ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1651
Liquid Membrane Electrode for Direct Potentiometry and Potentiometric Titration of Strychnine Saad S. M. Hassan" and M. B. Elsayes Research Microanalytical Laboratory, Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt
A new electrode sensitive and reasonably selective for strychnine Is developed. It is based on the use of strychnine-picrolonate ion-pair complex In nitrobenzene as a liquid membrane. The electrode shows Nernstlan response down to M strychnine solutions over a pH range of 2-7 with a cationic slope of 57 mV/concentration decade. Direct potentiometric determination of samples down to 5 ppm shows an average recovery of 101.3% and a mean standard deviation of 2.4 YO. Potentiometric tiratlon wlth either picrolonic acid or sodium tetraphenylboron shows an average recovery of 99.1 YO and a mean standard deviation of 1.2%. No interferences are caused by some of the other alkaloids, and many of the substances normally occuring as natural contaminants with strychnine.
Strychnine is one of the most important alkaloids related
to t h e indole group. It is used, in small doses, as a tonic and stimulant of t h e central nervous system as well as the respiratory, muscular, and cardiovascular centers, but large doses are fatal. Thus, accurate determination of strychnine is greatly needed in veterinary, pharmaceutical, medicinal, and forensic sciences. Direct spectrophotometry by measuring absorbance at 262 n m has been recommended by t h e British Pharmacopoeia ( I ) . However, many organic impurities and other alkaloids produce substantial extraneous absorption in the same region and require prior separation ( I , 2). T h e need for a rapid specific method applicable to the determination of strychnine on the micro- and ultramicroscales, in the presence of basic organic impurities, is evidenced by the fact that most of the reported methods are susceptible to environmental reaction conditions and suffer from lack of specificity, sensitivity and stoichiometry. Moreover, a time-consuming extraction of strychnine from all other interferents is a common step in many of the analytical procedures dealing with impure samples (2). Recently, several sensitive potentiometric sensors selective for some organic compounds have been reported (3). Electrodes have been prepared that are selective for the following alkaloids: nicotinate ( 4 ) ,methadone ( 5 ) ,methacholine (6), neostigmine (6), diphenhydramine (7), novocaine (8) and ephedrine (9). Tetraphenylboron (6, 8, 9 ) , dipicrylamine (6, 8), or quaternary ammonium ( 4 , 5 ) derivatives of such compounds were used as electroactive materials in polymeric ( 4 , 5 , 9) or liquid ( 4 , 6, 8, 9) membranes. However, the complexes of strychnine have not yet been investigated. T h e present work describes a new potentiometric sensor sensitive and reasonably selective for the determination of strychnine down t o the ppm scale, with acceptable accuracy and free from many of the difficulties usually encountered in the previously published methods.
EXPERIMENTAL Apparatus. Potentiometric measurements were made with an Orion Microprocessor Ionalyzer (model 901) using the 0003-2700/79/0351-1651$01.OO/O
strychnine electrode in conjunction with an Orion double junction reference electrode (model 90-02) with 10% potassium nitrate in the outer compartment. Argentimetric titration was carried out with an Orion silver/silver sulfide electrode (model 94-16) in conjunction with an Orion double junction reference electrode. The pH adjustment and titration were conducted with an Orion combined glass/calomel electrode (model 91-02). Reagents and Materials. All the reagents used were of analytical reagent grade, unless otherwise stated, and doubledistilled water was used throughout. The alkaloid test samples were obtained from the British Drug Houses (B.D.H., Poole, England) as standard samples of purity not less than 99%. Picrolonic acid and sodium tetraphenylboron (Laboratory Reagents) were purchased from B.D.H. Standard 0.005 M Picrolonic Acid. Dissolve 1.32 g of picrolonic acid in 200 mL ethanol, filter, and dilute to 1 L with double-distilled water. Standardize the solution by potentiometric titration of 10-mL aliquots with standard 0.005 M potassium hydroxide solution, using an Orion combined glass/calomel electrode system. The solution is stable for at least one month without change in strength. Standard 0.005 M Sodium Tetraphenylboron. Dissolve 1.71 g of sodium tetraphenylboron in a minimal amount of doubledistilled water, filter, and dilute to 1L with double-distilled water. Standardize the solution by potentiometric titration of 10-mL aliquots with standard 0.005 M silver nitrate using an Orion silver/silver sulfide electrode in conjunction with an Orion double junction reference electrode. Strychnine-Picrolonate Ion-Pair Complex. Mix 20 mL of a 0.01 M aqueous strychnine sulfate solution with 50 mL of a 0.01 M ethanolic picrolonic acid solution. Filter the precipitate on a sintered glass crucible no. G3, wash with double-distilled water followed by ethanol and dry a t 100 "C for 1 h. Procedure. Electrode Preparation. An Orion liquid membrane electrode barrel (model 92) is used as the electrode assembly with an Orion 92-81-04 porous membrane to separate the organic phase from the test solutions. The organic phase is prepared by dissolving 20-30 mg of strychnine-picrolonate ion-pair complex in 10 mL of nitrobenzene. The aqueous reference phase is 0.01 M solution in each of potassium chloride and strychnine sulfate. The electrode is preconditioned by soaking in aqueous 0.01 M strychnine sulfate solution for 3 days after preparation. Potentiometric Titration of Strychnine. Pipet a 10-mL aliquot of the strychnine solution (containing down to 0.1 mg/mL), or transfer a weight equivalent to 1-5 mg of strychnine to a 100-mL beaker. Adjust the pH to 4-7 with either 0.1 M potassium hydroxide or 0.1 M sulfuric acid. Immerse the strychnine electrode and the Orion double junction reference electrode in the solution. Titrate with a 0.005 M standard solution of picrolonic acid and carry out a blank. Alkaloid mixtures containing strychnine are similarly titrated with picrolonic acid for strychnine, followed by titration of the total alkaloid with 0.005 M standard sodium tetraphenylboron in separate aliquot. Direct Potentiometric Measurement of Strychnine. Immerse the strychnine electrode and the Orion double junction reference electrode in the aqueous strychnine solutions of pH 4-6. Allow to equilibrate, by stirring, for 30 s and record the potential readings. Compare with standard solutions containing 5.0,50.0, and 500.0 ppm strychnine using Orion Microprocessor Ionalyzer (model 901) or with an expanded-scale pH meter and semilogarithmic paper. 0 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979 c
U 4
'0 c
C
-0 L 4
I
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>
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c
-VI 0
c Tj
0 L
." u
->o 10-2
10-3
Membrane solute
10-4
-&:
-2c
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iz PS'BPt
concent r a t )o n , (M)
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cc
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Response of the strychnine electrode toward strychnine sulfate: Measurements in pure water (0)and In 0.1 M potassium sulfate background ( 0 )
Figure 2.
Electrode slope as function of strychnine picrolonate concentration in the membrane solution Flgure 1.
2 . The electrode shows Nernstian response with an average cationic slope of 57 mV/concentration decade over the Membrane Material. Strychnine is known (IO)to react concentration range from 5 x IO-' M to 5 x M, and a slope with picrolonic acid (3-methyl-4-nitro-l-p-nitrophenyl-5- of 52 mV/concentration decade over the range from 5 X pyrazolone) to form a stable 1:l water insoluble complex (i). M, in potassium sulfate background and pure M to 7 x water, respectively. Selectivity of the Membrane. Strychnine naturally occurs as a salt of either organic or inorganic acids admixed with indoles, amines, and a-amino acids. These substances constitute a serious problem in the various methods of analysis, and should be adequately removed prior to strychnine determination (2). However, the response of the electrode toward many of these compounds was tested, since the electrode response in the presence of foreign substances is expressed by the equation:
RESULTS AND DISCUSSION
Strychnine
Ag, AgCl 0.01M KC1
0.01M strychnine sulfate
E = Constant
picrolanate
M strychninepicrolonate in nitrobenzene
test s o ht ion
+ 2.303 R T / F log [astrychnine + Ks,(a,)nl (iii)
SCE
(ii)
where Ks, is the selectivity coefficient and a, is the activity of the foreign substance. In estimating selectivity, the molar concentrations were used instead of activity, and the mixed approach has been followed (11). This involved potential measurements of a series of solutions each containing lo-* M of the foreign substance and variable strychnine concentrations (i.e., 10-2-104 M). The results obtained with some substances, normally occurring as natural contaminants (Table I), indicate that if the interference concentration is less than l/lo that of strychnine, then the interference is negligible. Membrane Responses to Other Alkaloids. In a field as large as that of natural products, where the number of the known alkaloids and their derivatives exceeds a few hundreds ( E )it, is impossible to test the behavior of all these substances with the proposed electrode. However, it suffices to test its response toward simple compounds containing the main alkaloid nucleus and alkaloid samples representing most of the main alkaloid classes-ephedrine, caffeine, nicotine, codeine, pilocarpine, atropine, and cinchonine (i.e., alkylphenylamine, xanthine, pyridine, isoquinoline, pyrazole, pyrrolidine, and quinoline groups, respectively). The electrode responds to pilocarpine, atropine, and cinchonine, though with a small selectivity coefficient, but it does not significantly respond to the remaining aformentioned alkaloids (Table I). Effect of pH. The effect of pH on the potential readings of the electrode was examined by measuring the emf of the cell in strychnine solutions with varying acidity. The pH was varied by adding the appropriate amounts of sulfuric acid, and/or potassium hydroxide solutions. At pHs from 2 to 7 , no significant change of the membrane potential was observed,
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Table I. Selectivity Coefficients for the Strychnine-Picrolonate Membrane substance, j 4-aminopyridine aniline sulfate piperidine triethanolamine 4-aminoantipyrine A;-methylpyrrolidone urea adrenaline 8-hydroxyquinoline tryptophan leucine succinic acid oxalic acid maleic acid disodium hydrogen phosphate potassium nitrate potassium chloride ammonium chloride sodium sulfate indole acetic acid nicotine caffeine codeine theobromine ephederine pilocarpine atropine cinchonine
1653
t
selectivity coefficient, K,j 2.5 X 1.4 X 9.5 x 8.5 X 9.4 x l o - ’ 5.5 x 7.4 x 10-3 8.5 X 10.’ 1.0 x l o - ’ 4.9 x 10-3 5.8 x 10-3 1.4 X 4.8 X lo-’ 1.6 X lo-’ 2.3 x lo-’ 1.1 x 10-3 1.7 x 10-3 1.4 x 10-3 2.2 x 10-3 4.5 x 9.5 x 2.4 x lo-’ 2.5 X 2.3 x 9.5 x 10-1 1.73 1.59 1.91
0
20
60
LO
80
Time, (sec)
Flgure 4. Static response of the strychnine-picrolonate membrane
with time at various strychnine concentrations
\10-2 M --1020 -
100
-30-40-
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I
I
1
1
I
,
’
2
3
4
5
6
’
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Flgure 5. Typical titration curves of strychnine. ( 0 )Tetraphenylboron titrant, (0)Picrolonic acid titrant -401
1
1
1
2
3
I
4
5
,
6
I
7
PH
Figure 3. Effect of pH on potentials of strychnine-picrdonate membrane system at various strychnine concentrations
and the potential difference did not exceed 10 mV within the entire range of p H over the concentrations to M (Figure 3). At pHs higher than 7, the strychnine base in the aqueous test solution precipitates. Response Time. The response time of the electrode was tested both by measuring the time required for the electrode to attain a steady potential, after successive immersion in different strychnine solutions, each having a tenfold difference in concentration (static response) (Figure 4), and by measuring the time required to achieve a steady potential by rapid tenfold increase of the strychnine concentration in the same solution (dynamic response) (13). Both results indicate fast response as it ranged from 10-20 s for solutions > M to 20-40 s for solutions C M. Electrode Life-span. The reproducibility of the potential measurements in lo-* M standard strychnine solution was periodically checked over a period of two months. After 20 days, a shift of about 3 mV in the absolute millivolt value was
Table 11. Potentiometric Titration of Strychnine with Picrolonic Acid and Sodium Tetraphenylboron Strychnine, mg/mL added found recovery, 7% 0.203 0.201 99.0 0.295a 98.3 0.300 0.403 99.3 0.406 98.3 0.600 0.590a 0.598 98.0 0.610 0.806 99.1 0.813 o.90la 100.1 0.900 1.027 101.1 1.016 1.126 1.100a 97.1 1.209 99.2 1.219 1.596 98.2 1.626 2.060 101.4 2.032 a Sodium tetraphenylboron was used as a titrant. observed which increased to 7 mV after 40 days, but the electrode slope remained constant over a period of 60 days. Potentiometric Titration. Among the possible applications of the electrode is the potentiometric titration of strychnine with either picrolonic acid or sodium tetra-
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Table 111. Potentiometric Titration of Some Binary Mixtures of Strychnine and Other Alkaloids Strychnine weight, mg
alkaloid mix t u res strychnine
Alkaloid weight, mg
added found recovery, % added 3.77 3.70 98.1 7.32 t 5.90 5.92 100.3 5.00 pilocarpine 7.76 7.81 100.6 3.27 strychnine 4.00 3.98 99.5 6.38 + 6.72 6.74 100.3 4.27 atropine 8.37 8.30 99.2 3.04 strychnine 3.88 3.91 100.8 6.72 + 5.82 5.81 99.8 4.33 cinchonine 7.77 7.78 100.1 3.69 Based on the consumption of 2 mol of tetraphenylboronimol of cinchonine. Table IV. Determination of Strychnine o n the Milligram Scale by Direct Potentiometry Using the Strychnine Electrode Strychnine, mg/mL added found 0.190 0.189 0.240 0.238 0.318 0.320 0.400 0.398 4.140 4.160 5.130 5.150 6.000 6.200 7.700 7.820 8.730 8.560 9.540 9.420 10.270 10.300 11.100 11.300 ~~
found 7.33 5.10 3.25 6.29 4.29 3.00 6.73a 4.40a 3.7aa
recovery, 7% 100.1
102.0 99.4 98.6 100.5 98.7 100.2 101.6 102.4
Table V. Determination of Strychnine on the ppm Scale by Direct Potentiometry Using the Strychnine Electrode Strychnine, ppm added 4.5 6.0 9.0 12.0 24.0 32.0 40.0 48.0 56.0 80.0 98.9 114.0
recovery, % 99.5 99.2 99.4 99.5 100.5 100.4 103.3 101.6 102.0 101.3 100.3
found 4.6 6.3 8.9 11.5 24.4 31.9 40.9 50.0 57.0 80.8 100.5 116.0
recovery, 70 102.2 105.0 98.9 95.8 101.7 99.7 102.3 104.2 101.8 101.0
101.6 101.8
101.8 ~~~~~~
~
phenylboron. Typical titration curves with both titrants are shown in Figure 5. Table I1 demonstrates the results obtained with strychnine samples in the range of 0.2 to 2 mg/mL. The average recovery is 99.1% and the mean standard deviation is 1.2%. Fortunately, the reaction of many amines and alkaloids with picrolonic acid does not lead to the formation of precipitates or give precipitates much more soluble than t h a t of strychnine. This permits selective titration of strychnine in the presence of many interfering substances. Tetraphenylboron, however, is known to form precipitates with a large number of alkaloids and basic substances (14). Thus, it is possible to determine binary alkaloid mixtures containing strychnine by successive titration with the two titrants. The results obtained for the analysis of some of these mixtures (Table 111) show average recoveries of 99.8% (standard deviation 0.8%) and 100.4% (standard deviation 1.4%) for strychnine and the other alkaloids, respectively. Some multicomponent alkaloid mixtures containing strychnine are similarly, satisfactorily, assessed for both strychnine and total alkaloid-nitrogen. Direct Potentiometry. The results obtained for the direct potentiometric determination of strychnine on the milligram level, using the calibration method are quoted in Table IV. The average recovery is 100.7%, the standard deviation being 1.3%. As the electrode provides a stable and reproducible potential a t low concentrations, it can be used for strychnine measurement on the ppm scale. The average recovery of 12 samples containing from 5 to 100 ppm is 101.3%,the standard
deviation being 2.4% (Table V). The data obtained for various weights of strychnine in the presence of 50-fold amounts of many of the substances listed in Table I (except pilocarpine, atropine, and cinchonine) indicate no significant interferences.
LITERATURE CITED (1) "British
Pharmacopoeia"; The Pharmaceutical Press: London, 1973;p
328. (2) Higuchi, T.; Brochmann-Hanssen, E. "Pharmaceutical Analysis"; Interscience: New York, 1961;pp 313-543. (3) Buck, R. Anal. @em. 1978, 50, 17R. (4) Campanella, L.; DeAngelis, C.; Ferri, T.; Gozzi, D. Ana/yst(London) 1977, 102,723. (5) Srianujato, S.;White, W.; Higuchi, T.; Sternson, L. Anal. Chem. 1978, 50, 232. (6) Kina, K.; Maekawa, N.; Ishibashi, N. Bull. Chem. SOC.Jpn. 1973, 46, 2772. (7) Higuchi, T.; Illian. C.; Tossounian, J. Anal. Chem. 1970, 42, 1674. (8) Hopirtean, E.; Kormos, F. Stud. Unlv. Babes-Bolyal Ser. Chem. 1977, 22,35. (9) Fukamachi. K.: Nakaaawa. R.: Morimoto. M.: Ishibashi. N. BUnsekiKaaaku 1975, 24, 428. (IO) Warren, W.; Weiss, R. J . Biol. Chem. 1907, 3 ,327. (11) Moody. G.; Thomas, J. "Selective Ion Sensitive Electrodes", Merrow: Watford (U.K.), 1971;p 14. (12) Glasby, J. "Encyclopedia of the Alkaloids", Plenum: New York. 1975. (13)Moody, G.;Thomas, J. Lab. Pract. 1974, 23,475. (14) Ashworth, M. "Titrimetric Organic Analysis", Part 11; Interscience: New York, 1965;pp 853-855.
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RECEIVED for review January 18,1979. Accepted May 7,1979. This paper was presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 5-9, 1979.
