henomena at Glass Electrodes 6. A. Rechnitz Department of Chemistry, State University of New York, Buffalo, N . Y. 14214
.
C. Kugler Department of Chemistry, University of Pennsylvania, Philadelphia, Pa. 19104 The emf vs. time behavior of two types of cation-sensitive glass electrodes was studied with a newly-designed apparatus involving the rapid-mixing, continuousflow technique. The data indicate that the electrical characteristics of the electrodes are rate-limiting at times 510 mseconds, but also reveal a slowing of electrode response in certain binary mixtures. A pronounced transient response is found for several ions, including Ca+2and Sr+2,for which the glass electrode displays no appreciable equilibrium selectivity. A tentative explanation for the observed effects is proposed.
THECURRENT THEORIES of glass electrode response (1) do not explicitly treat the time dependence of the electrode potential. Only the slow changes in “asymmetry” potentials (2)and other potentials arising from structural modifications of the hydrated glass layer as a function of time have been subjected to careful study. An earlier theoretical expression of Rechnitz and Hameka (3) has been successful in describing the time course of such slow potential changes for several cation-sensitive glass electrodes. If cation-sensitive glass electrodes are to be used effectively to monitor rapidly changing reaction systems ( 4 ) and to carry out continuous analyses (5), it is necessary to have information regarding both the response time and potential Z.S. time course of such electrodes when subjected to a change in ionic environment. Preliminary studies (6, 7) of commercial cation-sensitive glass electrodes have shown that a more or less steady potential is established within a few seconds after a sudden change in ion activity, Because of the relatively rapid establishment of the potential, experimental methods must be devised which will allow the electrode to be exposed to a reproducible change in ionic environment within a few milliseconds. This paper describes the design of such an experimental system and summarizes the emf cs. time characteristics of two widely used types of cation-sensitive glass electrodes under varying solution conditions. The results should be useful in determining the practical application limits of such electrodes in rapidly changing systems. EXPERIMENTAL
Reagents and Solutions. The lithium, sodium, and potassium salts used in this study were of reagent grade (Baker Analyzed, obtained from Baker Chemical Co., Phillipsburg, N. J.). Their solutions were prepared by accurately weighing appropriate amounts of the dried salts and then diluting to (1) “Glass Electrodes for Hydrogen and Other Cations,” G. Eisenman, ed., Marcel Dekker, New York, 1967, Chaps. 4-7. ( 2 ) W. H. Beck, J. Candle, A. K. Covington, and W. F. K. WynneJones, Proc. Chem. SOC.,1963, 110. (3) 6. A. Rechnitz, and H. F. Hameka, 2.A n d . Chem., 214, 252
(1965). (4) J. E. McClure, and G. A. RecHnitz, ANAL.CHEM., 38,136 (1966). ( 5 ) H. Jacobson, Ibid.,38, 1951 (1966). (6) G. Mattock, Analyst, 87,930 (1962). (7) 6. A. Rechnitz, Tulanru,11,1467 (1964). 1682
m
ANALYTICAL CHEMISTRY
volume. Lower concentrations were realized by quantitatively diluting the highest concentration solutions. Silver and ammonium salts were handled in the same manner as the alkali metal salts, and were obtained from Baker Chemical Co. (Baker Analyzed). In all cases, except for silver ion, the anion was chloride. With silver ion present in the test solution, nitrate was used as the anion. pH Control. In many cases, buffering or pH control was not required in view of the favorable selectivity ratios. When buffering was required, however, 2-amino, 2-(hydroxymethyl)-l,3-propanediol (primary standard grade, obtained from Fisher Scientific Co., King of Prussia, Pa.), which has the trivial name tris-(hydroxymethyl) amino methane, or THAM, was added to the proper amount of hydrochloric acid to yield a buffer with a pH of 8.9. Tetraethylammonium hydroxide (obtained from Eastman Kodak, in the form of a 10% aqueous solution) was used to raise the pH of test solutions when necessary. Glass Electrodes. All electrodes used in this study were obtained from Beckman Instruments, Inc., Fullerton, Calif. Most steady-state measurements were made with the 39137 “Cationic Glass Electrode,” which has a length of 5 inches, and a lead length of 30 inches. The exact composition of the glass has not been given by the manufacturer, although it is acknowledged to be a sodium aluminum silicate glass. Occasionally the 39278 “Sodium Ion Glass Electrode” was used when sodium measurements were made. Response time measurements were carried out with Beckman micro electrodes Nos. 39046 and 39047. The 39047 has a glass composition identical with the 39137, and is hereafter referred to as the “cation-sensitive” glass electrode. The 39046 is a sodium-responsive glass composed primarily of lithium aluminum silicate, like the 39278 electrode. These micro electrodes are 2.5 inches long, with a lead length of 30 inches, and are provided with a tapered ground glass joint which mounts into the Beckman Micro Blood Assembly (see below). Measurements of pH were carried out with a 41263 glass electrode, which has an effective pH range of zero to 11. Reference Electrode. In all cases, the reference electrode used was a Beckman 39170 saturated calomel electrode (SCE). This model is of the restricted fiber junction variety, and only those particular electrodes with a very low leak rate were utilized. Leak rate tests were carried out by immersing the electrodes in a 1M AgN03 solution and then observing the formation of AgCl due to the leakage of saturated solution from the electrode. Individual cases ranged from those which gave no visual formation of Age1 after several minutes of immersion to those which immediately displayed a heavy “tail” of AgCl. The SCE used in the response time measurement work was modified by removing the unshielded cable with which it was equipped, and replacing it with 30 inches of Belden 8259 shielded cable, This was done to reduce possible pick-up of extraneous electrical signals fed into the electronic rneasuring and recording system. Salt Bridges. During steady-state emf measurements, the SCE was immersed directly into the test solution, unless Ag+ was present, The distance between the tip of the SCE and glass electrode was kept to a minimum and rigidly maintained during the series of measurements.
F1ow System
Shorting loop C liolotrd from gnd.)
\
High I Sprrd Elrotromrtrr
\/
\ //
Charnirr
Glarr Elrotrodr
Figure 1. Schematic of rapid-mixing continuous-flow apparatus The flow system apparatus used for the fast emf-time measurements necessitated the use of a bridge between the SCE itself and the flowing stream. This bridge was kept filled with concentrated (almost saturated) aqueous KC1, and was restricted at the end connected to the stream by a sand-blasted palladium wire fused through the glass. When Ag+ was measured, however, the KC1 solution was replaced by 1M KN03. Whenever Ag+ was measured outside the flow system, a 1M KNO, bridge was also used. This bridge was fabricated by modifying a Leeds and Northrup 1199-31-B salt bridge tube with a limited flow glass junction by attaching a tube to the top of sufficient diameter to accept the Beckman SCE. TIME INDEPENDENT (STEADY STATE) MEASUREMENTS-EQUIPMENT AND TECHNIQUE Electronic Instruments. The principal device used to measure the emf between the glass electrode and the SCE was a Beckman Model 101900 research pH meter. This instrument is essentially a line-operated electronic potentiometer of null-balance operation. It incorporates an acstabilized solid-state amplifier with a range of -1400 to +1400 mV. The readout is graduated in increments of 0.2 mV, and can be easily read to the nearest 0.1 mV. The input is chopped at line frequency to provide an alternating-current carrier for the amplifier circuit. A standard cell is incorporated into the input circuit which can be used as a secondary input to calibrate the readout. This gives a relative accuracy of ~ t 0 . 3mV when considered over the entire millivolt range of the instrument. The balancing network in the circuit is powered by two series-connected mercury batteries. pH measurements were made either with the above instrument, which is capable of measuring to the nearest 0.002 pH unit, with a Beckman Model 76004 Expandomatic pH meter, or a Beckman Model 9604 Zeromatic pH meter. These measurements were made either to check the background pH or to prepare buffers, and did not demand great accuracy.
TIME DEPENDENT (RESPONSE TIME) MEASUREMENTS-EQUIPMENT AND TECHNIQUE Flow System. During early attempts to measure the response time of glass electrodes when subjected to a concentration change of electroactive species, it became obvious that one of the main difficulties existed in effectively producing the concentration change itself in a short time. Transferring the electrode from one soIution to another is not feasible as the interval between immersions would break the circuit, in addition to destroying solvent equilibrium at the electrode-solution interface. Concentration changes effected by dumping or otherwise injecting solution of a different concentration into the test solution proved too slow for many of the systems which were to be studied. The use of syringes to exchange solutions is also electronically noisy, and produces unwanted signal artifacts. Dumping was used, however, for the certain systems amenable to slower techniques. Figure 1 shows a flow system designed and built specifically to achieve a rapid concentration change of electroactive species about the responsive portion of a given electrode. Chamber A was filled with a solution of a given concentration, C1, while chamber B was filled with a solution containing the same species, but at different concentration, CZ. Stopcocks I and 2 were closed, 3 and 4 opened, and the three-way stopcocks 5 and 6 were set to allow passage of the solutions along the route of the dashed lines. The two streams meet the mix at the mixing chamber, which is a specially-designed glass stopcock (see Figure 2). This apparatus, the heart of the system, allowed one of the streams to be interrupted rapidly at the very point of mixing. The proximity to the actual location of mixing is critical, as any “dead volume,” which would be present if the stopcock plug was located at a distance from the mixing site, would produce a gradual “washout,” and thus prevent a clean, sharp concentration change upon termination of the flow. During mixing, the concentration becomes approximately VOL. 39, NO. 14, DECEMBER 1967
1683
cE1 n
BORE
=
Imm
MICRO
BLOOD ASSEMBLY
TO S C E
Figure 2. Detail of mixing chamber
+
+
+
e
ANALYTICAL CHEMISTRY
P
Figure 3. Schematic of flow assembly showing electrode positions
(CI C2)/2, and this solution then flows past the electrode to be tested and the connection to the SCE reference electrode. The glass tubing from the SCE connection back to a few inches either side of the mixing chamber is heavy-walled capillary, with a uniform bore of 1 mm. Chambers A’ and B’ serve as reservoirs to facilitate the recharging of containers A and B once the solution in them has been depleted. The system was connected through a regulator and a safety valve to a nitrogen tank and put under a pressure of 25 psi to assure a rapid and reproducible flow rate. Upon turning the stopcock affiliated with the mixing chamber from the open to the closed position, the stream passing the electrode C2)/2 to Cz. undergoes a concentration change from (C, By resetting the three-way stopcocks 5 and 6, the solution from vessel B (C2)may be turned off at the mixing chamber, which then gives a concentration change from (CI C2)/2to Cl. Because C1 z Cp, this enables the operator to effect a concentration increase or decrease at will. The portion of the system below the mixing chamber was purchased from Beckman Instruments, Inc. (Micro Blood Assembly No. 14846) and carefully fused onto the outlet from the mixing chamber. A detailed picture of this assembly is given in Figure 3. The electrode socket is a female groundglass T 7/25 joint, which accepts the micro electrodes previously described, The bottom of the electrode socket is designed to match the shape of the electrode end, assuring a very small “hold-up” volume of solution about the electrode itself. Connection to the SCE salt bridge is effected by a sand-blasted palladium wire fused through the glass, which allowed electrolyte solution wetting from the salt bridge to the flowing stream, with no appreciable leakage. The SCE salt bridge and the electrode receptor compartment were both shielded by Faraday cages and these cages were electrically connected to the outer shield of the cables leading to the SCE and the glass electrode. This circuit was then grounded to the measuring devices and a common ground. Such shielding reduced extraneous ac signals generated by various devices in the laboratory, and thus increased the signal-to-noise ratio to an acceptable level. The solution in the flow system was not grounded, and the outlet tube was placed high enough from the spent solution receiver to allow the stream to break up in the air before reaching the receiver. This effectively isolated the solution from ground. It was found that this was necessary to eliminate spurious signals from the solution induced by charging effects. As can be seen in Figure 1, a shorting strap was connected to the solution to points in front of and behind the electrodes. Connection to the solution was made by imbedding platinum wire through the walls of the flow tubes in the locations shown. This shunt was shielded, and the shield connected to ground. The purpose of this shorting strap was to reduce any streaming potential effects about the electrodes. Portnoy, et al. (8) have devised an elaborate flow system involving a 7 684
ELECTRODE
TO MlXlWG CHAMBER
pressure-resistant nonmetallic conductive membrane tube (carotid artery of dog) connected in series with the cation electrode and surrounded by saline solution to minimize the streaming potential, which they claim is enhanced by any porous plug type connection to the reference electrode. Meier and Schwarzenbach (9) found good agreement between readings of a glass electrode in both flow and static systems if the conductance of the solution is high. This last finding has been borne out in our work, and it has been found that with the flow system described above, streaming potentials do not cause any troublesome flow artifacts as long as the ionic strength of the solution is maintained above approximately 5 X 10-3M. Below this value charging is observed, and changes in ionic strength bring about changes in the streaming potential. Electronic Instruments-Fast Times. The signal from the glass electrode-SCE couple was fed into a Keithley Model 610B Electrometer. This instrument is capable of measuring wide ranges of dc voltages, currents, resistances, and charges. It is a highly improved form of a conventional vacuum tube volt meter that uses an electrometer tube input to provide greater than lO1*-ohm input resistance; thus it can make measurements without loading circuits. It was used primarily to measure the voltages (emf) of the electrode cell, and to provide an output to a recording device, although its resistance measuring capability was used to compute the R C time constant of the system. Its response speed was very high, as it does not incorporate a chopper-operated circuit. When measuring emf, the electrometer exhibits an accuracy of I1 full scale, average noise of =t10 pV, and an average drift of less than 200 pV per hour. The scale was set on the 100-mV full scale setting for all flow system measurements. The amplifier output was 3 volts for a full scale input. The output from the electrometer was fed through a shielded cable to a Tektronix Type 564 storage oscilloscope equipped with a 3A3 dual-trace differential amplifier and a 3B3 time base generator. This combination provided a means of recording on the oscilloscope screen a single sweep of input voltage us. time. The display could be stored for viewing, photographing, or copying up to at least an hour after application of the input signal, and erased at any desired time. The time b’ase was capable of sweeping at calibrated rates from 1 second per cm (on the CRT graticule) to 0.5 psecond per cm. In practice, the stored display was copied from the 10- X 8-cm screen onto tracing paper placed over the graticule. The oscilloscope was programmed to trigger when the electron beam crossed a preselected period on the vertical (8) H. D. Partnoy, L. M. Thomas, and E. S . Gurdjian, J. Appl. Physiol., 17, 175 (1962). (9) J. Meier, and G. Schwarzenbach, Helv. Chim. Acta, 40, 907 (1957).
(emf) axis of the screen. This triggering resulted in a recorded single sweep of emf us. time. In practice the electron beam was set at a major division on the CRT graticule while the flow system was running. If a concentration decrease was expected, a division toward the top of the screen was chosen, and the oscilloscope was programmed to trigger on a negative slope (downward) mode when the beam reached a point immediately below the original rest position. Amplification and sweep rate were chosen to yield a pictorial representation of the desired portion of the waveform. For calibration, the sensitivity of the oscilloscope amplifier channel used was adjusted so that the vertical displacement of the electron beam relative to the graticule and dial settings exactly corresponded to the expanded output from the Keithley electrometer when a known input signal was applied. The sweep rates of the oscilloscope time base are typically within 1 and in all cases within 3 of the timeldivision switch settings, This degree of accuracy, when coupled with that of the Keithley electrometer, gave an overall error maximum of j=4z of the input signal. Electronic Instruments--Slow Times. When the response times to certain solutions did not warrant the use of the storage oscilloscope--e.g., when 90 of the equilibrium value was reached in times greater than 5 or 10 seconds-the Beckman Research pH Meter was used as an amplifier, and a strip chart recorder (Photovolt Linear/Log Varicord Model 43) was used to register the results. This system was also used to measure the final equilibrium value of the emf for some fast-time systems. In most cases the sensitivity control of the p H meter was set at about 70z of maximum, and the amplification of the recorder was adjusted so that a 50-mV input signal to the pH meter caused a full-scale displacement of the recorder pen. Electrode Handling and Measurement Procedure. The operation of the flow system and allied electronic equipment is described above. The micro electrodes were preconditioned for a period of at least 24 hours in 0.1M solutions of the ion to be studied. Electronic instruments were warmed up for at least an hour for stabilization. The electrometer and research pH meter were left on continuously in the standby mode. Thermostating was not used because of the large volumes expended by the system, and the ambient temperature varied from 25’ to 30” C. Both arms of the system were opened, so that mixing occurred, and a concentration change was not effected until a stable output was observed. In practice, several single sweeps were recorded on the oscilloscope, and jn all cases these perfectly superimposed on the screen. When the flow system was used for slow-time measurements, the stream was left flowing for at least 3 minutes after the concentration change. Several slow-time measurements were also made by reducing the concentration of a solution outside the flow system. A 25-ml portion of a solution of the exact concentration of that which eminated from the flow system during mixing was placed in a container, and a sufficient volume of solvent was then dumped in to yield a concentration decrease identicai with that of the flow system. In such instances the pH meter and Photovolt recorder were used. The container was conically shaped, with a flattened bottom which accommodated a 7/&1ch Kel-F coated magnetic stirrer revolving about 450 rpm. This particular form was chosen to yield a small change in height of solution when the volume was increased. It was necessary to prevent the relatively short micro electrode from being completely inundated by the solution, and at the same time to permit the original 25-ml portion to cover effectively both the glass and reference electrode tips. Mixing was particularly effective in the container, and runs were made on a known fast system to ascertain that mixing times would not adversely affect the measurements. A fiber junction SCE was used as previously described. Several systems were compared utilizing both the flow and dumping methods with identical results.
z
z
z
-__.. 1
b
110
A0
120
160
2h) msec
Figure 4. Response to voltage step change (a)
Electronic circuit alone
(6) Circuit plus “sodium-sensitive” glass electrode (c) Circuit plus “cation-sensitive” glass electrode
Calibration of Flow System. The limiting response characteristics of the circuitry were tested by applying a rapid dc voltage step of 38 mV to the input stage of the measuring system. The resulting potential us. time plot was recorded on the oscilloscope and is reproduced in Figure 4a. It can be seen that the monitoring system has a response time of well under 1 msecond and, therefore, will not be the limiting factor in the electrode response experiments. The limiting RC time constant curves for the “cationsensitive” and “sodium-sensitive” glass electrodes were determined by applying the same 38-mV potential step in series with the glass electrode-test solution-SCE system shown in Figure 1. The resulting curves are shown in Figure 4c and 4b, respectively. The difference between curves b and c is due to the difference in electrical resistance of the two types of glass electrodes used. The dc electrical resistance of the glass electrode-solution-SCE system was found to be 3.2 X lo7 ohms with the “cation-sensitive” glass electrode in the system and 3.2 X 108 ohms with the “sodium-sensitive” glass electrode in place. The difference in the R C time constants is, thus, in accord with the difference in electrode resistance. It will be seen below that the electrode response time is limited by this factor in many simple measurement situations. The possibility of spurious signals resulting from the mechanical operation of the flow system itself was checked by filling both sides of the system with an identical solution (0.01M KCI) and monitoring the cell emf during operation of the mixing chamber stopcock. No signals greater than the usual noise level (0.5 mV, max.) were noted. The net volume of the flow system after the mixing chamber was measured with a micrometer operated syringe. The volume from the mixing chamber to the sensitive portion of the glass VOL. 39, NO. 14, DECEMBER 1967
0
1685
0.0538g X t
-+
period of several months. In all cases, curves obtained in the direction of concentration increase were mirror images of curves obtained in the direction of concentration decrease for solutions containing a single potential determining cation. To test the overall effectiveness of the total measuring system, a silver metal electrode of similar geometry was substituted for the glass electrode in the flow system and subjected to a sudden fivefold change in Ag+ concentration. The resulting emf US. time traces recorded on the oscilloscope yielded an average value for t l / 2 (the time required to achieve one half of the expected Nernstian emf change) of approximately 10 mseconds. We, thus, take this time to be an estimate of the time required to effect a complete concentration change at the sensitive tip of the indicator electrode in our flow system and regard 10 mseconds as the lower limit for meaningful measurements with the present geometry and mixing characteristics of the system.
0.0100s K
N s Glass Electrode
I
RESULTS AND DISCUSSION
From the experimental limitations set by the RC time constants of the glass electrodes, on the one hand, and the mixing time of the flow system, on the other hand, it is clear that no meaningful response data can be obtained at half-times shorter than about 10 mseconds and about 15 mseconds, respectively, for the “cation-sensitive” and “sodium-sensitive” glass electrodes. With these limitations in mind, an extensive series of experiments was carried out where both types of glass electrodes were subjected to a sudden change in activity of the ions Naf, K+, Li+, Ag+, NH4+,Ca+z, and Srf2, both individually and in various mixtures.
t--+
I
0
1
I
2
I 3
Figure 5. Transient response of “sodium-sensitive”glass electrode to sudden change in K+ activity (0.0538 to 0.01OOM)
electrode was found to be 87.5 pl, while the volumes required to cover completely the sensitive portions of the glass electrodes were 28.0 pl for the “cation-sensitive” electrode and 19.0 p1 for the “sodium-sensitive” electrode. The small volume difference is due to imperfections in the geometry of the electrode tips. The flow rate of the system was determined by measuring the volume of solution expelled, under a constant Nt pressure of 25 psi, during a carefully measured time interval. The rate of flow, so measured, is 5.80 ml/second. Using the volumetric information determined above, one can calculate that a complete concentration change at the tips of the “cation-sensitive” and “sodium-sensitive” electrodes would require 4.8 and 3.3 mseconds, respectively. Mixing efficiency was checked by filling one side of the flow system with phenophthalein-colored base and the other side with a dilute acid solution. During flow, the indicator disappeared well before reaching the glass electrode tip, thus showing that mixing is complete. Mixing is facilitated by the establishment of turbulent (rather than laminar) flow. The critical velocity required for turbulent flow in our system is approximately 1.8 meters/second; the actual flow rate determined above corresponds to 7.4 meters/second and is more than sufficient to ensure turbulent flow. In practice, the actual composition of the effluent solution was also determined for each set of experiments in order to establish the ideal (Nernstian) potential change to be expected in dilution and concentration experiments. Because of slight imperfections in the bore of the flow tubes and the geometry of the mixing chamber, the actual composition of the solution seen by the glass electrode is slightly different than that expected from the 1 :1 mixing of C1and Cz, but the actual concentration changes remained constant during a 1686
ANALYTICAL CHEMISTRY
0.0538g KT+O.O1OOM X T Varping NaS baskgrounds
I 0
LO
120
00
160
200
milliseconds
1
T
io mv
I
G
= 0.100Og
[NaS]
[NaS] -0.OSOOg
L [Na-]
0
=0
I
I
2
L
6
8
10
aeconds
Figure 6. Response of “cation-sensitive” glass electrode to sudden changes in K+ activity in the presence of various background concentrations of Naf
Single Univalent Cation. The “cation-sensitive” glass electrode yielded flow system limited emf L’S. time curves for sudden activity changes involving the ions Ag+, Li+, Nar, K+, and NH4+ with t112values of about 10 mseconds and f 9 0 % values of less than 200 mseconds in all cases. For these simple solution-electrode systems, the kinetics of the potential-determining mechanism are not accessible to the present experimental arrangement. The response of the “sodium-sensitive” glass electrode was found to be RC time constant limited for activity changes involving the ions Agf, Li+, and Na+-Le., those ions for which this electrode has appreciable equilibrium selectivity. Anomalous behavior was found in the electrode’s response to K+ and NH4+-i.e., those ions for which the electrode has negligible equilibrium selectivity. When subjected to a sudden change in activity of K+ or NH4+,the “sodium-sensitive” glass electrode displayed an emf transient, well within the time scale of the experimental system, which decays to the expected equilibrium emf within approximately 30 seconds (see Figure 5 ) . These transients are highly reproducible and approach, at their point of maximum excursion, the emf change produced by a similar activity change of Ag+, Lif, or Na+. In fact, the early portion of the emf transients is indistinguishable from the emf cs. time curves produced by Ag+, Lif, or Na+, and appears to be also R C time constant limited. The fact that the maximum excursion of the transient approaches the Nernstian value suggests that the effect may be of analytical utility, perhaps for the simultaneous determination of two ions. Two Univalent Cations. Further dynamic differences between the “cation-sensitive” and “sodium-sensitive” glass electrodes are revealed when the activity of one univalent cation is rapidly changed in the presence of varying background concentrations of a second univalent cation. In the case of the “cation-sensitive” glass electrode, the emf cs. time curves are not affected except that the final equilibrium potential reflects the combined contribution of the two cations. Figure 6 illustrates this point for a sudden change in K+ activity in the presence of various Na+ background concentrations. The response characteristics of the “sodium-sensitive” electrode to Na+, for example, are also unchanged in the presence of background concentrations of ions which do not produce emf transients-Le., those ions for which the electrode shows equilibrium selectivity. The presence of background concentrations of those ions which produced transients above (K+ and NH4+)has a striking effect upon the response of the “sodium-sensitive” electrode to changes in Naf activity. As can be seen from Figure 7, the electrode’s response is considerably slowed under these circumstances ; the time required for attainment of the equilibrium potential is increased by a factor of about 100. These observations are of considerable analytical importance because measurements of Na+ activity using this electrode are often carried out in Na+-K+ mixtures. Clearly, sufficient time must be taken in such measurements to ensure that the equilibrium potential has been attained. Analytical measurements in rapidly changing systems may well be rendered meaningless by this effect. Divalent Cations. Glass electrodes usually do not show any appreciable response to multivalent cations. Eisenman (10) has examined the equilibrium selectivity of cation-sensitive glasses to several alkaline earth ions and found negligible response. Our experiments on the response of the “cation-
(10) G. Eisenman, in “Advances in Analytical Chemistry and Instrumentation,” Vol. 4, C. N. Reilley, ed., Wiley, New York, 1965, p. 213.
0.0538M Nat
-
0.GlOOlj Na’
Varying K+ backgrounds Na Glass Electrode _----
[K+!
[K
”1
-0.1000~
= 0.0500g
\
0
h0
0
2
80 120 milliseconds
6
ll
160
200
8
10
neconds
Figure 7. Response of “sodium-sensitive” glass electrode to sudden changes in Na+ activity as function of background concentration
sensitive” glass electrode to Ca+2 and Sr+Z substantiate these results but, surprisingly, we find a pronounced transient response to these cations. Figure 8, for example, shows the transient response to changes in Ca+a and Sr+Z activity measured in the presence of 10-4M K+ to provide a stable background potential and at pH 9 to suppress the H+ response of the glass electrode. In contrast to the transients observed above for univalent cations, the Cafz and Sr+2transients decay to an equilibrium value within about 100 mseconds. At their point of maximum excursion, the observed transients correspond to an emf change which is very nearly the Nernstian LIE value for a divalent cation; thus, these short-term transients may be useful for analytical purposes in the determination of alkaline earth ions. Interpretation of the Results. The origin of the potential of cation-sensitive glass electrodes has been well substantiated as involving a phase-boundary (ion exchange) and a diffusion contribution (1). Thus, the kinetics of the step or steps involved in the establishment of the overall potential must involve the rates of forward and backward reactions of the ion exchange equilibrium, the mobilities of the diffusing ion or ions, or both effects. While we had originally hoped to measure the kinetics of the ion exchange process at the phase boundaries in our experiments, it is now apparent that these processes, at least for the electrodes tested, are too rapid to be followed with the present measuring system even in those cases where the RC time constant of the glass electrode is not rate limiting. The transient response of the “sodium-sensitive” glass electrode to K+ and NH4+probably involves an interaction with VOL. 39, NO. 14, DECEMBER 1967
a
1687
I 0.L
0.6
0.8 t
\
c/ rO]
40
2
1.L
E at t = 0 arbitrarily taken as 100 mV ~
80
120
160
6
8
10 sec
Figure 8. Transient response of the “cation-sensitive” glass electrode to sudden changes in Ca+2 or Sr+2 activity the anionic sites on the glass surface (11) via a temporary disturbance of the ion exchange equilibrium. The relatively slow decay of these transients suggests that structural or hydration changes at the glass surface are necessary to restore the equilibrium condition. Fortunately, the slowing of electrode response to Na+ by the presence of transient-producing ions is amenable to mathematical analysis because the emf us. time curves are neither RC time-constant nor mixing-time limited. That the potentialdetermining mechanism involves diffusion as the rate-limiting step is shown by the fact that the potential function varies with the square root of time. This point is demonstrated in Figure 9 with plots of log I Eeq - E t //Eequs. t1Izwhere E,, is the final potential and Et the potential at time, t. This observation is in agreement with the results of Eisenman ( I ) who found that the uptake of ions by glass is diffusion-controlled and shows a linear dependence on t l i z . We interpret the response-slowing effect of the transient producing ions as resulting from the ability of these ions to decrease the mobility of Na+ in the hydrated layer of the glass electrode. This interpretation seems to be borne out by the work of Sendt (12),who claims that the motion of sodium in certain glasses is decreased drastically with increasing potassium ion concentration. Further indication of the effect of potassium ion on the mobility of ions in (11) J. 0. hard, “Glass Electrodes for Hydrogen and Other Cations,” G. Eisenman, ed., Marcel Dekker, New York, 1967, p. 96. (12) A. Sendt, Glastechn. Ber., 37, 116 (1964).
1688
1.2
(min)
1 2M)mseo
10 mV L
I 0
1.0
Figure 9. Test for diffusion control in response of “sodium sensitive” glass electrode to sudden changes in Na+ activity in the presence of varying concentrations of NHlf or K*
I
/
’
ANALYTICAL CHEMISTRY
glass is given by Eisenman, Sandblom, and Walker (13), who report a considerably higher electrical resistance for glass membranes interposed between identical solutions of Na+-K+ mixtures than for pure solutions of either K+ or Na+ at comparable activity levels. Sendt also points out that while K+ decreases the diffusion coefficient of Na+ substantially in glass, Na+ does not appreciably change the diffusion coefficient of K+. This is in agreement with our further observation that the electrode response to K+ is not slowed by increasing background concentrations of Na+. The pronounced transient response of the “cation-sensitive” glass electrode is quite unexpected. With the benefit of hindsight, however, it is possible to find in the literature some background data relative to this phenomenon. Truesdell and Christ (14) and Eisenman ( I ) , for example, conclude that the poor equilibrium response of glass electrodes to C@ results from the low mobility of Ca+2 in the hydrated glass layer even though Ca+2 competes for the ion exchange sites on the glass surfaces as well as do the alkali metal ions. It has further been shown with tracer experiments ( I ) that the rate of exchange of Ca+2 for Na+ at glass electrodes is initially rapid but decreases markedly as the exchange process progresses. In view of these findings, we tentatively propose that the short-term (-100 msecond) transients observed in our study correspond to the initial portion of an ion exchange process involving Ca+2 (or Sr+2)and K+ (the background ion). RECEIVED for review June 28, 1967. Accepted August 22, 1967. Based on a paper presented at the Summer Symposium, Division of Analytical Chemistry, ACS, Pomona, Calif., June 1967. Also taken, in part, from the Ph.D. Thesis of G. C . Kugler, University of Pennsylvania, 1967. Work supported by the Alfred P. Sloan Foundation and National Science Foundation Grant GP-6485.
(13) G . Eisenman, J. P. Sandblom, and J. L. Walker, Jr., Science, 155,965 (1967). (14) A. H. Truesdell and C. L. Christ, “Glass Electrodes for Hydrogen and Other Cations,” 6. Eisenman, ed., Marcel Dekker, New York, 1967, p. 318.
