid and 
 •diluted to Hi after disproportionation had yielded sufficient Am(Vl) to 
 permit measurement s. 
 
 The first series of measurements 7;ere -oade with Am(V)-Am(VI) in 0.3V 
 HCIO^ solution (prepared by the jieroxydisulf ate method). A shiny platinum 
 electrode was used in the americium solution and the potential was measured 
 versus the saturated calomel electrode. Contact with the reference electrode 
 
 •This document is based on work j*rformed at the Los Alamos Scientific 
 Laboratory of the University of California under ARC Contract '.Y-7/»05-Rng-36. 
 
 AECU-936 1 
 
2 AECU-936 
 
 was made by means of an agar-potasslum chloride and en agar-sodlum perchlor- 
 
 ate bridge. The potential was measured with a Rubicon Type B potentiometer. 
 
 The cell was teste"! with cerium prior to the aosricium measurement. 
 The magnitude of the junction potentials was tO.037 volts at 0.0OI cerium, 
 and ♦O.OIJ volts at 0.002 11 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 aoericium potential in 
 thie oeasurement is of the order of 0.01 volts. Concentrations of Am(V) 
 and A<n(VI) were determined with the Beckman DU spectrophotometer. Results 
 appear in Slid* 1. 
 
 Slide 1 Potential of the Am(V)-Am(VI) Couple in 0.3* IKIO^ vs. Sat. Calomel 
 Electrode 
 
 If - B^a, ♦ 8° (sat. calomel) ♦ 0.059 log ^FV * K 1 
 
 Junction 
 MVI) o 05Q lo* (VI) ^m (measured)* Ei(foraal) Potential E* forml 
 StvT 5 * * WT (volts) ^(volts) (volts) ^(volts) 
 
 -1.633 
 
 ♦ .015 
 
 -1.61B 
 
 -1.632 
 
 ♦ .015 
 
 -1.617 
 
 -1.631 
 
 ♦ .015 
 
 -1 .616 
 
 -1.624 
 
 ♦ .015 
 
 -1.609 
 
 -1.624 
 
 ♦ .015 
 
 -1.609 
 
 2.19 ♦0.020 -1.407 
 
 1.93 ♦0.017 -1.4Q3 
 
 0.338 -0.026 -1.357 
 
 0.167 -0.046 -1.332 
 
 0.122 -0.054 -1.324 
 
 Ave. -1.614 
 
 Pleasured against saturated calomel electrode taken as -0.246 volts. 
 
 The second series of measurements was made on a similar solution of 
 
 A«(VI) using cerous-ceric perchlorate in 1M HCIO^ as the reference electrode. 
 
 In this case a two -compart rent- 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 
 
aecu-936 3 
 
 matohed each other within 0.1 millivolts when us ed in the reference solution. 
 Both compartments were filled with the reference solution, the electrodes 
 inverted and the potential determined. One compartment was drained anc 
 rinsed several times with Hi HC10, , and the electrodes cleaned. The 
 •mericiua solution was then added and the measurements iiade. From analytical 
 values of the ceric-cerou^ ratio, the reference cell was calculated to have 
 a value of -1.695 volts. Results appear in Slide 2. 
 
 7.45 
 
 4.33 
 
 0.0375 
 
 0.052 
 
 -1.605 
 
 7.09 
 
 2.66 
 
 0.0269 
 
 0.062 
 
 -1.606 
 
 5.61 
 
 1.61 
 
 0.0124 
 
 0.084 
 
 -1.599 
 
 4.93 
 
 1.24 
 
 0.0055 
 
 0.090 
 
 -1.600 
 
 3-58 
 
 0.631 
 
 -0.0118 
 
 0.108 
 
 -1.599 
 
 2.27 
 
 0.358 
 
 -0.0263 
 
 0.124 
 
 ^,597 
 
 Slide 2 Am(V)-Ara(VI) Potential in 1M HCIO^ vs. Ce( Ill)-Ce(IV) 
 
 A»(V) Am(Vl) 
 
 (moles x 1CP) (moles x 1CP) (VI)/(V) O.U59 log(Vl)/(V) ^eas. *f 
 
 1.72 
 2.48 
 3.48 
 3.96 
 5.68 
 6.35 
 
 Ave. -1.601 
 *The potential of the cerium solution was -1.695 volts. 
 
 The potential measurenents made on an Am(V)-Am(VI) solution derived 
 from dispropcrtionation of Am'V) are presented in Slide 3. 
 
 Slide 3 Am(V)-Am(VI) Potential in 1M HCIO^ vs. Hg£\ Hg** 
 
 Am(V) Am(VI) 
 
 (moles x 1CH) (moles xlO^) (V1)/(V) 0.059 log( VI )/(V) Sneas . E f 
 
 0.82 
 1.67 
 2.13 
 
 Ave. -1.598 
 
 *The value of the reference cell was -0.921 volts calculated from B° 3 -0.907, 
 using analytically determined values of Hg** and HgJ*. This cell gave a 
 value agreeing within one millivolt of the calculated potential when 
 ■easured vs. the caric-cerou* ; or chlorate reference cell. 
 
 2.84 
 
 3.46 
 
 0.0319 
 
 -0.711 -1.600 
 
 1.89 
 
 1.13 
 
 0.0032 
 
 -0.680 -1.598 
 
 1.31 
 
 0.615 
 
 -0.0125 
 
 -0.662 -1.596 
 
h AECU-936 
 
 Immediately after taking the last reading, solid ammonium sulfate waa 
 added to sake the solution lM In sulfate . A shift In the measure* potential 
 froBi -0.662 to -0.620 occurred, a decrease of 0.01*2 volts, Indicating complex 
 ion fomation. Magnueson, Hindman, and LaChapelle found that the 
 neptunium (V)/(VI) couple is shifted tO.05 volts in 111 H^O, from its value 
 in perchloric acid. The same optical density was observed for the Am(V) 
 and Am(VI) peaks as before sulfate addition. Howevor, the Am(Vl) peak was 
 shifted 4 millimicrons towards the red. 
 
 A check on the value of the potential was made using eerie perchlorate 
 to partially oxidise Am(V) to Am(Vl). From the amount of Am(VI) formed, 
 the ratio of Ce(IV)/(IIl) was calculated, assuming that one Ce(lV) 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 show that values for the 
 potential are in essential agreement. 
 
 Slide 4 The Potential of the Am(V)-Am(VI) Couple in 0.3M "ClO^ Using a 
 Standard Solution of Ce(lV)-Ce(IIl) 
 (Values after Oxidation) 
 
 Ce(lV) B Celll/lV Am(Yl) Bf Am(V)-Am(VI) 
 
 Ce(III) (volts) AmTv) (volts) 
 
 0.136 -1.643 0.776 -1.65 
 
 0.168 -1.648 0.520 -1.66 
 
 No direct measurement of the Ce(lV), Ce(IlI) ratio was made after the 
 solution bad reached equilibrium. Since the solution tmd stood overrd«ht, 
 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 
 
AECU-936 5 
 
 the value o obtained by direct measurement against three different reference 
 electrodes. 
 
 From the series of direct measia- omenta , the value of the Am( V)-Jkm(VI) 
 couple may be taken aa -1.600 * 0.(X 5 volts in HI HC1Q, . Similar measure- 
 ments on U. Np, and Pu *« that the (V)-(VI) couple is independent of H* 
 ion concentration ir. the range C.113 to 1M. The coraplexing effect of sulfate 
 on americyl ion is about the sarae as for neptunyl ion. 
 
 Taking the value of the Am(IIl)-Am(VI) potential reported earlier, and 
 the value of the Am(LIl)-Am(IV) potential reported by Cunningham and co- 
 workers, it is possible to construct a potential scheme for the anericium 
 oxidation btates. 
 
 Slide 5 Oxidation Potentials of Anericium in Acid 
 
 -1.9 
 
 Am(III) -2.3 Ati(IV) -1.8 Am(V) -1.60 Am(Vl) 
 
 -2.0 
 
 END OF DOCUMENT 
 
UNIVERSITY OF FLORIDA 
 
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