Work is in progress to clarify this interesting question.
LITERATURE CITED (1) A. L. Underwood and R. W. Burnett, “Electrochemistry of Biological Compounds”, Elecfroanal. Chem., 8, 1-85 (1973). (2) J. E. Falk, “Porphyrln and Metalloporphyrins”, Elsevier, Amsterdam, 1964; 2nd ed., K. M. Smith, Ed., Elsevier, Amsterdam, 1975. (3) W. M. Clark, “OxkIation-Reductlon Potentials of Organic Systems”, William and Wilkins, Baltimore, Md., 1960. (4) D. G. Davis and D. J. Orleron, Anal. Chem., 38, 179 (1966);D. G.Davis and R. F. Martln, J . Am. Chem. SOC.,88, 1365 (1966). (5) T. M. Bednarski and J. Jordan, J . Am. Chem. SOC., 86, 5690 (1964);
89, 1552 (1967). (6) S.B. Brown and I. R. Lanske, Biocbem. J., 115,279 (1969). (7) K. M. Kadish and J. Jordan, Anal. Lett., 3, 113 (1970).
I
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0
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I
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40
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1
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(8) B. A. Feinberg, M. Gross, K. M. Kadlsh, R. S. Marano, S. J. Pace, and J. Jordan, Bloelectrochem. Bloenerg., 1, 73 (1974). (9) H. R. Gygax and J. Jordan, Discuss. Faraday Soc., 45, 227 (1968). (IO) D. W. Clark and N. S. Hush, J . Am. Chem. Soc., 87, 4238 (1965). (11) G. Peychal-Heillng and G. W. Wllson, Anal. Chem., 43, 545 (1971). (12) A. C. Censullo, J. A. Lynch, D. H. Waugh, J. Jordan, “Biochemical and Clinical Applications of Titration Calorimetry and Enthalpimetric Analysis” in “Analytical Calorimetry”, R. S. Porter and J. F. Johnson, Eds., Plenum Press, New York, N.Y., Vol. 111, 1974,pp 217-235. (13) J. Juillard and R. Loubinoux, C.R . Acad. Sci. Paris, 264, 1680 (1964). (14) J. Jordan, J. K. Grime, D. H. Waugh, C. D. Miller, H. M. Collis. and D. Lohr, Anal. Chem., 48, 427A (1976). (15) G. P. Kurnar and D. A, Pantony in “Polarography 1964,Proceedings of the Third International Congress”, Macmilian, London, 1966, p 1061. (16) R. S. Nicholson, Anal. Chem., 37, 1351 (1965). (17) R. S. Nicholson and I. Shain, Anal. Chem., 38, 706 (1964). (18) J. Koutecky, Cbem. Listy, 47, 323 (1953). (19) J. Weber and J. Koutecky, Cbem. Listy, 49, 562 (1955). (20) J. F. Swindells, C. F. Snyder, R. C. Hardy, and P. E. Golden, Natl. Bur. Stand. ( U . S . ) ,Clrc. Suppl., 440, 700 (1958). (21) H. Kojima and A. J. Bard, J . Electroanal. Chem., 63, 117 (1975). (22) J. E. Prue and P. J. Sherington, Trans. Faraday Soc., 57, 1795 (1961). (23) H. Strehlow, Z. Nekfrochem., 56, 827 (1952). (24) D. G. Davis and L. M. Bynurn, Bloelectrochem. Bioenerg., 2, 184 (1975). (25) M. Olsztajn, P. Turq, and M. Chemia, J . Cblm. Phys., 87, 217 (1970). (26) P. G. Sears, R. W. Wolford, and L. R. Dawson, J . Nectrochem. Soc..
!
8 0
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Flgure 7. Variation of the entropies of Reaction 9 as function of solvent composition
constants or formal potentials) are not necessarily the same. The point we wish to make is merely the qualitative observation that changes in physical solvent properties appeared to be most drastic in certain domains of solvent composition where electrochemical thermodynamic and kinetic parameters also changed a great deal. The observed solvent property effects are indicative of changes in liquid structure which are known to be due to solvent-solvent interactions (27-29). Our findings suggest that they were matched by corresponding solvent-solute interactions reflected in electrochemical behavior. Indeed, the trends in the electrode kinetics on the one hand (Figure 5, top), and in redox thermodynamics on the other hand (Figure 5, bottom, and Figure 7), both appear to point to the involvement of the solvent-solute interactions. The kinetic effects are entirely consistent with an electron transfer path via the porphyrin ring while the entropy effects can reasonably be accounted for by the solvation of the periphery of the equatorial porphyrin ligands (8). It should be noted, however, that the findings reported in the present paper (per se!) do not necessarily preclude supplementary effects due to the involvement of axial coordination orbitals of heme-iron.
103, 633 (1956). (27) R. Paul, P. S. Guraya, and B. R. Sreenathan, Ind. J. Cbem., 1, 335 (1963); 3. .. 300 11965). (28) B. G. Cox,ATJ. Parker, and W. E. Waghorne, J . Phys. Chem., 78, 1731 (1974). (29) 0.D. Bonner and U. S. Choi, J . Phys. Cbem., 78, 1723 (1974). ~~~
RECEIVED for review June 7, 1977. Accepted June 29, 1977. Presented in part before the 4th International Conference on Chemical Thermodynamics, Montpellier, France, August 26-30, 1975. Supported by the National Science Foundation (Research Grant CHE 76-21666), the National Institutes of Health (Research Grant 5R01 HL 02342 from the National Heart, Lung, and Blood Institute), and the North Atlantic Treaty Organization (NATO Research Grant RG 794).
Assay of Phenobarbital with an Ion-Selective Electrode Gary D. Carmack and Henry Freiser“ Department of Chemistty, University of Arizona, Tucson, Arizona 8572 1
A rapid and reliable phenobarbital tablet assay method was developed based on the potentiometric sensing of the phenobarbltal anion uslng a coated-wire electrode. The results obtained are in agreement with the standard USP method.
Phenobarbital (5-ethyl-5-phenylbarbituric acid) is conventionally assayed in pharmaceutical preparations using the extractive-spectrophotometric procedure specified in the U.S.
Pharmacopeia (1). A number of other analytical methods are also available including fluorimetry (21, coulometry (31, and liquid ChOmatograPhY ( 4 ) . However, these rmd.hods generally involve the Use Of more sophisticated inStI’UmentatiOn Or more complex procedures and are perhaps best suited to the analysis of complex mixtures, e.g., human sera. The development of ion-selective electrodes based on ion association systems in this laboratory (5-7) and other (8)have demonstrated that a wide variety of simple and economical analyses are possible with these sensors. ANALYTICAL CHEMISTRY, VOL. 49,NO. 11, SEPTEMBER 1977
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This study deals with the application of a coated-wire phenobarbital electrode based on the ion-pair complex between phenobarbital anion and the quaternary ammonium cation, tricaprylylmethylammonium, for the analyses of phenobarbital solutions. The advantages of using this electrode instead of previously mentioned methods are simplicity, speed of analysis, and economy. EXPERIMENTAL Materials. ACS Reagent grade chemicals were used except as noted. Aliquat 3368 (tricaprylylmethylammonium chloride) was obtained from General Mills Chemicals, Inc. Chromatographic grade poly(viny1chloride) powder, epoxy resin (Epon 826), and curing agent (diethylenetriamine) were from Polysciences, Inc. Mallinckrodt sodium phenobarbital was used for preparation of standard solutions. Decyl alcohol, melting point 5.5-6.5 "C, was from Eastman Kodak. Chloroform, spectrophotometric grade, was obtained from Aldrich Chemical Co. Platinum wire (0.07-cm diameter) was used without special preparation of the metal surface. Conversion of Aliquat 3368 to the Phenobarbital Form. Five milliliters of Aliquat 3368 were dissolved in approximately an equal volume of decyl alcohol and equilibrated with four separate 10-mL aliquots of 0.5 M sodium phenobarbital raised to pH 9.0 by addition of 0.01 M NaOH. After each shaking, the aqueous phase was separated and tested for the presence of chloride with acidified AgN03 The absence of chloride indicated complete exchange. The organic phase was washed twice with deionized water and then centrifuged until a clear liquid was obtained. Construction of Electrodes. Coated-wire electrodes were constructed using the technique as reported previously (5-7). Weighed amounts of the electroactive material were dissolved either in a 5% (w/w) solution of PVC in tetrahydrofuran or mixed with the epoxy mixture (containing equal weights of resin and curing agent) and the end of a platinum wire repeatedly dipped into the mixtures until a uniform coating was obtained. This usually required about three dippings. A greater number could be tolerated as long as the electrode impedance does not exceed lo7 Q. The electrodes were cured or air-dried overnight. The exposed portion of the wire was wrapped tightly with Parafilm (American Can Co.). All electrodes were subjected to initial conditioning by soaking for an hour in a lo-' M sodium phenobarbital solution. Immediately before use, the electrodes were soaked in a dilute (ca. 10" M) phenobarbital solution for approximately 15 min. When not in use, they were stored in air. An Orion Model 701 digital pH meter was used for all measurementa and a Beckman Fiber Junction calomel electrode served as reference electrode. Calibration curves were obtained using sodium phenobarbital solutions; the pH was adjusted to 9.6 by the addition of 0.01 M sodium hydroxide. All potentiometric measurements were made at 25.0 "C. Interference Studies. These were carried out by the mixed solution procedure described previously (5-7). The concentration M while the of sodium phenobarbital was fixed at 5 X concentration of interfering anions was varied between to lo-' M. A pH of 9.6, chosen t o retain the phenobarbital quantitatively in the anionic form without unduly raising the concentration of the interfering hydroxide ion, was maintained for all solutions. Electrode Analysis of Phenobarbital Tablets. Phenobarbital tablets (Eli Lilly and Co.) were analyzed by finely powdering a batch of not less than 20 tablets. A portion of the powder, equivalent to about 50 mg of phenobarbital, was transferred to a 100-mL volumetric flask and diluted to volume with water. The pH was adjusted to 9.6 with 0.01 M NaOH. Potentiometric measurements were alternately made on this solution and a standard solution having approximately the same phenobarbital concentration, until a reproducible difference in readings was obtained (i0.1 mV). USP Procedure. The USP procedure (1) for the assay of phenobarbital tablets involves successive chloroform extractions from an acidic phenobarbital solution. Following evaporation of the chloroform solution, the residue was taken up in an alco1578
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
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Table 1, ,c+lectivity Ratios for various~~i~~~ with Coated-Wire Phenobarbital Electrodes Foreign ion
Selectivity ratio, KAli'
Nitrate 0.2 Chloride 0.1 Acetate 0.02 Salicylate 0.7 Sulfate See text Phosphate See text a As defined by the equation: E A = EAo t 59.2 log( U A + K A l a i ' i n i )where E , is the potential of the phenobarbital efectrode, a A its ionic activity, and ai the activity of an anion i of charge n. holic-borate solution and the UV absorbance determined at 240 nm. RESULTS AND DISCUSSION The response characteristics of the electrodes were tested using solutions of from 0.1 M to M sodium phenobarbital, pH 9.6. The optimal membrane composition was 70 w t 7'0 Aliquat salt in the PVC films and 50 w t % for the epoxy films. The PVC coated-wire electrodes gave a linear response (slope = 55 f 2 mV/log a) from 0.1 M to approximately M. Coated-wire electrodes using the epoxy films gave slightly greater sensitivity (slope = 57 mV/log a) and were therefore used for the tablet analyses. In both types, the sensitivity of the electrodes rapidly decreases beyond a concentration of ca. lo4 M phenobarbital because of hydroxide ion interference. The response time of the coated-wire electrodes was fast, being nearly instantaneous a t higher concentrations and requiring less than 1min with a M phenobarbital solution. The potential readings could be reproduced to better than fl mV over the entire concentration range, but the absolute potential varied daily from 5 to 15 mV necessitating a onepoint restandardization before each run. The useful lifetime of these electrodes is a t least three months. The interference by other anions was determined from selectivity studies in which the calculated selectivity ratio, K (9), is used in evaluating the degree of interference. The results, summarized in Table I, show that most of the ions interfered moderately. Sulfate and phosphate gave negligible interference when present in approximately the same concentration as that of phenobarbital but, unexplicably, the electrode response became unstable when large excesses (>lo-fold) of either ion was present. The ions listed in Table I were chosen as representative of potentially low level contaminants in the phenobarbital tablet preparations. The bulk of the excipient, usually consisting of a lactose diluent and maize starch or gelatin binders (IO), should not show any interference. High precision (relative standard deviation of i1.37'0) quality control-type analyses are made possible with these electrodes because the approximate phenobarbital content of a tablet is known beforehand. A standard solution containing this concentration can be prepared and measurements can be performed repeatedly on it and the sample solution until a reproducible ( f O . l mV) potential difference is obtained between the two solutions. Usually, the difference in potential was within 1-2 mV. So despite an electrode drift of approximately 1mV during the course of several determinations, the relative potential difference remained constant. Samples exhibiting readings differing from the standard by more than 2 mV can either be rejected or redetermined by preparing a more closely matched standard. The results of the potentiometric analyses of phenobarbital tablets using an epoxy coated-wire electrode are reported in Table 11. In contrast to the 4 h required for assay by the USP
Table 11. Comparison of Conventional and Electrode Method for the Analysis of Phenobarbital Tablets Tableta 16 mg 32.5 mg
Electrode method, USP method, mg/tablet mg/ table t 15.7 c 0.2b 31.9 + 0.4
15.8 c 0.2 32.1 c 0 . 2
a Concentration as stated by manufacturer. Standard deviation (at least 4 determinations with electrode and 2 with USP method).
method (2), an electrode assay can be accomplished within 20 min. The rapidity with which the assay can be carried out using the coated-wire electrode makes it practical to perform the procedure on single tablets, so that tablet-to-tablet variation could be followed if desirable.
These results clearly show that ion-selective potentiometry using a coated-wire electrode sensitive to sodium phenobarbital is a useful and accurate method of analysis.
LITERATURE CITED (1) United States Pharmacopeia, XVIII Ed., American Pharmaceutical Association, Washington, D.C., 1970, p 490. (2) C. I. Miles and 0.H. Schenk, Anal. Lett., 4, 61 (1971). (3) J. R. Monforte and W. C. Purdy, Anal. Chlm. Acta, 52, 25 (1970). (4) P. Menyharth, A. L. Levy, and D. P. Lehane, Chromatogr. News/., 4, 15 (1976). (5) R. W. Cattrall and H. Frelser, Anal. Chem., 43, 1905 (1971). (6)H. J. James, G. D. Carmack, and H. Frelser, Anal. Chem., 44, 856 (1972). (7) B. M. Kneebone and H. Frelser, Anal. Chem., 45, 449 (1973). (8) N. Ishibashi, K. Klna, and N. Mackawa, Acta. h m . S m . , 9, 641 (1972). (9) A. K. Covington, Crlf. Rev. Anal. Chem., 3 , 355 (1974). (10) H. Burlinson, "Tablets and Tabietting", Heinemann, London, 1968.
RECEIVED for review May 12,1977. Accepted June 16,1977. Work supported by a grant from the Office of Naval Research.
Silver-I 10 Microgram Sulfate Analysis for the Short Time Resolution of Ambient Levels of Sulfur Aerosol Joseph Forrest * and Leonard Newman Atmospheric Sciences Division, Department of Applied Science, Brookhaven National Laboratory, Upton, New York 1 1973
Atmospheric particulate samples collected on glass flber or quartz fllters have been routlnely analyzed for total sulfur at the mllllgram level wlth "'Ag tracer. The method has been reflned to permit sulfate analyses of
