2168
Anal. Chem. W Q I , 63,2168-2174
(14) Connors, T. F.; Arena, J. V.; Rusllng, J. F. J . mys. Chem. 1988, 92, 28 10-28 18. (15) Owlla, A.; Wang, 2.; Rusllng, J. F. J . Am. Chem. SOC. 1989, 7 1 7 , 5901-5908. (16) Rusling, J. F.; Miaw, C. L.; Couture, E. C. Inwg. Chem. 1990, 29, 2025-2027. (17) Osteryoung, J.; O’Dea, J. J. In Electroanalyticel Chem/stty;Bard, A. J., Ed.; Marcel Dekker: New York, 1986; Vol. 14, pp 209-308. (18) Webber, A.; Shah, M.; Osteryoung, J. Anal. Chim. Acta 1984, 757, 1-16. (19) Owlla, A,; Rusling, J. F. J . Nectroanal. Chem. Interfaclal Nectrochem. ioa?, 234, 297-314. (20) Zeng, J.; Osteryoung, R. Anal. Chem. 1988, 5 8 , 2766-2771. (21) Rudzinski, W. E.; Bard, A. J. J . Electroanal. Chem. InterfacielElectrochem. 1988, 799, 323-340. (22) King, R . D.; Nocera, D. G.; Pinnavaia, T. J. J . Nectroanal. Chem.
Interfackl Ekhochem. 1987, 236, 43-53. (23) Andrieux, C. P.; Blocman, C.; Dumas-Bouchlat, J.-M.; M’Halla, F.; Saveant, J. M. J . Electroanel. Chem. Inteffackrl Electrochem. 1980, 773, 19-40. (24) Rusling, J. F. Acc. Chem. Res. 1991, 24, 75-81. (25) Halbert, M. K.; Baldwin, R. P. J . Chromatogr. 1985, 345, 43-49. (26) Halbert, M. K.;Baldwin, R. P. Anal. Chem. 1985, 57, 591-595. (27) Santos, L. M.; Baldwin, R. P. Anal. Chem. 1988, 58, 848-852.
RECEIVED for review March 1, 1991. Accepted July 1, 1991. This work was supported by U.S. PHS Grant ES03154 awarded by the National Institute of Environmental Health Sciences.
Measurement of Concentration Profiles inside a Nitrite Ion Selective Electrode Membrane Xizhong Li and D. Jed Harrison* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
The behavlor of bromo(pyrldlne)(5,10,15,20-tetraphenylporphyrlnato)cobattate(CoTPP(py)Br) as a NO,--selectlve Ion carrier Is reported. I n dloctyl adlpate plastlclred poly(vlnylchlorlde) membranes the carrier Induces NO,- sensltlvlty wlth a slope of -57 mV/decade and selectlvlty coefflclents for CI-, Br-, H,P04-, and HP04?-of 5 X lo4, 1.6 X 2.1 X and 6.1 X lo4, respectively. These membranes respond to pH changes above pH 6 wlth a slope of -25 to -34 mV/pH unit. A spatlal Imaging photometer has been developed to Image the concentratlon proflle changes Inside the membrane In the dlrectlon of Ion transport wlth a spatlal resolutlon of less than 5 pm. The uptake of H20 Is observed by the formatlon of llght scattering centers and exhlblts a nonunlform dlstrlbutlon across the membrane. The transport of NO2- Is also observed when a CoTPP(py)Br-contalnlng membrane Is exposed to NO2- at one membrane/solutlon Interface. The Internal concentration proflle Is descrlbed by Flck’s laws of dlffuslon, glvlng a dlffuslon coefficient of 5 X lo-’ cm2/s In the membrane. Evldence for lnhomogenelty In the membrane In the dlrectlon of charge transport Is also obtalned.
INTRODUCTION Considerable effort has been invested in theoretical analysis of membrane potentials during the past century (1-17). Much of the behavior of membrane potentials can be satisfactorily described; however, a number of somewhat different assumptions can be made about the internal concentration profile of electroactive species within a membrane (9,12).In the case of ion-sensitive membranes differentiation between the models is based on deductions from the membrane potential and current response to external stimuli (9,12, 13, 15-1 7). There is very little direct evidence from measurements of the internal concentration profiles themselves, and this means the extent to which a membrane obeys the ideal models is difficult to determine. Transport of K+ and H+ across a HzO/CHC13/Hz0liquid membrane has been examined by
* To whom correspondence should be addressed. 0003-2700/91/0363-2168$02.50/0
O’Brien et al. using a holographic technique based on the dependence of refractive index on solute concentration (18). Their study suggested convective effects were present during the initial stage of transport, but the steady-state distribution showed a linear concentration gradient across the CHC13 phase, indicating Ficks laws of diffusion adequately describe transport in the liquid membrane. Most liquid membrane electrodes in active use are based on a polymer membrane structure consisting of poly(viny1 chloride) (PVC), a H20-immiscible organic solvent such as dioctyl adipate, an ionophore such as valinomycin, and lipophilic salts to enhance Donnan exclusion (12).These materials are far more complex than a pure liquid membrane, but there is very little detail on the internal concentration profiles. Simon’s group have used a radiotracer technique to analyze transport across a membrane divided into five 40 pm thick segments, providing a spatial resolution of -40 pm (11). In that work an applied voltage was used to drive ions across a valinomycin-containing membrane. The distribution of K+ and valinomycin observed were qualitatively in agreement with the models developed by Morf et al. (10)and later by Buck, Pungor, and their co-workers (14-17). However, the radiotracer study offered insufficient spatial resolution and precision for quantitative comparison to theory. Further, Thoma et al. (11)did not obtain data under open circuit conditions with asymmetrical bathing solutions, which is in fact the common mode of use of these membranes. We have developed a spatial imaging photometer (SIP)to measure changes in absorbance in the direction of transport across an ion-sensitive membrane, with a nominal spatial resolution of 1.25 pm. The apparatus is similar to those described by Fukanaka et al. (19)and later by McCreery and co-workers (20,21)for imaging the electrochemical diffusion layer at a solid electrode. Those authors have reported the spatial resolution due to diffraction from the lenses and refractive index effects is limited to 2 pm (19,21), and due to edge diffraction effects a resolution of less than 5 pm is estimated near the electrode edge (21). We have recently reported the details of the design of the instrument and the electrochemical cells used to probe the distribution and diffusion rate of HzO inside a PVC-based membrane, using water-sensitive (solvatochromic) dyes (22).In the study of the distribution of electroactive species, the method is limited 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
Table I. Molar Absorptivity:
e,
2169
of CoTPP(py) Derivatives in Several Solvents
wavelength, nmb solvent
440
544
CHCl,' dioctyl sebacate PVC, DOA membraned
135000
8100 7100 7200
CoTPP(py)Br 556 10700
594
633
432
5010
1300 1600 1460
140200
CoTPP(py)N02 544 548 12000
633
12250
1200 1960
10700 10000
1300
OMolar absorptivity in L/(mol.cm). bAbsorbance peak maxima, except for 544 and 633 nm. CAveragesfor concentrations in the range (0.6-1.4)X lo-' M dye in chloroform. dMembrane composition as given in the Experimental Section. to ions or ionophores that are also chromophores. Metalloporphyrins are good candidates for the present study, as they have recently been shown to be useful ionophores for anions, since they exhibit selectivities different than classical ion exchangers (23-26) and they are well-known to have large molar absorptivities. We have prepared a NG2- ionophore, bromo(pyridine)(5,10,15,20-tetraphenylporphyrinato)cobalt(111) (CoTPP(py)Br), not previously used for this purpose, which differs modestly from a cobalt porphyrin derivative earlier reported by Ammann e t al. (23). The S I P has been used to characterize changes in the concentration profile of CoTPP(py)Br and its nitrite derivative in a PVC-based membrane when the NO2- content in the contacting solution changes. This corresponds to the situation common to many anion exchangers, where the membrane is first equilibrated with a solution of the target anion t o incorporate it in the membrane (27) and so allows a detailed study of transport during this process.
EXPERIMENTAL SECTION Instrumentation. A diagram of the SIP is shown in Figure 1. Details of the instrument design and the electronic interface with a Compaq 286 computer and Data Translation DT2801-A analog-to-digital converter are presented elsewhere (22). With the magnification used in this study the nominal spatial resolution was typically between 2 and 5 pm/diode. The optics have been modified since that report to allow use of two He:Ne lasers (Melles Griot) operating at 633 and 544 nm in a coplanar, concentric geometry. This was done to correct for light-scattering effects due to the uptake of H20 in the membrane following equilibration with aqueous solution. The data reported are calculated from the difference in absorbance of these two wavelengths as discussed later. Electrochemical cell voltages were measured by using an Orion 701A meter, and a linear strip chart recorder was used to record potential versus time. A Hewlett-Packard 8451A photodiode array spectrometer was used to record spectra of solutions and of membranes. The latter were obtained by transmission through the membrane bulk parallel to the direction of transport, and so any concentration gradient across the membrane was averaged. Reagents. Water was deionized and doubly distilled, while various salts were reagent grade and were used as received. The cobalt porphyrin derivative was prepared by using a method similar to that of Ammann et al. (23), viz. (5,10,15,20tetraphenylporphyrinato)Co(II) was used as received (Aldrich) and dissolved (200mg) in 40 mL of pyridine (BDH, distilled from BaO under Ar). The mixture was reacted at 100 "C in air for 1 h, after addition of 2 mL of HBr (48%). After cooling, this was poured into 100 mL of CH2C12,extracted with 0.1 M HBr (100 mL), separated, and washed briefly four times with 100 mL of H20. The organic phase was dried over MgSO,, filtered, and evaporated to dryness with a Rotovap. The residue was taken up in 4 mL of CH2C12and precipitated by addition of 100 mL of hexane. The very fine violet powder was collected on a fine glass frit (type D)and redissolved by using 35 mL of ethyl acetate to which a few drops of pyridine was added. The volume was reduced to -20 mL at 60 "C and a first crop of crystals was collected after 3 days at -5 "C. Following collection of a second crop, 92.4 mg of product was obtained. A second recrystallization of 70 mg of this product using the same procedure gave 43 mg, which was dried in vacuo at 50 "C for 16 h to be used for various
Laser
Cell
3n I
I
4
86.7 mm
Dlode Array
710 2 0 X
1 I sarnl I -I!-
W
633nM
i
.-.
11
I
I
544nM
Figure 1. Spatial imaging photometer with two laser sources (544 and 633 nm) to allow for compensation for light scattering effects in membranes. The source beams are concentric and are expanded by using two cylindrical lenses with f , = 6.67 mm and f , = 80 mm. The membrane cell dimensions are distorted for clarity. The optical path length is 0.1-0.3 mm, and the distance for transport from one solution to the other is 0.5-1.5 mm. The cell is placed at the object plane of a Pentax 35" lens (f3),and a 5 12 element photodiode array serves as the detector in the image plane. A piece of membrane is occasionally placed in front of 50 of the diodes in the image plane to act as an intensity fitter, as described in the text.
analyses. If a trace of pyridine was not present in the crystallization liquor, elemental analysis indicated less than the theoretical N content in the product. W-vis data (CHCI,), see Table I. 'H NMR: 6 0.91 (2 H, o-H of pyridine), 4.93 (2H, m-H of pyridine), 5.95 (1 H, p-H of pyridine), 7.70 (12 H, m-p-H of phenyl), 8.10 (8H, o-H of phenyl), 9.00 (8H, tetrapyrrole ring). Infrared (CH,C12): 700 (s),751 (s), 794 (s),995 (m), 1006 (s), 1069 (s), 1150 (m)(doublet), 1173 (m), 1210 (m) (doublet), 1309 (m), 1350 (s), 1440 (s), 1485 (m), 1541 (m), 1599 (s), 1804 (w), 3040 (m) (broad), 3105 cm-' (m). FAB-MS (ion, observed average m a s weighted by isotope distribution, theoretical average, abundance relative to M - Br - py+): M - Br+, 750.7,750.8,7;M - Br - py', 672.0,671.7,100. Theoretical elemental analysis: C, 70.85;H, 4.00;N, 8.43;Br, 9.62. Experimental: C, 71.4;H, 3.94;N, 8.46; Br, 9.31. These samples were hygroscopic and gained weight while being weighed for analysis, leading to some error. The nitrite derivative nitrito(pyridine)(5,10,15,2O-tetraphenylporphyrinato)Co(III) (CoTPP(py)N02)was prepared by ion exchange. CoTPP(py)Br in CHC13was shaken with several aliquots of aqueous 1.0 M NaN02and 0.1 M pyridine and dried over Na2S04,and the organic phase was evaporated to dryness. Pyridine was present as a precaution against loss of this ligand from the complex, although experiments without pyridine gave identical results. Potentiometric analysis of the aqueous phase as described below showed quantitative extraction of Br- from the organic phase. The data and standard deviations given in the Results and Discussion on the extraction experiments are the result of threefive trials each. CoTPP(py)N02was recrystallized from ethyl acetate as described above for the Br- complex. UVVIS data (CHC13),see Table I. 'H NMR: 6 1.01 (2 H, o-H of pyridine), 4.94 (2H, m-H of pyridine), 5.96 (1H, p-H of pyridine), 7.70 (12H, m-p-H of phenyl), 8.10 (8H, o-H of phenyl), 9.00 (8 H, tetrapyrrole ring). Infrared (CH2C12):700 (s), 751 (s), 794 (s), 817 (s), 995 (m), 1006 (s), 1070 (s), 1150 (m) (doublet), 1177 (m), 1208 (m) (doublet), 1313 (s), 1351 (m), 1440 (vs) (doublet), 1487 (m), 1541 (m), 1599 (s),1804 (w), 3015 (w), 3055 (w) (broad), 3105
2170
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
cm-' (m). FAB-MS: M - NOz+,751.0,750.8,4.4;M - NOz - py', 672.1,671.7,100. Theoretical elemental analysis: C, 73.86;H, 4.17;N, 10.55;0,4.02.Experimental: C, 73.85;H, 4.14;N, 10.30 0,4.08. Electrochemical Cells. Membranes were prepared by dissolving in tetrahydrofuran (BDH, distilled from K) 0.075 g of PVC (Polysciences,chromatographic grade),0.150 g of bis(2-ethylhexyl) adipate (DOA, Fluka, ionophore grade), and 0.6-1.2 mg of CoTPP(py)Br. The membrane was cast in a highly polished Teflon well (1.5 in. diameter) and covered over with filter paper, following the method of Craggs, Moody, and Thomas (28). Concentrations in the membrane were determined by first measuring the density of PVC/plasticizer membranes to be 1.130 g/cm3 and then calculating the concentration from the weight percent of dye added, assuming no density change with the added dye. Conventional ion-sensitive electrodes were prepared by cutting 1/4 in. diameter circles from the master membrane and mounting them on PVC tubing, giving membrane electrodes that were typically 0.15-0.20 mm thick in the direction of ion transport. The preparation of the spectroelectrochemical cells has been described previously (22);the optical path length through the cell was -0.1-0.3 mm and was determined by using a micrometer, while the membranes were typically 0.8-1.5 mm thick in the direction of ion transport (90' to the optical path). The potentials developed by these cells were compared to membranes mounted conventionally to ensure that there were no leakage paths or changes in electrochemical behavior as a result of the unusual cell configuration. Most of the cells for which potentials were measured had the following configuration: SCE// 1 M NH4Cl//sample solution/ membrane/O.Ol M NaNOZ,0.1 M NaCl/AgCl/Ag, for both conventionally mounted membranes and the optical cells. The electrodes were conditioned for 2 h in 0.01M NaNOZ,0.1 M NaCl before calibrationcurves were measured in solutions of either pure NaNOZor 0.1 M NaCl with varying NaNOZ content. Several measurements were made of potential versus time from the first contact of the membrane with aqueous solution. The cell configuration was Ag/AgC1/0.2 M NaC1, 0.01 M NaN02/membrane/0.2 M NaC1,O.Ol M NaN02/AgC1/Ag, and it was sealed to prevent evaporation losses and thermostated at either 19 or 24 OC by using a Colora water bath. A box was placed over the apparatus to reduce thermal gradients and spurious photovoltages developed by the AgCl reference electrodes. Selectivity coefficientswere measured by using the method of mixed solutions, by varying the NO; concentration with a fixed concentration of interfering ion: 0.1 M for C1-, 0.1 and 0.01M for Br-, and 0.2 M for H2P04-. Interference due to OH- was determined in solutions buffered at pH 9.3 with 0.05M NH4Cl, 0.05 M NH3 and at pH 9-01 with 0.067 M Na2HPOI. The interference due to HP04* was measured in 0.05 M H2P04-,0.05 M HPOP at pH 7.2,and the interference due to H2POi was taken into account. All selectivity data were analyzed with the Nicolsky equation (29). The effect of pH variation was determined in a 0.01 M NaN02 solution by adjusting the pH with NaOH or HCl solutions of the same NO2- concentration and measuring the potentials of the NO; electrode and a glass pH electrode (Fisher). The AgCl electrodes were prepared by dipping a Ag wire in concentrated HN09 to clean it, and then in 1 M HN03, 1.03 M NaCl to form AgCl. To measure the Br- content of aqueous solutions AgBr electrodes were prepared by electrochemical oxidation in NaBr. A Ag wire was cleaned in 3 M HN03 (3 min), then cycled from -0.3 to +0.3 V VB SCE at 100 mV/s for 3 min in 0.1 M NaBr, and finally held at +0.3 V for 20 min. The cell configuration used was SCE//sample solution/ /0.01M NaBr, 1,O M NaN02/AgBr/Ag at 28 OC. When calibrated in solutions of NaBr with 1.0 M NaNO2 added, these cells gave a slope of -58.4 mV for Br-, with a limit of detection of 3 X lod M Br- due to NO2-interference. The detection limits were slightly lower when calibrated in pure NaBr solutions or with 0.1 M NaNOz added. The Br- electrodes were calibrated in solutions of the same ionic strength a8 the samples to be measured.
RESULTS AND DISCUSSION The electrochemical behavior of membranes prepared from PVC, dioctyl adipate, and the ion carrier bromo(pyridine)(5,10,15,20-tetraphenylporphyrinato)cobalt(III), was examined
in terms of their response to the NOz- anion. The electrodes respond logarithmically to changes in NO2- activity over a range of IO-' to M in solutions of NaN02, with a slope of -57 mV/decade. In solutions containing 0.1 M NaCl as an ionic strength buffer, the interference by Cl- reduced the linear range of response to NOz-, indicating a selectivity coefficient of 5 X The bromide ion also interferes with NO2-, exhibiting a selectivity coefficient of 0.016 in 0.1 and 0.01 M NaBr. Interference due to the buffer phosphate was also examined by using the method of mixed solutions at pH 4.24 and gave a value of 2.1 X lo4 for HzP04-and 6.1 X lo4 for HPOZ- at pH 7.2. The membrane potential responded to pH changes with a slope of -25 to -34 mV/pH unit at pH values between 6 and 11 and was constant below pH 5. Measurement of the selectivity coefficient for OH- interference at constant OH- concentration gave values of about 1000 for NO2- concentrations less than 0.01 M, increasing to about 5000 at 0.1 M NOz-. The effect of pH is clearly more complex than that predicted by theories of membrane selectivity (9,12), since a non-Nernstian pH response and a variable selectivity coefficient are found. The carrier's response to NO2- compares favorably with that of the corrinoid Co(1II) complexes derived from vitamin B-12 described by Schulthess et al. (24),giving similar slopes and selectivity for NOz- relative to C1- and to pH. Perhaps surprisingly, both the slope of response and the selectivity versus C1- for the derivative reported here are much better than that of the rather similar bromo(pyridine)(5,10,15,20-tetrakis[4((hexyloxy)carbonyl)phenyl]porphyrinato)cobalt(111) complex described by Ammann et al. (23). This may arise from the choice of plasticizing agent, since a high molecular weight, high viscosity plasticizer was used in the cited work, and the differences in the plasticizers are greater than the differences in the complexes. It has been suggested that the tetraphenylporphyrin derivatives may have insufficient lipophilicity to produce long-lived, stable sensors (23). Consequently, the potential drift of membranes prepared with CoTPP(py)Br was evaluated over the first 50 h of contact with aqueous 0.1 M NaC1, 0.01 M NaN02, as described in the Experimental Section. In the direction of ion transport the membranes studied were 0.15-0.20 mm thick. The response varied somewhat from one electrode to the next but exhibited a consistent trend. During the first 10 min a large drift occurred at a rate of 13-36 mV/h, this was followed by a drift in the opposite direction of -3 to -5 mV/h for 1-6 h. After this initial period the electrodes exhibited a more modest drift of h0.2 mV/h. T o ensure the drift was not due to the reference electrodes, cells with the confiiation Ag/AgCl/O.l M NaC1,O.Ol M NaNOz/AgCl/Ag were also studied. These showed drifts of only h0.03 mV/h over 50 h. Since the membranes were symmetrically bathed, the sign of the potential is arbitrary; however, the appearance of a potential indicates that asymmetry develops across the membrane in the initial stages of conditioning. The final drift for these NOz- membranes is about 5-10-fold higher than for K+-selective valinomycin-based membranes (301,indicating they would require more frequent calibration. The source of this drift is not clear, although we observed no evidence for leaching of the ionophore from the membrane. As a general practice the membranes were equilibrated at least 2 h before calibration curves were obtained in nitrite solutions. When this was done the time required to reach a stable potential was on the order of seconds above 1 mM NOz-, and minutes at lower concentrations. The cobalt porphyrin complexes are not noticeably water soluble, however reaction with NO2- and H20 is easily effected in a two-phase system, as can be determined from the spectral characteristics of CoTPP(py)X, which vary with the ligand
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
w
3
9
D
w
~
D
51
% i
WAVELENGTH (nM)
Flgure 2. Absorbance spectrum of CoTPP(py)Br in CHCi, after extraction with H,O, 0.1 M &NO2, and then H20 again. The spectra are normalized to the absorbance at 633 nm.
X. Figure 2 shows the changes in the spectrum of CoTPP(py)Br in CHC13 after shaking first with HzO, then with 0.1 M NaNOZ,and then with HzO again. Exposure to HzO causes at least partial displacement of Br- by HzO (or OH-), as evidenced by the spectral shift of the peak a t 558 nm, and the decrease in absorbance at 594 nm. Figure 2 shows that after reaction with NOz- the peak at 594 nm disappears, and a hypsochromic shift of the peak at 556 to 549 nm is accompanied by an increase in the molar absorptivity. The spectral response of the CoTPP(py) moiety to NOz- and HzO confirms that the dye can in principle be used to monitor changes in the membrane concentration of these species. The two He:Ne lasers at 633 and 544 nm are convenient sources in the spectral region of interest, and Table I indicates the molar absorptivities a t these wavelengths and a t the wavelengths of peak absorbance. The extent of reaction of CoTPP(py)Br with H 2 0 and NOzis of interest, as is the rate of reaction since Co(II1) complexes are usually nonlabile. Thusius (31) showed that the Co(II1) corrinoids exhibit unusually high rates of ligand exchange for Co(II1) derivatives. Schulthess et al. (32)showed that Co(II1) derivatives of corrinoids gave functional electrodes, while non-porphyrin complexes with a similar ligand structure but poor ligand-exchange kinetics, such as the cobaloxime derivative chloro[ bis(cholestane-2,3-dionedioxime)pyridine]cobalt(II1) (32),did not yield functional membrane electrodes. Extraction experimentk were performed to establish the ligand-exchange characteristics of the complex used here. When a CHC13 solution of CoTPP(py)Br was shaken with 1.0 M NaNOZ,0.1 M pyridine for 1 h, the aqueous phase was found to have extracted essentially all of the Br- ([Br-1, = 9.6 X M, 96 f 6 % ) . Further extraction with fresh aqueous phase caused no further spectral change, and potentiometry showed no detectable Br- in the aqueous phase (detection limit of 3 X M). When a CHC13solution of CoTPP(py)Br was extracted with aqueous 0.1 M NaN02, 5 mM pyridine, the use of Br- potentiometry showed 64 f 10% conversion to the NO2complex after 2 min and 92 f 10% conversion after 1 h of shaking. These results indicate the exchange of NOz- for Bris relatively rapid and essentially quantitative for the porphyrin complex. I t should be noted that the CoTPP(py)NOz complex isolated following ion exchange can also be incorporated in a PVC membrane. The resulting electrode gives slopes and selectivity coefficients identical with those prepared with CoTPP(py)Br after they have been conditioned in a NOz- solution, and this indicates that the bromo derivative is a convenient precursor for the active form of the ion carrier. The extent of reaction of CoTPP(py)Br with water was evaluated by shaking a CHC1, solution with aqueous 20 mM
6oo
t
4
,
~
500
I
~
~:
200
~,
300
~,
400
~:
500
2171
, 600
,
,
DIODE NUMBER
Flgure 3. Plot of intensity (arbitrary units) versus diode number for a CoTPP(py)Br-containing PVC-based membrane, immediately after introducing H,O to both sides of the cell. See text for a discussion of the different regions of transmission of 544-nm light through the cell.
pyridine. After 3 h, potentiometry showed only 30% of the Br- was extracted into the aqueous phase. Further extraction with 0.1 M NO;, 20 mM pyridine gave quantitative (*lo%) recovery of Br- in the aqueous phase. This may indicate the reaction with HzO is slower than for NOz-. Alternatively, Brmay remain in the organic phase as a counterion for the cationic aquo complex, while 30% of the Co complex is present in the neutral form ligated by OH-. Using the spatial imaging photometer shown in Figure 1, it is possible to image changes in the absorbance profile across a membrane, and hence the concentration profile of a chromophore in the membrane. The change in absorbance, AA, following a change in NOz- concentration on one side of the membrane can be obtained by ratioing the intensities measured at each diode before and after the perturbation. Figure 3 shows a plot of intensity versus diode number (distance across a membrane) shortly after a buffer solution was introduced into the cell. The membrane edge is fairly well defined; it extends over five diodes (9 pm) for the cell shown, and varies between three and seven diodes (5-13 pm) depending on the cell. The lower value is in agreement with the edge resolution reported by Jan and McCreery (21),indicating similar spatial resolution (
