Voltammetry of neptunium(VI) malonate at the rotated glassy carbon electrode

J. inorg,nucl.Chem..1968,Vol.30, pp. 3023to 3032. PergamonPress. Printedin Great Britain

VOLTAMMETRY OF NEPTUNIUM(VI) AT THE ROTATED GLASSY CARBON

MALONATE ELECTRODE*

C. E. P L O C K The Dow Chemical Company, Rocky Flats Division, Golden, Colorado

(First received 27 March 1968; in revised form 24 April 1968) A l ~ t r a e t - T h e voltammetric behavior of the neptunium(Vl)-malonate has been investigated over widely varying conditions of ligand concentration and pH. Under the conditions investigated, an irreversible wave was obtained when the pH was greater than 3.3 and the ligand concentration was less than 6.0 × l0 -2 M. In all other pH ranges and ligand concentrations, a reversible wave corresponding to the reduction of neptunium(VI) complex to the neptunium(V) complex was obtained. At acid concentrations greater than 1 M and less than a pH 3.3, the half wave potential was found to be independent of acid concentration. Inside this range the half wave potential is pH dependent, and one hydrogen ion is involved in the reduction. The metal-ligand ratio was found to be 1 : 2 by conductometric titrations. The limiting current is proportional to the concentration of the neptunium(Vl) from 3.92 x 10-5 to 1-96 x 10-5 M. The diffusion coefficient is 0.28 × 10-5 cm2/sec at pH 1.3 and 0.22 x l0 -~ cm2/sec at pH 4.6. INTRODUCTION

THE ELECTROANALYTICAL investigation of the neptunium(VI/V) couple has been confined to controlled potent;.al coulometry[1-3] and to the voltammetric determination of neptunium at a glassy carbon electrode [4]. The investigation of the malonate complexes of the actinide elements has been limited to thorium[5], protactinium [6], and uranium [7, 8] complexes. Glassy carbon, a new form of carbon, was introduced by Yamada and Sato [9]. It is electrically conductive, highly resistant to chemical attack, gas impermeable, and obtainable in a relatively pure state. Glassy carbon is an exclusively owned preparation of the Tokai Electrode Manufacturing Co.[10]. It has many properties in common with pyrolytic graphite[11]. It does not, however, need to be oriented as does pyrolytic graphite. Zittel and Miller[12] reported on the possible analytical use of glassy carbon as an indicator electrode. This report describes the investigation of the neptunium(VI) malonate complex at the rotated glassy carbon electrode. *Work performed under U.S. Atomic Energy Commission Contract AT(29-1)- 1106. I. R.W. Stromatt, USAEC Rep. No. HW-59447(1959);Analyt. Chem. 32, 134 (1960). 2. M. O. Fulda, USAEC Rep. No. DP-673(1962). 3. G. W. C. Milner, Anal. Chem. Proc. Intern. Syrup., Birmingham University, Birmingham, England, 1962, p. 225. Publ. 1963. 4. C. E. Plock,J. Electroanal. Chem. lnpress. 5. K. B. Yatsimuskii and Yu. A. Zhukov, Zhr. Neorg. Khim. 8, 295 (1963). 6. R.T. Kolarich, V. A. Ryan, and R. P. Schuman,J. inorg, nucl. Chem. 29, 783 (1967). 7. T.T. Lai and C. C. Hsieh, J. inorg, nucl. Chem. 26, 1215 (1964). 8. C. E. Plock and F. J. Miner, Anal. chim. A cta 38, 553 (1967). 9. S. Yamada and H. Sato, Nature, Lond. 193, 261 (1962). 10. Tokai Electrode Mfg. Co. Ltd., 20 Ahasaka. Tameike-Cho. Minato-Ku. Tokyo, Japan. 11. F.J. Miller and H. E. Zittei, A nalyf. Chem. 34, 45 (1964). 12. H. E. Zettel and F. J. Miller, Analyt. Chem. 37, 200(1965). 3023

3024

C . E . PLOCK EXPERIMENTAL

Apparatus All voltammograms were obtained at 25-.+0.1°C with a calibrated Sargent Model X V recording polarograph. None of the measurements were damped. The electrolysis cell was a two-piece " H " form electrolysis cell which contained a saturated calomel electrode. The sample compartment was connected to the saturated calomel electrode by means of an agar saturated potassium nitrate bridge. The glassy carbon electrode was prepared by cutting a disk from a glassy carbon "plate". The disk was sealed into the end of a glass tube using an epoxy resin (Dow Epoxy Resin 331 and Dow Epoxy Hardner 24). A small amount of mercury was poured into the glass tube, and electrical contact was made by means of a copper wire inserted into the mercury. The d.c. electrical resistance through the electrode was 2 II, and the surface area of the electrode exposed to the solution was 0-14 cm 2. The electrode was mounted in a Sargent synchronous rotator that had a rotation speed of 600 rev/min. The electrode was cleaned at the end of each measurement by allowing the electrode to rotate for 15-30 see in concentrated nitric acid. The pH values of the solutions were measured using a Beckman Model 76 Expanded Scale pH meter and a glass electrode. The pH was adjusted with perchioric acid or sodium hydroxide. The conductometric titrations were made using a Model RC- 16 (Industrial Instruments, USA) line operated conductivity bridge.

Reagents The neptunium stock solution was prepared by dissolving neptunium dioxide in concentrated nitric acid and taking the solution to an incipient dryness. The neptunium salt was dissolved in 2.5 M nitric acid and diluted to volume with water. The neptunium in this solution was present as neptunium (Vl). Portions of this solution were transferred to a couiometric electrolysis cell [13], and the neptunium was reduced to the (V) oxidation state by electrolysis. These neptunium solutions were standardized using a controlled potential coulometric titration[l]. The nitric acid concentration in each of these solutions was approximately 0.5 M. Disodium malonate stock solutions were prepared from the monohydrate salt (Matheson, Coleman, and Bell, USA). All other chemicals were reagent grade and prepared in the usual manner. Purified nitrogen was used to deaerate the test solutions. DISCUSSION

AND RESULTS

Effect of pH The effects of changes in acidity on the half wave potential and on the limiting current of the neptunium(VI)-malonate complex were determined in solutions 7.83 × 10-4 M in neptunium(VI), 1.0 M in disodium malonate, and 0.5 M in sodium perchlorate. The influence of the pH on the half wave potential is apparent if the half wave potential values are plotted versus pH (Fig. 1). The plot can be divided into two straight line portions, corresponding to the pH regions 0.0-3.3 and 3.37.5. In the pH range 0.0-3.3, the half wave potential is a function of the pH, and the slope can be represented by the equation: E l l 2 ----- 0"872--

0-060 pH.

This indicates that one hydrogen ion is involved in the reduction of each neptunium(VI) ion in this pH range. In the pH range 3.3 to 7.5, the slope of the curve is zero which indicates that above pH 3.3, the half wave potential is independent of the pH, and that no hydrogen ions are involved in the reduction of neptunium(VI). Above pH 7.5, the 13. W. D. Shuits, Talanta 10, 833 (1963).

Voltammetry of neptunium(VI) malonate

0.9

I

I

I

3025 I

\

7.83 x 10"IM Neptunium COT) 0.SM NaCIO 4 1.0 M Nat Ca Ht 04

\\o

-~ aa

\ oi tO

0.7 o ___~_o

o

o o-.-~--.~ooo

\ x

0.6

I 0

I 2.0

\

pH

o o o

o I 4.0

I 6.0

8.0

Fig. 1. Variation of half wave potential with pH. 7.83 × 10-4 M neptunium(VI) 0.5 M NaCIO4 1.0 M Na~C3H204.

neptunium begins to precipitate. When the pH is less than zero (acid concentration greater than 1.0 M), the half wave potential is constant with the change of acid concentration, indicating that the half wave potential is independent of the acid concentration and that no hydrogen ions are involved in the reduction of neptunium(VI) (Table 1). The half Table 1. Variation of the half wave potential and limiting current with acid concentration (7.83 x 10-4 M neptunium(Vl);0.5 M NaCIO4; 1.0 M Na~C~H204) AcidConc. moles/l.

E,~vs. SCE, (V)

il, tatrnp

1"0 l "5 2"0 2"5 3"0

0"882 0"879 0"883 0"881 0-880

22"5 22"5 22"3 22"7 22"4

wave potential at acid concentrations from 1 to 3 M averages 0.881 V vs. saturated calomel electrode (SCE). This is in good agreement with the potential reported for the reduction of the simple neptuniu(VI) ion[4, 14, 15], indicating that in 14. L. Meites, Handbook of Analytical Chemistry, (Edited by L. Meites), pp. 5-10. McGraw-Hill, New York (1963). 15. R.W. Stromatt, R. M. Peekema, and F. A. Scott, USAEC Rep. No. HW-58212(1958).

3026

C . E . PLOCK

this concentration range no complexation has taken place. This is expected since the high concentration of acid would tend to suppress the ionization of malonic acid. The effect of pH on the limiting current is shown in Fig. 2. The limiting current is independent of the hydrogen ion in the pH range 0.0-2.3 indicating a complex

0 0 o

0 0 °~x

I 7.83 x 10"4M Neptunium('tn) 0.5M NoClO4 I.OM No2C3H=O4

~

¢k

o

I-Z UJ

re -,j

/

O Z

l-

i

o

0 0

I 4.0

0

pH

/

O

O

O

8.0

Fig. 2. Variation of limiting current with pH. 7.83 x 10-4 M neptunium(VI) 0.5 M NaClOa 1"0 M Na~CaH204.

species with a constant composition exists in this pH range. As the pH is increased, the limiting current decreases indicating the competition between the hydroxyl ions and the malonate ions for the neptunium(VI). In the pH range 5.1 to 6.4 the limiting current is again constant and independent of the hydrogen ion concentration. From a pH of 6.4 to 7.5, the limiting current increases as the pH increases. This increase in the limiting current may be the result of a breakdown of the neptunium(VI) malonate complex.

Effect of malonate concentration The effects of changes in the disodium malonate concentration on the half wave potential of the neptunium(Vl)-malonate complex were determined in solutions 7.83 × I0 -a M in neptunium(VI), 0.5 M in sodium perchlorate, and at pH 1.40 and 4.00. At pH 1.40 the half wave potential is independent of the malonate concentration over the range of 1.0 × l0 -4 to 3.0 × l0 -2 M (Fig. 3). In the malonate concentration from 3.0 × 10-2 to 1.0 M, the half wave potential is a function of the malonate concentration. In the malonate concentration range 1.0 × 10-4 to 3-0 x 10-2 M, the half wave potential has an average value of 0.884 V vs. SCE which compares very favorably

V o l t a m m e t r y of n e p t u n i u m ( V l ) malonate 0.90

o

o O_o

--~- ~ ° - 0 - °

°',\o

""

o "o

3027

_

o °~o

\

°\o

"0 0-,

ui d t6

D []

0.75

lad

\

7.83 x 10-4M Neptunium(~/.) % 0.5 M NoClO4 0 pH 1.40 O pH 4.00

--[] \[]

0.65

-

\

-

-'ID~-D~[]--D~D xx

0.63

-4.0

-t -O

I

-2.0

O.n

LOG C em.oN~rE

Fig. 3. Variation of half wave potential with disodium malonate concentration. 7.83 x 10 -4 M n e p t u n i u m ( V I ) 0.5 M NaCIO4.

with the potential reported for the reduction of non-complexed neptunium(VI) [4, 14, 15]. This would indicate that no complexation has taken place or that the dissociation constant of the neptunium(VI)-malonate is equal to the dissociation constant of the neptunium(V)-malonate. In the malonate concentration range 3.0 × 10-2 to 1.0 M, the slope of the plot is --0.060 V which indicates that neptunium(Vl) has one more malonate ligand bound to it than does neptunium(V). Reversibility of the reduction of neptunium(VI) was demonstrated by the value of the slopes for the plots of log i/(il - i) vs. ERoc. The average value of these slopes in the malonate concentration range 1.0 × 10-4 to 3.0 x 10-2 M was 0.0585 V. This compares very well with the theoretical value of 0.0591 V for a one electron, reversible reduction. In the disodium malonate concentration range 3.0 × 10-2 to 1.0 M, the average value of the slope of the plots was 0.0575 V which again agrees well with the theoretical value for a one electron, reversible reduction. To confirm the reversibility of the electrode reaction, solutions were prepared which were 7-83 × 10-4 M in neptunium(V), 0.5 M in sodium perchlorate, with various concentrations of malonate at pH 1-40. The neptunium(V) in these solutions was oxidized, and the half wave potential of the anodic waves was compared with the half wave potential of the cathodic waves (Table 2). The anodic half wave potential compares well with the cathodic half wave potential. This indicates that the electrode reduction of the neptunium(VI) in the malonate concentration ranges 1.0 × 10-4 to 3.0 × 10-2 M and 3.0 × 10-2 to 1.0 M and in the pH range 0.0-3.3 is reversible. At p H 4.00 the plot of the logarithm of the malonate concentration versus the half wave potential (Fig. 3) revealed that the half wave potential is a function of

3028

C. E. P L O C K

the malonate concentration. In the malonate concentration range 1.0 × 10-4 to 6.0 × 10-2 M, the slope of the plot is 0-086 V; whereas in the malonate concentration range 6.0 × 10-2 to 1.0 M, the slope of the plot is essentially zero. The reversibility of the electrode reaction was determined by a log i/(il- i) vs. ERGcplot. The slope of the plots in the malonate concentrations from 1"0 × 10-4 to 6.0 × 10-~ M varied from 0.090 to 0.067 V, respectively. These values are not in agreement with the theoretical value of 0-0591 V for a one electron, reversible electrode reaction. However, for the malonate concentration range 6.0 × 10-2 to 1-0 M the slope of the plots varied from 0-062 to 0.056 V. This does agree quite well with the theoretical value for a one electron, reversible electrode reaction. Table 2. Comparison ofanodic half wave potential with cathodic half wave potential at various malonate concentrations and at pH 1"40 and 4.00 (7.83 × 10-4 M neptunium; 0.5 M NaCIO4) pH 1.40

Na~CsH204 Conc. moles/I, 1'0 x 10-4 5"0 X 10-4 1"0 x 10-3 5-0 × 10-~ 1.0 × 10-~ 5"0 × 10-~ 0"1 0-5 1.0

(El;2)c

(El;2)a

vs. SCE (V) 0'886 0"889 0.888 0.889 0.897 0"885 0.860 0-812 0.783

0-902 0"889 0.900 0.888 0.901 0"881 0"865 0"816 0.785

pH 4.00

(Eln)c

(E~;2).

vs. SCE (V) 0.876 0"832 0.813 0.737 0"712 0"656 0.644 0"628 0-622

0.912 0"887 0-878 0.799 0.745 0.669 0"651 0"631 0.621

To confirm the irreversibility and reversibility of the electrode reaction, solutions were prepared which were 7.83 x 10-4 M in neptunium(V), 0.5 M in sodium perchlorate, with various concentrations of malonate at pH 4.00. The neptunium(V) was oxidized, and the anodic half wave potential was compared with the cathodic half wave potential. The anodic half wave potential and the cathodic half wave potential do not agree in the malonate concentration range of 1.0 x 10-4 to 6.0 x 10-2 M (Table 2). This indicates that the electrode reaction is irreversible. In the malonate concentration range of 6.0 × 10-z to 1.0 M, the anodic half wave potential and the cathodic half wave potential do agree quite well, thus indicating that the electrode reduction is reversible in this malonate concentration range. Since the reduction of neptunium(VI) in the malonate concentration range of 1-0 × 10-4 to 6.0 x 10-2 M at pH values greater than 3.3 is irreversible, no positive conclusions can be drawn from this data concerning the number of ligands attached to neptunium(VI) and neptunium(V). However, at these same pH values, but in the malonate concentration range of 7.5 × 10-2 to 1.0 M, the neptunium(VI) reduction is reversible, and the above data indicate that the same number of ligands are attached to the neptunium(V) as are attached to the neptunium(VI).

Voltammetry o f neptunium(Vl) malonate

3029

Reversibility at higher pH To determine if the reduction of the neptunium(VI) malonate complex was always reversible in 1.0 M malonate, the anodic half wave potential of neptunium(V) was compared with the cathodic half wave potential. The solutions were 7-83 x 10-4 M in neptunium, 0.5 M in sodium perchlorate, and 1.0 M in disodium malonate at pH 5.0, 6.0, and 7.0. In all cases the half wave potentials compared very well which indicated that the electrode reaction is reversible at these higher pH values.

Composition of neptunium( Vl ) malonate complex To determine the composition of the metal ligand ratio, a conductometric titration was used. The titrations were performed at pH 1-5 and 5.0 by using 5 ml of 19.58 x 10-a M neptunium(VI) solution diluted to 330 ml with water. The titrant was 41.5 x I0 -a M disodium malonate. Figure 4 shows the result of those titrations. At pH 1.5 the titration curve indicates the metal ligand ratio is 1:2, whereas at pH 5.0 the titration curves show that when neptunium(VI) is in excess, the metal ligand ratio of the complex formed is 1 : l, and is 1 : 2 when the malonate ion is in excess.

Electrode reactions Based on the foregoing data and discussion, the electrode reactions are postulated as follows when the disodium malonate concentration is 1"0 M:

1.30

5ml 19.58 x IO"~M Neptunium ('~.) 41.5 x 10"3M NotC~H204 325 ml Water 0 pH 1.5 [] pH 5.0

I0.0 )~. 0~

-r o

E

°~^

>: I.-

/

/

D//

O

IO "IQ,

D .25

E

/~

F--

9.5o

I-0 Q Z 0 t..3

b0 Z

8 '1.20

[

°"°"°":--D..~- ~ - o - o - o ,-ZD-S9.00(

2

4 6 ml of 41.5 x IC)'SM NozCsHzO 4

8

Fig. 4. Conductometric titrations of neptunium(VI). 5 ml 19-58 x 10-2 M neptunium(VI) 41-5 × 10 -2 M Na~CaHzO4 325 ml water. (© = pH 1.5; [] = pH 5"0.)

l0

3030

C.E. PLOCK p H < 2-3

and

q=p--1

NpO2(HA)2 + H + + e- --> NpO~HA + H2A pH > 2 . 3 ,

but

<3.3

and

q=p-1

NpO2Az-2+ H + + e- ---> NpO2A-I+ HA-1 pH >3-3,

but

<5.1

and

q=p

NpO2A2 -2 + e- --> NpOzA2 -a pH>5.1,

but

<6.4

and

q=p

NpO2(OH)A2 -3 + e- --> NpO2(OH)A2 -4. When the pH is greater than 6.4, the half wave potential is independent of the hydrogen ion concentration indicating that no hydrogen ions or hydroxyl ions are involved in the neptunium reduction, and the half wave potential is independent of the malonate concentration indicating that neptunium(VI) and neptunium(V) have the same metal-ligand ratio. The limiting current, however, increases with the increase of pH. It is difficult to see what the electrode reaction may be. Since the electrode reaction is reversible, it seems apparent that neptunium(V) hydroxide is not being formed at the electrode and that both neptunium (VI) and neptunium(V) are still complexed with the malonate; although it would appear that the neptunium(VI) malonate complex might be breaking down. When the malonate concentration is less than 6-0 × 10-2 M (pH > 3.3), the reduction of neptunium(VI) is irreversible. It is therefore impossible to use this data to determine what the reduction mechanism of the neptunium(VI) may be. One can make a relatively good assumption from the data available, however. This author feels that the electrode reaction of the neptunium(VI) must be proceeded by the following: NpO2 +2+ 2H20 ---> NpO2(OH) + + H30 + NpO~(OH) ÷ + A -2 ___>NpO2(OH)A-1, and the electrode reaction is therefore, NpO2(OH)A -1 + e- ---> NpO2(OH) + A -e. This would account for the irreversibility of the neptunium(VI) at low malonate concentrations in the high pH range. At low disodium malonate concentrations there is very little free malonate ion available to compete with the hydroxyl ions for the neptunium(VI), resulting in an irreversible electrode reaction. As the disodium malonate concentration is increased, the malonate ion begins to compete with the hydroxyl ion for the neptunium(VI). When the disodium malonate concentration is high enough, it prevents any reaction between neptunium(VI) and hydroxyl ions, and therefore at the higher disodium malonate concentrations, the electrode reaction is reversible.

Diffusion coefficient The diffusion coefficient of the neptunium(Vl) malonate complex was

V o i t a m m e t r y of n e p t u n i u m ( V I ) malonate

3031

determined at 25°C in solutions which were 7.83 × 10-4 M in neptunium(Vl), 0.5 M in sodium perchlorate, 1.0 M in disodium malonate, and at pH 1.3 and4.6 using the equation[16]: il v 1/6

D2/3 =

1"500 × 105nACN 1/2 where il = limiting current,/xamp v = kinematic viscosity n = number of Faradays A -------electrode area, cm 2 C = concentration, mmoles/1. N = number of revolutions per second. The value for the diffusion coefficient was 0.28 × 10-5 cm2/sec at pH 1.3 and 0-22 × 10-5 cm2/sec at pH 4.6. Calibration

curve

A calibration curve for neptunium was prepared by pipetting the proper aliquot of the standard neptunium stock solution into a 25-ml volumetric flask. To the volumetric flask were added 5 ml of 2-5 M sodium perchlorate, 10 ml of 2.5 M disodium malonate, and sufficient perchloric acid to adjust the pH of the final solution to the desired value. The solution was then diluted to volume with water. A portion was transferred to the electrolysis cell, and the solution was purged of oxygen by bubbling purified nitrogen through the solution for five minutes. Neptunium solutions of eight different concentrations were prepared, and two voltammograms were recorded for each concentration. The results given in Table 3 show that the limiting current is proportional to Table 3. Variation of limiting current with neptunium(V1) concentration at p H 1-50 and p H 5.50 (1.0 M disodium malonate; 0"5 M s o d i u m perchlorate) p H 5.50 N p O z +2 conc. (C) moles/l. 3.92 × 7-83 × 1"96 x 3-92x 7"83 × 1"18 × 1-57 × 1"96 ×

16. L. Meites,

10 -5 10 -~ 10 -4 10 -4 10 -4 10 -3 10 -3 10 -3

p H 1.50

il (/xA)

il/C (ttA/mM)

il (ttA)

6/C 0xA/mM)

0.95 1"89 4-56 9"45 18"8 28"4 39"3 46.3

24"3 24"1 23"3 24"2 23.9 24"2 25.1 23-6

1' 11 2'18 5-60 11-1 22"0 33"3 44-7 55"2

28.4 27'8 28'6 28"3 28"1 28"4 28"6 28"2

Polarographic Techniques, 2nd

Edn, p. 424. lnterscience, N e w Y o r k (1965).

3032

C. E. PLOCK

the concentration of neptunium at pH 1-5 and 5-5 under the conditions noted. The relative standard deviation of the limiting current quotient, il/C, at pH 1.50 was 2.25 per cent and at pH 5.50, 2.81 per cent.