Anal. Chem. 1980, 52, 2105-2108
2105
Influence of Ion Exchanger and Diluent Structure on Uranyl Ion Selective Electrode Response Ilia Goldberg Nuclear Research Centre-Negev, P. 0.Box 900 1, Beer-Sheva, Israel
Dan Meyerstein Chemistry Department, Nuclear Research Centre-Negev and Ben-Gurion University of the Negev, Beer-Sheva, Israel
A series of uranyl complexes were tested as the active substance in the membranes of uranyl Ion selective electrodes, ISEs. Complexes with phosphttes as ligands, ion exchangers, give better electrodes than those with phosphates and phosphonates. This observation seems to be related to the fact that phosphites are weaker ligands and therefore more labile ligands for uranyl. A simple technique for the preparation of PVC-based membranes for ISEs is described.
Recent analytical requirements have caused an increased effort toward the development of ion selective electrodes, ISEs (1-12). However, little progress has been reported on the development of new electrodes for the heavy and multivalent elements. Only two studies dealt with uranyl ion selective electrodes. At first Dietrich (13) reported that a PVC membrane becomes sensitive to uranium when it contains the uranyl complex with DEHPA (diethylhexylphosphoricacid). In a second, more comprehensive study, Manning et al. (14) examined membranes containing 2 1 organophosphorus compounds and established that six of them exhibited favorable response characteristics and approached a near Nernstian response (24-26 mvldecade). Manning's membranes were prepared from uranyl complexes with bis(2-ethylhexy1)phosphoric acid or mono-n-butylphosphoric acid or bis(2ethyl-4-methylpenty1)phosphoricacid as ligands, ion exchangers. The membranes contained also diluents as diamyl amyl phosphonate, bis(2-ethylhexyl) ethylphosphonate, tris(2-ethylbutyl) phosphate, or tributyl phosphate. Both studies were based on the experience gained during the development of calcium selective PVC membranes ( 1 5 , 16) and on the knowledge of uranium extraction by organophosphorus solvents. But, whereas the preparation of the membranes, by the Moody-Thomas technique (9),is the fruit of systematic studies, the ion exchangers chosen as UOZz+ligands were selected a t random. In the present work an attempt is made to correlate the response of the membrane with the structural characteristics of its chemically active constituents and to establish criteria for the selection of the ion exchanger and the diluent in order to improve the membrane response toward uranium and give it the required mechanical strength. for this purpose a series of PVC membranes containing uranyl complexes was used. Phosphites were applied here for the first time in connection with ISE. EXPERIMENTAL SECTION Reagents. Poly(viny1 chloride) powder, Rhovinyl F (RhonePoulenc), was used. All other chemicals were reagent grade. Phosphates, phosphonates, and phosphites were used without further treatment. the organophosphorus compounds were examined spectroscopically. The phosphites were tested according to Thornburn-Burns (17). Membrane Preparation. The uranylorganophosphorus complexes (ion exchangers) were prepared according t o Manning 0003-2700/80/0352-2 105$0 1 .OO/O
(14). Preparation of the uranyl complexes PVC membrane solution was accomplished by mixing 45 mg of the uranyl complex and 450 mg of the organophosphorus diluent with 6 mL of PVC solution (1.75 g of PVC in 60 mL of tetrahydrofuran, THF). A hole with a diameter of 19 mm was cut out of the bottom of a glass vessel (of an inside diameter of 33 mm). The vessel is glued to a glass plate by means of a PVC adhesive (7 g of PVC in 60 mL of THF). The gluing is performed as follows: PVC adhesive is applied to the outside of the bottom of the vessel, which is placed on the plate. Additional adhesive is pipetted on the outside of the vessel. The adhesive descends by gravitation and dries, forming a ring around the bottom. The uranyl ion exchanger PVC solution is then poured into the vessel. The vessel is covered with a filter paper on which a weight is placed for about 48 h, during which the THF evaporates. A solid membrane forms, which adheres to the bottom of the vessel (Figure 1). The glue ring around the vessel is cut by means of a sharp knife, so that the vessel containing the membrane can be detached from the glass plate. The vessel with the membrane can then serve as a base for an uranyl-selective electrode without any further treatment. Aqueous Solutions. A 34.1-g sample of uranyl chloride is dissolved in about 1L of double distilled water. The uranium concentration of the solution which is brought to pH 3.0 by the addition of NaOH and/or HC1, 1 M, is determined both gravimetrically and potentiometrically (18). The solution is then diluted to 0.100 M. Solutions of U02C1, in the concentration range to 1 X lo-' M were obtained by appropriate dilution of 1 X solutions with water and brought to pH 3.0. In addition 1 X of different ions were prepared, and each of them was mixed in ratio 1:l with a series of UOzClzsolutions at the concentrations 2.0 X to 2.0 x lo-' M. The selectivity coefficients were determined by using these solutions. Electrode System. Cells of the type Hg,Hg2Cl2,KC1 M, pH (satd.)lsample solutionlmembranelUOzClz 1 X 3.OIAgC1,Ag have been used (Figure 2). The external reference electrode was a Metrohm EA-404 SCE electrode. The EMF measurements were performed at 22 & 1 "C using a Keithley electrometer Model 610c. The immersion depth of the electrodes was -1 cm, and the sample solution was stirred slowly.
R E S U L T S A N D DISCUSSION Membrane Properties. The different membranes obtained by using this technique are uniform and transparent, they have yellow shades and absorb light in the 4000-A region. Their thickness varies from 0.3 to 1.2 mm although they are made of identical quantities of material (the thickness seems to be determined mainly by the properties of the diluent). The membranes are not soluble in examined solutions and do not swell in them. They may be destroyed mechanically by the pressure exerted by the inner electrolyte during the measurements. They may also be destroyed chemically due to leaching of the phosphate esters from the membranes or the formation of a rigid PVC crust that is impermeable to the solutes. Choice of Diluent. As diluents for preparation of membranes we checked the properties of the following compounds: (a) bis(2-ethylhexyl) phosphate, (b) tributyl phosphate, (c) 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
Table I. The E M F Response of Membranes Containing Uranyl Complexes with Phosphates and Phosphonates sum of electro-
membrane
nega-
solvent
no. 1 2 3 4 5 6
7
8 9
10 11 12
Assembly for membrane casting: (1) glass plate, (2) membrane, (3) vessel, (4) filter paper, (5) weight. Figure 1.
13 14
15 16 17
18
:-: ; ; ; ; ; : : ; ;E;
a
Zj,
- -. - - - -- - - - - - - - - - - - - - - - - - - - - .. - - - - - - - - - - -.
;:;1
- - - -- - - - - -
- - - -
Cell arrangement for EMF determination: (1) AgIAgCI, Metrohm electrode: (2) internal solution (UO,CI,: M; pH 3): (3) membrane; (4) Keithley electrometer, Model 610c; (5) SCE, Metrohm EA-404 refence electrode: (6) sample solution (UO,CI,, to lo-' M, PH 3). Figure 2.
dibutyl butylphosphonate, (d) bis(dichloropropy1) (dichloropropyl)phosphonate, (e) bis(chloroethy1) (chloroethy1)phosphonate, and (f) bis(2-ethylhexyl) (2-ethylhexy1)phosphonate. The diluents were checked together with all the uranyl complexes listed in Table I. The tests showed that only bis(2-ethylhexyl) (2-ethylhexy1)phosphonate gives membranes with good chemical and mechanical properties. This observation seems to be due to the fact that the ion exchanger remains saturated with uranium if no interactions occur between the uranyl and the diluent. If the diluent itself is a strong uranyl solvent, the uranium will pass partially or completely into the diluent. Consequently, the behavior of the membrane is uncontrolled as these act as solid extractants (19-20). This is the reason compounds a, b, and c are not suitable diluents. In addition (c) is a strong plasticizer and reduces the mechanical strength of the membrane. The diluents d and e have high viscosity and are thus incompatible with the other constituents of the membrane. The diluent, plasticizer, should be a weak ligand to uranyl, therefore its phosphorylic oxygen has to be masked in order to minimize its polar interaction with UO?+. This masking can be achieved by long and branched substituents on the phosphonates. Choice of Active Complex. After bis(2-ethylhexyl) (2ethylhexy1)phosphonate was chosen as the diluent, a series of membranes was prepared, in which complexes of uranyl nitrate with phosphates and phosphonates served as the active material. The potentials of cells containing these membranes and solutions of uranyl chloride in the concentration range of 1.0 X lo-' to 1.0 X M were measured. All the phosphate
Phosphates monobutyl dibutyl tributyl di( 2-ethylhexyl) tris( dimethylphenyl) diphenylmethyl trimethyl triphenyl tributoxyethyl triethyl triethylthio dieth ylchlorothio trichloroethyl triisooctyl Phosphonates dibutyl butyl bis( dichloropropyl) dichloropropyl bis(chloroethy1) chloroethyl bis( 2-e thylhexyl) 2-ethylhexyl
tivi ties A E M F , (ZX)
mV
7.6
25
8.3 9.0
22 12
8.3
20
9.0 9.4 9.0 9.6 9.0 9.0 9.0 9.0
17 14 11 12 10 12 8 17 7 7
8.0 8.0
a
8.0
a
8.0
10
10
N o t responsive.
membranes except monobutyl phosphate, as well as all the phosphonate ones are far from having a Nernstian behavior. The dependence of the measured values of the potentials on the activity does not obey the relationship d(EMF)/a(log ai). This does not, however, mean that the membranes behave similarly. Indeed, they differ greatly in their electrochemical behavior, response time, mechanical strength, color, and thickness. We decided therefore to examine these membranes and compare them with each other in the following way: for every UOzClz concentration and for every membrane the equilibrium potential was measured, when the potential became stable within f 2 mV. The potential differences (AJ3MF) for consecutive solutions differing in uranium concentration by a factor of 10 were calculated. (Unexpected discrepancies were neglected according to the McCutchen (21) criterion.) The average potential difference, AEMF, was calculated. This parameter characterizes the properties of nonideal membranes. The values of AEMF are given in Table I with the sums of the electronegativities of the ligand substituents, Ex. The sums of the electronegativities were calculated according to Rozen (22) and Bell (23). The data in Table I indicate that the following factors affect the response of the electrodes: (1) Phosphate and phosphonate membranes with the exception of monobutyl phosphate are unsuitable for uranium ISE. (2) Membranes of organophosphorus acids (OPA) are more sensitive to uranium than neutral organophosphorus compounds (NOPC). (3) The chain length of the substituents and steric hindrances in OPA do not affect significantly the membrane response. (4) For NOPC, solvents of greater extraction strength give membranes of lesser response. (5) The thiophosphate membranes respond less than the corresponding phosphate ones. (6) The substitution of hydrogen by chlorine reduces the response still further. (7) The substitution of OC2H5by chlorine enhances the response. (8) The phosphonate membranes respond less than the phosphate ones. The solvent extraction strength increases with the electron density on the phosphorylic oxygen, and the latter rises as the
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
1
'
3
~
Flgwe 3. Phosphite membranes response: (a)bis(chloroethyi) phosphite membrane; (b) tris(ch1oroethyi)phosphite membrane; (c) bis(chloropropyl) phosphite membrane.
\
l
A
o
-
2107
l
1 %
Table 11. Phosphites as Ion Sensors for Uranium and Their Response membrane no.
phosphite
19 20 21 22 23
di(chloroethyl) tri( chloroethyl) di( 2-ethylhexyl) di( chloropropyl) triethyl
aE/a(log ai), mV/decade 29 29 26 30 20
electronegativities of the substituents falls. However none of the substituted complexes gives a membrane with the required properties. The same is true to a far greater degree when phosphonates are chosen as solvents. It may therefore be concluded that the use of phosphoric acid derivatives affects the membranes and their response adversely. Moreover, in view of the data obtained it was possible to predict that the membrane response would continue to fall, when the number of substitutions of OR by R is raised. This was confirmed when a membrane was prepared with trioctylphosphine oxide (TOPO) which does not contain any OR groups. This membrane did not respond at all to UO?'. It should be noted here, that the extraction strength of TOPO is greater by 5 orders of magnitude than that of a homologous phosphate. In brief the membrane response decreases in phosphoric acid derivatives as follows: (RO)3P0 > ( R 0 ) 2 R P 0> R3P0. At this stage we prepared membranes with uranyl complexes with phosphites (RO)3Pas ligands, ion exchangers, with the hope that due to the fact that these complexes are weaker and more labile, they will form better membranes. Very little is known about phosphites as uranium extractants. As for ion-specific electrodes, we have found no previous reference to them in the literature. The lack of interest in phosphites has several reasons. They are not sufficiently stable when uncomplexed and are therefore difficult to work with. Some of them have a strong and offensive odor, while others are toxic. The first experiments carried out with phosphites revealed that uranium reacts quickly with phosphites, as it is dissolved in them. The resulting complex can be incorporated into PVC containing the diluent, giving yellow, half-transparent membranes of a satisfactory mechanical strength and a common electrical resistivity, lo' a, and moreover respond immediately with near Nernstian slopes. Examination of Phosphite Membranes. As the phosphite membranes gave the best response to uranium, we carried out a series of examinations of these membranes according to Moody and Thomas (24). For determination of the relationship between the membrane potential and U02C12 activity (Figure 3), the activity coefficients of the U02C12 solutions were calculated within the range 1.0 x 10* to 1.0 X lo-' M by means of the equation log f = -Az2p112/(1 + p1i2). T h e corresponding slopes dE/a(log ai) are given in Table 11.
Figure 4. Response of U022+-bis(chloropropyi)phosphite membrane in mixed solutions of UO Ci2 and interfering ions. Concentration of interfering ion is 5 X 10-3 M.
130
IC"
Figure 5. Response of U0,2+-bis(chioroethyI) phosphite membrane in mixed solutions of UOpCipand interfering ions. Concentration of
interfering ions is 5
X
M.
The selectivities of phosphite membranes were first studied by the mixed solutions technique (Figures 4 and 5), but the calculation of the selectivity coefficients on the basis of these data is inaccurate, as the membranes are not ideal in the whole measured activity range. For all the membranes a flattening of the curves a t low U022+concentrations is observed. Therefore it is difficult to determine where the membrane ceases to be ideal and where the influence of the interfering ions begins to affect seriously the results. We decided therefore to measure the potentials obtained using the membranes in separate solutions of the main ion and of the interfering ion and calculate Kij according to the reduced Eisenman equation (25) log Kij = -
2.303RT
where E , is the potential measured in the uranyl chloride solution with activity a and charge n = 2 and E2 is the potential measured in the interfering ion solution with activity a and charge z. The values obtained for Kij and a = M are given in Table 111. From these values it may be seen that
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
Table 111. Selectivity Coefficients of Phosphite Membranes
interfering
ion di(ch1oro(IO-~M) ethyl)
phosphite tri( chloroethyl)
NiZ+ Mg2+ CaZ+ c1-
0.0032 0.0004 0.0018
0.041 0.022 0.0015
0.000006
so42-
0.0003 0.000002 0.047 3.15 3.49 202
0.00006 0.0005
NO; Cr 3+ Fe3+ Ce4+
u4+
0.0002
0.526
di(ch1oroP'OPYI) 0.0035 0.0008 0.0025 0.00008 0.0003 0.0003 0.049
0.166 11.3
I
I
I
1
331
40'3
530
650
7:
havelength (rm;
Flgure 7. UV-visible spectrum of the complex of UOz2+ with trls(chloroethyl) phosphite.
The possibility that the uranium in the phosphite containing membranes is in the form of a U4+ and not UOzz+ complex, due to reduction by the phosphite, was checked. The UV-visible spectrum of a typical membrane is plotted in Figure 7. The absorption band at 425 nm is typical for UOZ2+ complexes. Furthermore, the color of U4+ complexes with phosphites was found to be green; their spectra are not reported due to the low solubility of these complexes in hexane, CC14, THF, and water.
ACKNOWLEDGMENT The authors are thankful to Mr. Uri Zach for his technical assistance.
LITERATURE CITED
$11
Figure 6. Effect of pH on potential response of UO;+-bis(chloropropyl) phosphite membrane.
alkaline and alkaline earth cations have no effect at all on the response of these membranes. The same is true for bivalent transition-metal cations as well as for the anions C1- and SO;-. On the other hand trivalent and polyvalent cations interfere strongly. These results are in accord with those obtained by the mixed solutions technique (Figures 4 and 5). However it is of interest to note that though nitrate lowers the cell potential, KLjfor them is very small. A similar observation was reported for U022+/phosphatemembranes by Manning (14).
The electrical resistance of phosphite membranes is -10'
Q. The potential stabilizes when passing from dilute to concentrated solutions within -10 s and when passing from concentrated to dilute solutions within 1.5 min. The average
-
lifetime of membranes in air is over 3 months. The effect of H+ions on a typical membrane is shown in Figure 6. The potential is pH independent between pH 2.0 and 4.0. Above pH 4 there is a decrease in the measured potential, probably due to the hydrolysis of UOZ2+(14) and below pH -2 the hydrogen ion contributes to the charge transport process across the membrane (16) and the measurements are not reproducible.
Buck, P. R. Anal. Chem. 1972, 44, 270R-295R. Buck, P. R. Anal. Chem. 1974, 46, 28R-52R. Buck, P. R. Anal. Chem. 1976, 46, 23R-39R. Buck, P. R. Anal. Chem. 1970, 50, 17R-29R. Covington, A. K. CRC Crn. Rev. Anal. Chem. 1974, 3 , 355-406. Durst. A. R. NBS Spec. Publ. 1089, No. 374. Koryta, J. Anal. Chlm. Acta 1972, 67, 329-411. Koryta, J. Anal. Chim. Acta 1977, 97, 1-85. M o w , G. J.; Thomas, J. D. R. "Selective Ion Senslttve Electrodes"; Merrov Publishing Co.: Watford. Herts., 1971. (IO) Pungor, E.; Toth, K. Analyst(London) 1970, 95,625-648. (1 1) Pungor, E.; Toth, K. Pure Appl. Chem. 1973, 36, 441-455. (12) Rechnitz, G. A. Pure Appl. Chem. 1973, 36, 457-471. (13) Dletrich, D. C. Technical Progress Report No. Y 1174D V.12 Development Division, Aug-Oct 1971. (14) Manning, D. L.; Stokely, J. R.; Magouyrk, D. W.Anal. Chem. 1974, 46, 11 16-1 119. (15) Moody, G. J.; Oke, R. B.; Thomas, J. D. R. Analyst(London) 1070, 95, 9 10-918. (16) Griffkhs, G. H.; Moody, G. J.; Thomas, J. D. R. Analyst(London) 1972, 9 7 , 420-427. (17) Thornburn-Burns, D.; Lee, J. D.; Harris, L. G. Mlcrochlm. Acta 1972, 188- 193. (18) Sorantin, H. "Determination of Uranium and Plutonium in Nudear Fuels"; Verhg Chemie: Weinheim, 1975. (19) Bloch. R.; Finkelstein, A,; Kedem, 0.; Vofsi, D. Id. Eng. Chem. Recess D e s . D ~ v 1967, . 6 . 231-237. (20) Vofsi, D.; Kedem, 0.;Bloch, R.; Marian, S. J . Inorg. Nucl. a m . 1989, 37, 2631-2634. (21) McCutchen, R. L. Report TID-7015, Section 1; USAEC, April 1968. (22) Rozen, A. M.; Nikolotova, Z. I. Russ. J . Inorg. Chem. 1964, 9 . (1) (2) (3) (4) (5) (6) (7) (8) (9)
923-944. .- - . . . .
(23) Bell, V. J.; Heisler, J.; Tannenbam, H.; Gddeson. J. J. Am. Chem. Soc. 1954. 76. 5185-5189. (24) Moody, G: J.; Thomas, J. D. R. Talanta 1972, 19, 623-629. (25) Levins, J. R. Anal. Chem. 1971, 43, 1045-1047.
RECEIVED for review April 16,1980. Accepted August 11,1980. This work is in partial fulfillment of the requirements for receiving the M.Sc. degree from the Ben-Gurion University of the Negev.
