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Consequently, the standard Gibbs energies of transfer of these ligands from water to NB or 1,ZDCE are negative, AGtro(L) = -RT In P < 0, and those for complexed ions must be negative too, as one would expect for bulky hydrophobic ions. On the other hand, it is clear from Table I11 that the variations in transfer energies for both the complexed ions and the ligand must be considered in solvents of high polarity. A comparison of the cavity size of B218C6and the diameters of unsolvated ions shows that the optimal spatial fit is reached for the potassium cation (18,19). Actually, B218C6 exhibits the selectivity for K+ in polar solvents like water, methanol, dimethylformamide, or dimethyl sulfoxide, the selectivity sequence being K+ > Na+ > Rb+ > Cs+. However, the positions of Na+ and K+ are reversed in acetonitrile, and the same is true for nitrobenzene (Figure 6). In 1,2-DCE the selectivity sequence is quite different, Li+ > Na+ > K+ > Rb+, and it seems to follow mainly the change in the cation solvation; i.e. the cavity size effect no longer dominates. A similar behavior was found for the other two crown ethers studied (Figure 6). Thus, the results of the present study confirm the trends previously observed (18, 19) but show that in low polar solvents, inclusive of perhaps the hydrocarbon interior of biological membranes and their models, the desolvation and solvation processes play a key role.
LITERATURE CITED (1) Koryta, J.; Vanqsek, P.; Biezina, M. J. Elecfroanal. Chem. Interfaclal Nectrochem. 1078, 6 7 , 263. (2) Koryta, J.; Vanqsek, P.; Biezina. M. J. Electroanal. Chem. Interfacial Electrochem. 1077,75, 211. (3) Samec. 2.; MareEek, V.; Weber, J.; Homolka, D. J. Nectroanal. Chem. Interfacial Nectrochem. 1070,9 9 , 385. (4) Kakiuchi, T.; Senda, M. Bull. Chem. SOC.Jpn. 1083,56, 1322. (5) Kakiuchi, T.; Senda, M. Bull. Chem. SOC.Jpn. 1083,56, 1753. (6) Kihara, S.; Yoshitia, 2.; Fujinaga, T. Bunsekl Kagaku 1082,3 1 , 297. (7) Yoshida, 2.; Freiser, H. J. Nectroanal. Chem. Interfacial Nectrochem. 1084, 162, 307.
1015
(8) Yoshida, Z.; Freiser, H. Inorg. Chem. 1084,2 3 , 3931. (9) Lin, S.; Freiser, H. J. Electroanal. Chem. Interfacial Elechochem. 1085, 191, 437. (10) Lin, S.; Zhao, 2.; Freiser, H. J. flectroanal. Chem. Interfacial Electrochem. 1088, 210, 137. (11) Kihara, S.; Suzuki, M.; Maeda, K.; Ogura, K.; Umetani, S.; Matsui, M. Anal. Chem. 1088, 58, 2954. (12) Lin. S.; Freiser, H. Anal. Chem. 1087,59, 2834. (13) Yoshida, 2.; Kihara, S. J. flectroanal. Chem. Interfackl flectrochem. 1087,227, 171. (14) Kihara, S ; Suzuki, M.; Sugiyama, M.; Matsui, M. J. Nectroanal. Chem. Interfacial Electrochem. 1088,249, 109. (15) Koryta, J. Nectrochim. Acta 1088,33, 189. (16) Pedersen, C. J. J. Am. Chem. SOC.1067,8 9 , 2495, 7017. (17) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1088, 27, 89. (18) de Jong, F.; Reinhoudt, D. N. A&. Phys. Urg. Chem. 1080, 17, 279. (19) Izatt, R. M.; Bradshaw, J. S.: Nielsen, S. A,; Lamb, J. D.; Sen, D.; Christensen, J. J. Chem. Rev. lg85,85,271. (20) Hofmanovi, A.; Hung, Le Q.; Khalil, W. J. flectroanal. Chem. Interfacial Electrochem. 1082, 135, 257. (21) Homolka, D.; Hung, le Q.; Hofmanovi, A,; Khaiil, M. W.; Koryta, J.; MareEek, V.; Samec, 2.; Sen, S. K.; Vanfsek, P.; Weber, J.; Biezina, M.; Janda, M.; Stibor, 1. Anal. Chem. 1080,5 2 , 1606. (22) Samec, 2.; Homolka. D.; MareEek, V. J. Electroanal. Chem. Interfacial Electrochem. 1082, 135, 265. (23) Vanqsek, P.; Ruth, W.; Koryta, J. J. Nectroanal. Chem. Interfacial Electrochem. 1083, 148. 117. (24) Yoshida, 2.; Freiser, H. J. Nectroanal. Chem. Interfacial €/echochem. 1084, 179, 31. (25) Kakutani, T.;Nishiwaki, Y.; Osakai, T.;Senda, M. Bull. Chem. Soc. Jpn. 1088,59, 781. (26) Wandiowski, T.; MareEek, V.; Holub, K.; Samec, 2. J. Phys. Chem. 1080,93, 8204. (27) Samec, 2.; MareEek, V.; Colombini, M. P. J. €/echoanal. Chem. Interfacial Electrochem. 1088,257, 147. (28) Makriik, E.; Hung, Le Q. J. flectroanal. Chem. Interfacial flectrochem. 1083, 158, 277. (29) Wandiowski, T.; MareEek, V.; Samec, 2. Nectrochlm. Acta, in press. (30) Heyrovskq, J.; Kiita, J. principles of Poiarography; Publishing House of the Czechoslovak Academy of Sclences: Prague, 1965. (31) Marcus, Y. J. Solution Chem. 1084, 13, 599. (32) Takeda, Y. Bull. Chem. SOC.Jpn. 1083.56, 3600.
RECEIVED for review August 8, 1989. Revised manuscript received January 18, 1990. Accepted February 1, 1990.
Multiple Sensor Response in Segmented Flow Analysis with Ion-Selective Electrodes D. Brynn Hibbert,* Peter W. Alexander,* Sri Rachmawati, and Sylvia A. Caruana Department of Analytical Chemistry, University of New South Wales, P.O. Box I , Kensington, New South Wales, Australia 2033
Improved sensitivity in potentiometric analysis is achieved wlth multlple cells In a continuous flow system. Cells connected In series have been applied previously for Improved sensltlvHy In potentlometric detection, but only when the cell solutions are phydcally Isolated from one another. The novel use of air-segmented flow wlth appropriate cell design Is shown here to glve addltlve cell response, even though the cell electrolyte solutions are connected. A simple theory for two cells In serles Is developed to show that the total cell potentlal Is expected to be Ar$[(2a 2 ) / ( a 2 ) ] ,where Ar$ is the slngle cell potentlal and CY is the ratio of the resistance between sells to that within each cell. Experimentally,the sensltlvlty of a three-cell sensor system for detection of chloride ion is shown to glve up to 3 times the single Nemstlan slope. The detectlon limit In the sub-Nernstian region of the cell response Is 0.7 pM, an improvement of approximately 10 over the single cell system, and peak height measurements show a relative standard deviation of 1.5 YO for determination of a chloride sample of 20 pM concentration.
+
+
The response characteristics of ion selective electrodes in rapid flow air segmented analysis have been reported (I),with flow rates up to 9 mL/min required to produce very rapid electrode response. However, the sensitivity of the electrode detection method is limited by the Nernstian response characteristics of the potentiometric detectors sensitive to ionic species. A monovalent anion, for example, is theoretically detectable with a slope of 59.1 mV a t 25 "C, and an error of 1 mV in reading gives an approximate 4% error in ion concentration due to the logarithmic relationship with measured electrode potential. In flow analysis systems with ion selective electrode sensors, the sensor response is subject to a noise level created by the flow across the sensor surface where the noise level is often more than 1 mV. Hence, there is significant error expected in flow analysis with potentiometric detectors unless the noise level can be reduced or the sensitivity of the sensor response can be improved. Improvement in the sensitivity of potentiometric electrode sensors is therefore desirable in future developments on
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electrode technology, particularly when applied to automated analysis. With this aim in mind, an air-segmented flow analysis system has been developed (2) incorgorating a number of cells connected in series and based on a cell design used previously, for flow analysis, high-performance liquid chromatography (HPLC), and ion chromatography (3-6). When cells connected in series (CCS) are electrically isolated from each other, it has been shown (7-11) that for a set of N identical potentiometric cells the total emf is the sum of the emfs of each individual cell. As a result, the slope of the calibration plot for N cells connected in series is N times larger than that for a single potentiometric cell system. By definition, the sensitivity of a technique is the ratio of the change in the measured signal to the change in the concentration of the species under analysis. It follows, therefore, that the sensitivity of the CCS method must increase as a result of the increased signal. Theoretically, this increase in sensitivity should give a decrease in the relative standard deviation (RSD) of the millivolt signal, provided that the standard error of the signal does not increase. It has been shown both theoretically and experimentally by Parnewski and Stepak (8)that the relative standard deviation decreases when the CCS technique is applied. Hence the limit of detection, being dependent upon the relative standard deviation and the sensitivity, must also decrease. In the present study, the concept of using air segmentation to achieve the improved sensitivity and precision of the CCS method in a continuous flow system forms the basis for the development of an automated analysis system and is applied to the potentiometric determination of chloride. Previous flow-injection methods for the determination of chloride have been based upon the development of potentiometric techniques using ion-selective electrodes (12,13),but the sensitivity of these methods is limited by the response characteristics discussed above, especially so in the sub-Nernstian ranges. The segmented flow analysis system developed in this work is based on similar potentiometric flow cells developed for flow analysis, HPLC, and ion chromatography by Alexander e t al. (3-6), and more recently by Lockridge e t al. (12) who used small coated-silver electrodes in ion chromatography. In the present study, however, a multiple cell design was used in a segmented flow system, and the data obtained may then be compared with the flow-injection system described by Trojanowicz et al. (13, 14). Each cell consisted of small silversilver chloride indicator and reference electrodes. The system was interfaced to a microcomputer to collect, store and process the data. The interfacing of computers to automated ISE systems, as shown in many papers, for example refs 15-17, is of importance not only for the collection and processing of data but also for the control of the system. The aim of this paper is to investigate the response characteristics of multiple cells connected in series in a flow system that is air segmented. Any improvements in the sensitivity of this new potentiometric technique may then be observed by comparing the results obtained for single, double, and triple ion-selective electrode flow-through cells.
EXPERIMENTAL SECTION Instrumentation. The continuous-flowsystem used for ion determinations consisted of a peristaltic pump supplied by Watson-Marlow,Ltd., with 10-channels fitted with Tygon pump tubes of 1.0-mm internal diameter. Maximum flow rate through each pump tube channel was 1.2 mL/min. Potentiometric measurements were made with an Activon digital pH/mV meter supplied with an analog voltage output which was connected to a Houston Omniscribe strip chart recorder. The analog output from the mV meter was also connected to an amplifier with an offset controller and interfaced to an Apple IIe microcomputer via an eight-hit analog-to-digital converter. Data were acquired
Figure 1. Schematic diagram of the muhicell sensor showing three cel s connected in series to a mV meter and with three sets of eleG
trmes where 1 represents each indicator elenrode and R represents each reference electrode. in real time with the microcomputer and stored on floppy disk. The data were laler either printed out on a dot matrix printer or plotted out on a Watanahi X.Y IXgiplotter. Cell Design. The stnsing and reference electrodes used were silversilver lhloride clectrudes which were constructed from silver wire of U 5 mm diameter hy immersing in a 0.5M solution of ferric chloride accurding tu the mcthud descrihed by Hates (18). 'The electrodes were connected to the mV meter after fitting intn the flow cella descrihed previously 15.61.The electriden were fixed intu the flow-through cells as shown in Figure I , using Omnifit connectom threaded tn screw inlo the flow cell LO prevent leakage. The reference electrodes used were also silversilver chloride electrodes fitted into the flow cell in the same way except that a plug uf agar gel, prepared with 1 M KNOl solution, was used to isolate the electrode irum the flowing sulution. In the multiple cell design, the rells were connected in series. and the uuter electrdes were wnnecrpd t o the indiratnr and reference terminals of the mV meter, as shown in Figure 1. The flow-through cells were constructed frum Perspex hlocks 5 x 3 x 1 cm in dimensions in such a way that the indicator electrode and reference electnde were at right angles to each uther and approximately 1 rm apart. The inlet and outlet ports fur the flowing solution were also cumtructrd at right angles tn the electrode positions. The multiple fluw cell system was then assemhled a5 shown in Figure 1 simply by connecting the outlet of one cell to the inlet of another cell ofthe same design. The length of tuhing between each cell was one ofthe critical factors in determining the total cell response, and waF fixed at 20 rm fnr routine measurements of the cell potential. Reagents. All reagents were Analytical Reagent grade. and water was MilliQpurified. The chloride sample solutions varied in cmcentration from 1 x IO" to 2 x 1W M and were prepared
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1017
Time (min.) Flgure 2. Peaks recorded for different concentrations of chloride solutions introduced into the flow system with sampling and wash times of 20 s, and the following concentrations of chloride: (a) 10, (b) 20, (c) 50, (d) 80, (e) 100, (f) 200 gM.
by serial dilution of a stock solution of 0.1 M KCl in water. The blank reagent stream was 0.1 M KNO, prepared in water. Procedure. The blank reagent stream consisting of 0.1 M KNO, was initially pumped through the flow system shown in Figure 1 at a flow rate of 0.64 mL/min. After a steady base-line potential was observed on the chart recorder set at 100 mV full scale, chloride sample solutions were pumped into the system with a blank reagent wash between each chloride sample. The flowthrough system was operated initially in the single-cell mode, and then two-cell and three-cell modes. Between each sample and wash solution, an air segment was pumped into the system to separate the sample from the reagent wash solution. The sampling was performed manually without computer control by moving a plastic sample tube from wash to sample solutions and back again with approximate timing using a stop watch. Sample and wash times were usually 20 s with a 5-s air segment between each sampling and wash period. Longer segmenting times resulted in excessive noise on the recorder output. Figure 1shows a schematic diagram of the segmented flow system. During the sampling operation, the cell potential in millivolts was monitored continuously on the strip chart recorder and also by data logging using the microcomputer. Calibration plots and peak precision measurements were recorded for the three modes of cell operation.
RESULTS This study,was aimed at establishing the feasibility of using a multiple flow cell design in order to achieve the cells-in-series response observed by Stepak (7) who used a static solution system in which the cell solutions were connected only by the electrical connections between the electrodes. The concept here was to use air segments in the flowing stream to isolate electrically each flow cell in the multiple cell system. The series response might then be achieved. If the cells were not electrically isolated during sampling, then only single-cell response should be observed. Response Calibration. Figure 2 shows the shape of the peaks for the three-cell system, acquired in real time with the microcomputer. The peaks were recorded for chloride sample to 2 x solutions varying in concentration from 1 x M. A distinct shoulder was observed for each cell connected in series, and a peak occurred at the point where the blank wash solution reached the first flow cell. There was some noise observable on the millivolt output when the air bubbles passed the electrodes, but insufficient to cause serious peak distortions. Similar peaks were observed with the two- and one-cell modes of operation and with the number of shoulders equivalent to the number of cells connected to the mV meter. Hence, only a single peak was observed with the one-cell system and the peak height was equivalent to the first shoulder shown on the peaks in Figure 2.
Figure 3. Calibration plots for chloride solutions showing sample peak heights (mV) as a function of chloride concentration (M) introduced into the multicell configuration connected in series with (a)three cells, (b) two cells and (c) one cell.
Figure 3 shows the calibration plots observed when the peak heights are plotted against the chloride concentrations in the M for the one-, two-, and three-cell range 1 X IO4 to 2 x systems. The slopes for the linear portions of these plots were 56, 118, and 174 mV per decade change in chloride concentration, respectively, in close agreement with the slopes expected from the Nernstian constant, N X 2.303RT/F, where N is the number of cells connected in series. The sensitivity of the sensor system to chloride ion is therefore improved by use of the air-segmented flow method, which achieves the same result as for the static systems (7-11) where the improvement factor is equal to the number of cells connected in series. Response Shape. The effect of the sample front after the initial air segment reached the first electrode was that the observed electrode potential initially increased to a steadystate value equivalent to the single-cell potential. When however the air segment passed through the second cell, a second air segment now isolated the cells from each other. Hence the observed potential increased and a second shoulder was observed. This effect was repeated for each cell connected. It was found that only a single air bubble was required between each sample and the following wash solution. If however the cells were too far apart, then the observed potential changes were lower than Nernstian. The flow rate, tubing diameter, and tubing length between cells affected the response. In the present design, it was found that the theoretical Nernstian response could be achieved with a slow flow rate of 0.6 mL/min, and tubing of 20 cm length and 0.5 mm internal diameter. An increase in length to 40 cm caused a marked decrease in the potential change observed for the second and third cells connected in series. Response Time. The above design factors also control the rate of response of the cell system. The flow rate used here of 0.6 mL/min gave a response time of approximately 5 s for the potential reading to reach each shoulder. The response was therefore sufficiently rapid for a shoulder to be observed for each cell in series when using a 20-s sampling time with 5 s for air segmentation between each sample and wash. The total peak width depends on the total sampling, wash, and segmenting times. The response time for the cell design used here was approximately 90 s, as shown in Figure 2. The response time can be improved by using faster flow rates, but ultimately at the expense of poorer sensitivity. At
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990
Table I. Precision of Replicate Peak Measurements for a Three-Cell System chloride peak heights, mV 2 x 10-4 M 5 X lo6 M
mean
SDa RSD,b o/c
178 176 178 174 174 172 172
15.5 14.3 14.3 13.7 14.9 12.7
175 2.5 1.5
14.2 0.98 6.9
Standard deviation. Relative standard deviation.
where R1 and R2 are the intracell resistances, R3 is the intercell resistance (representing the "short circuit" resistance), and RM is the resistance of the meter. The small leakage current, i, is partitioned between the two cells in series (il) or the short-circuited cell (iJ. The terms represented by 41 and d2 are the Galvani potentials of the indicator and reference electrodes, respectively. Connection of two electrodes in the same electrolyte will cause a galvanic cell to be established with the potentials at each electrode deviating from their equilibrium values by the overpotentials, q1 and T ~ .In the multicell device discussed here, if the resistance of the solution between the connected electrodes is high, then vl = 0 and o2 = 0, and we assume that the cells are identical. The cell voltage is then obtained by summing the potentials through R1 and R2 through R3, and through the meter. The cell voltage, V , can be expressed in terms of the following three equations:
The leakage current, i, is given by
i = il
+ i2
(4)
Equations 1-4 may then be solved for V as a function of - &) and R,, RZ,R3, and RM giving
1 i2
Rl
I
'
R2
V=
I
a flow rate greater than 1 mL/min, the shoulders disappeared and the total peak height decreased due to the shorter time of exposure of the electrodes to the sample solution. We kept the flow rate at 0.6 mL/min for this study in order to indicate the response shoulders of each individual cell in the system. Precision and Detection Limit. The precision of the air-segmented method was tested for chloride concentrations of 2 x 10"' and 5 x lo4 M, respectively. Considerable noise was observed in the latter case, where the amplifier gain used to acquire the data was increased. However, the peak shapes were reproducible and the peak heights and standard deviations were determined, as shown in Table I. The results gave relative standard deviations of 1.5% and 5.4% respectively for the two concentrations tested. The detection limit calculated in Table I from 20 for the low concentration was 7 X M. This is approximately a 10-fold improvement over the recognized detection limit for the silver-silver chloride electrode (13, 14).
DISCUSSION In previous work describing the cell-in-series concept (7-111, it has been assumed that additivity is not possible unless the individual cells are completely electrically isolated. However, this present study has established that there are two limiting cases representing the extremes of a continuum in which the cells are (a) isolated completely from each other, and (b) in which they are close together in the same electrolyte. These extremes may be characterized by the ratio of the intercell resistance to the intracell resistance in the overall multicell design. An analysis of two cells connected in series may be made in terms of a network of resistances, as shown in Figure 4
(2R3 + R , + R J R M + Rl(R3 + R M )+ Rz(R3 + R M ) R~RM
(@l
(5)
where
J.4 = 41 - 42 Since the resistance of the meter, R M ,will be in megaohms, i.e. several orders larger than the solution resistances R1,RP, R3, we can write 2R3 + R 1 + R2 V = R3 + R1 + R2 A 4 In the case of matched cells (R, = R,) and writing a = R3/Rl = &/Rz
(7) Thus, a is the ratio of the short-circuit resistance to the internal cell resistance. The two limiting cases referred to above 0 (Le. complete short occur: (a) if R3 > R1 then a m (i.e. complete isolation of the two cells) and V = 2A4. For example, if a = 10, then V = 1.83A4 which would lead to a Nernstian slope of 108 mV for a one-electron process. A plot of V/&J against a is given in Figure 5 showing the relationship expected from eq 7 as the intercell resistance increases. The above analysis indicates that a device may be constructed in which there is a high resistance path between the cells, even if they are not isolated, as shown in the design used in this study (Figure 1). A detector using narrow-bore tubing to create a large resistance between cells to maximize R3 and with electrodes close together to minimize R, will show additivity according to eq 7 . The continuous flow multicell design may thus be applicable in a flow injection experiment without air segmentation, and a full analysis of cells with from 2 to 6 indicator electrodes will be published elsewhere. The concept may also be applicable to large numbers of cells in series and also to cells with different types of selective sensors connected in series. By use of computer methods for Calibrating the partly selective response of a variety of sensors ( I 7 ) , multicomponent determinations may thus become a
-
-
Anal. Chem. 1990, 62, 1019-1021
2.5
-
2.0
-
V
=
“y 1.0
0.5
0.0
F
0
to
60
40
80
LOO
a
Flgure 5. Relationship between measured cell voltage ( V I A @ )and the ratio of the short-circuit response to the internal cell resistance (a).
reality with cells connected in series.
LITERATURE CITED (1) Alexander, P. W.: Seegopaul, P. Anal. Chem. 1980, 5 2 , 2403-2406. (2) Alexander, P. W.; Hibbert. D. B. Australian Patent No. PI7728, April 13, 1988. (3) Alexander, P. W.; Maitra, C. Anal. Chem. 1981, 53, 1590-1594.
1019
(4) Alexander, P. W.; Haddad, P. R.; Maitra. C.; Lowe, G. J . Chromatogr. 1981, 209,29-39. (5) Alexander, P. W.; Haddad, P. R.; Trojanowicz, M. Anal. Lett. 1984, 77, 309-320. (6) Alexander, P. W.; Haddad, P. R.; Trojanowicz, M. Chromatographk, 1985, 2 0 , 179-186. (7) Stepak, R. Fresenius’ 2.Anal. Chem. 1983, 375, 629-630. (8) Parczewski, A.; Stepak, R. Fresenius’ 2.Anal. Chern. 1983, 376, 29-31. (9) Stepak, R . Fresenius’ 2.Anal. Chem. 1987, 328, 288. (IO) Suzuki, K.; Tohda, K.; Shirai, T. Anal. Lett. 1987, 2 0 , 1773-1779. (11) Stepak, R. Anal. Lett. 1988, 2 7 , 1945-1946. (12) Trojanowicz, M.; Matuszewski, W. Anal. Chim. Acta 1982, 738, 71-79. (13) Trojanowicz, M.; Matuszewski, W. Anal. Chim. Acta 1983, 757, 77-84. (14) Lockridge,J. E.; Fortier, N. E.; Schmuckler, G.;Fritz, J. S . Anal. Chlrn. Acta 1987. 792,41-48. (15) Simeonov, V.; Malissa, H. Fresenius’ 2. Anal. Chem. 1977, 287, 37-42. (16) Ariano, J. M.; Gutknecht, W. F. Anal. Chem. 1978, 48, 281-287. (17) Otto,M.; Thomas, J. D. R. Anal. Chem. 1985, 5 7 , 2847-2651. (18) Bates, R. G. Determination of pH. Theory and Practice; Wiley and Sons; New York, 1965; D 279.
RECEIVED for review October 13, 1989. Accepted February 15,1990. The authors are grateful for financial assistance from the Australian International Development Assistance Bureau in support of S.R. and for grants from the Special Research Grant Committee and UNISEARCH, Ltd., University of New South Wales.
Verification of the Approximate Equitransference of the Aqueous Potassium Chloride Salt Bridge at High Concentrations Paolo Longhi,* Fernando D’Andrea, Patrizia R. Mussini, Torquato Mussini, and Sandra Rondinini
Department of Physical Chemistry and Electrochemistry, University of Milan, Via Golgi 19, I-20133 Milan, Italy
Use of chlorine electrodes In the transference cell Pt-IrJC12)KCiaq, m,I)KCI aq, m ,lCi2)Pt-Ir, whose electromotive force was measured at 298.15 K at various KCI molalities m2 with fixed m allowed the cation and the water transference numbers, TK+and T-, respectively, to be determined up to the KCI saturation molality. The observed TK+and T , values are In good accord with the overall and single-ion primaryhydration numbers of KCI. I t turns out that TK+Is subdantlally constant at 0.486 In the usual molality range employed for KCI as a salt brklge in electrochemistry and electroanalysis. This imperfect equltransference of the popular, aqueous KCI salt bridge causes imperfect efficacy of the latter in minimizing liquid junction potentials and may, in turn, enlarge the uncertainty limits of analysis results. I t is, thereby, suggested that when high-ionic-strength, strongly acidic or strongly alkaline samples are submitted to pH-metric, plon-metric, or titrimetric analysis, the KCI salt bridge be replaced by the closely equitransferent (T-+ = 0.502) CsCl salt bridge, whose saturation molality is, furthermore, as high as 11.3 mol kg-‘ at 298.15 K.
INTRODUCTION Aqueous potassium chloride (KC1) a t concentrations of 0.001, 0.01, 0.1, and 1 mol dm-3 is well-known as the standard material for reference in conductometric determinations. Yet,
its greatest renown and popularity came from its applications as a salt bridge in pH-metric, pIon-metric, and titrimetric determinations where, however, it is normally used a t higher concentrations ( I , 2), e.g. 1, 1.75, 3.5, 4, and 4.804 mol kg-’, the last figure denoting the saturated KC1 at 298.15 K, so often associated with the equally popular HgZCl2(calomel) reference electrode. The present widespread choice of aqueous KC1 relies on the fact that it possesses to a good degree the essential qualifications of a salt bridge in terms of a concentrated equitransferent binary salt, a principle that rests on the evidence provided chiefly by Bjerrum’s (2,3)and Guggenheim’s (2, 4-6) experiments. The saturation concentration of KC1, though good, is not comparable with that of other equitransferent salts such as CsCl (11.3 mol kg-’ at 298 K) and constitutes a limitation to its use when in contact with sample solutions of high ionic strengths, especially strongly acidic or strongly basic. It is, however, surprising and disturbing that the KCl equitransference, which also is good but not excellent, was hitherto ascertained only for the range of low concentrations, leaving the range from 1mol kg-’ to saturation still unverified, in spite of the widespread and blindly confident use of KC1, taking for granted that KC1 would become exactly equitransferent upon approaching its saturation concentration. Such a verification is desirable and overdue, and it prompted the present investigation. The experimental basis was furnished by measuring the electromotive force (emf) E of the following concentration cell with transference
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