(8) W. N. Carr and J. P. Mize, "MOS/LSI Design and Application", McGrawHill Book Co., New York, N.Y., 1972, (9) L. Vadasz and A. S. Grove, E€€ Trans, ED-13, 893 (1966). (10) S. I. Raider, L. V. Gregor, and R. Flitch, J. Electrochem. SOC., 120, 425 (1973). (1 1) J. O'M. Bockris and A. K. N. Reddy, "Modern Electrochemistry", Vol. 2, Plenum Publishing Company, New York, 1973, p 644. (12) A. Wlkby, J. Electroanal. Chem., 38, 429 (1972). (13) S. D. Moss, J. Janata, and C. C. Johnson, unpublished results, 1974. (14) T. E. Burgess, J. C. Baum, F. M. Foukes, R. Holmstrom, and G. A. Shirn, J. Nectrochem. Soc.. 116, 1005 (1969). 115) P. Lauoer. Sclence. 178. 24 (1972). i16j A. A. lev. V. V. Malev,'and'V. V: Osipov. in "Membranes," G. Eisen-
man Ed., Vol. 2, Marcel Dekker, New York N.Y., 1973, Chapter 7, p 482. (17) R. W. Cattrall, S. Tribuzio, and H. Freiser, Anal. Chem., 46, 2223 (1974). (18) H. Freiser, University of Arizona, Tuscon, personal communication, 1975. (19) M. D. Smith, M. A. Genshaw, and J. Greyson, Anal. Chem., 45, 1782, (1973).
RECEIVEDfor review April 28, 1975. Accepted jUly 30, 1975. Support for this research has been provided by the University of Utah.
Potentiometric Response of the Calcium Selective Membrane Electrode in the Presence of Surfactants Ramon A. Llenado Procter and Gamble Company, Ivorydale Technical Center, Cincinnati, Ohio 4521 7
The response characteristics of the calcium selective membrane electrode were investigated in calcium test solutlons containing linear alkyibenzene sulfonate (LAS) or dilsobutylphenoxyethoxyethyi(dimethyl)benzyiammonium chioride (Hyamine). Both surfactants were found to interfere at the membrane electrode, albeit by different mechanisms. LAS interferes via competing reactions at the organophiilc membrane between LAS and Ca2+ on the one hand, and Ca2+ and the ton-exchanger site on the other. Hyamine interferes through the displacement of ca*+ in the ion-exchanger site. The interference effect of LAS can be overcome by loading the membrane phase with an equilibrium amount of LAS. This procedure desensitizes the sensing eiement from surfactant effects and permits the use of the calcium electrode for Ca*+-surfactant interaction studies. Hyamlne interference patterns cannot be overcome. Seiectivlty studies showed the calcium electrode to be -lo3 times more selective to Hyamlne than to calcium. This, however, permits the use of the electrode for LAS-Hyamine titration analysis and also for the direct potentiometric measurement of Hyamine and analogous species.
The past decade has seen the renaissance of the field of analytical potentiometry with the development of ion-selective membrane electrodes (1-17). During this time period, the field of ion-electrodes has progressed to a point where there are now commercially available about two dozen types and an equal number that are still in the laboratory curiosity stage (1-9). The popularity of membrane electrodes is due to their impressive list of advantages such as ease of measurement, low cost, sensitivity, selectivity, etc. (3, 5 ) . Perhaps, their most important advantage is their ability to measure thermodynamic ionic activity in a direct fashion by potentiometry ( 2 , 3 ) . In the soap and detergent industry, the calcium selective membrane electrode is probably the most useful. To a large extent, its uses include water hardness measurement and calcium-sequesterer interaction studies (18). Other investigations dealt with preparation and theory of operation (19),response times ( 2 0 ) ,response characteristics in highly acid solutions ( 2 1 ) ,and a variety of other studies (22-23). Despite the wealth of information on the calcium electrode, there is considerable variation and oftentimes conflicting
opinions on the suitability of using the calcium electrode for potentiometric measurement in the presence of surfactants. Generally, these opinions are based on theoretical expectations because of the lack of actual experimental data. Two recent papers included descriptions of possible surfactant effects on the potassium (34) and calcium ( 3 5 ) electrodes. In another paper ( 2 8 ) ,the response of the calcium electrode to cyclohexylammonium ion was shown to be Nernstian. Other than these three papers, there is a dearth of information on the surfactant effects at the calcium membrane electrode. In most cases, investigators failed to study further or completely ignored the effects. Because of the importance of the calcium electrode in a variety of detergency studies, experiments were carried out to characterize the effect of surfactants on the potentiometric response of the electrode. This paper describes the result of these experiments. I t will be shown that the effect of a typical anionic surfactant linear alkylbenzene sulfonate (LAS) is different from that of a cationic surfactant, Le., diisobutylphenoxyethoxyethyl(dimethy1)benzylammonium chloride (Hyamine). Explicit examples will also be given wherein advantage is taken of the mechanism of the surfactant contribution to the membrane potential for a variety of studies.
EXPERIMENTAL Apparatus. An Orion model 92-20 calcium electrode was used in conjunction with an Orion model 90-01 single junction reference electrode or a Corning model 476002 saturated calomel reference electrode. Potential measurements were carried out with an Orion model 801 digital meter. The output of the meter was connected to a Sargent-Welch recorder, model DSRLG. Recording of the potentiometric response was necessary to monitor drifts, noise levels, and response times of the electrodes. A special holder, Orion model 92-00-01, that inclines the electrodes about 20° from the normal was also used to minimize the possibility of air bubbles being trapped in the concave space a t the electrode. Reagents. Standard calcium solutions were prepared from Mallinckrodt analytical reagent grade CaC12.2H20 by dissolving 147.02 grams in enough distilled water to make 1 liter of solution. Standardization was carried out with EDTA titration. With careful quantitative techniques, solutions can be prepared to within 1.000 f 0.004M Ca2+ by the above procedure. The stock solution was sequentially diluted to prepare a series of calibrating solutionslo-' to 10-jM ea2+. NaCl solution was used to adjust ionic strength when necessary. For a cationic surfactant solution, 46.61 grams of Hyamine 1622 (98.8% diisobutylphenoxyethoxyethyl(dimethy1)benzylammonium chloride monohydrate) from Rohm and
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 13, NOVEMBER 1975
2243
01
.
-4
-5
-4
Log [Ca'J
-3
, moles/iiter
-2
-2
I
I
I
Figure 1. Typical calcium electrode calibration curves Upper curve: activity scale; lower curve: concentration scale
Haas was dissolved in enough distilled water to make 1 liter of solution. For an anionic surfactant, 23.70 grams of Sulframine from Witco Chemicals was similarly dissolved in distilled water. GC analysis showed Sulframine to be a mixture of various chain length linear alkylbenzene sulfonate with the alkyl portion being 42.5% CII, 34.6% Cil, 13.8%Clo, and 9.1% 5Cs and ?CIS. Procedure. All potentiometric measurements were carried out by immersing the indicator and reference electrodes in a 50-ml test solution a t ambient room temperature. Solutions were stirred magnetically making sure that no air bubbles were trapped a t the electrode sensing element. Potential (mV) readings were made when steady-state recordings on the chart paper were reached. Response times were fast, in the order of 10-100 seconds depending on the concentration of the test solution. Response times were generally faster a t higher concentrations and also when a change in concentration is from low to high. Newly constructed electrodes were slow to respond but can be conditioned to respond faster by soaking in 0.1M CaC12 solution overnight. New electrodes were also prone to noise pick-up and spurious signals. Such problems can be overcome by conditioning and also by using higher ionic strength (better conductance) test solutions. In between measurements, the electrodes were rinsed well with distilled water and carefully dried with tissue paper. This prevented sample carryover and cross contamination. Electrodes when not in use were stored in 0.1M Ca2+ solution. Other important operating and electrode assembly procedures can be found elsewhere ( 3 6 ) .
Flgure 2. Calcium electrode calibration curves in the presence of the anionic surfactant linear alkylbenzene sulfonate (LAS) ( A ) Pure CaC12 solutions, (6)Ca2+ in 0.1 M NaCI, ( C ) Ca2+ in 0.1 M NaCl X 10-4MLAS, (0) Ca2+ in O.1MNaCI -t 4 X 10-4MLAS
The cell potential would follow the modified Nernst-Nicholsky relationship
where n, R, T , F , and a would have the usual thermodynamic meanings and y would be the ionic charge of the in-
-
+2
terfering ion j . Kij would be an experimental quantity which relates the selectivity of the electrode for calcium over than of other ions, e.g., surfactants. With slightly or noninterfering ions, Kij is very small and Ki;(aj)"'Y 0 and the Nernst equation for calcium takes the form
-
2.303 RT log aca2+ (2) nF Equation 2 predicts that the observed cell potential would show a 29 mV difference for every decade difference in e a 2 +activity a t 25 "C. In all experiments in this paper, the calcium electrode exhibited near theoretical slopes. Analytical Calibration and Measurement. In the early phases of this work, typical precision of the calcium electrode measurement was estimated under conditions reflecting the actual performance characteristics of the electrode under routine laboratory use. The potentiometric signal evoked by the electrode was repeatedly measured in test solutions containing ea2+ in the range of 10-1 to 10-5M. The result for five replicate measurements is shown in Table I. Linear regression analysis shows that the concentration data fit the equation
Ecell = E o +
RESULTS AND DISCUSSION The calcium electrode (19, 36) used in this study is of the liquid membrane type of potentiometric sensor which measures e a 2 + activity in a manner analogous to pH measurement. Its operation depends on the establishment and measurement of a potential difference across a thin membrane which comprises the sensing element of any membrane electrode. In the Orion version of the calcium electrode, the sensing membrane consists of a water-immiscible solution of the ion-exchanging calcium salt of didecylphosphoric acid dissolved in dioctylphenylphosphonate (36). In this paper, the calcium selective membrane electrode is combined with a reference half cell to complete the potentiometric cell assembly of the form
2244
-3
Log [ca++], moi.s/liter
-00
E = -69.3
+ 23.2 log [ea2+]
(3)
with a correlation coefficient of 0.99656. Using activity rather than concentration, the data fit the equation
E = -88.3
+ 28.7 log aca2+
(4)
with a correlation coefficient of 0.99971. Figure 1 shows typical calcium electrode calibration curves. Good reproducibility can be obtained with the calcium electrode when measurements are carried out in a routine manner with no extra precautions and with a wide working range of 1000-fold calcium ion activity (concentration). A deviation from the true value of 1 mV would lead to an inaccuracy in calcium activity (concentration) of 8.1% relative. Better reproducibility can be attained by thermostating and better relative accuracy can be obtained by bracketting an unknown calcium solution within a narrow limit of known calcium ion activity. In Table I, the elapsed time between replicate measurement is about 10 minutes, hence, most of the potential uncertainty is due to drift which is roughly 0.05-0.1 mV/min. It must be recognized that under extremely controlled conditions, relative accuracies of 0.78% are possible. Furthermore, the precision and accuracy quoted above are only for equilibrium types
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
,pure 0 . 1 CO++ ~
Table I. Reproducibility of Potentials Evoked by a Calcium Electrode as a Function of Concentrationa Potential, E in m\'
Tri 11
1
10 l j f b
10 * ' I
c44.5 +22.0 2 +46.0 +23.0 3 +46.0 +23.5 4 t46.0 +24.0 5 +46.5 +23.5 +45.8 +23.2 Mean 0.76 0.76 Std dev Linear regression yields E = 0 99656 1M G lo5 ppm
3,~,~
-2.5 -1.5
-1.0 -1.0 -1.0 -1.4 0.65 -69.27
+
AI -28.0 -26.0 -27.0 -27.5 -27.0 -27.1 0.74 23.21 log
~ ~ - j ! . ca2f
45.5 -45.5 45.0 44.5 45.0 45.1 0.42 [CazL] r =
C 0-
of measurements and are not valid when electrodes are used as sensors in rate studies.
B
CO-
Ca'
I N T E R F E R E N C E ARISING F R O M THE ANIONIC S U R F A C T A N T L I N E A R ALKYLBENZENE S U L F O N A T E (LAS) Verification of LAS I n t e r f e r e n c e Effects. When calcium ion activity measurements are carried out in the presence of linear alkylbenzene sulfonate (LAS), the calcium electrode exhibits unusual signal behavior. A newly constructed calcium electrode when immersed in a freshly prepared calcium solution will evoke a stable response. However, when a small increment of the anionic surfactant LAS is added, there is a potential shift in the direction of the more negative potential. The signal then deteriorates into a noisy drifting potential. The magnitude of the initial potential shift is too large (-5-15 mV) to account for pure complex formation (Ca2+binding), ionic strength variation, or changes in the junction potential. Furthermore, the shifts are time-dependent resulting in unpredictable calibration curves as shown in Figure 2. The shapes of the calibration curves are also time-dependent and may shift in an erratic manner. If calibrating solutions were made u p to contain a background concentration of the anionic surfactant LAS (2 x 10-4M), the potential readings also drift, making it extremely difficult to measure calcium ion activity in the presence of this surfactant. Mechanism. The behavior described above is puzzling because the ion-exchanger in the calcium electrode is negatively charged and thus can carry only positively charged species across the organophilic membrane. Further experiments were therefore made to obtain a clear picture of the surfactant interference a t the membrane electrode. In a series of experiments, when the potential of a newly prepared calcium electrode immersed in a calcium test solution containing LAS is continuously monitored over a period of one hour, the following behavior is observed (see Figure 3). A stable potential is evoked in the presence of calcium (fast) (see A in Figure 31, a shift (fast, see B , Figure 3) in the potential is noted when the surfactant is added, then a gradual drift (slow, see C, Figure 3) back to the initial potential upon standing. When a fresh calcium solution is measured, the potential now remains the same within experimental error (see D ) . This phenomenon is graphically demonstrated in Figure 3. The effect is attributed to initial monolayer formation a t the membrane-solution interface (see B , Figure 3) followed by extraction into the membrane phase itself (see C, Figure 3). The initial surfactant monolayer formation is rapid causing the equally fast potential shift. The extraction process causes the drifting potential which continues until an equilibrium amount of LAS is extracted into the membrane phase. A t this point, the electrode becomes desensitized with respect to shifts and drifts caused
test solution
organophilic membrane
reference solution
Figure 3. Response characteristics of the calcium electrode in the presence of linear alkylbenzene sulfonate (LAS) Lower diagram shows mechanistic representation of the LAS interference effects. See text for the discussion
by the surfactant LAS. I t then becomes possible to measure calcium in the presence of surfactants without the attendant problems associated with a newly prepared calcium electrode. Desensitization Procedure. With an understanding of the LAS interference effect a t the calcium membrane electrode, one Can now desensitize the sensing element and overcome the interference. One simply conditions the electrode in the surfactant solution together with a 0.1M Ca2+ solution, or better still, a t concentrations a t which the electrode is to be used. The conditioning operation may take between 15-60 minutes. I t is best to monitor the electrode potential until a steady-state reading is reached. Alternately, one can shake the calcium liquid exchanger with a solution of anionic surfactant LAS prior to introduction into the electrode assembly. With this procedure, water must then be completely removed by centrifugation, otherwise noisy signals are obtained because of microemulsions forming in the membrane phase. I t is thus easier to construct the calcium electrode in the normal way and then condition it in the manner described above. Response of a Desensitized Electrode. T o test the validity of our explanation above, measurements were carried out with a calcium electrode before and after conditioning. Potentiometric readings were made in calcium solutions between the concentration range 10-1 to 10-5M with and without a background concentration of 2 X lO-*M LAS. The low concentration of LAS was chosen because it was previously observed that even low concentrations of this surfactant would cause a large shift in potential. At the same time, such a low concentration of LAS would not tie up a measurable quantity of Ca2+ above 10-3M. Any change in potential, therefore, should be due to surfactant interference alone and not due to calcium binding. The
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2245
10' M Cd' , ,A,,
;%"1
'.'
-1
Flgure 4. Desensitized calcium electrode response calibration
curves in the presence of linear alkylbenrene sulfonate (LAS) From top to bottom: 1st curve, with 2 X 10-4M LAS: 2nd curve, with 4 X 10-4M LAS; 3rd curve, with 6 X iO-4M LAS; 4th curve, with 8 X 10-4M LAS
10 'M \,
'
I'
Figure 5. Noisy calcium electrode signal after excessive exposure (>48 hours) to 1 X 10-2MLAS in the absence of calcium
Table 11. Effect of Conditioning of the Calcium Electrode Prior to Measurement in the Presence of Surfactants
10.'M Cd'
E , m\ before conditioning
Conc? C a * - ,
U
IVithodt U S , mV
lo-' 10-2 10-3 10-4 10-5
+99.5 +72.5 +44.5 +24.5 +13.5
lo" 10'2 10-3 10-4 10-5
+99.5 +72 .O +45.0 +22.0 +13.0
\I t t h LAS at
A E difference
2 x 10-4.U, m\'
in c a l i b r a t i o n , mV
+96.5 +67.5 +40.0 +13.5 -2 .o
3 .O 5 .O 4.5 11.0 15.5
,1
-,-
IV'M
E , m\.' a f t e r condttioning
+100.0 +71.5 +44 .O +21.5 +1.5
0.5 0.5 1.o 0.5 11.5
data are summarized in Table 11. I t can be seen that the effect of conditioning is important for the proper operation of the calcium electrode in the presence of the surfactant LAS. The large AE observed between calibration before and after conditioning is too large to be due to calcium complexation by LAS. Such large AE shifts are non-existent after conditioning the electrode. Figure 4 illustrates a series of Ca2+ measurements carried out in the presence of various levels of LAS. With a conditioned calcium electrode, one gets improved curves (see Figure 4) which are vastly superior to those found in Figure 2. Response i n t h e Absence of Calcium. When the calcium electrode is immersed in a solution of LAS (>10-3M) without calcium ions present for an appreciable length of time (>48 hours), the electrode exhibits a noisy signal as depicted in Figure 5. This is due to the loading up (extraction) of LAS into the membrane phase and subsequent interaction with the calcium didecylphosphate salt itself when external Ca2+ is absent from the test solution. At sufficiently high LAS concentration (>10-2M), precipitation or gelling is observed at the sensing element (white deposit) and this is attributed to the formation of calcium-LAS salt. Fortunately, the effect is reversible and the electrode response is re-established after soaking the electrode in a stirred 0.1 M Ca2+ solution (see Figure 6). This conditioning process is essentially a conversion of the exchanger ma2246
Figure 6. Reconditioning of a calcium electrode after excessive ex-
posure to LAS Response curves shown were made after conditioning the electrode in 0.1 M CaZ+. Compare this figure to Figure 5
terial into its calcium form and a back extraction of the excess LAS out of the membrane phase into the aqueous test solution phase. A plot of the calcium electrode response to aqueous solutions of LAS in the absence of Ca2+ is interesting as shown in Figure 7 . Surprisingly, a response is obtained between to 10-?-M LAS when theoretically no response should be observed at all since the carrier ion-exchanging site is negatively charged. Above 10-2M LAS, the curve drops off instead of maintaining a constant slope. This drop is not presently understood, but it may be due to micellar solubilization of the membrane or a more complex mechanism (37). Further experiments have shown that the mechanism of the LAS response is similar to that operating a t the AgZS, AgX, MX crystal membrane electrodes (22). At the calcium electrode membrane-solution interface, LAS interacts with the infinitesimal amount of Ca2+ a t the interface effectively lowering the Ca2+ activity according to the solu-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
change solution -123.
4
-'OC-
? 3 -8C.
m
a
> E
--6G.
W
1 min.
D -5
-3
-4
Log [LAS],
-2
motes/Iiter
Figure 7. Calibration curves showing calcium electrode response t o LAS in the absence of Ca2+ ( A ) Newly constructed electrode, (6) electrode conditioned 1 hour, electrode conditioned 4 hours trode conditioned 2 hours, (0)
(0 elecFigure 8. A typical calcium electrode response when exposed to dilute solution of Hyamine
bility product or complex formation principle, i.e., TableIII. Hyamine Interference Effects a t the Calcium Membrane Electrode H) amine concrl,
and (7)
Substituting Equation 7 into Equation 2, we have
10-l.v c a 2 -
lo-*!,,
ca*
0
+ 58 .O
lo-@ lo-' lomb 10-5
+58.0 +58.0 +58.0 m . 0
lo-
