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Selectivity coefficients of univalent cations for liquid ion-exchange membrane electrodes based on nitrobenzene
31
J. Electroanal. Chem., 219 (1990) 31-41 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Selectivity coefficients of univalent cations for liquid ion-exchange membrane electrodes based on nitrobenzene
Czechoslovak Academy (Czechoslovakia)
of Sciences,
Institute
of Analytical
Chemistry,
Leninova
82, CS-61142
Brno
J. !hnk$f J.E. PurkynP University, Department (Received
of Analytical Chemistry, Kotkikkd
2.5 May 1988; in revised form 29 September
2, CT-61137 Brno (Czechoslovakia)
1989)
ABSTRACT
Liquid ion-exchange membrane electrodes, sensitive to potassium, caesium, tetramethyl-, tetraethyland tetrabutylammonium cations, were prepared and their response characteristics studied. The membranes of the electrodes were nitrobenzene solutions of tetrakis(4-fluorophenyl)borates of the respective cations. Selectivity coefficients were determined by the modified separate solution method and the reliability of such measurements is discussed. Values of the selectivity coefficients are given for 41 univalent cations in the range of almost 13 orders of magnitude. The values represent constants which are independent of the membrane composition and the activities of both primary and interfering ions in a sample solution. They give comprehensive information not only on the cation interference with the electrode response, but also on the extractability of the cations into nitrobenzene.
INTRODUCTION
A large number of papers have been published concerning liquid ion-exchange membrane ion-selective electrodes responsive to alkali metal and alkylammonium cations. Various lipophilic anions, such as tetraphenylborate [l-6], its 4-chloro- [7,8] and 3-trifluoromethyl derivative [9,10], dipicrylaminate [11,12] and dinonylnaphthalenesulphonate [13,14], were used as ion-exchange sites of these electrodes and a variety of water-immiscible organic solvents was used for the preparation of the liquid membranes. In a majority of the papers reported the electrode selectivities to other cations were determined. However, the results are not comparable to one another because of different experimental conditions. 0022-0728/90/$03.50
0 1990 Elsevier Sequoia
S.A.
32
Assuming complete dissociation of the liquid ion exchanger, it follows membrane potential theory [15] that the theoretical selectivity coefficient pot = k A3
K’”
A,B(
u,/'B)
from the
0)
where DA and 0, are the membrane mobilities of ions A and B, and KEB the ion-exchange constant, does not depend on the ion-exchange site, its concentration in the membrane or on the ionic activities in the test solution, but depends only on the properties of the membrane solvent used. The apparent selectivity coefficient krh(app.) [16] is defined by the equation E = E. + s log[ aA + k~;(app.)lZ,]
(2)
where S is the Nernstian slope; aA and aB are the activities of the primary ion A and interfering ion B, respectively; the charges of the ions are assumed zA = zB = 1. Its value is not a constant: it changes as a consequence of ion-exchange and diffusion processes at the membrane/aqueous solution interface [17], especially at low ionic activities in the aqueous solution or at a high concentration of the ion exchanger in the membrane [18,19]. The relationship between the apparent and theoretical selectivity coefficients can be expressed by the equation [20] krk(app.)
= 12( k xq-$~
where U, and U, are ionic mobilities in the aqueous phase and Q is the critical concentration [20,17], a parameter dependent on experimental conditions, particularly on the ion exchanger concentration in the membrane, on time and the stirring rate. If uA = 0 and U, = U,, the value of Q is identical with the activity uB, at which the maximum change in the apparent selectivity coefficient is observed; krA(app.) = ( k$$)‘i2 at aB = Q [20]. Equation (3) allows us to calculate k,!$,(app.) valid for the given conditions (composition of the membrane and sample solution, experimental arrangement), provided that the theoretical values of kr: are known. The concentration-independent values of the selectivity coefficient give us useful information on the extractability of single ions into polar organic solvents, since they are closely connected with the individual extraction parameters of the ions [1,15]. They enable us also to calculate the basic characteristics (detection limit, selectivity) of liquid ion-exchange membrane electrodes [21]. The purpose of the present paper is to determine accurate values of the selectivity coefficients kc; for a number of univalent cations following the previous papers [22,23] concerning selectivity of anion-sensitive electrodes. Selectivity measurements are reliable, if the logarithms of k,!?,$ lie approximately within the range ( - 2, + 3) [22]. Since the selectivity range is several times wider in the series of cations, more than one electrode must be used for its evaluation. The
33
log kri values measured by means of different into one another using the relationships [11,24] log kr;
= - log kr;
log kj$
= log kr;
electrodes
can easily be converted
(4 + log kr;
(5)
For selectivity measurements the cation-exchange site used should fulfil the following requirements: (i) low interference of Hf ions in acidic medium (some of the cations studied are derived from weak bases); and (ii) high lipophilicity and, consequently, low detection limit of the corresponding electrode [25] (extraction of the ion exchanger into the aqueous phase is not considered in eqns. l-5). Tetrakis(4-chlorophenyl)borate and tetrakis(4-fluorophenyl)borate anions appear to be very suitable sites for such measurements [26]; increased stability is an advantage of the fluorinated derivative [27]. Equations (1) (3)-(5) are valid for a fully dissociated ion exchanger. As follows from conductivity [28,29] and extraction [30,31] measurements in nitrobenzene solutions, the ion association of tetraphenylborates at the concentrations used in this work is almost negligible. An even lower association of tetrakis(4-fluorophenyl) borate salts can be expected from the increased solubility of these salts in water in comparison with unsubstituted tetraphenylborate or the commonly used tetrakis(4chlorophenyl)borate [32]. EXPERIMENTAL
Chemicals and solutions For the preparation of potassium tetrakis(4-fluorophenyl)borate (KTFPB) a filtered aqueous solution of the sodium salt (kindly supplied by Dr. I. Kolowos, Chemistry Section, Karl-Marx-University, Leipzig, G.D.R.) was precipitated with an excess of potassium chloride solution. The precipitate was separated and dried, then dissolved in a small amount of anhydrous acetone. An excess of benzene was added and the solution was concentrated under reduced pressure. Very fine crystals formed, which were washed with benzene and vacuum dried. Caesium and tetramethylammonium tetrakis(4-fluorophenyl)borate (CsTFPB, TMATFPB) were prepared analogously; the tetraethylammonium salt (TEATFPB) was recrystallized from ethanol. Tetrabutylammonium tetraphenylborate (TBATPB) was recrystallized from an acetone + ethanol mixture (1 : 1) after precipitation. Membrane solutions were prepared by dissolving the above mentioned salts in nitrobenzene purified according to ref. 23. NaCl, NH,Cl, KCl, RbCl, CsCl, TlCl and AgNO, salts, analytical grade, were used without further preparation. Choline, acetylcholine, hydroxy-, methyl-, dimethyl-, diethyl- and tetramethylammonium chloride were dried at 110 o C before weighing. Aminoguanidinium sulphate, tetramethyland tetraethylammonium bromide were recrystallized from ethanol and dried. Chlorides of the other cations were prepared by neutralization of the bases with hydrochloric acid; the salts were
34
recrystallized from a suitable solvent (water, ethanol, acetone + ethanol mixture). They were dried at 11O’C before use, and especially hygroscopic salts were desiccated over P,O, in vacuum. Stock solutions of lithium and tetraethylammonium chloride were standardized by argentometric titration. A solution of tetrabutylammonium sulphate was prepared by neutralization of the hydroxide with sulphuric acid, using potentiometric indication of the end point. Trimethylphenylammonium and tetrapropylammonium iodide, tetraphenylphosphonium bromide and tetraphenylarsonium chloride (a.r.) were converted to the sulphates by reaction with an equivalent amount of silver sulphate in an aqueous medium. The concentrations of these solutions were checked potentiometrically. The test solutions, containing salts of the cations studied, were prepared by serial dilution of the stock solutions with doubly distilled water with a specific conductivity of ca. 1 @/cm. If cations strongly adsorbtive on glass were present (especially octyl-, dodecyl- and tetrabutylammonium, tetraphenylphosphonium and tetraphenylarsonium ions), the test solutions were prepared in volumetric flasks which had been rinsed repeatedly with a solution of the same concentration of the corresponding cation. The measuring vessels were also rinsed repeatedly. Electrodes and apparatus The dependence of the membrane potential on the test solution obtained by measuring the electromotive force (emf) of the cell Hg IHg,Cl,,
satd. KC1 11test solution
liquid membrane
1inner solution,
(bridge),
test solution
composition
was
(vessel) 1
AgCl 1Ag
(A) A Radiometer K 401 calomel electrode, used as an external reference electrode, was separated from the test solution in a measuring vessel by a bridge, which was filled with the same test solution. This setup prevented contamination of the test solutions with KC1 and interfering phenomena at the salt bridge (satd. KCl)/stirred solution interface. If the electrode response in Ag+ solutions was measured, an additional bridge filled with saturated sodium formate solution was placed between the test solution and the calomel electrode; only a minimum liquid-junction potential was observed in this case. Liquid membranes were nitrobenzene solutions of KTFPB and CsTFPB at a concentration of 1 X 10e4 mol dme3, and TMATFPB, TEATFPB and TBATPB at a concentration of 2 x lop4 mol dmp3. The inner standard solutions of the K+-, Cs+-, TMA+- and TEA+-sensitive electrodes contained chlorides of the respective cations at a concentration of 1 X 1O-3 mol dme3, the inner solution of the TBA+-ISE contained 5 x 1O-5 mol dmp3 (TBA),SO,, and 5 x 1O-5 mol dm-3 Ag,S% For the determination of the selectivity following junctionless cell was used: Ag IAgCl, HCl or KC1 soln. (bridge), 1 X
10P4 mol dmm3 KTFPB
coefficient
of H+ ion towards
K+ ion, the
HCl or KC1 soln. (vessel) I
in NB 11 X 10e3 mol dme3
KCl, AgCl IAg
(B)
35
The electrode body, which enables the membrane surface to be renewed prior to each measurement, was described previously [22]. The electrode design prevents irreversible poisoning of the membrane and increases the reproducibility of the measurements. A Radiometer G 202 B glass electrode was used for pH determination. The emf and pH values were measured at 25 _t 0.5”C with an MV-88 precision pH-meter (Pracitronic, Dresden). The emf was read 100 s after the electrodes had been dipped into the test solution. A cylindrical stirrer, 12 mm in diameter, at 2000 rpm, ensured reproducible stirring of the solutions. The geometrical setup of the stirrer and electrodes was fixed for all the measurements. Determination of selectivity coefficients Apparent selectivity coefficients were determined [33], according to the equation log k!$(app.)
= (En - &J/S
by the separate
+ log(aJan)
solution
method
(6)
where EA and E, are the potentials of an indicator A+-ISE in a solution containing solution), and in a solution the primary ion A+, at the activity aA (calibrating containing the interfering ion B+, at the activity aB, respectively; S is the slope of the calibration curve of the A+-ISE (plot of E vs. log aA). The potential measureseveral times, always with the ments at different activities aB were repeated membrane surface renewed. In the Nernstian region of the E vs. log aB plot, the kri(app.) values calculated from eqn. (6) correspond directly to the theoretical selectivity coefficients kc;. Points situated in a slightly curved part of the E vs. log aB plot were also used for the evaluation of krh, provided that the electrode potential at these points was reproducible and independent of time. The k$$(app.) values calculated from these points were converted to kr; values using eqn. (3), arranged for aA = 0 and u, = u,:
Q/ad + Q/a,]
(7)
The selectivity coefficient for hydrogen ion, krh, from the data measured in cell (B) using mean activities individual ones.
was evaluated analogously of HCl and KC1 instead of
kX$ = [k~~(app.)]*/[k~~(app.)(I
-
Calculation of ionic activities Ionic concentrations ci were converted a, = c,y,, where yi is the activity coefficient from the extended Debye-Hiickel equation log y, = -0.509J7/(1
+ J7)
to activities ai using the relationship of ion i. The y, values were calculated
(8)
where I is the ionic strength of the test solution. Activity coefficients of K+, Na+, Li+ and Cs+ ions at concentrations c > 0.1 mol dmm3 were taken from ref. 34, mean activity coefficients of HCl from ref. 35.
36
RESULl S AND DISCUSSION
Indicator electrodes Nitrobenzene-based ion-exchange membrane electrodes, sensitive to potassium, caesium, tetramethyl-, tetraethyl- and tetrabutylammonium cations, were prepared and their response characteristics studied. Tetrakis(4-fluorophenyl)borate was used as an ion-exchange site for the K+-, Cs+-, TMA+- and TEA+-sensitive electrodes, unsubstituted tetraphenylborate was used for the preparation of the TBA+-ISE. The response time of these electrodes in stirred solutions was less than a minute throughout the concentration range of the primary ion, and the slopes of the calibration curves were 59.3 k 0.4 mV. The other characteristics of the electrodes applied are summarized in Table 1. The detection limits of the tetraalkylammonium-sensitive electrodes can be expected to be ca. 1 X lo-’ mol dmP3 and lower [21]. It is very difficult to prepare defined solutions of tetraalkylammonium salts at such low concentrations, since the cations are adsorbed on the walls of the glass vessels used. Therefore, the calibration of the tetraalkylammonium electrodes was stopped at a concentration of 1 x low6 mol dmm3 without observing any curvature of the linear E vs. log aA plot due to extraction of the ion exchanger into the aqueous phase. On the other hand, co-ion interference [36] with tetraalkylammonium ISEs causes a curvature of the calibration plots at high activity levels. The interference of halide anions with the TEA+-ISE response became evident by their reducing the linear region of the calibration curve at TEA+ concentrations higher than 1 x 1O-2 mol dme3 (see Table 1). The tetrabutylammonium electrode with the given membrane composition could be used at a concentration of TBA+ ions as high as 1 x 10e2 mol dme3, if sulphate co-ions only were present in the test solution (the potential deviation due to sulphate interference was - 2 mV at ~-,.a*+ = 1 x 10e2 mol dm-3).
TABLE 1 Response characteristics of the indicator electrodes used for the determination of selectivity coefficients in stirred solutions. Ion exchanger concentrations: 1 X 1O-4 mol dme3 of KTFPB and CsTFPB, 2 X 10m4 mol drnm3 of TMATFPB, TEATFPB and TBATPB Ion exchanger
Linear range /mol drnm3
KTFPB CsTFPB TMATFPB
5x10-‘-5x10-5 1x10-‘-5x10~6 1X10-‘-1X10-6~ 5x10-*-1X10-6b 5x10-2-1x10-6a 1X10-2-1x10-6b 5x10-3-1x10~6c
TEATFPB TBATPB a Co-ion: chloride. b Co-ion: bromide. ’ Co-ion: sulphate.
106 ao /mol dm -3 3 0.6
lo6 Q /mol drne3 4 4
12
il
18 9
37
The presence of univalent co-ions caused considerable deviations from Nernstian response even at relatively low TBA+ activities [37]. Critical concentrations Q for time t = 100 s (Table 1) were determined by measuring the potential response of the K+- and Cs+-ISEs to tetramethylamto tetrabutylammonium ions, and of monium ions, of Th4A+- and TEA+-ISEs TBA+-ISE to tetraphenylarsonium ions, according to ref. 20.
Determination of selectivity coefficients According to our experience, the determination of selectivity coefficients by the separate solution method is sufficiently reliable (with a maximum deviation of ~0.02 in the logarithmic scale), if the activity of the interfering ion B’ in the test solution conforms to the conditions a, 2 1OQ (for kri I=-1) or a, > lOQ/kri (for k$‘$ < 1). The critical concentration value Q in a stirred solution is considerably lower than in an unstirred one [20]; this enables the selectivities in an extended range of activities aB to be measured. Therefore, all measurements of selectivity coefficients were performed in vigorously stirred solutions.
E mV
3oa
200
100
5
4
3
2
-log (aa/mol
dme3)
Fig. 1. Potential responses of the K+-sensitive electrode to tetramethylammonium+ (2), diethylammonium+ (3), Cs+ (4), guanidinium+ (5), Rb+ (6), methylammonium+ (9), NH30H+ (lo), Na+ (11) and Lit (12) chloride, stirred solutions. Membrane: KTFPB, in nitrobenzene.
(l), piperidinium+ (7), K+ (S), NH: 1~10~~ mol dme3
38
Figure 1 shows the plot of the potential response of the K+-senstive electrode vs. the activities of some interfering ions selected. The electrode potential was stable and well reproducible in all points of the graph (except for the response to TMA+ ions at cTMA+ < 5 x low5 mol dme3). These points were used for the calculation of log kri values, using eqn. (6) and, if necessary, eqn. (7). The data measured with the Cs’+-, TMA+-, TEA+- and TBA+-ISEs were processed in the same manner. Since the experimental slope values differed by max. kO.5 mV from the theoretical one, the Nernstian value 59.2 mV was used in all calculations by eqn. (6). The logarithms of kc; calculated for different activities uB differed by maximally f0.02. Mean log kri values were converted to Cs+ reference ion by means of relationships (4) and (5) using the parameters: log kg& = - 1.40, log kg:T,, = 1.91, log kg;&, = 3.39, log kg;,,, = 7.02. Except for the marginal values log kgYB < - 1.4 and log kCs,B pot > 8 all selectivity coefficients were measured with two or even three different electrodes. The correspondence of the results obtained was very good, the logarithms of k,TB for an ion B+ differing by more than 0.01 only exceptionally. Table 2 presents the resulting set of log k,$. The sequence of B+ ions represents a lyophilic series in the water/nitrobenzene system. TABLE 2 Concentration-independent
selectivity coefficients of univalent cations for nitrobenzene-based
change membrane electrodes B + cation
Log @‘$I
B+ cation
Log St*
Li+ Tt.is+ Na+
- 3.80 - 3.36 - 3.30
cs+ Diethylammonium+ DEAE+ d
- 3.19 - 2.87 -2.17 - 1.93 - 1.91 - 1.85 - 1.70 - 1.40 -1.18 - 0.97 - 0.78 - 0.72 -0.59 - 0.39 - 0.38 - 0.05 - 0.01
Cyclohexylammonium+ Trimethylammonium+ Piperidinium+ Choline + Tetramethylammonium+ Triethylammonium+ Acetylcholine+ Dibutylammonium+ n-Octylammonium + Tetraethylammonium+ DicyclohexylammoniumC Trimethylphenylammonium+ Tributylammonium+ n-Dodecylammonium+ Tetrapropylammonium+ Tetrabutylammonium+ Tetraphenylphosphonium+ Tetraphenylarsonium+
0.00 0.64 0.82 0.84 0.89 0.98 1.03 1.91 1.96 2.29 2.59 2.85 3.39 3.44 3.62 4.82 4.9 5.23 7.02 8.75 8.92
a.b
H+ NHsOH+ ’ N,H; b NH: AB+ Ethanolammonium+ b b Diethanolammonium+ K+ b Triethanolammoniumf Methylammonium+ Tl+ Rb+ Ethylammonium + Allylammonium + Guanidinium+ Dimethylammonium+ Aminoguanidinium +
a b ’ d
Tris(hydroxymethyl)methylammonium+. pH = 5.OkO.l. pH = 4.0+0.1. (N, N-diethyl-2-aminoethanol). Hf.
ion-ex-
39
-
E
mV
500
400
3
log
( ag/moi
dmw3 )
Fig. 2. Potential responses of the TBA+-sensitive electrode to tetrabutylammonium+ (0), tetraphenylphosphonium+ (CD)and tetraphenylarsonium+ (0) sulphate, stirred solutions. Membrane: 2 x 10m4 mol drne3 TBATPB, in nitrobenzene. Dashed line denotes the theoretical Nernstian slope, 59.2 mV.
The least lipophilic univalent cation is indisputably Li+ ion. A lower extractability can be expected only for polyvalent cations, e.g., alkaline earth metal ions. Since the values of their Gibbs energies of hydration are very negative, the probability that free ions are extracted into the organic phase is low [38]. The function E vs. log uM2+ was not linear even at concentrations higher than 0.1 mol dme3, and an accurate evaluation of the selectivity coefficients was not possible. The selectivity coefficients of tetraphenylarsonium (TPAs+) and tetraphenylphosphonium (TPP+) ions represent the limiting values, which can be determined accurately with a nitrobenzene-based membrane electrode. Although our measurements were carried out in the presence of very little extractable sulphate co-ions, at concentrations higher than 5 X lop4 mol dmp3 deviations from Nernstian response appeared, which were caused by co-ion interference (Fig. 2). Only the potential data measured in the concentration range 5 X 10e4-1 X lop4 mol dmp3 TPP+ and 3 x 10e4-1 X lop4 mol dmp3 TPAs+ could be considered for accurate evaluation. Selectivity determinations in solutions of cations more extractable than TPAs+ (e.g.,
cations of some triphenylmethane dyes and cationic surfactants [l]) are not possible by means of the electrode type given. The co-ion interference can be suppressed by increasing the ion exchanger concentration in the membrane [36,39]; however, this results in the same increase in the critical concentration Q simultaneously [20]. In the E vs. log uB plot no linear region, which is necessary for the evaluation of selectivity coefficients, can be found. Only approximate values are given for the selectivity coefficients of octylammonium and dodecylammonium cations. These ions behave as surfactants and are adsorbed on the membrane surface. The electrode response was over-Nernstian in solutions of these ions (slope 62-65 mV), and the potential was time-dependent and poorly reproducible. CONCLUSION
The modified separate solution method presented appears to be accurate and reliable for the determination of selectivity coefficients of liquid-membrane ionselective electrodes. The data in this work, determined for 41 univalent cations, will be useful not only for those who work with such ion-selective electrodes. The selectivity coefficient values enable one to calculate basic characteristics of ISEs under given conditions, and they give also information on the lipophilicity of the ions and their extractability into polar organic solvents. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
R. Scholer and W. Simon, Helv. Chim. Acta, 55 (1972) 1801. R. Geyer and I. Preuss, Z. Chem., 14 (1974) 29. E. Hopirtean and E. Stefaniga, Rev. Roum. Chim., 19 (1974) 1265. C.J. Coetzee and A.J. Basson, Anal. Chim. Acta, 83 (1976) 361. E.W. Baumann, Anal. Chem., 48 (1976) 548. T. Kakiuchi, T. Obi and M. Senda, Bull. Chem. Sot. Jpn., 58 (1985) 1636. G. Baum and M. Lynn, Anal. Chim. Acta, 65 (1973) 393. J.E.W. Davies, G.J. Moody, W.M. Price and J.D.R. Thomas, Lab. Pratt., 22 (1973) 20. C.J. Coetzee and A.J. Basson, Anal. Chim. Acta, 92 (1977) 399. C.J. Coetzee and A.J. Basson, Anal. Chim. Acta, 126 (1981) 217. A. Jyo, M. Torikai and N. Ishibashi, Bull. Chem. Sot. Jpn., 47 (1974) 2862. E. Hopirtean and E. Stefaniga, Chem. Anal. (Warsaw), 22 (1977) 845. C.R. Martin and H. Freiser, Anal. Chem., 52 (1980) 562. L. Cunningham and H. Freiser, Anal. Chim. Acta, 132 (1981) 43. J. Sandblom, G. Eisenman and J.L. Walker, Jr., J. Phys. Chem., 71 (1967) 3862. R.P. Buck, Anal. Chim. Acta, 73 (1974) 321. W.E. Morf, The Principles of Ion-Selective Electrodes and of Membrane Transport, Akademiai Kiado, Budapest, and Elsevier, Amsterdam, 1981, Ch. 11, Sect. 4. 18 A. Hulanicki and R. Lewandovski, Chem. Anal. (Warsaw), 19 (1974) 53. 19 N. Yoshida and N. Ishibashi, Bull. Chem. Sot. Jpn., 50 (1977) 3189. 20 J. Senkj+ and J. Petr, Symposium Analytiktreffen 1979, Neubrandenburg, 1979, Chemische Gesellschaft der DDR, p. 49; J. Senkjrf and J. Petr in E. Pungor and I. Buzas (Eds.), Ion-Selective Electrodes, Vol. 3, Akademiai Kiadb, Budapest, and Elsevier, Amsterdam, 1981, p. 327.
41 21 J. Set@?, J. Petr, I. Kolowos and G. Werner, Wiss. ‘2. K~l-Marx-U~v. Leipzig, Math.-Natu~ss. R., 35 (1986) 62. 22 J. Senkyi and K. Kouiil, J. Electroanal. Chem., 180 (1984) 383. 23 J. Senkj? and Z. Mala, J. Electroanal. Chem., 233 (1987) 267. 24 J. Senkj? and J. Petr in E. Pungor and I. Buzz& (Eds.), Ion-Selective Electrodes (Conference on ISEs, Budapest, Hungary, 5-9 September 1977) Akademiai Kiado, Budapest, and Elsevier, Amsterdam, 1978, p. 559. 25 N. Ishibashi and A. Jyo, Microchem. J., 18 (1973) 220. 26 V. SustaEek and J. Senk@. Ser. Fat. Sci. Nat. Univ. Purkynianae Brun., No. 4 (Chemia), 19 (1989) 177. 27 M. Meisters, J.T. Vandeberg, F.P. Cassaretto, H. Posvic and C.E. Moore, Anal. Chim. Acta, 49 (1970) 481. 28 K. Selinger, Chem. Anal. (Warsaw), 27 (1982) 51. 29 R.M. Fuoss and E. Hirsch, J. Am. Chem. Sot., 82 (1960) 1013. 30 T. Sekine and D. Dyrssen, Anal. Cbim. Acta, 45 (1969) 433. 31 M. Pivohkova and M. KyrS, J. Inorg. Nucl. Chem., 31 (1969) 175. 32 C.E. Moore, F.P. Cassaretto, H. Posvic and J.J. Lafferty, Anal. Chim. Acta, 35 (1966) 1. 33 IUPAC, Commission on Analytical Nomenclature, Ion-Set. Electrode Rev., 1 (1979) 139. 34 R.G. Bates, B.R. Staples and R.A. Robinson, Anal. Chem., 42 (1970) 867. 35 A. Ogg, Z. Phys. Chem., 22 (1897) 536; International Critical Tables, Vol. 7, McGraw-Hill, New York, London, 1930, p. 233. 36 A. Jyo, K. Fukamachi, W. Koga and N. Ishibashi, Bull. Chem. Sot. Jpn., 50 (1977) 670. 37 V. &tst%ek and J. Senkj+, unpublished results. 38 WE. Morf in ref. 17, p. 229. 39 F.S. Stover and R.P. Buck, J. Electroanal. Chem., 94 (1978) 59.
J. Electroanal. Chem., 219 (1990) 31-41 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Selectivity coefficients of univalent cations for liquid ion-exchange membrane electrodes based on nitrobenzene
Czechoslovak Academy (Czechoslovakia)
of Sciences,
Institute
of Analytical
Chemistry,
Leninova
82, CS-61142
Brno
J. !hnk$f J.E. PurkynP University, Department (Received
of Analytical Chemistry, Kotkikkd
2.5 May 1988; in revised form 29 September
2, CT-61137 Brno (Czechoslovakia)
1989)
ABSTRACT
Liquid ion-exchange membrane electrodes, sensitive to potassium, caesium, tetramethyl-, tetraethyland tetrabutylammonium cations, were prepared and their response characteristics studied. The membranes of the electrodes were nitrobenzene solutions of tetrakis(4-fluorophenyl)borates of the respective cations. Selectivity coefficients were determined by the modified separate solution method and the reliability of such measurements is discussed. Values of the selectivity coefficients are given for 41 univalent cations in the range of almost 13 orders of magnitude. The values represent constants which are independent of the membrane composition and the activities of both primary and interfering ions in a sample solution. They give comprehensive information not only on the cation interference with the electrode response, but also on the extractability of the cations into nitrobenzene.
INTRODUCTION
A large number of papers have been published concerning liquid ion-exchange membrane ion-selective electrodes responsive to alkali metal and alkylammonium cations. Various lipophilic anions, such as tetraphenylborate [l-6], its 4-chloro- [7,8] and 3-trifluoromethyl derivative [9,10], dipicrylaminate [11,12] and dinonylnaphthalenesulphonate [13,14], were used as ion-exchange sites of these electrodes and a variety of water-immiscible organic solvents was used for the preparation of the liquid membranes. In a majority of the papers reported the electrode selectivities to other cations were determined. However, the results are not comparable to one another because of different experimental conditions. 0022-0728/90/$03.50
0 1990 Elsevier Sequoia
S.A.
32
Assuming complete dissociation of the liquid ion exchanger, it follows membrane potential theory [15] that the theoretical selectivity coefficient pot = k A3
K’”
A,B(
u,/'B)
from the
0)
where DA and 0, are the membrane mobilities of ions A and B, and KEB the ion-exchange constant, does not depend on the ion-exchange site, its concentration in the membrane or on the ionic activities in the test solution, but depends only on the properties of the membrane solvent used. The apparent selectivity coefficient krh(app.) [16] is defined by the equation E = E. + s log[ aA + k~;(app.)lZ,]
(2)
where S is the Nernstian slope; aA and aB are the activities of the primary ion A and interfering ion B, respectively; the charges of the ions are assumed zA = zB = 1. Its value is not a constant: it changes as a consequence of ion-exchange and diffusion processes at the membrane/aqueous solution interface [17], especially at low ionic activities in the aqueous solution or at a high concentration of the ion exchanger in the membrane [18,19]. The relationship between the apparent and theoretical selectivity coefficients can be expressed by the equation [20] krk(app.)
= 12( k xq-$~
where U, and U, are ionic mobilities in the aqueous phase and Q is the critical concentration [20,17], a parameter dependent on experimental conditions, particularly on the ion exchanger concentration in the membrane, on time and the stirring rate. If uA = 0 and U, = U,, the value of Q is identical with the activity uB, at which the maximum change in the apparent selectivity coefficient is observed; krA(app.) = ( k$$)‘i2 at aB = Q [20]. Equation (3) allows us to calculate k,!$,(app.) valid for the given conditions (composition of the membrane and sample solution, experimental arrangement), provided that the theoretical values of kr: are known. The concentration-independent values of the selectivity coefficient give us useful information on the extractability of single ions into polar organic solvents, since they are closely connected with the individual extraction parameters of the ions [1,15]. They enable us also to calculate the basic characteristics (detection limit, selectivity) of liquid ion-exchange membrane electrodes [21]. The purpose of the present paper is to determine accurate values of the selectivity coefficients kc; for a number of univalent cations following the previous papers [22,23] concerning selectivity of anion-sensitive electrodes. Selectivity measurements are reliable, if the logarithms of k,!?,$ lie approximately within the range ( - 2, + 3) [22]. Since the selectivity range is several times wider in the series of cations, more than one electrode must be used for its evaluation. The
33
log kri values measured by means of different into one another using the relationships [11,24] log kr;
= - log kr;
log kj$
= log kr;
electrodes
can easily be converted
(4 + log kr;
(5)
For selectivity measurements the cation-exchange site used should fulfil the following requirements: (i) low interference of Hf ions in acidic medium (some of the cations studied are derived from weak bases); and (ii) high lipophilicity and, consequently, low detection limit of the corresponding electrode [25] (extraction of the ion exchanger into the aqueous phase is not considered in eqns. l-5). Tetrakis(4-chlorophenyl)borate and tetrakis(4-fluorophenyl)borate anions appear to be very suitable sites for such measurements [26]; increased stability is an advantage of the fluorinated derivative [27]. Equations (1) (3)-(5) are valid for a fully dissociated ion exchanger. As follows from conductivity [28,29] and extraction [30,31] measurements in nitrobenzene solutions, the ion association of tetraphenylborates at the concentrations used in this work is almost negligible. An even lower association of tetrakis(4-fluorophenyl) borate salts can be expected from the increased solubility of these salts in water in comparison with unsubstituted tetraphenylborate or the commonly used tetrakis(4chlorophenyl)borate [32]. EXPERIMENTAL
Chemicals and solutions For the preparation of potassium tetrakis(4-fluorophenyl)borate (KTFPB) a filtered aqueous solution of the sodium salt (kindly supplied by Dr. I. Kolowos, Chemistry Section, Karl-Marx-University, Leipzig, G.D.R.) was precipitated with an excess of potassium chloride solution. The precipitate was separated and dried, then dissolved in a small amount of anhydrous acetone. An excess of benzene was added and the solution was concentrated under reduced pressure. Very fine crystals formed, which were washed with benzene and vacuum dried. Caesium and tetramethylammonium tetrakis(4-fluorophenyl)borate (CsTFPB, TMATFPB) were prepared analogously; the tetraethylammonium salt (TEATFPB) was recrystallized from ethanol. Tetrabutylammonium tetraphenylborate (TBATPB) was recrystallized from an acetone + ethanol mixture (1 : 1) after precipitation. Membrane solutions were prepared by dissolving the above mentioned salts in nitrobenzene purified according to ref. 23. NaCl, NH,Cl, KCl, RbCl, CsCl, TlCl and AgNO, salts, analytical grade, were used without further preparation. Choline, acetylcholine, hydroxy-, methyl-, dimethyl-, diethyl- and tetramethylammonium chloride were dried at 110 o C before weighing. Aminoguanidinium sulphate, tetramethyland tetraethylammonium bromide were recrystallized from ethanol and dried. Chlorides of the other cations were prepared by neutralization of the bases with hydrochloric acid; the salts were
34
recrystallized from a suitable solvent (water, ethanol, acetone + ethanol mixture). They were dried at 11O’C before use, and especially hygroscopic salts were desiccated over P,O, in vacuum. Stock solutions of lithium and tetraethylammonium chloride were standardized by argentometric titration. A solution of tetrabutylammonium sulphate was prepared by neutralization of the hydroxide with sulphuric acid, using potentiometric indication of the end point. Trimethylphenylammonium and tetrapropylammonium iodide, tetraphenylphosphonium bromide and tetraphenylarsonium chloride (a.r.) were converted to the sulphates by reaction with an equivalent amount of silver sulphate in an aqueous medium. The concentrations of these solutions were checked potentiometrically. The test solutions, containing salts of the cations studied, were prepared by serial dilution of the stock solutions with doubly distilled water with a specific conductivity of ca. 1 @/cm. If cations strongly adsorbtive on glass were present (especially octyl-, dodecyl- and tetrabutylammonium, tetraphenylphosphonium and tetraphenylarsonium ions), the test solutions were prepared in volumetric flasks which had been rinsed repeatedly with a solution of the same concentration of the corresponding cation. The measuring vessels were also rinsed repeatedly. Electrodes and apparatus The dependence of the membrane potential on the test solution obtained by measuring the electromotive force (emf) of the cell Hg IHg,Cl,,
satd. KC1 11test solution
liquid membrane
1inner solution,
(bridge),
test solution
composition
was
(vessel) 1
AgCl 1Ag
(A) A Radiometer K 401 calomel electrode, used as an external reference electrode, was separated from the test solution in a measuring vessel by a bridge, which was filled with the same test solution. This setup prevented contamination of the test solutions with KC1 and interfering phenomena at the salt bridge (satd. KCl)/stirred solution interface. If the electrode response in Ag+ solutions was measured, an additional bridge filled with saturated sodium formate solution was placed between the test solution and the calomel electrode; only a minimum liquid-junction potential was observed in this case. Liquid membranes were nitrobenzene solutions of KTFPB and CsTFPB at a concentration of 1 X 10e4 mol dme3, and TMATFPB, TEATFPB and TBATPB at a concentration of 2 x lop4 mol dmp3. The inner standard solutions of the K+-, Cs+-, TMA+- and TEA+-sensitive electrodes contained chlorides of the respective cations at a concentration of 1 X 1O-3 mol dme3, the inner solution of the TBA+-ISE contained 5 x 1O-5 mol dmp3 (TBA),SO,, and 5 x 1O-5 mol dm-3 Ag,S% For the determination of the selectivity following junctionless cell was used: Ag IAgCl, HCl or KC1 soln. (bridge), 1 X
10P4 mol dmm3 KTFPB
coefficient
of H+ ion towards
K+ ion, the
HCl or KC1 soln. (vessel) I
in NB 11 X 10e3 mol dme3
KCl, AgCl IAg
(B)
35
The electrode body, which enables the membrane surface to be renewed prior to each measurement, was described previously [22]. The electrode design prevents irreversible poisoning of the membrane and increases the reproducibility of the measurements. A Radiometer G 202 B glass electrode was used for pH determination. The emf and pH values were measured at 25 _t 0.5”C with an MV-88 precision pH-meter (Pracitronic, Dresden). The emf was read 100 s after the electrodes had been dipped into the test solution. A cylindrical stirrer, 12 mm in diameter, at 2000 rpm, ensured reproducible stirring of the solutions. The geometrical setup of the stirrer and electrodes was fixed for all the measurements. Determination of selectivity coefficients Apparent selectivity coefficients were determined [33], according to the equation log k!$(app.)
= (En - &J/S
by the separate
+ log(aJan)
solution
method
(6)
where EA and E, are the potentials of an indicator A+-ISE in a solution containing solution), and in a solution the primary ion A+, at the activity aA (calibrating containing the interfering ion B+, at the activity aB, respectively; S is the slope of the calibration curve of the A+-ISE (plot of E vs. log aA). The potential measureseveral times, always with the ments at different activities aB were repeated membrane surface renewed. In the Nernstian region of the E vs. log aB plot, the kri(app.) values calculated from eqn. (6) correspond directly to the theoretical selectivity coefficients kc;. Points situated in a slightly curved part of the E vs. log aB plot were also used for the evaluation of krh, provided that the electrode potential at these points was reproducible and independent of time. The k$$(app.) values calculated from these points were converted to kr; values using eqn. (3), arranged for aA = 0 and u, = u,:
Q/ad + Q/a,]
(7)
The selectivity coefficient for hydrogen ion, krh, from the data measured in cell (B) using mean activities individual ones.
was evaluated analogously of HCl and KC1 instead of
kX$ = [k~~(app.)]*/[k~~(app.)(I
-
Calculation of ionic activities Ionic concentrations ci were converted a, = c,y,, where yi is the activity coefficient from the extended Debye-Hiickel equation log y, = -0.509J7/(1
+ J7)
to activities ai using the relationship of ion i. The y, values were calculated
(8)
where I is the ionic strength of the test solution. Activity coefficients of K+, Na+, Li+ and Cs+ ions at concentrations c > 0.1 mol dmm3 were taken from ref. 34, mean activity coefficients of HCl from ref. 35.
36
RESULl S AND DISCUSSION
Indicator electrodes Nitrobenzene-based ion-exchange membrane electrodes, sensitive to potassium, caesium, tetramethyl-, tetraethyl- and tetrabutylammonium cations, were prepared and their response characteristics studied. Tetrakis(4-fluorophenyl)borate was used as an ion-exchange site for the K+-, Cs+-, TMA+- and TEA+-sensitive electrodes, unsubstituted tetraphenylborate was used for the preparation of the TBA+-ISE. The response time of these electrodes in stirred solutions was less than a minute throughout the concentration range of the primary ion, and the slopes of the calibration curves were 59.3 k 0.4 mV. The other characteristics of the electrodes applied are summarized in Table 1. The detection limits of the tetraalkylammonium-sensitive electrodes can be expected to be ca. 1 X lo-’ mol dmP3 and lower [21]. It is very difficult to prepare defined solutions of tetraalkylammonium salts at such low concentrations, since the cations are adsorbed on the walls of the glass vessels used. Therefore, the calibration of the tetraalkylammonium electrodes was stopped at a concentration of 1 x low6 mol dmm3 without observing any curvature of the linear E vs. log aA plot due to extraction of the ion exchanger into the aqueous phase. On the other hand, co-ion interference [36] with tetraalkylammonium ISEs causes a curvature of the calibration plots at high activity levels. The interference of halide anions with the TEA+-ISE response became evident by their reducing the linear region of the calibration curve at TEA+ concentrations higher than 1 x 1O-2 mol dme3 (see Table 1). The tetrabutylammonium electrode with the given membrane composition could be used at a concentration of TBA+ ions as high as 1 x 10e2 mol dme3, if sulphate co-ions only were present in the test solution (the potential deviation due to sulphate interference was - 2 mV at ~-,.a*+ = 1 x 10e2 mol dm-3).
TABLE 1 Response characteristics of the indicator electrodes used for the determination of selectivity coefficients in stirred solutions. Ion exchanger concentrations: 1 X 1O-4 mol dme3 of KTFPB and CsTFPB, 2 X 10m4 mol drnm3 of TMATFPB, TEATFPB and TBATPB Ion exchanger
Linear range /mol drnm3
KTFPB CsTFPB TMATFPB
5x10-‘-5x10-5 1x10-‘-5x10~6 1X10-‘-1X10-6~ 5x10-*-1X10-6b 5x10-2-1x10-6a 1X10-2-1x10-6b 5x10-3-1x10~6c
TEATFPB TBATPB a Co-ion: chloride. b Co-ion: bromide. ’ Co-ion: sulphate.
106 ao /mol dm -3 3 0.6
lo6 Q /mol drne3 4 4
12
il
18 9
37
The presence of univalent co-ions caused considerable deviations from Nernstian response even at relatively low TBA+ activities [37]. Critical concentrations Q for time t = 100 s (Table 1) were determined by measuring the potential response of the K+- and Cs+-ISEs to tetramethylamto tetrabutylammonium ions, and of monium ions, of Th4A+- and TEA+-ISEs TBA+-ISE to tetraphenylarsonium ions, according to ref. 20.
Determination of selectivity coefficients According to our experience, the determination of selectivity coefficients by the separate solution method is sufficiently reliable (with a maximum deviation of ~0.02 in the logarithmic scale), if the activity of the interfering ion B’ in the test solution conforms to the conditions a, 2 1OQ (for kri I=-1) or a, > lOQ/kri (for k$‘$ < 1). The critical concentration value Q in a stirred solution is considerably lower than in an unstirred one [20]; this enables the selectivities in an extended range of activities aB to be measured. Therefore, all measurements of selectivity coefficients were performed in vigorously stirred solutions.
E mV
3oa
200
100
5
4
3
2
-log (aa/mol
dme3)
Fig. 1. Potential responses of the K+-sensitive electrode to tetramethylammonium+ (2), diethylammonium+ (3), Cs+ (4), guanidinium+ (5), Rb+ (6), methylammonium+ (9), NH30H+ (lo), Na+ (11) and Lit (12) chloride, stirred solutions. Membrane: KTFPB, in nitrobenzene.
(l), piperidinium+ (7), K+ (S), NH: 1~10~~ mol dme3
38
Figure 1 shows the plot of the potential response of the K+-senstive electrode vs. the activities of some interfering ions selected. The electrode potential was stable and well reproducible in all points of the graph (except for the response to TMA+ ions at cTMA+ < 5 x low5 mol dme3). These points were used for the calculation of log kri values, using eqn. (6) and, if necessary, eqn. (7). The data measured with the Cs’+-, TMA+-, TEA+- and TBA+-ISEs were processed in the same manner. Since the experimental slope values differed by max. kO.5 mV from the theoretical one, the Nernstian value 59.2 mV was used in all calculations by eqn. (6). The logarithms of kc; calculated for different activities uB differed by maximally f0.02. Mean log kri values were converted to Cs+ reference ion by means of relationships (4) and (5) using the parameters: log kg& = - 1.40, log kg:T,, = 1.91, log kg;&, = 3.39, log kg;,,, = 7.02. Except for the marginal values log kgYB < - 1.4 and log kCs,B pot > 8 all selectivity coefficients were measured with two or even three different electrodes. The correspondence of the results obtained was very good, the logarithms of k,TB for an ion B+ differing by more than 0.01 only exceptionally. Table 2 presents the resulting set of log k,$. The sequence of B+ ions represents a lyophilic series in the water/nitrobenzene system. TABLE 2 Concentration-independent
selectivity coefficients of univalent cations for nitrobenzene-based
change membrane electrodes B + cation
Log @‘$I
B+ cation
Log St*
Li+ Tt.is+ Na+
- 3.80 - 3.36 - 3.30
cs+ Diethylammonium+ DEAE+ d
- 3.19 - 2.87 -2.17 - 1.93 - 1.91 - 1.85 - 1.70 - 1.40 -1.18 - 0.97 - 0.78 - 0.72 -0.59 - 0.39 - 0.38 - 0.05 - 0.01
Cyclohexylammonium+ Trimethylammonium+ Piperidinium+ Choline + Tetramethylammonium+ Triethylammonium+ Acetylcholine+ Dibutylammonium+ n-Octylammonium + Tetraethylammonium+ DicyclohexylammoniumC Trimethylphenylammonium+ Tributylammonium+ n-Dodecylammonium+ Tetrapropylammonium+ Tetrabutylammonium+ Tetraphenylphosphonium+ Tetraphenylarsonium+
0.00 0.64 0.82 0.84 0.89 0.98 1.03 1.91 1.96 2.29 2.59 2.85 3.39 3.44 3.62 4.82 4.9 5.23 7.02 8.75 8.92
a.b
H+ NHsOH+ ’ N,H; b NH: AB+ Ethanolammonium+ b b Diethanolammonium+ K+ b Triethanolammoniumf Methylammonium+ Tl+ Rb+ Ethylammonium + Allylammonium + Guanidinium+ Dimethylammonium+ Aminoguanidinium +
a b ’ d
Tris(hydroxymethyl)methylammonium+. pH = 5.OkO.l. pH = 4.0+0.1. (N, N-diethyl-2-aminoethanol). Hf.
ion-ex-
39
-
E
mV
500
400
3
log
( ag/moi
dmw3 )
Fig. 2. Potential responses of the TBA+-sensitive electrode to tetrabutylammonium+ (0), tetraphenylphosphonium+ (CD)and tetraphenylarsonium+ (0) sulphate, stirred solutions. Membrane: 2 x 10m4 mol drne3 TBATPB, in nitrobenzene. Dashed line denotes the theoretical Nernstian slope, 59.2 mV.
The least lipophilic univalent cation is indisputably Li+ ion. A lower extractability can be expected only for polyvalent cations, e.g., alkaline earth metal ions. Since the values of their Gibbs energies of hydration are very negative, the probability that free ions are extracted into the organic phase is low [38]. The function E vs. log uM2+ was not linear even at concentrations higher than 0.1 mol dme3, and an accurate evaluation of the selectivity coefficients was not possible. The selectivity coefficients of tetraphenylarsonium (TPAs+) and tetraphenylphosphonium (TPP+) ions represent the limiting values, which can be determined accurately with a nitrobenzene-based membrane electrode. Although our measurements were carried out in the presence of very little extractable sulphate co-ions, at concentrations higher than 5 X lop4 mol dmp3 deviations from Nernstian response appeared, which were caused by co-ion interference (Fig. 2). Only the potential data measured in the concentration range 5 X 10e4-1 X lop4 mol dmp3 TPP+ and 3 x 10e4-1 X lop4 mol dmp3 TPAs+ could be considered for accurate evaluation. Selectivity determinations in solutions of cations more extractable than TPAs+ (e.g.,
cations of some triphenylmethane dyes and cationic surfactants [l]) are not possible by means of the electrode type given. The co-ion interference can be suppressed by increasing the ion exchanger concentration in the membrane [36,39]; however, this results in the same increase in the critical concentration Q simultaneously [20]. In the E vs. log uB plot no linear region, which is necessary for the evaluation of selectivity coefficients, can be found. Only approximate values are given for the selectivity coefficients of octylammonium and dodecylammonium cations. These ions behave as surfactants and are adsorbed on the membrane surface. The electrode response was over-Nernstian in solutions of these ions (slope 62-65 mV), and the potential was time-dependent and poorly reproducible. CONCLUSION
The modified separate solution method presented appears to be accurate and reliable for the determination of selectivity coefficients of liquid-membrane ionselective electrodes. The data in this work, determined for 41 univalent cations, will be useful not only for those who work with such ion-selective electrodes. The selectivity coefficient values enable one to calculate basic characteristics of ISEs under given conditions, and they give also information on the lipophilicity of the ions and their extractability into polar organic solvents. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
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