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Coulometric determination of organic compounds with methylviologen cation-radical
MICROCHEMICAL
JOURNAL
Coulometric C. MARrfNEz Department
39, %-64
(1989)
Determination Methylviologen
of Organic Compounds Cation-Radical
LOZANO,' T. PEREZ RUIZ, V. of Analytical
Chemistry,
Received
Faculty
TOMAS,
of Science,
June 6, 1988; accepted
July
with
AND M. I. ROMERO
University
of Murcia,
Spain
5, 1988
The scope of coulometrically generated viologen radical-cation as a reagent for the determination of organic compounds such as o-nitrophenol, p-nitrophenol, p-benzoquinone, and dichloroindophenol was investigated. Samples of the order lo-’ to 10m4 pmol could be determined with about 1% precision and accuracy. These organic compounds react quantitatively and rapidly with the viologen radical-cation, which can be electrolytically generated with 100% current efficiency. A sensitive biamperometric end point detection can be applied. a 1989 Academic press, IIIC.
INTRODUCTION
The methylviologen radical-cation (MVt) is one of the most powerful reductants but, surprisingly, it has been used scarcely in titrimetric analysis (I, 2), because MVi- reacts rapidly with oxygen and elaborate precautions are necessary in preparing and storing standard solutions. These circumstances suggest the application of coulometric electrogeneration of MV t as a titrant. A study describing the coulometric titration of oxygen with the viologen radical-monocation has been reported (3). We have investigated the possibility of generating MV -f with a constant current in order to develop a reliable and sensitive method for the determination of several organic compounds of great interest, such as aromatic nitrocompounds, quinones, and dichloroindophenols. MV t has a low redox potential (4,5), - 0.44 V vs NHE, and reacts rapidly with these organic substances. The usefulness of aromatic nitrocompounds in a variety of commercial products and as intermediates in chemical synthesis make the determination of this functional group very interesting. Analytical methods for the nitro group have centered on the reduction to the corresponding amine generally polarographically (6) or by addition of excess standard reductant (Ti3+ or C?‘) and back titration of the excess with a standard oxidant (7, 8). Electrochemical generation of strong analytical reductants by constant-current coulometry eliminates calibration curves and avoids the difficulties of handling and standardizing oxygen-sensitive solutions. Among the reducing agents coulometrically generated, chromium(I1) is one of the most often employed (9-22). A variety of methods has been reported for the determination of quinone, some with solution or electrogenerated C?’ and Ti3+ (23-19). r To whom
correspondence
should
be addressed. 59 0026-265X/89
$1.50
Copyright 0 1989 by Academic F’ress, Inc. All rights of reproduction in any form reserved.
60
LOZANO
ET AL.
The conventional methods for the assay of 2,6-dichloroindophenol are the iodometric, spectrophotometric, potentiometric, and polarographic (20, 22). This paper describes the conditions for the quantitative electrochemical generation of MV+ at mercury and graphite electrodes and the suitable conditions for the titrations of o-and p-nitrophenol, p-benzoquinone, and 2,6-dichloroindophenol. MATERIALS
AND METHODS
Reagents. An aqueous solution, 5 x 10m3 M methylviologen (l,l’-dimethyl4,4’-bipyridinium dichloride, Sigma), was prepared for use as the catholyte in the coulometric cell. o-Nitrophenol (Merck) was recrystallized from ethanol until the melting point was within +l”C, and the reported value was obtained. p-Nitrophenol (Merck) was recrystallized from benzene. The aqueous stock solutions (10m3 M) of these substances were standardized with electrogenerated Cr(I1) (II). A 5 x 10m4 M solution of p-benzoquinone (Merck), purified by either sublimation or stream distillation, was standardized coulometrically with Sn(II) (22). A 10m3 M aqueous solution of 2,6-dichloroindophenol (BDH) was titrated with ascorbic acid. A 2 M acetate buffer (pH 5.6) was prepared from reagent grade chemicals. A solution of potassium sulfate was used as electrolyte. Apparatus. The constant-current source was a Metrohm E-524 Coulostat. Potential-current curves were obtained with a Metrohm E-506 Herisau polarograph. The mercury pool cathode was 28.28 cm2 in area. A vitreous carbon SargentWelch s-30441 was used also. A spiral of platinum wire served as the anode. A pair of identical platinum electrodes (1 cm2) served as indicating electrodes. The indicating current was monitored with a Radiometer PO4 polarograph. Procedure. Titrations with coulometrically generated viologen radical-cation were performed in solutions containing 1.25 x 10m3 M in MV2+, 0.05 M potassium sulfate and with 0.2 M acetate buffer, buffered at pH 5.6. The solutions in the cell were deaereated for 15 min. Titrations were carried out with generating current from 0.19 to 0.48 mA. The potential difference applied across the two platinum electrodes was 100 mV. The titration was started by generating MV f- in small increments of time and measuring the current after each increment by disconnecting the coulometric titrator and waiting for a few seconds until a stable signal of current is obtained. The readings of current were plotted vs electrogenerated microequivalents and the end point located by lineal extrapolation. Of considerable interest was the number of samples that could be titrated successively in a single portion of analyte solution, because it is possible to reoxidate MV* by bubbling air through the solution. Five or six samples of oxidant compounds could be determined in one portion. RESULTS Electrolytic
Generation
AND DISCUSSION
of the Methylviologen
Radical-Cation
The current efficiency for the electroreduction of methylviologen at a vitreous carbon or mercury cathodes can be estimated from the current density vs cathode potential curves in Figs. 1 and 2 and is shown as a function of the total current
COULOMETRY
OF ORGANIC
61
COMPOUNDS
a
b
Potential
“4
5.2.t.,
‘J31ts
FIG. 1. Current density-cathode potential curves for reduction of methylviologen. Supporting electrolyte, 0.05 plus 0.2 M acetate buffer (pH 5.6). [MV’+]: 1, 0; 2, 0.3; 3, 0.6; 4, 1.3; 5, 1.6 X 10m3 M. (a) Mercury cathode; (b) vitreous carbon cathode.
density and concentration. These curves were derived from the current density vs potential curves on the assumption that the current efficiency is given by (i i,)/i, where i is the total current at a given current density (or cathode potential) and iu is the current at the same cathode potential for the supporting electrolyte alone. Figure 2 indicates that current effkiencies of up to 100% should be obtainable using current densities ranging from 0.05 to 0.5 mA/cm2 for a graphite electrode and from 0.05 to 0.35 mA/cm2 for a mercury pool electrode. Under this condition, methylviologen ion is reversible and electrolytically reduced to the MV t, without side reactions, and can be used as a coulometric reagent. Oxygen must however be completely removed from the solution.
0.2
0-4 Current
rJ,6 density,
0.5
1
mAlcm*
2. Current efftciency for electrogeneration of the methylviologen cation radical as a function of the current density and solution composition. [MI@+]: 1, 0.3; 2, 0.6; 3, 1.3; 4, 1.6 x low3 M. (a) Mercury cathode; (b) vitreous carbon cathode. FIG.
62
LOZANO
ET AL.
TABLE 1 Coulometric Titration of o-Nitrophenol o-Nitrophenol
(mg)
Taken
Found
0.0579 0.1158 0.2317 0.28%
0.0561 0.1171 0.2298 0.2872
No. determinations 6 9 8 7
SD b&
Relative SD (%)
0.21 0.97 0.16 1.34
0.37 0.83 0.07 0.47
Nom. Generating cathode: vitrous carbon. Supporting electrolyte (ca. 10 ml): 0.05 M potassium sulfate; 0.2 it4 acetate buffer, pH 5.5; and 1.2 x 10V3 M methylviologen. Generating currents from 0.193 to 0.48 mA (current density from 0.06 to 0.15 mA/ cm’) to provide titration times of 50&1500 s. TABLE 2 Coulometric Titration of p-Nitrophenol p-Nitrophenol
(mg)
Taken
Found
0.1436 0.1914 0.2873 0.3591 0.4309
0.1454 0.1940 0.2885 0.3584 0.4269
No. of determinations 5 8 6 7 8
SD (CLg)
Relative SD c%
0.13 0.14 0.10 1.17 0.18
0.09 0.07 0.04 0.33 0.04
Note. Conditions are as in Table 1. TABLE 3 Coulometric Titration of p-Benzoquinone p-Benzoquinone Taken
(mg) Found
0.1015 0.2538 0.5076 0.7614
No. of determinations
0.1022 0.2532 0.5103 0.7629
6 8 9 7
SD (ug)
Relative SD 6%
0.60 0.32 0.80 1.67
0.59 0.13 0.16 0.22
Note. Conditions are as in Table 1. TABLE 4 Coulometric Titration of 2,6Dichloroindophenol 2,6Dichloroindophenol Taken 0.0843 0.2108 0.4216 0.8432
Found 0.0837 0.2114 0.4238 0.8381
(mg)
No. of determinations 7 8 6 8
Note. Conditions are as in Table 1.
SD Relative SD (pg) 6% 0.72 0.63 0.52 0.61
0.86 0.30 0.12 0.07
COULOMETRY
50
100
OF ORGANIC
50
63
COMPOUNDS
100
50
100
96 reduced 3. Plots of endpoint biamperometric in the coulometric titration of 5 p.eq of o-nitrophenol p-benzoquinone (b), and 2,6dichloroindophenol (c) with MV f electrogenerated. FIG.
It is evident that vitreous carbon is preferred as the electrode material of the high hydrogen over-potential when acid media are necessary.
(a),
because
Detection of the Viologen Radical-Cation In order to have a complete coulometric titration method, a detection system for the viologen radical-cation must be added. Because of the reversible behavior of the MV2+h4V+ redox couple (23, 24) a sensitive biamperometric endpoint detection can be used. A small voltage (100 mV) is therefore applied across a pair of identical platinum electrodes and the resulting current monitored. The indicating current is proportional to the generated viologen radical-cation concentration down to IO-’ M. Coulometric Titration The result of coulometric titration of o- and p-nitrophenol, p-benzoquinone, and 2,6-dichloroindophenol are summarized in Tables l-4. The characteristics of biamperometric curves are shown in Fig. 3. The shape is governed by electrochemical behavior of the titrant and titrant couples. Under the test conditions, the redox systems with o-nitrophenol/o-aminophenol and p-nitrophenollp-aminophenol are irreversible, whereas those with p-benzoquinonelhydroquinone and dichloroindophenol,,/dichloroindophenol,~ are reversible. It can be concluded that the reduced form of methylviologen is easily generated electrolytically and the reported data show that MV t is a powerful reducing agent and a useful reductimetric titrant of organic compounds. The results clearly demonstrate the usefulness of the method. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support given by CAICYT (Project 374/84).
REFERENCES 1. 2. 3. 4. 5. 6.
Perez-Ruiz, T.; Martinez-Lozano, C.; Tomas V. Quim. Anal., 19854. 369-377. Perez-Ruiz, T.; Martinez-Lozano, C.; Tomas, V. Mikrochim. Acta, 1985, 367-377. Leest, R. E. Electroanal. Chem. Interfncial Electrochem., 1973,43, 251-257. Sweetser, P. E. Anal. Chem., 1%7, 39, 972-982. Ward, M.; White, J.; Bard, D. J. J. Amer. Chem. Sot., 1983, 105, 27-31. Wolff, G.; Nuemberg, H. W. Fresenius’s 2. Anal. Chem., 1965, 216, 169473.
64
LOZANO
ET
AL.
S.; Furman, N. H. Anal. Chew, 1955,27, 1182-I 184. D.; Sharma, J. P. Anal. Chem., 1963, 35, 1307-1308. 9. Aikens, A.; Carlita, S. C., Sr. M. Anal. Chem., 1965, 37, 459-462. 10. Bourg, Astrue, M.; Bonastre, J. Analysis, 1975, 3, 252-257. II. Al-Daher, I. M.; Kratochvil, B. G. Anal. Chem., 1979, 51, 1480-1483. 12. Kratochvil, B. G.; Al-Daher, I. M. Analyst (London), 1981, 106, 796-799. 13. Cheronis, N. D.; Ma, T. S. Organic Functional Group Analysis, p. 212. Wiley, New York, 1964. 14. Stuzka, V.; Lucas, I.; Jilek, J. A. Acta Univ. Palacki Olomuc Fat. Rerum Nat., 1976, 49, 93-105 7. Bottei,
8. Tiwari,
R. R. D. P.;
(Chem. Abst., 87, 779742). 15. Amin, D.; El&unman, F. M. Mikrochim. Acta, 1983, 1, 467-472. 16. Murty, N. K.; Murty, P. M. P. Indian .I. Chem. Sec. A, 1982, 756-757. 17. Amin, D.; El-Samman, F. M. J. Indian Chem. Sot. 1983,60, 502-504. 18. Sastry, C. P. S.; Rao, B. G. Natl. Sci. Acad. Lett., 1983, 6, 127-128. 19. Zaki, M. T. M.; Abdel-Rehiem, A. G. Microchem. J., 1984, 29, U8. 20. Patrick, R. A.; Svehla, G. Anal. Chim. Acta, 1977, 88, 363-370. 21. Patrick, R. A.; Cardwell, T. J.; Svehla, G. Anal. Chim. Acta, 1977, 88, 155-162. 22. Bard, A. J.; Lingane, J. J. Anal. Chim. Actu, 1959, 20, 463-471. 23. Yafiez, P.; Pingarr6n, J.; Polo, L. Mikrochim. Acta, 1985, 3, 279-287. 24, Elofson, R.; Edsberg, R. Canad. .I. Chem., 1957, 35, 646-650.
JOURNAL
Coulometric C. MARrfNEz Department
39, %-64
(1989)
Determination Methylviologen
of Organic Compounds Cation-Radical
LOZANO,' T. PEREZ RUIZ, V. of Analytical
Chemistry,
Received
Faculty
TOMAS,
of Science,
June 6, 1988; accepted
July
with
AND M. I. ROMERO
University
of Murcia,
Spain
5, 1988
The scope of coulometrically generated viologen radical-cation as a reagent for the determination of organic compounds such as o-nitrophenol, p-nitrophenol, p-benzoquinone, and dichloroindophenol was investigated. Samples of the order lo-’ to 10m4 pmol could be determined with about 1% precision and accuracy. These organic compounds react quantitatively and rapidly with the viologen radical-cation, which can be electrolytically generated with 100% current efficiency. A sensitive biamperometric end point detection can be applied. a 1989 Academic press, IIIC.
INTRODUCTION
The methylviologen radical-cation (MVt) is one of the most powerful reductants but, surprisingly, it has been used scarcely in titrimetric analysis (I, 2), because MVi- reacts rapidly with oxygen and elaborate precautions are necessary in preparing and storing standard solutions. These circumstances suggest the application of coulometric electrogeneration of MV t as a titrant. A study describing the coulometric titration of oxygen with the viologen radical-monocation has been reported (3). We have investigated the possibility of generating MV -f with a constant current in order to develop a reliable and sensitive method for the determination of several organic compounds of great interest, such as aromatic nitrocompounds, quinones, and dichloroindophenols. MV t has a low redox potential (4,5), - 0.44 V vs NHE, and reacts rapidly with these organic substances. The usefulness of aromatic nitrocompounds in a variety of commercial products and as intermediates in chemical synthesis make the determination of this functional group very interesting. Analytical methods for the nitro group have centered on the reduction to the corresponding amine generally polarographically (6) or by addition of excess standard reductant (Ti3+ or C?‘) and back titration of the excess with a standard oxidant (7, 8). Electrochemical generation of strong analytical reductants by constant-current coulometry eliminates calibration curves and avoids the difficulties of handling and standardizing oxygen-sensitive solutions. Among the reducing agents coulometrically generated, chromium(I1) is one of the most often employed (9-22). A variety of methods has been reported for the determination of quinone, some with solution or electrogenerated C?’ and Ti3+ (23-19). r To whom
correspondence
should
be addressed. 59 0026-265X/89
$1.50
Copyright 0 1989 by Academic F’ress, Inc. All rights of reproduction in any form reserved.
60
LOZANO
ET AL.
The conventional methods for the assay of 2,6-dichloroindophenol are the iodometric, spectrophotometric, potentiometric, and polarographic (20, 22). This paper describes the conditions for the quantitative electrochemical generation of MV+ at mercury and graphite electrodes and the suitable conditions for the titrations of o-and p-nitrophenol, p-benzoquinone, and 2,6-dichloroindophenol. MATERIALS
AND METHODS
Reagents. An aqueous solution, 5 x 10m3 M methylviologen (l,l’-dimethyl4,4’-bipyridinium dichloride, Sigma), was prepared for use as the catholyte in the coulometric cell. o-Nitrophenol (Merck) was recrystallized from ethanol until the melting point was within +l”C, and the reported value was obtained. p-Nitrophenol (Merck) was recrystallized from benzene. The aqueous stock solutions (10m3 M) of these substances were standardized with electrogenerated Cr(I1) (II). A 5 x 10m4 M solution of p-benzoquinone (Merck), purified by either sublimation or stream distillation, was standardized coulometrically with Sn(II) (22). A 10m3 M aqueous solution of 2,6-dichloroindophenol (BDH) was titrated with ascorbic acid. A 2 M acetate buffer (pH 5.6) was prepared from reagent grade chemicals. A solution of potassium sulfate was used as electrolyte. Apparatus. The constant-current source was a Metrohm E-524 Coulostat. Potential-current curves were obtained with a Metrohm E-506 Herisau polarograph. The mercury pool cathode was 28.28 cm2 in area. A vitreous carbon SargentWelch s-30441 was used also. A spiral of platinum wire served as the anode. A pair of identical platinum electrodes (1 cm2) served as indicating electrodes. The indicating current was monitored with a Radiometer PO4 polarograph. Procedure. Titrations with coulometrically generated viologen radical-cation were performed in solutions containing 1.25 x 10m3 M in MV2+, 0.05 M potassium sulfate and with 0.2 M acetate buffer, buffered at pH 5.6. The solutions in the cell were deaereated for 15 min. Titrations were carried out with generating current from 0.19 to 0.48 mA. The potential difference applied across the two platinum electrodes was 100 mV. The titration was started by generating MV f- in small increments of time and measuring the current after each increment by disconnecting the coulometric titrator and waiting for a few seconds until a stable signal of current is obtained. The readings of current were plotted vs electrogenerated microequivalents and the end point located by lineal extrapolation. Of considerable interest was the number of samples that could be titrated successively in a single portion of analyte solution, because it is possible to reoxidate MV* by bubbling air through the solution. Five or six samples of oxidant compounds could be determined in one portion. RESULTS Electrolytic
Generation
AND DISCUSSION
of the Methylviologen
Radical-Cation
The current efficiency for the electroreduction of methylviologen at a vitreous carbon or mercury cathodes can be estimated from the current density vs cathode potential curves in Figs. 1 and 2 and is shown as a function of the total current
COULOMETRY
OF ORGANIC
61
COMPOUNDS
a
b
Potential
“4
5.2.t.,
‘J31ts
FIG. 1. Current density-cathode potential curves for reduction of methylviologen. Supporting electrolyte, 0.05 plus 0.2 M acetate buffer (pH 5.6). [MV’+]: 1, 0; 2, 0.3; 3, 0.6; 4, 1.3; 5, 1.6 X 10m3 M. (a) Mercury cathode; (b) vitreous carbon cathode.
density and concentration. These curves were derived from the current density vs potential curves on the assumption that the current efficiency is given by (i i,)/i, where i is the total current at a given current density (or cathode potential) and iu is the current at the same cathode potential for the supporting electrolyte alone. Figure 2 indicates that current effkiencies of up to 100% should be obtainable using current densities ranging from 0.05 to 0.5 mA/cm2 for a graphite electrode and from 0.05 to 0.35 mA/cm2 for a mercury pool electrode. Under this condition, methylviologen ion is reversible and electrolytically reduced to the MV t, without side reactions, and can be used as a coulometric reagent. Oxygen must however be completely removed from the solution.
0.2
0-4 Current
rJ,6 density,
0.5
1
mAlcm*
2. Current efftciency for electrogeneration of the methylviologen cation radical as a function of the current density and solution composition. [MI@+]: 1, 0.3; 2, 0.6; 3, 1.3; 4, 1.6 x low3 M. (a) Mercury cathode; (b) vitreous carbon cathode. FIG.
62
LOZANO
ET AL.
TABLE 1 Coulometric Titration of o-Nitrophenol o-Nitrophenol
(mg)
Taken
Found
0.0579 0.1158 0.2317 0.28%
0.0561 0.1171 0.2298 0.2872
No. determinations 6 9 8 7
SD b&
Relative SD (%)
0.21 0.97 0.16 1.34
0.37 0.83 0.07 0.47
Nom. Generating cathode: vitrous carbon. Supporting electrolyte (ca. 10 ml): 0.05 M potassium sulfate; 0.2 it4 acetate buffer, pH 5.5; and 1.2 x 10V3 M methylviologen. Generating currents from 0.193 to 0.48 mA (current density from 0.06 to 0.15 mA/ cm’) to provide titration times of 50&1500 s. TABLE 2 Coulometric Titration of p-Nitrophenol p-Nitrophenol
(mg)
Taken
Found
0.1436 0.1914 0.2873 0.3591 0.4309
0.1454 0.1940 0.2885 0.3584 0.4269
No. of determinations 5 8 6 7 8
SD (CLg)
Relative SD c%
0.13 0.14 0.10 1.17 0.18
0.09 0.07 0.04 0.33 0.04
Note. Conditions are as in Table 1. TABLE 3 Coulometric Titration of p-Benzoquinone p-Benzoquinone Taken
(mg) Found
0.1015 0.2538 0.5076 0.7614
No. of determinations
0.1022 0.2532 0.5103 0.7629
6 8 9 7
SD (ug)
Relative SD 6%
0.60 0.32 0.80 1.67
0.59 0.13 0.16 0.22
Note. Conditions are as in Table 1. TABLE 4 Coulometric Titration of 2,6Dichloroindophenol 2,6Dichloroindophenol Taken 0.0843 0.2108 0.4216 0.8432
Found 0.0837 0.2114 0.4238 0.8381
(mg)
No. of determinations 7 8 6 8
Note. Conditions are as in Table 1.
SD Relative SD (pg) 6% 0.72 0.63 0.52 0.61
0.86 0.30 0.12 0.07
COULOMETRY
50
100
OF ORGANIC
50
63
COMPOUNDS
100
50
100
96 reduced 3. Plots of endpoint biamperometric in the coulometric titration of 5 p.eq of o-nitrophenol p-benzoquinone (b), and 2,6dichloroindophenol (c) with MV f electrogenerated. FIG.
It is evident that vitreous carbon is preferred as the electrode material of the high hydrogen over-potential when acid media are necessary.
(a),
because
Detection of the Viologen Radical-Cation In order to have a complete coulometric titration method, a detection system for the viologen radical-cation must be added. Because of the reversible behavior of the MV2+h4V+ redox couple (23, 24) a sensitive biamperometric endpoint detection can be used. A small voltage (100 mV) is therefore applied across a pair of identical platinum electrodes and the resulting current monitored. The indicating current is proportional to the generated viologen radical-cation concentration down to IO-’ M. Coulometric Titration The result of coulometric titration of o- and p-nitrophenol, p-benzoquinone, and 2,6-dichloroindophenol are summarized in Tables l-4. The characteristics of biamperometric curves are shown in Fig. 3. The shape is governed by electrochemical behavior of the titrant and titrant couples. Under the test conditions, the redox systems with o-nitrophenol/o-aminophenol and p-nitrophenollp-aminophenol are irreversible, whereas those with p-benzoquinonelhydroquinone and dichloroindophenol,,/dichloroindophenol,~ are reversible. It can be concluded that the reduced form of methylviologen is easily generated electrolytically and the reported data show that MV t is a powerful reducing agent and a useful reductimetric titrant of organic compounds. The results clearly demonstrate the usefulness of the method. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support given by CAICYT (Project 374/84).
REFERENCES 1. 2. 3. 4. 5. 6.
Perez-Ruiz, T.; Martinez-Lozano, C.; Tomas V. Quim. Anal., 19854. 369-377. Perez-Ruiz, T.; Martinez-Lozano, C.; Tomas, V. Mikrochim. Acta, 1985, 367-377. Leest, R. E. Electroanal. Chem. Interfncial Electrochem., 1973,43, 251-257. Sweetser, P. E. Anal. Chem., 1%7, 39, 972-982. Ward, M.; White, J.; Bard, D. J. J. Amer. Chem. Sot., 1983, 105, 27-31. Wolff, G.; Nuemberg, H. W. Fresenius’s 2. Anal. Chem., 1965, 216, 169473.
64
LOZANO
ET
AL.
S.; Furman, N. H. Anal. Chew, 1955,27, 1182-I 184. D.; Sharma, J. P. Anal. Chem., 1963, 35, 1307-1308. 9. Aikens, A.; Carlita, S. C., Sr. M. Anal. Chem., 1965, 37, 459-462. 10. Bourg, Astrue, M.; Bonastre, J. Analysis, 1975, 3, 252-257. II. Al-Daher, I. M.; Kratochvil, B. G. Anal. Chem., 1979, 51, 1480-1483. 12. Kratochvil, B. G.; Al-Daher, I. M. Analyst (London), 1981, 106, 796-799. 13. Cheronis, N. D.; Ma, T. S. Organic Functional Group Analysis, p. 212. Wiley, New York, 1964. 14. Stuzka, V.; Lucas, I.; Jilek, J. A. Acta Univ. Palacki Olomuc Fat. Rerum Nat., 1976, 49, 93-105 7. Bottei,
8. Tiwari,
R. R. D. P.;
(Chem. Abst., 87, 779742). 15. Amin, D.; El&unman, F. M. Mikrochim. Acta, 1983, 1, 467-472. 16. Murty, N. K.; Murty, P. M. P. Indian .I. Chem. Sec. A, 1982, 756-757. 17. Amin, D.; El-Samman, F. M. J. Indian Chem. Sot. 1983,60, 502-504. 18. Sastry, C. P. S.; Rao, B. G. Natl. Sci. Acad. Lett., 1983, 6, 127-128. 19. Zaki, M. T. M.; Abdel-Rehiem, A. G. Microchem. J., 1984, 29, U8. 20. Patrick, R. A.; Svehla, G. Anal. Chim. Acta, 1977, 88, 363-370. 21. Patrick, R. A.; Cardwell, T. J.; Svehla, G. Anal. Chim. Acta, 1977, 88, 155-162. 22. Bard, A. J.; Lingane, J. J. Anal. Chim. Actu, 1959, 20, 463-471. 23. Yafiez, P.; Pingarr6n, J.; Polo, L. Mikrochim. Acta, 1985, 3, 279-287. 24, Elofson, R.; Edsberg, R. Canad. .I. Chem., 1957, 35, 646-650.