Investigation of the Factors Affecting the Response Time of a Calcium Selective Liquid Membrane Electrode Bernard Fleet a n d T.
H. Ryan
Chemistry Department, Imperial College, London S. W. 7., U.K.
M. J. D. Brand' imperial Chemical lndustries Ltd., Agricultural Division, Billingham, Teeside. U.K
Measurements on the response time of a Calcium Selective Liquid Membrane Electrode are reported and a new parameter f 9 5 is proposed as being a more representative measure of electrode performance than the more widely used tlI2. The former value is of more importance in assessing the performance of an electrode in real analytical situations. The effect of various interfering ionic species on the response time of the electrode is investigated and ions tested have been divided into three groups according to their effect on the response time. An interpretation of this division is proposed on the basis of the kinetics of the reaction of the interferent ion with the membrane site species and conventional selectivity ratios.
The increasing importance of ion-selective electrodes for use in continuous monitoring situations (1-9) has led to the need for a more exact definition of the performance characteristics of these devices. Although the importance of selectivity has been stressed by several groups of workers (10-15), in many applications, especially in the field of continuous analysis, it is the response time of the electrode which is the critical limiting factor. In view of the importance of the calcium liquid membrane electrode (and related immobilized liquid membrane configurations) in biomedical and water quality monitoring, the present study has attempted to evaluate factors influencing the response time and specifically to define the role of interfering species in their influence on this response. When an ion-selective membrane electrode is subjected to a rapid change in activity of the primary ion, a finite time will be required for the electrode to assume a potential characteristic of the new activity level. This "response time," though a complex function of several contributory factors, is a fundamental property of the electrode system and can, in some cases, be the limiting factor in the application of an electrode to analytical problems. Present address. T e c h n i c o n I n s t r u m e n t Corporation, T a r r y town, N.Y.
(1) B. Fleet and A. Y. W. Ho, "ion Selective Electrodes," E. Pungor, Ed. Akademia Kiado. Budapest, 1973,p 1 . (2)T. S . Light, "Ion Selective Electrodes," R. A. Durst, Ed., Nat. Bur. Stand. (U.S.) Spec. Pub/., 314, 349 (1969). (3) B. Fleet and H. VonStorp, Anal. Chem., 43, 1575 (1971). (4)T. G. Lee, Anal. Chem., 41, 391 (1969). (5) H. H. Webber and A. L. Wilson,Ana/yst (London), 94, 209 (1969). (6) P. J. Milham,Analyst (London), 95, 758 (1970). (7)G. W. Neff, W. A. Radke, C. J. Sambucetti, and G. M. Widdowson. Clin. Chem., 16, 566 (1970). ( 8 ) J. Ruzicka and C. J. Tjell, Anal. Chim. Acta., 47, 475 (1969). (9) B. Flee! and G. A. Rechnitz, Anal. Chem.. 42, 690 (1970). (10) W. E. Morf. D. Ammann. E. Pretsch, and W. Simon, Proc. IUPAC. Symp. Ion Selective Electrodes. Cardiff 1973,in press. (11) K. Srinivasan and G. A. Rechnitz,Anal. Chem., 41, 1203 (1969). (12) G. J. Moody and J. D. R. Thomas, Lab. Pract., 20, 307 (1971). (13) E. Pungor and K. Toth, Anal. Chim. Acta, 47, 291 (1969). (14) M. Whitfield and J. U. Leyendekkers, Anal. Chern., 42, 444 (1970). (15) J. Bagg and W P. Chaung, Aust. J. Chem., 42, 1963 (1971). 12
The importance of response times of electrodes was recognized in the early work on glass electpodes (16-18) and much effort was devoted to developing a theory relating response times with response mechanisms (19-24). Variations in response times were linked to such factors as washing (16, 25), pre-treatment (18, 26), composition (17), construction (18), and the influence of interfering ions (16, 27). Various authors have made measurements of electrode response times and much data on the different electrode systems are scattered throughout the literature. With few exceptions (18, 22, 23) including the work of Pungor et al. (28) on the dynamic response of precipitate based membrane electrodes, most measurements of response time have been incidental to the evaluation of a particular electrode or membrane system and so have been carried out under a wide range of experimental conditions. Without attempting a t this stage a rigorous definition, it is possible to visualize the overall response time as a composite quantity with contributions from hydrodynamic mixing, membrane-solution equilibrium, and membrane response. Time variable potentials arising from establishment of a liquid junction, e.g., a t a reference electrode, are not considered part of the membrane electrode response. I t is apparent that the various classes of ion-selective membranes have fairly well-defined response times, a t least in pure solutions. "Fixed site" membranes, e.g., glass (22, 23) and single-crystal (29-31), have response 100 msec while "mobile site" times of the order of 10 membranes e.g., liquid ion-exchangers ( 3 2 ) , neutral carrier complexes (33), and their immobilized analogs (34, show response times varying from a few seconds to several minutes.
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(16) G. Mattock, Analyst (London), 87, 930 (1962). (17)J. A. Savage and J. 0. Isard, J. Phys. Chem. Glasses, 3, 147 (1962). (18) G. A. Rechnitz, Talanta, 11, 1467 (1964). (19) G. Eisenman, Biophys. J., 2, 259 (1962). (20) G. Eisenman, D . C. Rudin, and J. U. Casby, Science. 126, 831 (1957). (21) G. A. Rechnitz and H. F. Hameka, Z. Ana/. Chem., 214, 252 (1965). (22) G. Johansson and K. Norberg, J. Electroanal. Chem., 18, 239 (1968). (23) G. A. RechnitzandG. C. Kugier,Anal. Chem., 39, 1682,(1967). (24) M. J. D. Brand and G. A. Rechnitz, Anal. Chem., 41, 1788 (1969). (25)T. M. Hseu and G. A. Rechnitz,Anal. Chem.. 40, 1054 (1968). (26) G. Mattockand R. Uncles, Analyst (London),89,350 (1964). (27)J. Rao, M. Pelavin, and S. Morgenstern. "Advances in Automated Analysis," Mediad, Inc., White Plains, New York. N . Y . , 1972, in press.
(28) K. Toth, I. Gavaller, and E. Pungor, Anal. Chim. Acta, 57, 131 (1 971). (29) R. P. Buck, J. Electroanal. Chem.. 18, 363 (1968). (30) R. P. BuckJ. Electroanal. Chem., 18, 381 (1968). (31) R. P. Buck and I. H. Krull, J. Electroanal, Chem., 1 8 , 387 (1968). (32)G. A. Rechnitz and Z. F. Lin, Anal. Chem.. 40, 696 (1968). (33) W. Simon, Eidg. Tech. Hochschule. Zurich, Switzerland, private communication, January 1973. (34) J. Pick, K. Toth. M. Vasak, E. Pungor, and W. Simon, "Ion Selective Electrodes,'' E. Pungor, Ed., Akademia Kiado, Budapest, 1973, p 245.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 1, J A N U A R Y 1974
Thus, in addition to providing critical parameters for the design of automated analyzers or continuous monitoring systems, response time measurements are significant in the elucidation of the mechanism of electrode response. EXPERIMENTAL
mV, vs.
Measuring System. The electrode used in the present study was a Model 92-20 Calcium Selective Liquid Membrane Electrode (Orion Research Inc., Cambridge, Mass.) which was screened by means of an earthed copper foil sheath. A single-junction reference electrode (Philips, Eindhoven) was used and potential measurements were made with a Digital Electrometer Model 101 (Corning-EEL, Corning, N.Y.). The voltage output from the meter was displayed on a potentiometric recorder uia a "backing off" device to increase scale sensitivity. The response time of the measuring system to a step input was just less than 1 sec for full scale deflection ( 5 mV/cm) corresponding to ca. 0.5 sec for halfscale, This performance was considered to be adequate since none of the systems studied had response times of less than 1.0 sec. All measurements were made a t 20 f 0.5 "C in a thermostated cell. Activity Changes. All activity changes were effected by means of a "dumping technique." A small volume (usually 1.0 ml) of a concentrated solution of the primary ion was injected by means of a syringe into 10.0 ml of the rapidly stirred test solution. The finite mixing time for this type of technique is of the order of 0.5 sec and is, in any case, a constant factor in all measurements. Formation of B a r i u m a n d Lead Ion Exchangers. The lead form of the Orion Liquid Ion-Exchanger was prepared by dissolving 2.0 ml of the calcium form in 50 ml of chloroform and extracting with three aliquots of a 2M aqueous solution of lead nitrate. Separation, drying, and evaporating gave a pale yellow, translucent waxy solid which melted at ca. 30-40 "C. The barium form of the exchanger required more drastic conditions because of the unfavorable exchange reaction. The calcium form was first converted to the free acid by extracting with 2M hydrochloric acid instead of lead nitrate; the resultant chloroform solution was then re-extracted with 2M barium hydroxide. Separation, drying, and evaporation yielded a yellow, transparent, very viscous fluid which became quite mobile at ca. 60 65 "C. Materials. All chemicals used were of Analytical Reagent grade. A constant ionic strength background was maintained in each test solution with 0.1M Tetraethyl-ammonium chloride. Interfering ions studied were used as their chlorides or nitrates. Calibration of Calcium Electrode. The performance of this electrode has already been extensively reported (32). Daily calibration gave an average slope of 25.5 f 1.0 mV/decade. When the slope decreased to 23 mV. the membrane was renewed.
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RESULTS AND DISCUSSION One of the major problems in the discussion of response times of ion-selective electrodes is the lack of a universally accepted definition of the term. Various quantities have been used ranging from t l l z to tloo. In the present work, the use of the parameter t g 5 is proposed, and is defined as the time required for the electrode to reach 95% of its equilibrium steady-state potential, after a rapid change in ion activity. The primary reason for choosing this parameter rather than the more widely used t 1 , ~is that the former is a far more realistic measure of the practical performance of the electrode. Response in Pure Calcium Solution. A typical potential/time profile of electrode signal when subjected to a rapid increase in calcium ion activity is shown in Figure 1. The time required for the potential to reach the new steady state varied randomly between 4 and 6 sec owing to the uncertainty in measurement because of the small slope of the curve near equilibrium. Measurements of the suggested parameter t 9 5 , however, show much more reproducible values (Table I) and further were independent of the initial calcium activity of the solution and depend only on the magnitude and direction of the activity change. No significant influence on response time was observed due to changes in ionic strength over the range 1 0 - 4
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S.C.E.
TIME
i n seconds
Potential-time profile of the electrode after a sudden change in calcium activity. No interfering ions present Figure 1.
Table I. Response Times of Ca Electrode in Pure Ca Solutions Activity change t 9 5 , sec 1 0 - 4 4 1 0 - 3 ~ 2.3 f 0 . 2 10-3IO-*M 2.2 f 0.2 10-2+ 1 0 - ' M 2 . 2 f 0.1 2.3 f 0 . 2
No buffer
10-1A4, but the electrode did show the characteristic dim-. inution of performance with aging. These effects were not significant until the electrode slope had decreased below 23 mV/decade, a t which time the membrane was renewed. The response profile in pure calcium solutions (Figure 1) has the same general shape as those previously reported in time response studies (25, 28) and can be described by an equation similar to that derived by Rechnitz and Hameka (21)for the glass electrode.
V,
= V r q u l l , b r l u r-ePy1' n(~
- ep52''
where y1,yz . . . are a series of empirical parameters. Electrode Response in Multi-Ionic Solutions. Extensive studies on electrode selectivities (10-12, 35) have clearly defined the role of interfering ions on the equilibrium potentials of electrode systems. Analogous effects on the dynamic electrode response, although widely observed, have been less well documented. Rechitz and Lin (36, 37) have described the general deterioration in the performance of the calcium electrode in the presence of magnesium ions and have clearly demonstrated the slowing effect of the interferent on the electrode response profile. These authors also made the interesting observation that the electrode shows a rapid transient response when the magnesium ions are added. In the present work, the influence of a range of interfering cations on the response of the calcium electrode has been studied. Figure 2 shows the effect of varying background levels of magnesium ion on the response of the (35) J. Bagg, 0. Nicholson, and R. Vinen, J . Phys. Chern., 7 5 , 2138 (1971). (36) G . A. Rechnitz. "Ion Selective Electrodes," R. A . Durst, Ed., Nat. Bur. Stand. ( U . S . ) Spec. Pub/., 314, 313 (1969). (37) G. A . Rechnitz and Z.F. Lin, A n a / . Chem., 40, 696 (1968)
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1 , JANUARY 1974
13
*
4
I
mV v s S.C.E. 10
t
5 mV
ri
I
r
-
I
I/ /
i
L
.
.
:
2
-
3
5
L
6
P Z” .
.
>
TIME in s e c o n d s
Figure 2. Effect of various levels of magnesium ion activity on the response profile of the electrode after a change in calcium activityfrom 1 0 - 4 4 l 0 - W
Figure 4. Effect of zinc interference on the response time of the electrode
+, Ca =
e , Ca
=
___-
Table II. Response Times of Ca Electrode in Presence of lnterferents Ion
S.C.E.
Selectivity Ratio
Mg2Sr’f
0.040 0.026
0a2+
0.009
Z++
0.003
Pb2*
r
1.8 1.7
Cd2+
5 mV
L
CU2+
0.13
Na+
0.001
Hf N (Meh’
2 x 104 0.0006
ACEAa
2.5 x 3.6 x 1.1 x 3.3 x 5.6 x
10-3 10-3 10-2 10-2 10-5
x
10-5 10-4
5.9
t95,
sec
4.0
8.5 22.4 9.6
3.2
3.0
7.7 x 8 . 3 X lo-’
2.2
2 x 10-8
2.3 2.2
0.15
3.0
“Apparent calcium-equivalent activity”; see text.
T I M E in s e c o n d s
Figure 3. Comparison of the effects of different interfering ions, all at 10-’M level, on the response of the electrode to a calciu m ion change 10-3M electrode to an increasing step change in calcium activity. The effect on t 9 5 as the concentration of interferent is increased, is readily apparent as is the effect on equilibrium potential which would be predicted from normal selectivit y considerations. A consequence of the above results is that even at levels of interferent where no significant effect on equilibrium potential is observed, i.e., a tolerable analytical level predicted from selectivity values, a marked effect on t 9 5 is apparent. The response profiles for a series of alkaline earth interferents is shown in Figure 3, and shows a general trend toward increasing response time with increasing atomic weight of interfering ion. In order to compare the effect of interferents on the dynamic response pattern of the electrode with conventional interference studies, selectivity ratios were measured using the general Eisenman-Nicolsky equation
E
=
Eo
+ S log(a, + K,,a,)
where a, and a, are the activities of primary and interfering ion and K,, is the selectivity ratio of the electrode towards interfering ion j . 14
The selectivity of the calcium electrode toward zinc was far less serious than previously reported (38) and in reasonable agreement with other observations (37). Selectivity values ,measured by several methods (12) gave values of 0.007 0.65. Despite the low value of measured selectivity ratios, zinc exhibited a significant effect on the calcium electrode response time. In a background zinc level of 3.10-2M, a t g 5 value of over 9 sec was observed for a calcium activity change from to lO-3M. This value was in excess of those for magnesium and strontium which show less favorable selectivity values. Figure 4 shows the effect of zinc on the calcium electrode response time for two initial levels of calcium. A plot of this type serves a useful function in that it enables the tolerable threshold level of interferent to be predicted. Table I1 shows the effect on response time of a range of cations. The alkaline earth metals were chosen as being similar to calcium; the group Zn, Pb, Cd, and Cu were chosen as being serious Ca-electrode interferents while the last group Na+, N(CH3)4+, H+ were tested to ensure that possible supporting electrolyte components had little effect. The third column of Table I1 lists a parameter, “the apparent calcium-equivalent activity,” which is defined, for any interferent, as the quotient of initial calcium ion activity and the average measured selectivity ratio of that interferent. Hence the response time values in the final
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(38) Orion Research Inc
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 1 , J A N U A R Y 1974
Sheet, 1957
Cambridge, M a s s , Model 92-20 Application
-
~~~
column show the values of t95 for the change in Ca2+ or 10-4 lO-3M in the presence of that background level of interferent which would, in pure solution, give the same potential response as 10-4M Ca2+. The alternative to this approach-namely, the measurement of response times at fixed ca1cium:interferent ratios-was not practicable owing to the wide variation in selectivity of the ions studied. The results shown in Table I1 suggest that the interfering ions studied fall into one of three main groups. First, there are those ions which have a negligible small effect on response time and a correspondingly small selectivity coefficient, e.g., N a + . Second, there is a group of ions which also have a negligibly small effect on response time but have a significantly large selectivity constant, e.g., Pb2+. Finally, there are a group of ions having a measurable effect on response times and an intermediate selectivity constant, e . g . , BaZ+. Qualitative interpretation of these observations can be provided in terms of the mechanism of the electrode response. The ion exchanger of the electrode used in this investigation is known to be a substituted phosphoric acid (39) which is present as a weakly dissociated metal salt in the membrane phase: MS,(memb.) e M2+(memb.) + 2S-(memb.) (1) Considering first the single ion response of a membrane in equilibrium with a solution, when the concentration of M2+ changes in the aqueous solution, the system equilibrium will alter so as to maintain equality of the electrochemical potentials of M2+ in membrane and aqueous phases. This can be achieved by two processes: a ) a change in the equilibrium position of Equation 1 and/or b) transport of M2+ across the membrane solution interface M'+(aq) e M2+(memb.) (2) The electrode response time will be determined by the kinetics of one of these two processes but it is not possible to say which is the rate determining step. Consider now the response of the membrane to a change in concentration of M2+ when an interfering ion M I n + is present in solution. In this system, in addition to Equations l and 2, the following equilibria will exist M,S,(memb.)
e M,"'(memb.)
-tnS-(memb.)
M,"-(aq) 1-M,"+(memb.)
(3) (4)
Simultaneous occurrence of reactions 1 to 4 is equivalent to the overall true ion exchange reaction nMS,(memb.) i- 2M,"'aq S 2M,S,(memb.) -I- M'+(aq)
This exchange process will continue until a state of thermodynamic equilibrium exists across the entire membrane cell. In practice, this equilibrium will rarely be achieved and it may be noted that a constant electrode potential is obtained when the ion-exchange reaction (39)J. W . Ross in "Ion Selective Electrodes." R. A . Durst, Ed.. Mat. Bur Stand. ( U . S . ) Spec. P u b / . . 314, 70 (1969).
Table 111. Single Ion Response Times for a Change in Primary Ion Concentration Primary ion
Ca2+ Ba2+ Pb2+
t95,
to 10-3M
sec
2.3 14 1.7
reaches a steady state (40). A change in concentration of Mz+ in the aqueous phase results in a change in the equilibrium position of Equation 5 uia the intermediate reactions 1 through 4. The presence of M,n+ in the solution will influence the kinetics of this process when a ) the kinetics of Equation 3 and/or 4 become rate limiting, and b) the ion exchange (and therefore potentiometric) selectivity constant is favorable. Thus in the limiting case when MIn+ (memb.) = 0, the system reverts to the single ion case and Mzn+has no effect on response time. It may be noted that reactions 3 and 4 are equivalent to the single ion response to ion M,n+. In order to test the above deductions, the calcium ion exchanger was converted to its lead and barium forms, and the single ion response of these membranes measured. Table 111 shows the results of these measurements which would seem to support the proposed mechanism. Thus a rationale for the three groups of interferents proposed above can be given in mechanistic terms as follows: a) the interferent ion exchange selectivity is so small that the system is equivalent to the single ion case; no effect on response time is observed. b) Although the interferent ion exchange selectivity is large, the kinetics of the interferent ion reactions are faster than the primary ion reactions; no effect on response time is observed. c) The interferent ion exchange selectivity is significant but the kinetics of the interfering ion reactions are rate limiting; here an increase in response time is observed.
CONCLUSIONS It has been shown that a calcium liquid membrane electrode responds rapidly and reproducibly to step changes in calcium ion activity, with response time of the order of a few seconds. Also interfering ions can have a marked effect on the electrode response time and this cannot always be predicted from a knowledge of the selectivity constant. Where the selectivity constant is very small, it may be assumed that no effect on response time will be observed. For interfering ions with moderate to large selectivity constants, an increase in electrode response time will be observed when the kinetics of the interferent ion reactions become rate limiting. Received for review March 26, 1973. Accepted July 16, 1973. THR thanks the Science Research Council for a CAPS award. The support of Imperial Chemical Industries Limited, Agricultural Division, is also gratefully acknowledged. (40) G. Eisenman, ibid, p 42
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1 , J A N U A R Y 1974
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