Journal of Nuclear and Radiochemical Sciences, Vol. 2, No. 1-2, pp. 11-15, 2001

ArticlesVoltammetric Study on Transfer of UO22+ Across the Interface Between Aqueous andNitrobenzene Solutions in the Presence of Octyl(phenyl)-N,N-diisobutylcarbamoyl-methylphosphine Oxide (CMPO)Min Ying,a, b Yoshihiro Kitatsuji,a Takaumi Kimura,a and Zenko Yoshida*, aaAdvanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Department of Applied Chemistry, East China Geological Institute, Fuzhou 344000, Chinab

Received: June 9, 2001; In Final Form: December 5, 2001
The characteristics of the phase transfer of UO22+ across aqueous(w)/nitrobenzene(nb) solutions interface in thepresence of octyl(phenyl)-N,N-diisobutylcarbamoyl-methylphosphine oxide (CMPO) as an ionophore were investigated by cyclic voltammetry (VITIES) and polarography (PITIES) for ion transfer at the interface of two immiscible electrolyte solutions. The transfer of UO22+ from w to nb is remarkably facilitated by CMPO and the iontransfer reaction exhibits an irreversible nature. The rate-determining step of the UO 22+ transfer is assigned toadsorption / desorption process of the UO22+-CMPO complex at the w/nb interface. The ion transfer potential of
UO22+ shifts more positively when the two phase system is allowed to stand stationarily for a given time before themeasurement of the cyclic voltammogram. The potential shift with the standing time is attributable to a decreasein the interfacial concentration of “free” CMPO due to the protonation of CMPO followed by the accumulation of
the ion pair between HCMPO+ and TPhB− at the w/nb interface.

Introduction
An ion transfer across the interface of two immiscible electrolyte solutions (ITIES) such as aqueous(w)/organic(org)solutions interface has become an increasingly attractive
subject on account of its feasibility to direct utilization for the
separation technology and its informative significance in not
only equilibrium but kinetic studies of liquid/liquid interfacial
phenomena. The first electrochemical study of ITIES wasconducted on the transfer of ions in the water/phenol/water
system. 1 The aim of the work was primarily to develop
methods for the measurement of the transport number in theorganic solvents.2, 3 After these preliminary electrochemical
investigations, interests in the ITIES has spread to physiologists who pointed out an analogy between the water/oil/watercell and biological membranes.4, 5 The oil-water interface thenbecame a model in studying bioelectrical potentials andcurrents, fascinating the scientific community.6, 7
Voltammetry (VITIES) and polarography (PITIES) for
ITIES are very powerful methods for elucidating the iontransfer phenomena with a unique feature in that they candetermine simultaneously the transfer energy (as a potential )of the ion and the amount (as a current ) of the ion transferred.
The VITIES and PITIES have furnished a deeper insight into
the thermodynamics and kinetics of a solvent extraction reaction, a phase transfer catalysis, an ion-selective electrode and abiological membrane transport, etc.8−17
In studying the transfer of actinide ions from w to org there
is an inevitable difficulty due to a strong hydrophilicity ofthese ions. It is hard to measure a current-potential relationship for the transfer of actinide ions between w and org bymeans of the conventional VITIES or PITIES. Therefore,
appropriate ionophors such as multidentate phosphine oxide
derivatives18 that form stable complexes with these metal ionswere employed to facilitate the ion transfer from w to org. In
the present study, the transfer of UO22+ has been studied using
octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide*Corresponding author. E-mail: zyoshida@popsvr.tokai.jaeri.go.jp.
FAX: +81-29-282-5927.

(CMPO) as the ionophore by VITIES and PITIES withaqueous/nitrobenzene (w/nb) solvent pair. Though CMPO has
been extensively studied in the solvent extraction and has
become an important extractant for the separation of actinide
ions, 19, 20 interfacial phenomena involved in the extraction
system, especially in the kinetic aspects, has not been fully
understood.
Experimental
Chemicals. A stock solution of UO22+ was prepared by dissolving uranium metal (JAERI-U4) with nitric acid. After the
addition of 0.5 M H2SO4, the solution was heated to remove an
excess of acid, and the residue was dissolved with 0.5 M
H2SO4. A working solution of UO22+ was prepared by diluting
the stock solution in 0.1 M (NH4)2SO4 (pH = 3.0).
CMPO, the product of Wako Pure Chemical Industries,
Ltd., was used without further purification. Nitrobenzene as
the organic solvent and crystal violet - tetraphenylborate(CV + TPhB − ) as the organic supporting electrolyte were
employed.
Other chemicals were of reagent grade.
Polarography (PITIES) and Voltammetry (VITIES) for the
Ion Transfer at the Interface of Two Immiscible Electrolyte
Solutions.11 Polarograms were recorded with an aqueous electrolyte dropping electrode (AEDE) and cyclic voltammograms
were recorded with a stationary w/nb interface electrodehaving an interfacial area (A) of 0.38 cm 2 . The potential
difference at the w/nb interface was measured using a Ag/AgClelectrode in w and a tetraphenylborate ion-selective electrode(TPhBE) in nb. The potential in the present paper is referredto the standard potential denoted as TPhE, which is equivalent
to a transfer energy of zero. The potential of TPhBE referred
to TPhE is 0.350 V for w/nb interface at 25 ˚C. Equipments
such as an HA 501 potentiostat, an HB 105 function wave
generator, an HI 203 ohmic potential drop compensator(Hokuto Denko) and an X-Y recorder were employed. The pH
was measured by an HM-266 pH meter (TOA) with a GST5311C pH combination electrode.

© 2001 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2001

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Ying

J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

Results and Discussion
Characteristics of UO22+ Transfer Studied by PITIES.
The transfer of UO22+ from w to nb in the presence of CMPO
was investigated by PITIES. The results are shown in Figure
1. Polarogram 1 is a residual polarogram observed with w of
0.1 M (NH4)2SO4 (pH = 3.0) without UO22+ and nb of 5 × 10−2
M CV+TPhB− + 5 × 10−2 M CMPO. The final rise of anodic
current of the residual polarogram is due to the transfer of H+
from w to nb which is facilitated by CMPO. The transfer
potential of H+ with CMPO shifted to more negative side sothat the potential at which 10 µA of anodic current flows shifted from +0.18 V (without CMPO) to +0.032 V (with 5 × 10−2
M CMPO ). In polarogram 2 recorded using w containing 1 ×
10−3 M UO22+, an anodic wave due to the transfer of UO 22+
from w to nb is observed at half-wave potential ∆V1/2 of –0.057
V, though the wave is not a typical S-shape polarogram predicted from Nernst equation.
Polarograms were recorded under various conditions of theconcentration of UO 2 2+ in w, C U(VI), w , the concentration of
CMPO in nb, C CMPO, nb , and the flow rate of w to prepare
AEDE, f. Figure 2 shows the results of logarithmic analysis,i.e., ∆V vs log[i / (i l –i )] plot, where il and i denote a limiting
current and an instantaneous current, respectively, of thepolarogram for the transfer of 1 × 10−3 M UO22+ from w to nb.
The plots show clear linearity. The slopes 56 – 81 mV, which
decrease with an increase of CCMPO, nb , are larger than that
expected for the reversible transfer of a divalent cation, 29
mV. The UO22+ transfer facilitated by CMPO is concluded to
be not reversible.
The ∆V1/2 and il of the polarographic waves are summarizedin Table 1. The ∆V1/2 depends strongly on CCMPO, nb, when theratio of CCMPO, nb to CU(VI), w enough high, and the ∆V1/2 shifts tomore negative side with increasing CCMPO, nb. The dependenceof ∆V1/2 on CCMPO, nb was found to be −98 mV/dec. The il of thepolarogram with w of 1.0 × 10 −3 M UO 2 2+ is influenced by
CCMPO, nb in the range of 5 × 10−3 to 1 × 10−2 M and approaches
a constant, 42 ± 1.5 µA, when CCMPO, org is higher than 1 × 10−2
M. This result indicates that the amount of transferring UO22+
is virtually independent of the diffusion of CMPO when the

Figure 1. Polarograms recorded by PITIES for the transfer of UO22+
from w to nb facilitated by CMPO.
w; 0.1 M (NH4)2SO4, pH = 3.0 + (1) none ( residual polarogram ), (2)1 × 10−3 M UO22+, nb; 5 × 10−2 M CV+ TPhB− + 5 × 10−2 M CMPO, f =1.0 ml min−1.

ratio of CCMPO, nb to CU(VI), w is larger than ca. 10. A linear relationship between i l and CU(VI), w is clearly observed at CU(VI), w in
the range of 2 × 10−4 to 2 × 10−3 M when CCMPO, nb = 5 × 10−2 M.
It is also indicated in Table 1 that i l was almost independent off in the range of 0.25 to 1.5 ml/min. Both of the relationshipbetween i l and CU(VI), w and that between i l and f are contradictory to each other if only a diffusion-controlled process istaken into account. Alternative reaction steps determining thelimiting current other than the diffusion process need to beelucidated for interpreting these phenomena. A similar
phenomenon was observed in the polarograms for the transfer
of UO22+ facilitated by multi-dentate phosphine oxide derivatives,18 which was explained based on an assumption that the
UO 2 2+ transfer reaction involves the adsorption/desorption
processes of the UO22+-phosphine oxide complex as well as thephosphine oxide itself at the w/nb interface. In that work the
TABLE 1: Characteristics of Polarograms for the Transfer
of UO22+ from w to nb Facilitated by CMPO
flow rate, f∆V1/2(ml/min) (V vs TPhE)

CU(VI), w(M)

CCMPO, nb
(M)

il
(µA)

1.0 × 10−3
1.0 × 10−3
1.0 × 10−3
1.0 × 10−3
1.0 × 10−3

5.0 × 10−3
1.0 × 10−2
2.0 × 10−2
5.0 × 10−2
1.0 × 10−1

0.5
0.5
0.5
0.5
0.5

0.047
0.012
-0.025-0.057-0.080

31.5
41.5
43.0
43.0
42.5

2.0 × 10−4
5.0 × 10−4
2.0 × 10−3

5.0 × 10−2
5.0 × 10−2
5.0 × 10−2

0.5
0.5
0.5

-0.059-0.057-0.055

8.5
20.0
88.0

1.0 × 10−3
1.0 × 10−3
1.0 × 10−3
1.0 × 10−3
1.0 × 10−3

5.0 × 10−2
5.0 × 10−2
5.0 × 10−2
5.0 × 10−2
5.0 × 10−2

0.25
0.75
1.0
1.25
1.5

-0.062-0.054-0.050-0.048-0.045

40.5
42.0
41.2
40.5
40.3

Figure 2. Logarithmic analysis of the polarographic wave for thetransfer of UO22+ from w to nb facilitated by CMPO.
w; 0.1 M (NH4)2SO4 + 1 × 10−3 M UO22+, pH = 3.0, nb; 5 × 10−2 M
CV+ TPhB− + (1) 5 × 10−3 M, (2) 1 × 10−2 M, (3) 2 × 10−2 M, (4) 5 ×
10−2 M, (5) 1 × 10−1 M CMPO, f = 1.0 ml min−1.

Voltammetric Study on Transfer of UO22+

J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

adsorption/desorption characteristics were investigated by themeasurement of the drop time, t d , of the AEDE under thepolarographic condition. The same experimental approach as
above was conducted for the UO22+ transfer reaction facilitatedby CMPO. The results of the td vs ∆V are shown in Figure 3.
The td decreases remarkably by the addition of CMPO into
nb, indicating an adsorption of CMPO at the w/nb interface (cf.
plots 1 and 2 in Figure 3). The td shows a further decrease but
to a less extent when UO22+ is added to w (plot 3). Further
decrease in td is attributed to an additional adsorption of thecomplex of UO 2 2+ -CMPO. These results indicate that theadsorbed species of UO22+-CMPO complex or CMPO molecule participate in the UO22+ transfer, and UO22+ transfer reaction from w to nb involves a desorption of the adsorbed species
from the interface. When the CU(VI), w increases, a diffusion
flux of UO 2 2 + from the bulk of w toward the interface
increases. The i l is proportional to CU(VI), w, if the formation of
UO22+-CMPO complex is sufficiently rapid.
A diffusion controlled limiting current, i l, d observed in thepolarogram obtained by PITIES can be expressed by eq 1,21
i l, d = (4.0×105) n D1/2 CU(VI), w td1/6 m2/3−(2.6×105) n D1/2 CU(VI),w td−1/2 r 2,

(1)

where n, D, m and r denote a charge of the ion transferred (n =2 for UO22+), diffusion coefficient of UO22+ in w, flow mass of
w to prepare the AEDE and the inner radius of the capillary to
prepare AEDE, respectively. The second term of eq 1 isusually neglected, because the cross section of the capillary ismuch smaller than the surface area of the AEDE. The size ofeach drop is independent of f, and therefore, t1/6m2/3 becomeslinear to f 1/2. When f = 1 ml/min and CU(VI), w = 1 × 10−3 M andassuming D = 4.26 × 10−6 cm2 sec−1,22 the theoretical diffusioncontrolled i l, d is calculated to be 117 µA which is 2 to 3 times
larger than the i l observed experimentally (cf. Table 1). This
result implies that the i l of the polarogram for the transfer of
UO22+ facilitated by CMPO is not fully controlled by a diffusion process but involves a kinetically controlled process. The
limiting current i l for the ion transfer voltammogram which is

Figure 3. Drop time, td, of the AEDE under the condition of PITIES
measurement.
w; 0.1 M (NH4)2SO4, pH = 3.0 + (1,2) none, (3) 1 × 10−3 M UO22+, nb;
5 × 10−2 M CV+ TPhB− + (1) none, (2,3) 5 × 10−2 M CMPO, f = 1.0 ml
min−1.

13

controlled both by a kinetic and diffusion process can be
expressed by eq 2 in a manner similar to an expression for aredox voltammogram observed at a rotating disk electrode.23
1/il = 1/i k + 1/i d.

(2)Here kf (E) is a rate constant of the rate-determining step, and ik
and id are the partial currents controlled by a kinetic and adiffusion process, respectively. When a transfer process isslow enough, 1/i k is much larger than 1/i d, and the i l equals
approximately to i k. The theoretical treatment of i l for the
kinetically controlled transfer of UO22+ studied presently has
not yet been completed.
Characteristics of UO22+ Transfer Studied by VITIES.
Cyclic voltammograms recorded by VITIES at a stationary
w/nb interface electrode are shown in Figure 4. Residual
voltammogram as curve 1 was recorded with w without UO22+
and nb containing 5 × 10−2 M CMPO. The final rise of theanodic current is due to the transfer of H+ from w to nb, which
is facilitated by CMPO as mentioned above, and the final
descent of the cathodic current is due to the transfer of SO42−
from w to nb or the transfer of CV+ from nb to w.
The anodic and cathodic peaks observed in voltammogram
2 correspond to the transfer of UO22+ from w to nb and from nb
to w, respectively. The characteristics of cyclic voltammograms observed under various conditions of CU(VI), w, CCMPO, nb,
and the potential scanning rate, v, are summarized in Table 2.
The peak potential, ∆Vp, for both anodic and cathodic waves at
a given v does not depend on CU(VI), w in the range examinedand depends on CCMPO, nb, when CCMPO, nb is sufficiently higher
than C U(VI), w . The ∆V p vs log C CMPO, nb plot shows a clear
linearity and the slopes of the plots for the anodic and cathodic
waves were determined to be –108 and –110 mV/dec, respectively. The ∆Vp also depends on v. The difference in ∆Vpbetween the anodic and cathodic waves of a cyclic voltammogram increases with an increase of v which is manifesting atypical characteristic of an irreversible electrode process.
Both of the peak currents, ip, of the anodic and cathodic
waves are proportional to CU(VI), w, if CCMPO, nb is sufficiently
higher than CU(VI), w. The i p of the cathodic wave increases with

Figure 4. Cyclic voltammograms recorded by VITIES for the transfer
of UO22+ between w and nb in the presence of CMPO.
w; 0.1 M (NH4)2SO4, pH = 3.0 + (1) none (residual voltammogram),(2) 2 × 10−3 M UO22+, nb; 5 × 10−2 M CV TPhB− + 5 × 10−2 M CMPO,
interfacial area A; 0.38 cm2, potential scanning rate v; 1 mV sec−1.

14

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J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

TABLE 2: Characteristics of Cyclic Voltammograms for the Transfer of UO22+ at a Stationary w/nb Interface in the
Presence of CMPO in nba)∆Vp (V vs TPhE)(M)

potential
scanning ratev (mV/sec)

anodicwave

cathodicwave

anodicwave

cathodicwave

5.0 × 10−4
1.0 × 10−3
2.0 × 10−3

2.5 × 10−2
2.5 × 10−2
2.5 × 10−211
1-0.007-0.007-0.007-0.043-0.044-0.044

5.1
10.4
21

4.5
8.5
17

2.0 × 10−3
2.0 × 10−3
2.0 × 10−3

1.0 × 10−2
5.0 × 10−2
1.0 × 10−1

1
1
1

0.034
-0.045-0.073-0.011-0.082
-0.12

19.5
21
22

12
18
18.5

2.0 × 10−3
2.0 × 10−3
2.0 × 10−3
2.0 × 10−3

2.5 × 10−2
2.5 × 10−2
2.5 × 10−2
2.5 × 10−2

2
3
4
6

0.002
0.010
no wavea)
no wavea)-0.058-0.068-0.078-0.08823
25
no wavea)
no wavea)

24
30.5
36
39

CU(VI), w

CCMPO, nb(M)

ip (µA)No clear wave was observed in the voltammogram.

an increase of v. The dependence of the ip of the anodic wave
on v could not be clarified, because no clear anodic current
peak was observed in the voltammogram recorded with v
larger than 4 mV/sec.
For the estimation of the irreversibility of the UO22+ transfer
reaction, the experimentally determined i p is compared withthe peak current predicted for a diffusion controlled reversible
reaction, i p, d ,by eq 3:
i p, d = 2.69 × 105 n3/2 A CU(VI), w D1/2 ν 1/2.

(3)

With A = 0.38 cm2, CU(VI), w = 2 × 10−3 M, and ν = 1 mV/sec,
ip, d is calculated to be 38 µA. The ip experimentally measured
is 21 µA. The UO 22+ transfer facilitated by CMPO is irreversible, and the transfer coefficient, αa, of the anodic process
is calculated to be 0.25 from the above results of ip, d and ip
according to eq 4:
i p = 2.99×105 n (αa n)1/2 A CU(VI), w D1/2 ν 1/2.

(4)

For the cathodic wave, the ∆Vp vs log v plot was found to belinear with a slope of -57.3 mV/dec. The slope corresponds to30/αc n mV where αc denotes the transfer coefficient of thecathodic process, and the αc for the electrode process of the
UO22+ transfer from nb to w is thus estimated to be 0.26.
Change of the Voltammetric Behavior of UO22+ Transfer
on Standing. The cyclic voltammogram was recorded after
the stationary w/nb interface electrode system was allowed tostand for a given time before the VITIES measurement. The
results obtained with standing time ts in the range of 5 to 120
min are illustrated in Figure 5. The anodic and cathodic waves
shifted positively with an increase of ts. The i p of both waves
decrease slightly with an increase of ts. The most remarkablefeature of this standing time effect is that the situation having
changed on standing is restored to the original state by disturbing the w/nb interfacial region. For example, the voltammogram essentially identical to voltammogram 1 in Figure 5 wasobserved when the w/nb interface was stirred gently for 1 minusing Pyrex glass stick after the measurement of voltammogram 3. This result gives an idea that an interfacial reaction
such as adsorption or accumulation of relatively slow kinetics
takes place during the standing.
A growth of an interfacial layer of the adsorbed species thatblocks the ion transfer is a possible origin of the standing timeeffect that UO22+ becomes harder to transfer from w to nb as
the system stands longer. This “blocking layer” hypothesis,
however, is unlikely from the following experimental result of

Figure 5. Effect of standing time, ts, of a stationary w/nb interface
electrode system on the cyclic voltammogram for the transfer of UO22+
in the presence of CMPO.
w; 0.1 M (NH4)2SO4 + 2 × 10−3 M UO22+, pH = 3.0, nb; 5 × 10−2 M
CV+ TPhB− + 5 × 10−2 M CMPO, standing time, ts, before the VITIES
measurement ; (1) 5 min, (2) 45 min, (3) 120 min, v; 1 mV sec−1.

the transfer of tetrapropylammonium ion, TPrA+ as a reference
ion. The standing time effect was not observed for the transfer
of TPrA+ from w to nb recorded with w of 0.1 M (NH4)2SO4 +1 × 10−3 M TPrA+ (pH = 3.0) and nb of 5 × 10−2 M CV+TPhB−+ 5 × 10−2 M CMPO. Here, the “blocking effect” is expected
to be observed irrespective of the kind of transferring ion. In
addition, the “blocking layer” hypothesis does not explain the
result that the cathodic wave due to the facilitated transfer of
UO22+ from nb to w shifted also to positive side with standing.
Alternative reason for the standing time effect must be
investigated. A potential shift of both anodic and cathodic
waves of the cyclic voltammogram is observed generally when
CCMPO, nb is varied as shown in Table 2. The standing timeeffect resulting the positive shift of ∆Vp shown in Figure 5 is
attributable to a decrease of the concentration of CMPO at theinterfacial region, CCMPO, int. Its decrease can be attributed to aVoltammetric Study on Transfer of UO22+

protonation of CMPO and successive ion-pair formation
between the protonated CMPO (HCMPO+) and TPhB− at the
w/nb interface. The ion pair HCMPO+TPhB− accumulates at
the interfacial region with increasing the standing time. If theinterface is disturbed mechanically, the interfacial layer of
HCMPO+TPhB− is decomposed restoring to the original stateof a “fresh” w/nb interface.
There is experimental evidence supporting the formation of
the interfacial layer of HCMPO+TPhB− ion pair, which is theresult of the measurement of pH of w after contacted with nb
for a given time. The w (50 ml) of 0.1 M (NH4)2SO4, pH =3.02 ± 0.01, was allowed to contact stationarily with nb (50ml) of various compositions for 24 h and then the pH of the w
was determined. With nb containing either 5 × 10 − 2 M
CV+TPhB− or 5 × 10−2 M CMPO, the pH of w after the contact
were 3.04 ± 0.01 or 3.02 ± 0.01, respectively, indicating thatan appreciable amount of H + was not consumed in thesesystems. With nb containing both 5 × 10−2 M CV+TPhB− and 5
× 10−2 M CMPO, the pH of w was 3.12 ± 0.01, which impliedthat approximately 2 × 10−4 M H+ in the aqueous phase wasconsumed by contacting with the nb phase. These results
suggest that H+ reacts only with the simultaneous presence of
CMPO and TPhB− to form the ion pair HCMPO+TPhB− at the
w/nb interface. The accumulation of the ion pair at the interfacial region is assumed to be very slow.
Based on the correlation between ∆V p and C CMPO, nb asshown in Table 2, the CCMPO, int was calculated from the resultsof ∆V p of the voltammograms in Figure 5. The results of
CCMPO, int calculated are plotted against ts in Figure 6, plot (1).
The positive shift of ∆V p with standing corresponds to a
gradual decrease of CCMPO, int with an increase of ts. The reciprocal of CCMPO, int was found to be linear with ts as shown byplot (2), which suggests that a bimolecular process between
HCMPO+ and TPhB− to form ion-pair takes place at the w/nb
interface.
The facilitated transfer of UO22+ by CMPO studied in thepresent work can be utilized as a basic reaction for the electrolytic ion-transfer separation of uranium.18 The formation of
the interfacial layer of HCMPO+TPhB− ion pair inhibits the

Figure 6. Decrease of the interfacial concentration of CMPO,CCMPO, int, with standing time, t s.
(1) CCMPO, int vs ts plot, (2) 1/CCMPO, int vs ts plot, VITIES measurement;
identical condition as shown in figure caption of Figure 5.

J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

15

2+

facilitated transfer of UO 2 from the aqueous phase to theorganic phase. The interfacial layer, however, is not mechanically stable and can be removed from the interface simply bystirring the interface.
Conclusion
The transfer of UO22+ between aqueous and nitrobenzene
phases in the presence of CMPO was studied by VITIES andPITIES. It is concluded that the UO2 2+ transfer from theaqueous to the organic phases is facilitated by CMPO
dissolved in the organic phase. The mechanism of UO 2 2+transfer with CMPO is found to be a kinetically controlled
process including the adsorption and/or desorption of UO22+ CMPO complex. The irreversibility of UO22+ transfer is determined to be 0.25 to 0.26 of the transfer coefficients for bothanodic and cathodic processes, respectively. The origin of astanding time effect on the transfer potential at a stationary
w/nb interface is attributed to the gradual decrease in the interfacial concentration of free CMPO caused by the ion-pairformation of protonated CMPO with TPhB− at the interface.
The reaction mechanism elucidated in the present work may
provide useful information of relevance to the solvent extraction reaction of actinide ions using CMPO as an extractant.
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