Anal, Chem. 1985, 57, 1397-1401
ACKNOWLEDGMENT We thank A. Rieker for helpful discussions. LITERATURE CITED (1) McDonald, D. D. "Transient Techniques in Electrochemistry"; Plenum Press: New York, 1977; p 185 ff. (2) Bard, A. J.; Faulkner, L. R. "Electrochemical Methods, Fundamentals and Applications"; Wlley: New York, 1980; p 227 ff. (3) Speiser, B. Chem. in uns. Zeit 1981, 15,62. (4) Feldberg, S. W. I n "Electroanalytical Chemistry"; Bard, A. J., Ed.; Marcel Dekker: New York, 1969; Vol. 3, p 199. (5) Spelser, B.; Rleker, A. J . Electroanal. Chem. 1979, 702, 1. (6) Speiser, B. J . Electroanal. Chem. 1980, 770, 69. (7) Pons, S. Can. J . Chem. 1981, 59, 1538. (8) Spelser, B.; Pons, S.;Rleker, A. Nectrochim. Acta 1882, 27, 1171. (9) Langhus, D. L.; Wilson, G. S.Anal. Chem. 1978, 51, 1139. (IO) DePalma, R. A,; Perone, S. P. Anal. Chem. 1979, 57,829. (11) Hanafey, M. K.; Scott, R. L.; Rldgway, T. H.; Reilley, C. N. Anal. Chem. 1978, 50, 116. (12) Meites, L. I n "Coulometric Analysis"; Pungor, P., Ed.; Budapest, 1979; p 113. (13) Meltes, L.; Lampugnanl, L. Anal. Chem. 1973, 45, 1317. (14) Elenkova, N. G.; Nedelcheva, T. K. J . Electroanal. Chem. 1980, 108, 239. (15) Strojek, J. W.; Thullie, W. Pol. J . Chem. 1981, 55, 1093.
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Rusllng, J. F.; Connors, T. F. Anal. Chem. 1983, 55,776. Rusling, J. F. Anal. Chem. 1983, 55, 1713. Rusllng, J. F. Anal. Chem. 1983, 55, 1719. Goto, M.; Oldham, K. B. Anal. Chem. 1973, 45,2043. Imbeaux, J. C.; SavOant, J. M. J . Electroanal. Chem. 1973, 4 4 , 169. Amatore, C.; Nadjo, L.; SavOant, J. M. J . Electroanal. Chem. 1978, 90,321. (22) "IMSL Reference Manual", 9th ed.; IMSL: New York, 1982; Routine
(16) (17) (18) (19) (20) (21)
RLSEP.
(23) "IMSL Reference Manual", 9th ed.; IMSL New York, 1982; Routine ZXSSQ.
(24) "IMSL Reference Manual", 9th ed.; IMSL: New York, 1982; Routine QQNML.
(25) "IMSL Reference Manual", 9th ed.; IMSL: New York, 1982; Routine QQUBS.
(26) Nicholson, R. S.; Shaln, I. Anal. Chem. 1964, 36, 706. (27) Speiser, B.; Rleker, A. J . Chem. Res., Synop. 1977, 314; J . Chem. Res., Miniprint 1977, 3601. (28) Sharp, M. J . Electroanal. Chem. 1978, 88, 193. (29) Draper, N. R.; Smith, H. "Applied Regression Analysis"; Wiiey: New York, 1966.
RECEIVED for review June 18,1984. Resubmitted December 26,1984. Accepted February 11, 1985.
Nitrite-Selective Liquid Membrane Electrode Peter Schulthess, Daniel Ammann, Bernhard Krautler, Christian Caderas, Ren6 Steplnek, and Wilhelm Simon* Department of Organic Chemistry, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland
A liquid membrane electrode based on a lipophilic derlvative of vltamin B,, is described which exhibits a seiectlvity sequence NO,- > SCN- >> CiO,- > CINO,-, with a preference for NO2- over NO3- and Ci- by >lo4., and lo4.', respectively. Response tlmes are on the order of seconds and the emf drift of the cell assembly is smaller than 5 pV/h. The potentiometricallyobserved reversible response of the membrane system can be correlated with the coordlnation of the substrate anion to the lipophilic Co( 111) complex using spectrophotometrlc techniques. The cationic Co( I I I ) complex acts as a very selective charged carrier for anions.
-
The determination of nitrite in the presence of nitrate is rather tedious ( 1 , 2 ) and even in sensors for nitrite (3-9) indirect methods are in use, as illustrated below: Nitrite is reduced to NH4+by the enzyme nitrite reductase (3). The pH of the sample is then raised and the ammonia is sensed by a gas electrode. Nitrite is oxidized to nitrate by nitrite oxidizing bacteria and the oxygen uptake is measured by an oxygen electrode (4,5). Nitrite is oxidized to nitrate with potassium permanganate and NO3- is determined by a conventional ion-exchanger electrode (6). The nitrite-containing solution is acidified and the nitrous acid produced is sensed by a gas electrode (7-9). Here we report on the characterization of a nitrite selective solvent polymeric membrane electrode which is based on a lipophilic derivative of vitamin B12 (see Figure 1). In a previous publication we described the synthesis of 1, Za, and 2b (10). We also showed that 1 and 2 exhibit an anion selectivity in membranes which clearly deviates from the selectivity sequence Clod- > SCN- > NO3- > NO; > C1- of classical anion exchanger membranes (Hofmeister series (11)).
The sensor described here can obviously be used for the determination of nitrite in unmodified solutions.
EXPERIMENTAL SECTION Reagents. For all experiments, double quartz distilled water and chemicals of puriss. or p.a. grade were used. Membranes. The solvent polymeric membranes were prepared according to ref 1 2 using 1 wt % carrier, 66 wt % bis(1-butylpentyl) adipate (BBPA; purum, Fluka AG, Buchs, Switzerland), and 33 wt % poly(viny1 chloride) (PVC; S704, high molecular, Lonza AG, Basle, Switzerland; an equivalent product is now available from Fluka AG, Buchs, Switzerland). The synthesis of the carriers 1 and 2 (2 being a mixture of 2a and 2b) is described in detail in ref 10. Electrode System. Cell assemblies of the following type were used: Hg; Hg2C12,KCl(satd)l3 M KCllsample solution11 membrane110.01 M NaC1, 0.01 M NaN02, AgC1;Ag The external reference electrode was a double-junction calomel electrode, Philips R44/2-SD/1 (N.V. Philips Gloeilampenfabrieken, Eindhoven, The Netherlands). The liquid membranes were mounted in Philips IS-561 electrode bodies. Emf Measurements (General). The emf values were measured at 20 l "C unless otherwise specified. The 16-channel electrode monitor used was equipped with FET operational amplifiers AD 515 KH (input impedance, 1013Q/2 pF; bias current, 0.25) could be obtained from the literature (18). Since these values were rather low, the activity coefficient of NaN3 for I = 0.1 was arbitrarily set to 0.73. Separate Solution Method (SSM) (19,20). The selectivity factors, log KEB, were obtained in 0.1 M solutions of the corresponding sodium salts. The solutions were buffered with 0.01 M Tris and adjusted to pH 7.45 f 0.05 with concentrated H2SOI. Fixed Interference Method (FIM) (19,20). The selectivity factors were graphically evaluated from the electrode functions of the measuring ion containing a fixed concentration of the respective interfering ion. Influence of pH (20). Fifty milliliters of a NaNOz solution of a given concentration (W3, or lo-’ M) was adjusted to an initial pH value of about 5 by a small amount of a dilute HCl solution. By the stepwise addition (between 1 pL and 100 wL) of NaOH solutions of varying concentrations, the pH was gradually increased to about 12.5. The emf values were corrected for changes in the liquid-junction potential and in the activity of NaNOz due to the dilution and the change of the ionic strength. Practical Response Time (19,20). The electrode was conM NaNOz solution for 3-5 min. ditioned in 40 mL of 3 X
Then the magnetic stirrer was turned on (for approximately 2 s) and 1 mL of 0.3 M NaNOz solution was injected immediately (resulting in 1.024 X M NaNOJ. Emf values were taken at 160-ms intervals. Stability/Drift (19,20). Emf stabilities and emf drift were examined using a flow-through measuring cell made of poly(methy1 methacrylate), into which the indicator electrode described above was inserted. In order to fit the reference electrode into the cell, the sleeve type diaphragm was replaced by a ceramic diaphragm. The channel diameter was 5 mm and the flow rate approximately 6-7 mL/h. UV/Vis Absorption Spectra. Spectra were recorded on a PE555 spectrophotometer (Bodenseewerk Perkin-Elmer & Co. GmbH, D-7770 Ueberlingen, GFR). For CHC1, solutions, 10-mm cells were used. Membranes of an approximate thickness of 0.2 mm were placed in contact with a quartz window. The absorbance scales of the spectra were arbitrarily expanded so that all y-bands exhibited the same relative intensity. The nitro-cyano complexes were obtained by shaking a solution of a,b,c,d,e,f,g-heptamethyl Co-aquo-Co-cyanocobyrinateperchlorate (aquo-cyano-cobester) (21) in CHC13with an aqueous 0.1 M NaNOz solution for 10 min or by immersing a membrane containing 1 (conditioned in water) in an aqueous 0.3 M NaNOz solution for 16 h. The formation of the nitro-cyano derivative of aquo-cyano-cobester is supported by its infrared spectrum (22). Extraction Procedure. In a preliminary study 10-g samples of finely ground cured dried meat (“Bundner-Binden-Fleisch”) were placed in a 300-mL flask and 200-mL of hot (80-90 “C) citrate buffer solution (0.2 M citric acid, 0.5 M NaOH, pH -5.4) was added. The flask was heated for 15 min on a steam bath and occasionally shaken. The mixture was then cooled, filtered (LS 14 1/2, Schleicher & Schull, D-3354 Dassel, GFR), and pumped through Sep-PAK Cu cartridges (Waters Associates, Milford, MA). The ion sensor was calibrated in the same buffer using a fixed M KNOB. background of 0.05 M NaCl and
RESULTS AND DISCUSSION A comparison of the selectivities of membranes based on the lipophilic a,b,d,e,f,g-hexamethyl c-octadecyl Coa-Copdicyanocobyrinate (1) and its aquo-cyano perchlorate derivative (2) (10) with those of a classical anion exchanger and a blank membrane is shown in Figure 2. Large differences with K p values were observed depending on the method used (SSM of FIM) to obtain the anion selectivities, but even when these differences are taken into account, there are substantial inversions in the selectivity sequence of the two sensor systems (columns 1and 2 as compared with 3 in Figure 2). The fixed
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985 EMF
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Flgure 3. Emf responses of a liquid membrane electrode cell assembty based on the carrier 2 to varylng activities of NOp- and SCN- in the presence of a fixed amount of an interfering anion (sodium salts, unbuffered solutions). The arrows indicate the detection limit for the primary ions in the = 4.7. = 4.8, and log presence of the intefering ion. The selectivity factors obtained are log @:3,No2 3 4.2, log qNo,
interference method FIM (19,20)indicates that the ion selective electrode described here is nitrite selective. Electrodes based on 2 (Figure 1) have a slightly lower preference for NOz- (FIM, Figure 2) then those based on 1 (Figure 1). However, the other characteristics of electrodes based on 2 are superior to those based on 1. Consequently, only results from electrodes based on 2 will be discussed. Since CN- is known to exhibit a very large equilibrium constant for the substitution of coordinated HzO in aquocobalamin (23),an attempt was made to obtain selectivity factors for cyanide a t pH 10 (SSM). However, when the electrodes were brought into contact with the solution containing CN-, the emf first dropped (to a value even lower than that observed for SCN- or NO2-) and then suddenly rose (by >120 mV) and was still drifting after 25 min. This disturbance is reversible. Experiments in buffered 0.1 M NaN02 solution at a pH value of 8 showed that a total cyanide concentration of up to lo4 M does not show significant interference. Under the usual experimental conditions (one electrode, 100 mL sample solution, 50% loading of the carrier with CN-) a total cyanide concentration of lov7M or less was estimated to be tolerable irrespective of the nitrite concentration. The very high selectivities of nitrite over nitrate and chloride and the modest selectivity over thiocyanate are illustrated in Figure 3. The graphs show that in solutions of 1 M NaCl and of 0.1 M NaN03 the detection limit (19,20) for nitrite is still below lod M (arrows in Figure 3). The slopes of the electrode response (relative standard deviation 1.1-3.4%) is slightly smaller than that predicted by the Nernst equation. The optimum pH working range of the sensor is about pH 5-8, as shown in Figure 4. For a stepwise change in the ion concentration from 3 X M to 10" M, the response time is 2 s and 7 s for t,, and tlmV, respectively (Figure 5). Membrane resistances (measured by the resistor-in-parallel method (24))were on the order of 30 MO, corresponding to specific resistances of about 700 MO cm (20 "C). In lo-* M NaN02 an emf drift of the cell assembly of
