364
Anal. Chem. 1983, 55, 364-367
(13) Routh, J. I. I n “Fundamentals of Clinical Chemistry”; Tletz, N. W., Ed.; W. B. Saunders, Philadelphia, PA, 1976; Chapter 16. (14) Perry, T. L., Hansen, S. Clln. Chlm. Acta 1969, 25, 53-58. (15) Matthews, D. M.; Muir, G. G.; Baron, D. M. J . Clln. Pathol. 1964, 17, 150- 153. (16) Goodwin, J. F. Clln. Chem. (Wlnston-Salem, N . C . ) 1966, 14, 1080- 1090. (17) Constantsas, N. S.: Danelatau-Athanassladon, C. c//n, chlm. Acta 1964, 9 , 1-12. (18) Masclnl, M.; Gullbault, G. G. Anal. Chem. 1977, 49, 795-798. (19) Fratlcelii, Y. M.; Meyerhoff, M. E. Anal. Chem. 1961, 53, 992-997. (20) Meyerhoff, M. E.; Robbins, R. H. Anal. Chem. 1980, 52, 2383-2387.
(21) ”Biochemica Information 11”; Boehringer: Mannheim, 1975; pp 34-35. (22) Slgma Chemlcal Co. Catalogue, 1982, p 138.
RECEIVED for review August 26, 1982. Accepted November 12,1982. Acknowledgment is made to the National Institutes Health (Grant No- l-RO1-GM 28882-01) for support of this research.
Of
Membrane Electrode for the Determination of Actinyl(V1) Cations Peggy A. Bertrand, Gregory R. Choppin,” and Lln Feng Rao Department of Chemlstv, Florida State University, Tallahassee, Florida 32306
Jean-Claude G. Bunzll Institut de Chimie Minerale et Analytique, Universite de Lausanne, 1 105 Lgusanne, Switzerland
A coated wlre speclflc electrode has been developed for actlnyl(V1) cations. The electrode responds In Nernstlan fashlon to actlnyl(V1) concentratlons of 105-10-2 M between pH 2 and 5. The Interference by M(I), M(II), M(III), and MO,’ catlons is small even when [M(III)]:[AnO;+] 10. However, Th( I V) does Interfere. The electrode can be used NpO;’, and In complexatlon and redox studles of UO;,’ Pu02,+ wlthln Its useful range of pH and An02’’ concentration.
-
Electrode membranes sensitive to UOZz+have been made by incorporation of uranyl compounds into organic matrices (1, 2). Recently Senkyr e t al. (3) reported U(V1)-sensitive ”neutral-carrier”/PVC/chloronaphthalenemembranes which are not constructed with macroscopic quantities of uranyl. Such membranes function by coordination of the neutralcarrier ligand t o the metal ion in solution. Because the membranes do not require the inclusion of significant amounts of metal ion, it seemed promising t o study their use with the more radioactive actinides such as plutonium. Electrodes for use with radioactive species should have certain features to make them practical; Le., they should be small and, if possible, should not incorporate significant amounts of the species. Both these constraints are met by electrodes constructed by using the coated-wire technique described by Freiser ( 4 , 5 ) . This paper reports on the development of a coated-wire electrode (CWE) by using the membrane system of Senkyr et al. and describes the response of these electrodes to actinide cations in oxidation states 111-VI. The voltage response of an electrode t o an ion in which i t is sensitive can be described by the Nernst equation a t constant ionic strength
E , = m log c
+ Eo
where E, is the voltage measured a t concentration c and Eo is the voltage for unit concentration. The slope m should have a value of 2.303RT/nF or 59.2 and 29.6 mvldecade change
in cation concentration for cation charges, n, of +1and +2, respectively. An experimental slope which agrees with the theoretical slope will be referred to in this paper as a “Nernstian” slope while a slope larger than the Nernstian value will be referred to as “over-Nerstian”. EXPERIMENTAL S E C T I O N Metal Solutions. Metal stock solutions were prepared by dissolving the nitrates or oxides in HCl or HC104. Working solutions were made by dilution of the stocks with appropriate amounts of aqueous NaCl or acetate buffer t o the desired metal concentration and 0.1 M ionic strength. The pH was adjusted with reagent grade NaOH and HC1 with a Beckman Model 1019 research pH meter equipped with a Corning combination pH electrode. The 2a7Npand 242Puoxides were obtained from Oak Ridge National Laboratory. Membrane Coating Solution. N,N’-Diheptyl-N,N’,6,6tetramethyl-4,8-dioxaundecanediamide(DTDD) was synthesized at the Institut de Chimie Minerale et Analytique and used without further purification. Poly(viny1 chloride) (PVC) from Aldrich was dissolved in tetrahydrofuran (THF) to prepare a stock solution (0.05 g/mL). The membrane coating solution was prepared by dissolving 0.026-0.039 g of the ligand in 6.0 mL of PVC/THF stock plus 0.46 mL of 1-chloronaphthalene. The 1-chloronaphthalene was added as a plasticizer and was necessary to produce a liquid membrane. The resulting viscous, cloudy solution separated into a clear yellow supernatant and a small amount of white gelatinous precipitate upon standing. Construction of Electrodes. One end of a coaxial cable (e.g., Belden RG-58/U) was stripped of 5 cm of its outer insulation and wire screening. At the end of the 5 cm, inner insulation was removed to expose 2 cm of bare copper wire. This was sanded until the end was flat and the surface no longer shiny. It was cleaned with soap and rinsed with distilled water and acetone. The membrane was applied by dipping the exposed end of the wire into the membrane coating solution to a depth of about 1.5 cm, taking care to avoid disturbing the gelatinous precipitate. This coated the wire with an organic film which was allowed to dry approximately 1 min prior to repeating the dipping procedure. In this manner, three to four membrane coats were applied to the wire. Following the final application of membrane/coating solution, the wire was air-dried 45-60 min and wrapped in parafilm so that only the small organic “bead”near the tip of the wire was exposed.
0003-2700/83/0355-0364$01.50/0 Q 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 2, FEBRUARY 1983
wire shielding inner insulation '
-copper
core
parafilm wrapping
1
365
COAXIAL
CABLE
> E
Y
membrane bead
2ool IjO/
Figure 1. Schematic representation of coated-wire electrode (CWE),, external reference electrode not shown. 3001
0 A
4 4
x
4 3
u
3
Np
0
v .
u Np Pu
Pu
3
2
U
c
50
.5
-4
-3
-2
log [ A n ] ';O 1 6M ~
Flgure 3. Response of the DTDD-CWE to concentration of UO,*+, 0 , O ) , and 4 (0, A, A). and PuO," at pH 2 (X), 3 (0,
NpO,';
M
O M
1001
;
1
1
4
3
1
5
PH Figure 2. Response of the DTDD-CWE to pH for different concentrations of UO,*+ in 0.1 N NaCI.
The CWE was soaked in 0.001 M uranyl chloride (pH 3) for at least 1 h prior to use. When not in use, electrodes were stored in this soak solution. Correct application of the membrane to the copper wire was critical to proper CWE funct ion. The procedure described resulted in membranes of proper thickness provided the membrane coating solution had not become overly viscous due to partial evaporation of the THF. Clouding of the organic coating occurred if laboratory humidity was high and could be avoided by drying the freshly coated wire over a desiccant. Allowing the membranes to dry too much prior to soaking in the 0.001 M uranyl chloride solution adversely affected CWE response time. It was found that approximatelyhalf of the electrodes prepared according to this procedure functioned satisfactorily. A schematic representation of a CWE appears in Figure 1. Experimental Equipment. In early experiments a Keithley Model 610A electrometer with lOI4-Qinternal impedance at low voltage was used for the potential measurements with a Fluke Model 8000A multimeter connected to the output port of the Keithley electrometer to provide digital readout. Later, a highimpedance digital voltmeter with measured impedance of 5 X 1014 Q and five-digitdisplay was wed (6). A Corning H4350-3 standard calomel electrode was used as the external reference electrode except in experiments involving neptunium, where a Corning H4350-15 miniature calomel electrode was used. All experiments were conducted at room temperature (22 f 2 "C). RESULTS AND DISCUSSION Response of Coated-WireElectrodes to Uranyl. Figure 2 shows voltage response curves for a typical CWE in solutions of ionic strength 0.1 M (NaCl) with p H varying from ca. 2 to 4.2; curves are shown for uranyl ion concentrations from 0 to
M. The CWE's responded to changes in uranyl concentration rapidly, giving constant voltage readings typically within 10 s. The values of the slope, m, varied somewhat with each CWE but usually was within the range of 50 f 5 mV between p H 3 and 4, decreasing to 35 f 5 mV for p H 2 and ([UO;'] 2 10-3.5M) (Figures 2 and 3). These "over-Nernstian" slopes were similar to those reported by Senkyr et al. and were quite reproducible over the lifetime of the C W E (approximately 1 week, barring physical damage). Voltage readings on the same solution taken within 15 min of each other were reproducible to within f 3 mV. Over longer periods of time, electrode (or electrometer) drift resulted in less precision; however, because of the reproducibility of the slope, calibration with one solution of known concentration could be used to correct for this long-term drift. Response to Neptunyl and Plutonyl. After soaking in uranyl solutions, CWE's were tested in the absence of UOzz+ for their response to NpOzz+and PuOzz+a t concentrations from M. The curves are shown in Figure 3 for pH to 3 and 4. At pH 2, the C W E showed little sensitivity below M and the curves for NpO$+ and PuO$+ resembled that shown for UOzz+. Obviously, the CWE's respond equally well and nondiscriminately to all three cations under these conditions of p H and cation concentrations. The slopes for NpOzz+and PuOzz+fell within the same range of 50 f 5 as for UOzz+. Interference by Other Ions. In Figure 4 the response of the CWE to Sm(III), Th(IV), and NpOz+cations a t p H 4 and 10-5-10-3 M concentrations in the absence of UOzz+is shown. From these curves, the selectivity factors, log Ki,, by the "separate solution method" (7) were calculated by using the equation Ej - Ei 1 log Kij = - log ai - - log aj (2) m Zj where i = UO$+ (Zi = 1) and j = intefering cation. The calculated selectivity factors together with the values reported in the literature for mono- and divalent cations are listed in Table I. The values for Sm(II1) and NpOz+represent upper limits of log Kij since the values of Esm and ENpin the absence of uranyl were very close to the value of EO,l"aCi a t pH 4 (ca. 130-150 mV).
+
I P
/"
YX6
P /"
x'"
/"
"1
I
I
-5
-4
80
-3
[uo;+]
log
Flgure 6. Response of the DTDD-CWE to varying concentrations of U O r in 0.1 N NaCl of pH 4 in the absence and in the presence of 10- M NpO,': UO+: only (X), UO+ : -t NpO,' (0). -'-'-'-.-.o-
.-.-.
0'-'b"o-.-
log [Th4+]
-3.0
log [Th
'1
Figure 4. Response of the DTDD-CWE to concentration for Sm(II1) (0)and Np(V) (0)In 0.1 N NaCl solution of pH 3 and Th(1V) (0)in 0.1 N NaOAc of pH 4. UO+: Is absent from these solutions. 2407
'
x'
I60
- 3.5
-4.0
log 160
-
-3.0
[uo;]'
Figure 7. Response of DTDD-CWE to varying concentrations of U O t f and Th(1V) in 0.1 M NaCl of pH 3: UO +: only (X), UOZ2+ Th(1V) (0)with a total U Th concentration of 1.0 mM.
I..
+
+
for Np02+. No value could be calculated for Sm(II1) as (Ei+, - Ei) was negative. Anion Complexation. In order to ascertain that the CWE Flgure 5. Response of the DTDD-CWE to varying concentrations of responds only to "free" uranyl, we measured the EMF in the U02*+ In 0.1 N NaCi solution of pH 3 in the absence and in the presence of M Sm(II1): UO,*+ only (X); UO+: + Sm(II1) (0). presence and absence of complexing acetate anion. The stability constants for formation of U02Ac+and U02Ac2(8) Table I. Selectivity Factors for DTDD Membrane System were used to calculate the concentration of uncomplexed UO;+ at different total concentrations of uranyl. We obtained metal AE," mV ref the same potential readings for the calculated "free" uranyl -1.8 -76 3 H+ M as for concentraconcentrations between lo4 M and
