Anal. Chem. 1991, 63,754-759
754
Selectivity of Membrane Electrodes Based on Derivatives of Dibenzyltin Dichloride Scott A. Glazier’ and Mark A. Arnold*
Department of Chemistry, ,University of Iowa, Iowa City, Iowa 52242 Selectivity properties are establlshed for membrane electrodes prepared by Incorporating bls(p-methylbenzyl)tln dlchlorlde, dlbenzyltln dlchlorlde, and bls(p -chlorobenzyl)tln dichloride In plasticized polymer membranes. These electrodes display an unusually hlgh level of selectlvlty for dlbaslc phosphate over many common anions. Electrodes prepared with the pthloro derlvatlve possess the best detectlon limit and the hlghest degree of selectivity for phosphate. Selectlvlty coefflclents are calculated for phosphate relative to the followlng group of anions: salicylate, benzoate, thiocyanate, lodlde, nitrate, bromlde, chloride, acetate, fluorlde, pyrophosphate, arsenate, adenoslne 5‘-cyclic monophosphate, adenosine S’monophosphate, adenoslne S‘dlphosphate, and adenoslne 5’4rIphosphate. lln-carbon hyperconlugation within the organotln compound Is hypotheslzedto be critically Important In the selective response to phosphate. Enhancement of tln-carbon hyperconlugation by Increasing the electron-withdrawing power of substituents on the benzyl ring Is predicted to provlde even higher levels of selectivity for phosphate. INTRODUCTION
Significant effort has recently been placed on the development of ion-selective membrane electrodes for anions (1-6). Much of this effort has centered around the incorporation of organometallic compounds in plasticized polymer membranes. Several interesting systems have been reported where the selectivity of these electrodes differs from the well-known Hofmeister series. Electrode selectivity, in these cases, is not governed by simple anion lipophilicity but by specific chemical interactions between the organometallic species in the membrane and the anions in solution. We are interested in characterizing the selectivity of membrane electrodes based on organotin compounds (7-11). Our interest in this work was sparked by a group of reports by Zolotov and co-workers (12-15) wherein a series of dialkyltin dinitrates (diheptyltin to didodecyltin) were shown to be excellent extracting agents for oxyanions of phosphorus, arsenic, and selenium. These researchers constructed liquid membrane electrodes with these dialkyltin compounds and found selective responses for phosphate and arsenate over many common anions (14). These electrodes clearly exhibited non-Hofmeister type selectivity patterns. Our initial work has shown that a selective response for dibasic phosphate is possible by using bis(p-chlorobenzy1)tin dichloride in plasticized poly(viny1 chloride) (PVC) membranes (7). These electrodes display the following selectivity pattern: HPOd2-> I- > NO3- > Br- > C1- > OAc- > SO,2Essentially, the tin compound brings phosphate from the bottom of the Hofmeister series to a level above iodide. In addition, the relative responses to dibasic phosphate and chloride have been measured for a series of electrodes prepared Current address: Chemical Process Metrolo y Division, Center for Chemical Technology, National Institute for Standards and Technology, Gaithersburg, MD 20899. 0003-2700/91/0363-0754$02.50/0
with bis(p-chlorobenzy1)tin dichloride, dibenzyltin dichloride, and bis(p-methylbenzy1)tin dichloride (8). Each of these electrodes displays greater selectivity for phosphate over chloride. The p-chloro derivative provides the highest degree of selectivity for phosphate, followed by the dibenzyl and then the p-methyl derivatives, respectively. More information about the selectivity of polymer membrane electrodes based on derivatives of dibenzyltin dichloride is reported here. Selectivity patterns are presented for electrodes based on bis(p-methylbenzy1)tin dichloride, dibenzyltin dichloride, and bis@-chlorobenzy1)tin dichloride for a series of common anions (acetate, chloride, bromide, nitrate, iodide, and thiocyanate). In addition, responses from electrodes based on bis(p-chlorobenzy1)tin dichloride are reported for a second group of organic and inorganic anions, which includes salicylate, benzoate, arsenate, fluoride, pyrophosphate, and several phosphorylated nucleotides. Finally, the current knowledge concerning the selectivity of membrane electrodes based on these and related diorganotin compounds is used to develop a working hypothesis that relates the chemical structure of the organotin complex and the selectivity of these membrane electrodes. EXPERIMENTAL SECTION Reagents. Trizma base, adenosine 5’-monophosphate (AMP,
sodium salt), adenosine 3’:5’-cyclic monophosphate (CAMP,sodium salt), adenosine 5’-diphosphate (ADP, potassium salt), and adenosine 5’-triphosphate (ATP, sodium salt) were purchased from Sigma Chemical Co. (St. Louis, MO). High molecular weight poly(viny1chloride), tetrahydrofuran (Gold Label), Aliquat 336 (a quaternary ammonium salt), p-chlorobenzyl chloride, and p-methylbenzyl chloride were obtained from Aldrich Chemical Co. (Milwaukee, WI). Benzyl chloride and dimethylformamide (Omnisolve)were purchased from EM Scientific (Gibbstown,NJ). Dibutyl sebacate was obtained from Eastman Kodak Co. (Rochester, NY). Tin mesh (-150 to +325 mesh) was purchased from Cerac, Inc. (Milwaukee, WI). All other chemicals were reagent-grade quality and were obtained from common suppliers. Distilled-deionized water was used to prepare all aqueous solutions. The bis(p-chlorobenzyl)tin, dibenzyltin, and bis@-methylbenzy1)tin dichlorides were synthesized and purified according to a reported method (16). Melting points were used to identify the compounds and to estimate their purities. Potential Measurements. Potentiometric measurements were made with a laboratory constructed data acquisition circuit (17) in conjunction with a IBM 9000 computer. All measurements were made in glass-jacketed cells thermostated to 25.0 OC with a Fisher Model 80 water bath. The solution in the cell was stirred by a small magnetic stir bar. A calibrated Corning pH electrode (Catalog No. 476216),connected to a Fisher Model 620 pH meter, was used for all pH adjustments. A simple holder suspended eight polymer membrane electrodes of the same type in solution. Each membrane electrode had an internal reference solution of 0.1 M potassium chloride in conjunction with a silver/silver chloride internal reference electrode. All potentials were measured relative to a Corning silver/silver chloride double junction reference electrode (Catalog No. 476067) with 1M lithium acetate in the outer compartment and 3 M potassium chloride in the inner compartment. The data acquisition circuit, glass cell, and all electrodes were housed in a grounded Faraday cage. Electrode Construction. Each polymer membrane electrode was constructed from a l-mL disposable pipet tip with a short 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
length of PVC tubing (3/32-in.i.d., 1/s2-in.wall) pressed onto the sensing end. Membranes were formed by dipping the free end of the PVC tubing in the membrane-casting solution three times. Membranes were allowed to dry between the first two dips and then they were stored overnight following the final dip. Membrane-casting solutions contained 70.2 mg of the tin compound, 131.’1 mg of poly(viny1 chloride), 49.5 mg of dimethylformamide, 141.3 mg of dibutyl sebacate, and 3 mL of tetrahydrofuran. Membrane Conditioning and Electrode Operation. Unless noted otherwise, each electrode was conditioned in the following manner. After adding the intemal reference solution, the polymer membrane was immersed for 20 h in a stirred liter of a pH 7,O.Ol M Tris-H$304 buffer. The electrode tip was then placed in a fresh 50-mL aliquot of the Tris buffer, and the potential was monitored. After a steady-statepotential was attained, a potassium phosphate standard solution was added to bring the total phosphate concentration to 10 mM. After a new steady-state potential was obtained, the electrode tip was removed from this solution,rinsed with water, and immersed in a fresh aliquot of buffer. This process was continued until consistent potentials were attained before and after the addition of phosphate. Generally, three exposures to phosphate were required. Electrode responses to all anions were obtained in 50 mL of 0.01 M Tris buffer that contained 0.0045 M sulfuric acid. The concentrations of the various anions were increased by making additions of an anion standard. After each addition, the pH of the buffer was adjusted to 7.00 A 0.01 with aqueous Tris or sulfuric acid. The potentials of the eight polymer electrodes immersed in solution were then measured. The reported potential changes were calculated as the difference between the electrode potential in the presence and absence of the anion of interest in the Tris buffer solution. The reported values are the means (fl standard deviation) over all electrodes tested. Generally, eight electrodes were tested in each experiment. In a few cases, an electrode stopped functioning during the experiment, and in these cases, data from the malfunctioning electrode were not used. A record was kept of all species added to the working buffer during the experiment. Concentrations and activities were calculated based on the resulting ionic strength. Activity coefficients were calculated from the Davies equation (18). Thermodynamic acid dissociation constants were corrected for ionic strength. RESULTS AND DISCUSSION Effect of Cations. In order to obtain reproducible anionic responses, freshly cast membranes must be conditioned prior to calibration. This conditioning procedure involves soaking the membrane in a pH 7 Tris-H,SO, buffer followed by exposing the treated membrane to a pH 7.00 potassium phosphate solution. The importance of this initial exposure to phosphate is demonstrated by the calibration curves presented in Figure 1. This figuregshows two calibration curves where, in one case, the electrode has been soaked only in the buffer and, in the other case, the electrode has been soaked in buffer and exposed to potassium phosphate. Without the initial exposure to phosphate, a slight, but reproducible, cationic response is observed a t low concentrations. The electrode begins to respond in an anionic fashion as the concentration increases. The magnitude of this cationic response decreases significantly when the electrode is calibrated a second time, and no cationic response is observed when the electrode is calibrated a third time. As shown in Figure 1, the cationic response is not present when the membrane is initially conditioned by exposure to potassium phosphate. The above described cationic response suggests that an ion-exchange reaction occurs between the cations in the solution and an unidentified anionic site in the membrane. The shape of the calibration curve when the electrode is not conditioned with phosphate (see Figure 1)indicates that there is a net uptake of cations relative to anions when the membrane is initially exposed to a solution of potassium phosphate. The net uptake of ions switches from cations to anions when the concentration of the phosphate salt reaches a critical value. The cationic response disappears after the membrane is ex-
755
2o
0
>
E e;
v
-20
C m
-40 0
-
0 .-c
5
-60
0
a -80
-100
1 1 -6
-5
-4
-3
-2
-1
0
Log Phos. Conc. (M)
Flgure 1. Response to dibasic phosphate by electrodes prepared with bis@-chlorobenzy1)tindichloride with (0)and without (m) prior conditioning of the membrane.
posed to a sufficient amount of the solution ions, presumably due to some alteration in the anionic sites. The magnitude of the cationic response depends on the cation in the phosphate standard. Larger cationic responses are observed when potassium phosphate is used as opposed to sodium phosphate. Furthermore, different cations affect the position of the calibration line. This line is slightly shifted to more positive potentials when potassium phosphate standards are used in comparison to sodium phosphate standards. Typically, the y intercept is shifted 8 mV more positive when potassium phosphate is used. These observations suggest that a simple cation-exchange process is present a t the solution/membrane interface and that this process contributes, at least to some extent, to the measured membrane potential. The membrane conditioning procedure reduces the effect of this interfacial process. Selectivity for Anions. Responses of electrodes based on bis(p-chlorobenzy1)tin dichloride, dibenzyltin dichloride, and bis(p-methylbenzy1)tin dichloride were measured for a series of common anions. Electrode responses to phosphate, thiocyanate, iodide, nitrate, bromide, chloride, and acetate were measured a t pH 7.00 in a Tris-HzS04 buffer solution. Representative response curves for this class of electrode are presented in Figure 2 where the responses of electrodes prepared with dibenzyltin dichloride are plotted. Inspection of this curve, and the corresponding curves for the p-chloro and p-methyl derivatives, reveals the following selectivity patterns for these three electrode systems: bis(pmethylbenzy1)tin dichloride
SCN- > I- > NO3- > Br- > HP04*- > C1- = OAcdibenzyltin dichloride
SCN- > I- > NO3- = HPOd2-> Br- > C1- = OAcbis(p-chlorobenzy1)tin dichloride
HP0,2- = SCN- > I- > NO3- > Br- > C1- = OAcA more quantitative treatment of the data requires the calculation of selectivity coefficients for each electrode system. Conventionally, such coefficients are measured by either the fixed interference or the separate solution methods (19). Both of these methods are based on the extended Nernst equation (19),which is an empirical relationship that works well in modeling the nonspecific response of a membrane electrode
756
ANALYTICAL CHEMISTRY, VOL.
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20
Table I . Selectivity Coefficients for Organotin Electrodes I
KPOTi,,
W
c
,"
-40
-
c)
-
? c
anion
p-methyl
HP0,2SCN1NOaBr-
1.00 3580 358.9 11.54 4.400
C1-
0.402
OAc-
0.1954
p-chloro
benzyl 1.00
1.00 4.936 0.255 8 0.019 I8 0.015 67 0.003 089 0.004 985
195.7 27.71 1.135 0.6133 0.1044 0.09408
c
3
-60
-
-80
-
0
c
-100
'
-6
1 I -5
-4
-3
-2
-1
L o g Anion A c t i v i t y (M)
Figure 2. Response of membrane electrodes prepared with dibenzyttin dichloride to the following series of anions: (0)dibasic phosphate, (A) thiocyanate, (0) iodide, (0)nitrate, (V)bromide, (V)chloride, and ( 0 ) acetate. when the ions under investigation have the same charge. This equation is not well suited, however, when ions of different charge are involved (6, 20-24). Coefficients are either deceptively large or small depending on whether the ion of higher charge is considered as the primary or interfering species. These distorted values are caused by the superscripts for the activity terms, which are necessary in the extended Nernst equation to account for the different response slopes for ions with different charges. Buck et al. (25)have recognized these problems in their latest recommendations for nomenclature of ion-selective electrodes. General recommendations are currently being formulated concerning the measurement of selectivity coefficients for systems with ions of different charge, based on critical evaluation of several methods including the one independent of the extended Nernst equation (26). Selectivity coefficients have been calculated for the three tin electrodes by the activity ratio method ( 7 , 8 , 2 7 ) .In this method, the selectivity coefficient is measured as the ratio of ion activities, or concentrations, that generate the same membrane potential when measured in a separation solution type experiment. For these measurements, the electrode response to the individual anions was measured over an ion activity range from approximately 0.1 to 10 mM. Unless otherwise noted, the data were modeled by the following function f(x) = E
+ s log ( x + B )
by using a nonlinear least-squares curve-fitting routine for the three parameters E , S, and B. Values used for f ( x ) were the mean potential changes measured for the group of electrodes tested. These mean values were weighted during the curvefitting procedure according to their standard deviations. Values of x corresponded to either ion activity or concentration. The resulting equations were then used to calculate the activity of dibasic phosphate that corresponds to the potential change predicted for an activity of 10 mM for the interfering ion. The selectivity coefficient was then calculated as the ratio of these activities (Le., KPoTi/j= ai/aj where ai and a, are the activities of the primary and interfering ions, respectively). The resulting selectivity coefficients are listed in Table I where the primary ion is dibasic phosphate. Each of these electrodes responds better to dibasic phosphate than to chloride. This fact demonstrates that even the poorest re-
-100
~
6
-5
-4
-3
-2
-1
Log Anion A c t i v i t y ( M )
Figure 3. Response of membrane electrodes prepared with (0,B) bis@-methy1benzyl)tindichloride, (0, 0 )dibenzyltin dichloride, and (V, V)bis@-chlorobenzy1)tindichloride to dibasic phosphate (closed symbols) and thiocyanate (open symbols). sponding of these electrodes has a superior response to phosphate than a typical membrane electrode based on a simple ion-exchange mechanism (28). The response pattern for anions other than dibasic phosphate, however, follows the conventional Hofmeister series. This second observation suggests the presence of a cationic site within the membrane that is capable of interacting with solution anions in a simple ion-exchange manner. The most striking feature of these data is the apparent relationship between electrode selectivity and the substituent in the para position of the benzyl ring. The response to phosphate ranks between chloride and bromide for the p methyl derivative. As the substituent is changed from a methyl group to hydrogen, the selectivity for phosphate improves by a factor of 10. The dibenzyltin dichloride electrodes have approximately equal selectivity for dibasic phosphate and nitrate. By changing the substituent from hydrogen to chlorine, selectivity improves by a factor of 100 and the electrode response to phosphate is about the same as that to thiocyanate. As the substituent in the para position goes from a methyl group to hydrogen to chlorine, the response for phosphate becomes better and the response for thiocyanate becomes worse. Figure 3 presents the responses to dibasic phosphate and thiocyanate for each of the tin electrodes, and Table I1 lists the parameters for the linear regions of these response curves. For phosphate, the linear region is extended and the detection limit is lowered. Conversely for thiocyanate, the linear range is shortened and the detection limit increases. As a result of these changes in the response curves, the selectivity for phosphate improves. Although the benzyl derivative provides a better response for phosphate than does
ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
757
Table 11. Response Properties of Bis(p-chlorobenzy1)tin Dichloride Electrodes to Dibasic Phosphate and Thiocyanate 0
response property
p-methyl
tin comud benzyl
p-chloro
h
>
-20
slope, mV/decade
phosphate thiocyanate beginning of linear
-29.8 (f0.3) -32.2 (10.4) -33.0 (f0.1) -52.8 (f0.6) -50.7 (10.9) -56.2 (f0.5)
w m t
-40 0
range, mM
-
phosphate 3.56 0.887 0.509 thiocyanate 0.0531 0.292 3.04 limit of det, pM" phosphate 350 (f10) 150 (f40) 32 (f5) 7.3 (fO.9) thiocyanate 22 (f4) 150 (f10) Limit of detection calculated according to IUPAC recommendations (18).
.-c0 C
+
-60
0
(L
-80
-100
Table 111. Response to Phosphate by Bis(p-chlorobenzyl)tin Dichloride Electrodes in the Presence of Selected Anions
anion activity, compd
I-
mM 0.00674
anionic-cationic NOc
Br-
Cl-
OAc'
0.660 1.21 0.0585 0.583 3.39 5.62 0.0611 0.609 4.37 5.86 0.182 1.81 8.49 17.0 0.183 1.81 10.3 17.0
response slope to phos, mV/decade -31.6 -30.7 -30.6 -28.9 -29.1 -28.3 -27.7 -26.3 -27.5 -27.3 -28.5 -27.8 -32.2 -32.4 -29.9 -29.3 -34.2 -29.5 -28.2 -26.7
(f0.2) (f0.2) (420.2) (fO.l) (f0.2) (f0.2) (fO.l) (f0.5) (fO.1) (f0.2) (f0.5) (fO.8) (f0.2) (f0.3) (f0.1) (f0.3) (fO.1) (fO.1) (f0.1) (f0.1)
phos det limit, p M 41 ( f l ) 110 (f30) 220 (f7) 204 ( f l ) 25 ( f l ) 40 ( f 2 ) 120 (f2) 240 (f10) 55 ( f l ) 113 (f4) 422 (f4) 280 (zk20) 44 ( f l ) 49 ( f 2 ) 66 ( f l ) 139 (ai601 49 ( f l ) 57 ( f l ) 86 ( f l ) 345 (f7)
the p-methyl derivative, in both cases the response is better for thiocyanate than for dibasic phosphate. With the p-chloro derivative, however, the electrode is actually more selective for dibasic phosphate at low activities. A crossover activity is observed where the electrode responds equally to phosphate and thiocyanate. After this crossover activity, the electrode responds better to thiocyanate than to dibasic phosphate. The responses from the p-chloro derivative dramatically illustrate the activity dependency of electrode selectivity when ions of different charge are considered. Separate solution type selectivity measurements are not able to uncover synergistic effects on the electrode response when phosphate and other anions are present in solution simultaneously. To test for such effects with the bis(p-chlorobenzy1)tin dichloride based electrodes, response curves have been measured for dibasic phosphate in the presence of different anions. The anions tested include iodide, nitrate, bromide, chloride, and acetate. In each experiment, the activity of the anion has been held constant and the electrode potential has been measured for a series of phosphate activities. The calibration slopes and limits of detection for the resulting phosphate response curves are listed in Table 111. Selectivity for phosphate is evident by the low detection limits for phosphate in the presence of these potential interferences. As expected, the detection limit for phosphate becomes worse
-6
-5
-4
-3
-2
-1
Log Anion Conc. (M)
Figure 4. Response of membrane electrodes prepared with bis@ch1orobenzyl)tin dichloride to the following anions: (0)dibasic phosphate (potassium salt), (0)dibasic phosphate (sodium salt), (A)cAMP (sodium salt), (V)AMP (sodium salt), (W) ADP (potassium salt), and (0) ATP (sodium salt).
as the amount of the interfering ion increases. The overall selectivity order is the same as that measured with the separate solution method. Lower detection limits are obtained a t higher concentrations of chloride and acetate, which indicates superior selectivity for phosphate over these anions. Iodide, on the other hand, has a strong influence on the detection limit a t relatively low concentrations. Responses from electrodes made with the p-chloro derivative have also been measured for CAMP,AMP, ADP, and ATP a t pH 7.00. These responses are presented in Figure 4 where the membrane potential is plotted with respect to the logarithm of the total nucleotide concentration. The corresponding responses to dibasic phosphate concentration are also presented for comparison. Responses to phosphate are given for the two cases where sodium and potassium salts were used to prepare the standard solutions. Accurate comparisons require that the counterion be consistent between responses. On the basis of detection limits, electrodes based on the p-chloro derivative respond to these adenosine-based nucleotides in the following order: ATP > ADP > AMP > cAMP The response to cAMP is essentially a horizontal line with only a slight response at high concentrations. The response to AMP is significantly better with a slope of -28.5 (f1.5) mV/decade at high concentrations and a detection limit of 220 pM. For ADP, the response slope is -21.4 (f1.6) mV/ decade and the detection limit is 13.2 pM. The response to ATP is characterized by a slope of -14.2 (k0.7) mV/decade and a detection limit of 0.77 pM. Both the response slope and the detection limit decrease as the charge on the nucleotide increases. Selectivity coefficients have also been calculated for these nucleotides with respect to dibasic phosphate. These coefficients have been calculated as concentration ratios (KPoT = [HP0,2-]/[Nuc.]) by fitting the measured membrane potentials to the model f ( x ) = E + S log ( x + B ) and using a concentration of 10 mM for the nucleotide. Values for CAMP, AMP, ADP, and ATP are 0.001 36, 0.0459, 0.655, and 0.203, respectively; hence, the selectivity order is
HP0,2- > ADP > A T P > AMP > cAMP These coefficientsindicate a preferential response to ADP over
758
ANALYTICAL CHEMISTRY, VOL. 63,
NO. 8, APRIL 15, 1991
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Table IV. General Response for Tested Organotin Compounds
0
compd
general response to potassium phosphate
h
>
diethyltin dichloride dioctyltin dichloride didodecyltin dichloride diphenyltin dichloride tribenzyltin chloride dibenzyltin dichloride bis@-methylbenzy1)tindichloride bisbchlorobenzv1)tindichloride
E
v
6
u-
-50
C
0
1 c)
-
?
+
-IC0
C
m
cationic anionic-cationic anionic-cationic slight anionic cationic anionic
anionic anionic
+ 0
c -150
-200 -6
-5
-4
-3
-2
-1
Log Anion Conc. (M)
Flgure 5. Response of membrane electrodes prepared with bis@chlorobenzy1)tin dichloride to the following anions: (0)dibasic phosphate, (e)pyrophosphate, (0)salicylate, ('I benzoate, ) (A)fluoride, and (W) arsenate.
ATP which contradicts the limit of detection data. This contradiction is caused by the concentration dependency of the selectivity coefficients. As shown in Figure 4, the response to ATP crosses that for dibasic phosphate at 3.43 X M. At concentrations below 3.43 X 10-4M, the electrode responds better to ATP, and at higher concentrations, the response is better for phosphate. By selecting an interfering ion concentration higher than 3.43 X M, the coefficient is less than unity. The responses to ADP and phosphate cross at approximately 1.2 X M. Figure 5 shows the responses obtained at pH 7.00 from electrodes based on the p-chloro derivative to salicylate, benzoate, arsenate, fluoride, and pyrophosphate. The response to dibasic phosphate has been included for comparison. In addition, the selectivity coefficients have been calculated for these anions relative to dibasic phosphate by using the procedure stated above. The response to salicylate is strong with a selectivity coefficient of 95.9. The response to benzoate and the selectivity crosses that of phosphate at 8.2 X coefficient for benzoate is 4.62. The response to arsenate closely parallels that for phosphate. The slope of the arsenate response is -30.2 mV/decade and the detection limit is 3.5 X M. The selectivity coefficient is 0.759, which indicates a slight preference for phosphate over arsenate. These electrodes respond in a non-Nernstian fashion to pyrophosphate and fluoride. A sharp decrease in the potential is measured at low pyrophosphate concentrations, and then the response levels off. For fluoride, on the other hand, little response is observed at low concentrations, but at high concentrations, the potential changes sharply with a slope of -82.1 mV/decade. Selectivity coefficients for pyrophosphate and fluoride are 173 and 0.279, respectively. The function used to model the response to pyrophosphate was f ( ~ = ) A + B exp(-x/C) where x corresponds to the negative logarithm of the anion concentration.
Relationship between Chemical Structure and Electrode Selectivity. Over the course of our work, many different organotin compounds have been screened by monitoring the response of the corresponding membrane electrode to solutions of potassium phosphate. The results of our screening experiments are summarized in Table IV. Each of the three dialkyltin compounds responded preferentially to potassium over phosphate. The dioctyl and didodecyl compounds responded slightly to phosphate at low concentrations before
a larger cationic response prevailed. The diphenyl derivative gave a weak anionic response, but the slope was low and the overall potential change for the entire concentration range was small. The tribenzyl derivative did not respond at all to phosphate. As detailed above, each of the dibenzyltin derivatives displays a significant response to phosphate, although to differing degrees. The key components of the active compound appear to be the benzyl group and the electron-withdrawing power of the substituents on the benzyl ring. Kitching et al. (29) used tin NMR spectroscopy to characterize the electron density around the tin center for a series of (benzyltrimethy1)tin compounds. They report a correlation between the position of the tin resonance and the electronwithdrawing power of the substituent on the benzyl ring. Evidence is given to suggest that a linear relationship exists between the chemical shift of the tin peak and the Hammett constant of the substituent. Higher Hammett constants, corresponding to stronger electron-withdrawing power, result in larger downfield chemical shifts, which indicates lower electron density about the tin center. In addition, Kitching and co-workers (29) give evidence that the preferred bond angle between the tin, the methylene carbon, and the first carbon in the aromatic ring is approximately 60". They further speculate that tin-carbon hyperconjugation is significant in these compounds. The cationic charge caused by pulling electron density from the tin center through the tincarbon bond is stabilized by the electron-rich r system of the aromatic ring. The preferred bond angle places this r system in close proximity to the tin center, thereby facilitating this stabilization. Our results have lead us to hypothesize that tin-carbon hyperconjugation is important for the selective response to phosphate. Of all the tin compounds tested, only the dibenzyl derivatives give significant responses to phosphate. Moreover, the Hammett constant of the substituent on the benzyl ring appears to be related to the quality and selectivity of the response to phosphate. Figure 6 shows how the limit of detection for phosphate relates to the Hammett constant of the substituent on the benzyl ring. Likewise, this figure shows the correlation between the measured selectivity coefficients and these Hammett constants. In both cases, the trend is clear. The quality of response to phosphate and the selectivity for phosphate are improved significantly by increasing the electron-withdrawing power of the substituent on the benzyl ring. A wide variety of organotins are possible to test the validity of this hypothesis. Swain and Lupton (30)have tabulated Hammett constants for a number of possible meta and para substituents. Compounds of particular interest include those based on a trifluoromethyl group in the meta and para positions and those with a methoxy group in these positions. The Hammett constants for the m- and p-trifluoromethyl groups are +0.430 and +0.540, respectively, and constants for the mand p-methoxy groups are -0.42 and -0.266, respectively. We predict that the trifluoromethyl derivatives will provide the
ANALYTICAL CHEMISTRY, VOL.
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63,NO. 8, APRIL 15, 1991 750
in the membrane are required before a selective response to phosphate is obtained. We have experimental evidence that hydrolysis of both tin-carbon and tin-chlorine bonds is considerable during both the initial conditioning procedure and normal electrode operation (31). These hydrolysis reactions will clearly alter the composition of these membranes. Complexation reactions between the tin compounds in the membrane and anions from solution will likewise effect the chemical composition of these membranes. At this point, the stoichiometry between the ionophore and phosphate is not established. Overall, many questions remain unanswered concerning the fundamental chemistry responsible for the selective response of these electrodes to phosphate.
LITERATURE CITED Chang, Q.; Park, S.B.; Kliza, D.; Cha, G. S.; Yim, H.; Meyerhoff, M. E. Am. Biotechnol. Lab. 1990, 8 , 10-21. Solsky, R. L. Anal. Chem. 1990, 62. 21R-33R. Chaniotakis, N. A.: Chasser. A. M.: Meverhoff, M. E. Anal. Chem. 1988, 6 0 , 185-188. Wuthier, U.; Pham, H. V.; Zund, R.; WeRi, D.; Funck, R. J. J.; Bezegh, A.; Ammann, D.; Pretsch, E.; Simon, W. Anal. Chem. 1984, 56,
0
535-538. - -- -- -.
-1
-2
I -0.2
-0.1
0.0 Hammett
0 1
0.2
0.7
Constant
Flgure 6 . Correlation between electrode response and Hammett constant for the substituent in the para position on the benzyl ring. Graph A shows the correlation with the detectlon limit for dibasic phosphate, and graph B shows the correlation with selectivity coefficient for (0)thiocyanate, (V)iodide, (V)nitrate, (0) bromide, (W) chloride, and (A)acetate.
best response of all compounds tested to date and that the methoxy derivatives will result in the worst response. Another aspect to be investigated is the combination of substituents on the benzyl ring to further enhance the electron-withdrawing character of the benzyl ring. At some point, however, we anticipate a maximum in the quality of response to phosphate as the electron-withdrawing power is increased. Hydrolysis of the tin-carbon bond must be considered as a potential mechanism for deactivating the membrane-active compound. Results from preliminary experiments with dibenzyltin dichloride reveal considerable hydrolysis of both the tin-carbon and the tin-chloride bonds under normal operating conditions of the sensor (31). Hydrolysis of the tin-carbon bond will certainly be promoted as electron density is further stripped away from the tin center. The organotin compound that possesses the proper electron-withdrawing power to balance between tin-carbon hyperconjugation and tin-carbon hydrolysis should provide the best response properties for a phosphate-selective polymer membrane electrode. Finally, we must stress that the interfacial chemistry responsible for the unusual selectivity of these membrane electrodes is still not well established. The chemical composition of the "active" membrane is not yet defined. In fact, the structure of the ionophore that provides the selective response to phosphate has not been identified. The fact that pretreatment is required to suppress the initial cationic response of these electrodes demonstrates that chemical changes
(25) (26) (27) (28) (29) (30) (31)
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RECEIVED for review October 15,1990. Accepted January 11, 1991. This work was supported by a grant from the National Institute for Dental Research (DE07996).
