Tin organic compounds as neutral carriers for anion selective electrodes

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Anal. Chem. 1084, 56,535-538

Tin Organic Compounds as Neutral Carriers for Anion Selective Electrodes Urs Wuthier, Hang Vi& Pham, Richard Ziind, Dieter Welti, Robert Jean Jacques Funck, Andras Bezegh, Daniel Ammann, Ern0 Pretsch, and Wilhelm Simon* Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH),CH-8092 Zurich, Switzerland

Trl-n-octyltln chlorlde acts as a neutral carrler for anlons. Llquld membranes contalnlng thls carrler, ( R ,R )-2,3dlmethoxysuccinic acld bls( 1-butylpentyl) ester as plastlclzer, and poly(vlny1 chlorlde) as membrane malrlx show selectlvlly patterns whlch are not In accordance with the sequence obtained for classlcal anlon exchangers. These neutral carrler membranes dlsplay Nernstian electrode functlons for dlfferenl anlons In the I O 9 M to IO-' M concentratlon range. NMR and vapor pressure osmometry studles lndlcate an lnteractlon of the tln organlc compound wlth the chlorlde anlon. The expected mlgrallon of the tin organic compound In the dlrectlon of the anode is demonstrated with an electrodlalytlc transport experlment.

A large number of anion-selective liquid membrane electrodes based on classical ion exchangers such as quaternary ammonium salts, phosphonium salts, complexes of 0phenanthroline, and of other complexing agents have been described (1-4). Electrodes have been proposed for the analysis of Cl- (5-9),NO, (3,9,IO),SCN- (11-13) and other inorganic as well as organic anions. For electrodes based on tetraalkylammonium salts, 35 different detectable anions have been mentioned in ref 1. Usually all these electrodes exhibit roughly the same selectivity sequence with a preference of lipophilic and a rejection of hydrophilic anions (14, 15). In these cases, the free energy of transfer of the anions from the aqueous sample phase to the membrane phase-and therefore the membrane selectivity-is controlled by the free energy of hydration of these ions (4,16-20). This is in contrast to neutral carrier based systems for cations, where the ion selectivity highly depends on the free energy of the interaction of the ions with the ligand (14,21). In the latter case, a wide variety of ion selectivities is made accessible (22). Tin organic compounds have been used as components in anion selective electrodes with response t o C1- (23), HAs0,2-IH2As04-(N), NO, (W), and other species. Although tripropyl tin chloride (26)and other triorgano tin compounds (27) have been found to influence anion transport, it has not yet been properly proved that such tin organic compounds may act as neutral carriers for anions. In this paper, we show that trioctyltin chloride displays anion carrier properties if incorporated in solvent polymeric membranes. EXPERIMENTAL SECTION Cell Assemblies for EMF Studies. For all EMF measurements, cells of the following type were used: Hg, Hg2C12;KC1 (satd)llM LiOAclsample solution llmembrane 110.01 M NaC1; AgC1, Ag The external reference electrode was a double junction saturated calomel electrode with a ceramic diaphragm (for details see ref 28). Measurements were performed with four different membranes of the following compositions: (a) 6 wt % methyltri-ndodecylammonium chloride (MTDDACl), 65 wt % di-n-butyl phthalate (DBP), 29 wt % poly(viny1 chloride) (PVC); (b) 3 wt % MTDDACI, 49 wt % (R,R)-2,3-dimethoxysuccinic acid bis(10003-2700/84/0356-0535$01.50/0

butylpentyl) ester (DMSNE), 48 wt % PVC; (c) 3 wt % tri-noctyltin chloride (TOTCl), 49 wt % DMSNE, 48 wt % PVC; (d) 20 wt % TOTCl, 40 wt % DMSNE, 40 wt % PVC. All four membranes were prepared as described in ref 29. They were mounted in Phillips electrode bodies IS 560 (N. V. Philips, Gloeilampenfabrieken, Eindhoven, The Netherlands) and conditioned overnight in about 2 mL of a 0.01 M NaCl solution, which corresponds to the internal filling solution of these electrodes. EMF Measurements. The EMF measurements were performed at 21 & 1"C by immersing four ion-selectiveelectrodes and the reference electrode into a PVC vessel containing about 25 mL of the sample solution. Reference and ion-selective electrodes were connected with FET operational amplifiers AD 515 KH (Analog Devices, Norwood, MA), and the analog signal of the difference amplifier (input impedance 1019Qll2 pF) was processed by an analog interface board DT 1744 (Data Translation, Natick, MA), and a single-board computer (SBC 80/20-4; SBC 116A; Intel Corp., Santa Clara, CA) served to store the data for further manipulation with the built-in arithmetic processor AMC 95/6011 (Advanced Micro Computers, Santa Clara, CA). A display terminal (ADDS Regent 20, Applied Digital Data Systems, Inc., Hauppauge, NY) and a printer (Matrix-Drucker Print Swiss, Wenger Print Swiss Matrix, Basle, Switzerland) were used for viewing the data, which were processed off-line on a HewlettPackard HP 85 calculator system. Electrode Functions and Selectivity Factors. The observed EMF differences were corrected for changes in the liquid junction potential for the various sample solutions according to the Henderson formalism (14, 30, 31). Single-ion activities were calculated with use of the Debye-Huckel theory; the equations and parameters are given elsewhere (32-34). EMF values were taken every minute over a period of 20 min. The average potential difference of the last 5 min (i.e., of six data points) was chosen for further evaluation. All electrodes showed response times (defied here as the time within which the potential change gets smaller than 0.1 mV/min) of less than 3 min, improving with increasing salt concentration. The stabilities of the EMF values were moderate (10.2 mV/min) for lo4 M solutions and good (10.1 mV/min) for more concentrated solutions during the remaining 17 minutes of each measuring period and all anions marked in Figure 1. The selectivity factors of the four membranes tested (see Figure 2) were obtained by the separate solution method (SSM), using 0.1 M solutions of the sodium salt of each anion. Since the various anions can cause the pH value to differ considerably from sample to sample, all solutions for the determination of the KcKpotvalues were buffered with 2-amino-2-(hydroxymethyl)-l,3-propanediol (Tris) to pH 7.50 0.04. The response times for selectivity factor measurements were less than 2 min in all cases, except for the membrane with 3 wt % TOTC1. For the latter, response times of up to 15 min were observed, and the EMF stabilities were poor (10.3 mV/min), which is in sharp contrast to the three other membranes used (10.02 mV/min). The selectivity pattern of this membrane is therefore somewhat uncertain. This is not surprising, considering the membrane resistance values: the membrane with 3 wt % TOTCl shows a resistance of about 1O1O Q , whereas the membrane with 20 wt % TOTCl displays a resistance of roughly 108 Q. These values have been obtained by EMF measurements in a 0.1 M sodium chloride solution. The unknown membrane resistance and an interchangeable reference resistance were arranged in a parallel circuit (35). Since the reference resistances are known with a tolerance of *lo% only, the accuracy of the resistance values given above is limited.

*

0 1984 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984

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EMf

[mv

-5

20% TOTCI 40 70 DMSNE 40 "10 PVC

-3

-4

-2

-1 log ax

Flgure 1. Electrode functions of a cell assmembly with a membrane based on tri-n-octyltin chloride for four different anlons. The spaces between the curves are not drawn to scale, but their order corresponds to the selectivity sequence (see Figure 2).

Iog

'ot

5

4

3

--1-

--

I-

2

1

0

-1

-2

6O/. MTDDACI 3 % MTDDACl 3% TOTCI 65% DBP 49% DMSNE 49% DMSNE 29% PVC 48% PVC 48% PVC

20% TOTCl 40% DMSNE 40% PVC

Figure 2. Selectivity factors, log KCIXPot for membranes based on a classical ion exchanger (columns 1 and 2) and on a tin organic compound (columns 3 and 4) as determined by the separate solution

method.

The electrode functions were obtained by measurements in solutions with four different concentrations of each anion. The slopes were then determined by linear regression over the activity M to lo-' M (see Figure 1). range from NMR Measurements. 13Cand ll9Sn FT NMR spectra were recorded at 50.32 MHz and 74.63 MHz, respectively, on a 4.70-T Bruker WP 200 SY spectrometer. A 10 mm 0.d. tube containing

2.5 mL of 0.2 M trioctyltin chloride in deuteriochloroform was prepared, and small amounts of dried 4,7,13,16,21,24-hexaoxal,l0-diazabicyclo[8.8.8]hexacosane (Kryptofix 222) potassium chloride salt were added in a drybox consisting of a Dri Lab DL-001-S-G, a Dri Train HE 493, and a Pedatrol HE 63 P (Vacuum/Atmospheres Co., Hawthorne, CA). The complex salt had been synthesized by separately dissolving equimolar amounts of potassium chloride and Kryptofix 222 in absolute methanol, pouring the two solutions together, and evaporating the solvent. The obtained crystalline salt showed a melting point of 189-190 "C. Initially, and after each addition, 13Cand ll9Sn shifts were measured. The coupling constant 1J(11gSn-13C)was determined from the I3C spectra. Conditions for 13CMeasurements: Spectral width 6024.10 Hz, acquisition time 2.7197 s, data table size 32K of 24 bit words, rf pulse duration 40 ps (approximately 86O), number of transients generally 500 to 1000. Broad-band proton decoupling of approximately 2 W with 250 L/h ambient temperature air cooling was applied during the acquisition period. In order to have similar temperatures for 13Cand lleSn measurements, the decoupler power was reduced to the minimum 0.5 W required to maintain the NOE for 3.0 s after each acquisition period. MelSi was used as an internal standard. Conditions for ll9SnMeasurements: Spectral width 25000 Hz, acquisition time 0.3277 s, data table size 16K, pulse duration 25 ps (approximately B O O ) , number of transients 400 to 1600 depending on the line width of the ll9Sn signal. In order to reduce potentially unfavorable NOE, the broad-band proton decoupling of 2 W was switched off for 4.0 s between the end of the acquisition period and the next pulse. The temperature for the l19Sn spectra was therefore slightly lower than for the 13Cspectra. A coaxial 1.2 mm 0.d. capillary containing tetramethyltin was used as an external standard at the beginning of the measurements, and all subsequent spectra were referred to the same absolute frequency. At the end of the series of measurements, the capillary was mounted again, and a shift of -0.46 ppm relative to the initial value of tetramethyltin was observed. This progressive difference of less than 0.23% of the total shift could safely be neglected. Vapor Pressure Osmometry (VPO). All measurements were performed on a vapor pressure osmometer constructed in this laboratory (36). For the processing of the signals, a dc amplifier Keithley Microvolt Ammeter type 105 B (Keithley Instruments Inc., Cleveland,OH), a digital voltmeter Solartron type LM 1440.2 (Solartron Electronic Group Ltd., Farnbourg, GB), and a W+W recorder (W+W Electronic, Basle, Switzerland) were used. A temperature of 30 f 0.001 OC was maintained by two Haake thermostats F423 (Gebriider Haake, Berlin, GFR). All solutions were prepared with methylene chloride (which had been purified by refluxing for 2 h in the presence of molecular sieves 4A 1/16 (Bender and Hobein AG, Zurich, Switzerland) and ensuing distillation) in a drybox to avoid water uptake. For all VPO measurements, solutions of about 3 X mol/kg of tetrabutylammonium chloride and the complex salt of potassium chloride with Kryptofix 222, respectively, were used as reference samples. In both cases, TOTCl was added to these solutions to give different tin compound concentrations over the range from mol/kg to 2 X lo-' mol/kg (37). The proportionality constant between the signal measured and the molality of the sample solution was determined by calibration with sulfonal. The readings were taken after 10 min of equilibration. All measurements were repeated three times, and the obtained values were averaged. Calibrations were made at the beginning and at the end of each measurement to allow for zero point shifts. The Bjerrum complex formation curves (37) for different stoichiometriesindicated that a 1:l complex only is formed. The equilibrium constants K were obtained by the least-squares method (38) with the experimental values of a one-parameter nonlinear model of a 1:l complexation (39) [LI = (K([Llo - [XI,) - 1 + [W([Llo - [XI,) - 112+ 4K[L101"')/2K The free and the total ligand concentrations are represented by [L] and [L],, respectively; and [XI, is the totalanion concentration. Transport Experiment. The cell assembly for the electrodialysis experiments has been specified earlier (40). The mem-

ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984

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brane system studied consisted of a stack of three equal membranes (20 wt % TOTCI, 40 wt % DMSNE, 40 wt % PVC) with a total weight of all three membranes of ca. 180 mg and an overall thickness of about 200 pm. The segments were prepared as follows: The three membrane components were dissolved in roughly 2 mL of freshly distilled tetrahydrofuran, and the solution was poured into a glass ring with 45 mm inner diameter, which was pressed on a clean glass plate by a rubber band. M e r evaporation of the solvent overnight, the membranes were carefully taken off the glass, and three disks of 18 mm diameter were cut out. In order to ensure quick and smooth detachment of the segments after the dialysis experiment (which is important to prevent a back diffusion induced change in the concentration gradient of the ligand built up during the experiment), Teflon rings (thickness, 50 pm; outer diameter, 18 mm; inner diameter, 10 mm) were placed between the single cm 1200 1000 800 600 400 200 membrane segments. Therefore, the membrane stack exhibited an actual electrodialysis-active diameter of 10 mm. Figure 3. Infrared spectra of three membrane segments alter the The potential of the electrodialysis was controlled by a Fairchild electrodiaiysls experiment. The Identical transmittance of the three Integrating Digital Meter, Model 7100A (Fairchild Cameras & segments at 323 cm-' after their restacking for 50 h Is also indicated. Instruments Corp., Mountain View, CA), and the current was measured by a Keithley Microvolt Ammeter type 150B (Keithley % D) were obtained from Ciba-Geigy AG, Basle, Switzerland, and Instruments, Cleveland, OH), the current-time curve being resulfonal was supplied by the British Drug Houses Ltd., Poole, corded on a W+W recorder Model 1100 (W+W Electronic, Basle, England. Switzerland). RESULTS AND DISCUSSION The electrodialysis was carried out at 25 f 1OC, using identical solutions in both cell compartments 0.1 M (C4H8)4NC1.The same The liquid membrane electrodes based on methyltri-n-dosolution ssnved to condition the membrane segments for 50 h decyl ammonium chloride (MTDDACl) show the selectivity before t?ie experiment in order to ensure equilibrium between sequence expected for classical anion exchanger membranes membrane and cell solutions. The lipophilic tetrabutylammonium (see Figure 2). This sequence is not heavily influenced by cation was chosen to guarantee sufficient uptake of chloride anions changes in the plasticizer and/or the concentration of the into the membrane stack. quaternary ammonium compound (1) (see columns 1 and 2 Applying a constant potential of 15 V throughout the experof Figure 2). Certain deviations observed for membranes iment, the current decreased exponentially during 90 min from an initial value of 160 pA to a constant value of 60 MA. When containing trioctylmethylammonium benzoate in o-dichlorothe electrodialylsis was stopped, the membrane stack was disbenzene/octylphenol have been attributed to hydrogen assembled in less than 90 s. bonding between plasticizer and anion (42-44). Before and after the experiment, the amounts of ligand in the Substantial changes in the selectivity sequences are induced, three membrane segments were measured separately by infrared if tri-n-octyltin chloride (TOTCl) is incorporated into memspectrometry, using the absorption band of the Sn-C1 stretching branes (see columns 3 and 4 in Figure 2). As expected, a vibration at 323 cm-l. In order to correct for changes in the 100% acid membrane with TOTCl in (R,R)-2,3-dimethoxysuccinic transmission, the spectra had to be standardized relative to an bis(1-butylpentyl) ester (DMSNE) exhibits an almost absorption band caused by poly(viny1chloride) and the plasticizer Nernstian electrode response for SCN-, ClO;, NO3-,and C1only. The absorption band at 1024cm-I proved to be most suitable for this purpose. Thus, the absorption spectra of the three seg(Figure 1)with slopes of -61.9 f 2.7 mV, -55.3 f 0.9 mV, -60.4 ments obtained after the electrodialysis were shifted until they f 3.0 mV, and -60.1 f 0.1 mV, respectively (theoretical, -58.36 showed the same absorption intensity at 1024 cm-I. The ligand mV). This is in agreement with the behavior of a neutral concentration profile obtained by this procedure is depicted in carrier for anions. Figure 3. Afterward, the membrane segments were restacked for An NMR study shows indeed that there is a direct inter50 h. After this relaxation period, the ligand concentration profile action of chloride anions with TOTCl, if chloride salts are displayed (after standardization as mentioned above) the same added in increasing concentrations to a fixed amount of absorption intensity at 323 cm-I for all three membrane segments, TOTCl. The three curves presented in Figure 4 for the I19Sn which is also indicated in Figure 3. chemical shift (6119sn), the chemical shift of the carbon atom For the infrared measurements and the spectra manipulations, a Perkin-Elmer infrared spectrophotometer, Model 283 and a in a-position to the tin center (61ac) a5 well as the coupling Perkin-Elmer Infrared Data Station, Model 3600, were used, constant between these two atoms (lJ(llOSn-lsC)) have been including a software (PECDS) specially designed for this purpose calculated with a stability constant of the interaction of C1(Perkin-Elmer Corp., Norwalk, CT). with TOTCl of 52 Lemol-' in CDC1, assuming a 1:l complex Reagents. All electrolyte solutions for the potentiometric formation. This interaction is corroborated by vapor pressure measurements were prepared with doubly quartz distilled water osmometry measurements. As indicated in Figure 5 C1- inand sodium salts of high purity (pro analysis, E. Merck, teracts with TOTCl in CH2C12with a stability constant of 151 Darmstadt, GFR); exception was sodium thiocyanate from Fisher f 30 kg-mol-l and 345 f 28 kgmol-I if the tetrabutylScientific Co., Fair Lawn, NJ). Tetrabutylammonium chloride (purum), 4,7,13,16,21,24-hexaoxa-l,l0-diazabicyclo[8.8.8]hexa- ammonium ion and a complex of the potassium ion with Kryptofix 222, respectively, are used as counterions. cosane (puriss.), tetramethyltin (purum), methanol (puriss.), 2-amino(2-hydroxymethyl)-1,3-propandiol (puriss.), and tetraConvincing evidence for a neutral carrier based anion hydrofuran (puriss.) were obtained from Fluka AG, CH-9470 transport can be obtained by an electrodialysis experiment. Buchs, Switzerland. Poly(viny1chloride) (PVC S704 hochmoleIf a potential gradient is applied on a membrane stack conkular) originated from Lonza AG, CH-3930 Visp, Switzerland; sisting of identical segments (see insert in Figure 3) in the and dibutyl phthalate (pract.) was provided by Bender & Hobein presence of an easily extractable chloride salt (tetrabutylAG, Zurich, Switzerland. (R,R)-2,3-Dimethoxysuccinicacid ammonium chloride), there must be a migration of the tin bis(1-butylpentyl)ester was kindly supplied by V. Prelog and K. organic compound if either a dissociation of the tin compound Kovacevic, ETH Zurich; its synthesis is described in ref 41. into Cl- and a positively charged Sn species or an association MTDDACl was obtained from Polysciences Inc., Warrington, PA; of C1- with TOTCl occurs. For these two situations, opposite and TOTCl was synthesized in this laboratory according to ref 39. Tetramethylsilane(Me,Si) and deuteriochloroform (99.8 atom migration directions are to be expected. In Figure 3 the

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ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984

Registry No. TOTC1,2587-76-0; DMSNE, 82052-75-3; PVC,

9002-86-2. LITERATURE CITED

- 480

- 440

- 400 - 360

- 320 05

0

10

15

20

25

30

MOLES CI- I MOLES LIGAND

Flgure 4. Chemical shifts 8"osn and 613, for the tin center and the carbon atom in the CY position, respectively, and one-bond coupling constants rJ(1'eSn-'3C) between these two nuclei for soiutlons with different ratios of Kryptofix 222 potassium chloride salt to tri-noctyitln chloride. FREE LIGAND CONCENTRATION l L 1 [m mol. k g ' ]

15

10 CH2C12 K. 151 1 3 0 k g mol-'

5

CH2C12 K - 3 4 5 2 2 8 kg mol' 0

0

5

10

15

20

[m mol. k g '

TOTAL LIGAND CONCENTRATION [ L

Flgure 5. Concentration of the free ligand vs. total l l n d concentration measured by vapor pressure osmometry at different molalities of trinoctyitin chloride. A constant background of tetra-n-butylammonium chloride in CH,Ci, (0)or Kryptofix 222 potassium chloride in CH2Ci, (0)was used. A comparison is given for pure ligand in CH,Ci, (0).

infrared spectra of the three membrane segments are presented. They were recorded after a steady state had been established. The accumulation of tin organic species in membrane segment C and the decrease of its concentration in segment A are compatible with a neutral carrier transport mechanism for anions and rules out the possibility of TOTCl being a classical exchanger. A restacking of the membrane segments actually restores the symmetrical distribution of the ligand concentration (50 h of relaxation in Figure 3), as expected for a neutral carrier. Since the tin organic compound is lost to some extent into the aqueous solutions of the chloride salt, some difference in the total amount of the ligand in the three membrane segments before and after the electrodialysisexperiment is observed. Fortunately,this loss has been proved to be virtually identical for all three segments. The concentration of TOTCl in segment C, however, was clearly above the initial level. The behavior of tin organic compounds as neutral carriers for anions opens up a wide variety of accessible and analytically relevant anion selectivities.

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RECEIVEDfor review August 30,1983. Accepted November 3, 1983. This work was partly supported by the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung and Orion Research. A.B. thanks the Scientific Exchange Agreement for a grant.