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Determination of periodate with photoreduced thionine
AnalyticaChimka A&z, 94 (1977)129-133 @ Ekvier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
DETERMNATION
OF PERIODATE
F. SIERRA,
C. S_WCmZ-PEDREP;IO.
Department
of Analytical
Chemidy,
WITH PHOTOREDUCED
T. PEREZ RUIZ and C. hlARTINEZ Uniuersity
of Murciu
THIONINE
LOZANO
(Spain)
(Received 25th April 1977)
SUMMARY A nethod for the microdetermination of periodate in the presence of iodate is presented It is based on reduction of periodate to iodate by leucothionine generated in situ by photochemical reaction between EDTA and blue thionine. Biamperometric curves are used for evaluation, representing reduction time for the periodate versus the current generated in the oxidation of the Tred in excess after the end-point. A polarographic study of the process is presented. An applied potential of 100 mV is suitable for the biamperometric measurements with Pt-Pt electrodes. Tbe method is applicable for 0.4-45 ppm periodate.
IodaB does not interfere up to IO; : IO; ratios of 1OO:l.
In a seriesof generai studies of photochemical
reactions in analytical 113, a spectrophotometric determination of chromium as DATAcr(III) was developed f2’j. The complex was formed by reductio= of Cr(Vf) with Ieucotoluidine generated in the photochemical reaction between toluidint blue and 1,24iiaminocycIohexanetetraacetic acid (DCTA). The present paper reports a rather similar procedure for the determination chemistry
of periodate, both alone and in the presenceof iodate and other anions. The reductant is leucothionine which is formed in situ by photoreduction of the blue oxidized dyestuff with EDTA in the absence of oxygen. Many workers have studied the photochemical reduction of thionine and other thiazine, oxazine and phenazine based dyestuffs by EDTA; the mechanisms are complex and ill-defined [3-Y]. The overall reaction is always irreversible,the final products being the leuco dyestuff and variousoxidized species from the aminopofycarboxylate, dependingon the experimental conditions. At appropriatepH v&es, the ~eu~o~ion~e (Tma) formed in the photochemical reaction between T,, and EDTA rapidly and stoichiometrically reduces periodate to iodate, the dyestuff being E-oxidized to its blue form. The cycle keeps repeatingas long as periodate is present,The evolution of the process can be folIowed biamperometrically,by determiningthe current correspondingto oxidation of Ieucothionineafter all the periodate has been reduced. Thus the periodatereduction time is a&ualIy tieasured, and this obviously depends on the periodate concentration.
130
The conventional methods for the determination of period&e in the presence of iodate are usually based on selective titrations, periodate being reduced to iodate in neutral medium. The micro method presented here is accurate and precise and is advantageous for periodate concentrations of 0.4-45 ppm in the presence of iodate. E:U’ERMENTAL
Apparatus A Radiometer PO4 polarograph was coupled to the illumination device (Arrosu Electromedidas, Murcia) described previously 181. A Radiometer rotating platinum wire electrode was used to obtain We voltammograms. Amperometric titrations were followed with two platinum electrodes of about l-cm2 area. Reagerzts The chemicals used were of reagent grade (Merck). A 0.02 M solution of potassium metaperiodate was standardized with sodium arsenite in the conventional way; 10e3 and lo4 M solutions were prepared by suitabIe dilution. Acetate buffers (1 M) were used for pH 4-6 and 1 M borax buffers for pH 8-10. The pH values were always checked with a pH meter. Distilleddeionizcd water was used for all solutions. Procedure The sample must always be prepared under diffuse light. To the reaction cell, add 5 ml of acetate buffer pH 5,5 ml of 0.2 M EDTA (disodium salt), 2 ml of aqueous lo-’ M thionine solution and different volumes of standard potassium periodate solution (low3 M or 10e4 M); the final concentration of periodate should be belzveen 0.5 and 45 ppm. Dilute to exactly 30 ml with deionized water. Thermostat cell to 30 F 0.5%. Remove oxygen from the solution by bubbling pure nitrogen (99.997%) through it for 20 min. Apply an e-m-f. of 100 mV between the two platinum indicator electrodes. Then, switch on the halogen lamp of.the illumination device [S] and the polarograph recorder, simul~ku~eously, and record the i vs. t curve until the sample is decolorked. The shapes of the curves correspond to the amperometric titration of periodate with photogenerated Trcdr and permit evaluation of the time required for total reduction of periodate (see Fig. 2). In order to obtain reproducible results, the reagent concentrations and *he light intensity must be kept constant throughout the series of measurements. RZSULTS
AND DISCUSSION.
When a solution containing thionine and EDTA in a medium of pH > 4 and in the absence of oxygen is exposed to white light, the dyestuff is reduced,
131 and the blue solution becomes colourless [ 53 . The photoreduction process is very fast at pH 5-7 under intense light. ‘When perioda’& is added to a solution of EDTA and thionine, the photoreduced thionine is oxidized T red + IO, == T,,
+ 10;
T, is photoreduced again by the EDTA, and the cycle is repeated until no periodate remains. At the end of the process the dyestuff is in its leuco form. The main factors affecting both redos processes involved are pH, temperature and light intensity. The rate of the photochemical reaction between EDTA and thionine depends strongly on pH, reaching its maximum value at pH 6.5 151. The reduction of periodate by leucothionine was shown to be fast in the pH range 4-9. In order to meet both requirements and to achieve a periodate reduction time which was properly measurable under the experimental conditions, pH 5 was selected. Because of kinetic considerations, unwanted photo-oxidation of EDTA and of thionine by periodate is negligible at pH 5. The rate of the overall redox process is affected by temperature changes. This influence is stronger above 5O”C, when the periodate reduction time is substantially shorter (Table 1). A temperature of 30 -c 0.5”C was selected. The light intensity was controlled by using ;GIall cases a lamp of the same characteristics [ 81. Electrochemical behauiour of the periodate/T,, reaction To appraise the electrochemical behaviour of the systems involved and to enable the optimum conditions for biamperometric measurements to be selected, voltammetic curves were ,-ecorded (Fig. 1). The residual current of the acetate buffer is negligible (Curve 1). Curves 2 and 3 show voltammetric curves for a solution of the reactants, and for the same solution after it had been exposed to light for long enough to achieve complete reduction of periodate and produce similar concentrations of T,, and T,,. Under the iest conditions, the periodate-iodate redox system appears to be irreversible whereas the T,,J&, system is reversib!e. These current-voltage curves indicated that a potential of 100 mV is most suitable for biamperometric determinations with two platinum electrodes. TABLE
‘I
Effect of temperature on the photoreduction of periodate (Experimental conditions: 2 ml of lo-’ M periodate solution, 10 ml of 0.1 M EDTA, 5 ml of 1 M acetate buffer pK 5 and 2 ml of 0.001 ?tl thionine and dilute to exactly 30 ml with deionized water.) Temp.
(” C)
Time (sl
25 383
30 279
35 273
40 265
50 232
60 199
70 175
132
Fig. 1. Current-voltage curres at a rotating platinum microelectrode (vs.SCE). Curve 1, residual current of acetate buffer pH 5. Curve 2, voltammogramfor lo-’ M periodate, 5 x 1iF M SDTA and lo* M thionine in acetate buffer pH 5. Curve 3. voltammogram for the solution used for curve 2 after exposure ta light. Fig. 2. Determination
of the end-point
for the determination
of periodate.
F’igure 2 shows a biamperometric determination of periodate with photogenerated T,, The total periodate reduction time can be observed accurately. The ascending portion of the curve measures the current increase after the periodate equivalence point, when the T,/T,,, ratio increases. Calibration graphs are prepared by plotting periodate concentration versus the time t, required for complete reduction of periodate. Application of appmpriat. light intensities, which can be adjusted by changiig the distance between the lamp and the reaction vessel, gives t, values ranging from 15 to 600 s. Linear calibration graphs were obtained with 550 lux for periodate concentrations of 0.5-10 ppm (t, = 15-550 s); under 1500 Iux, linear graphs were a&o obtained for periodate concentrations of 6-45 ppm (t, = 60-1000 s). Sensitivity, reproducibility and interferences The sensitivity of the method is ca. 0.4 ppm of periodate. This limit is imposed by the sensitivity or’.the instruments available (chronometers, etc.). Reproducibility in the given range of periodate concentrations (0.4 to 45 ppm) is good (Table 2). Oxidizing anions such as MnO; ,.Cr,O2,-, VOj, Fe(C&-;.etc., interfere, and also affect Td at pH 5. Reducing agents such as iron(U) and tin@) must be previously oxidized. The presetice of metal ions tails for an intie& in thC EDTA concentration.
133 TABLE 2 Reproducibiiity
at various periodate concentrations
Periodate
sr (%I) (N = 10)
2.34 x lo-* M (0.448 ppm) 6.70 x lo- M (12.80 ppm) 2.34 x lo* M (44.80 ppm)
4.0 1.2 2.6
TABLE 3 Determinations
of periodate in presence of iodate
10; taken (mg)
10; taken (mg)
10; found (mg)
Error (SC)=
0.382 0.382 0.382 0.382 0.0764
8.745 17.49 34.98 43.73 34.98
0.387 0.389 0.391 0.393 0.0789
i-l.3 +1.8 i2.3 +2.8 +3.2
0.0764
43.73
0.0793
+ 3.8
aBaxd on 3 samples at each level.
Table 3 shows the errors obtained in determinations of periodate in the presence of iodate. REFERENCES 1 F. Sierra, C. Sanchez-Pedreno, T. Perez and C. Martinez, An. R. Sot. Esp. Fis. Quim.. 68 (1972) 1091; 70 (1974) 595; 72 (1976) 456 and 72 (1976) 538. 2 F. Sierra, C. Sanchez-Pedreno, T. Perez and C. Martinez, An. R. Sot. Esp. Fis. Quim., 71(1975) 382. 3 G. Oster .md N. Wotherspoon, J. Am. Chem Sot, 79 (1957) 4836;81 (1959) 5543. 4 J. Joussot-Dubien and J. Faure, J. Chim. Phys_, 60 (1963) 1214; Bull. Sot. Chim. Belg., 71 (1962) 877. 5 J. Faure and J. Joussot-Dubien, J. Chim. Phys., 63 (1966) 621. 6 K Sbimada and N. Masse, Sei Rep. Tohoku Univ. Ser. 1,52(3) (1969) 119. 7 M. Koizumi and Y. Usui. Mol.Photochem., 4 (1972) 57. 8 F. Sierra, C. Sanchez-Pedrenc, T. Perez, C. Martinez and M. Hernandez, Anal. Chim. Acta, 78 (1975) 277 and 78 (1975) 498.
DETERMNATION
OF PERIODATE
F. SIERRA,
C. S_WCmZ-PEDREP;IO.
Department
of Analytical
Chemidy,
WITH PHOTOREDUCED
T. PEREZ RUIZ and C. hlARTINEZ Uniuersity
of Murciu
THIONINE
LOZANO
(Spain)
(Received 25th April 1977)
SUMMARY A nethod for the microdetermination of periodate in the presence of iodate is presented It is based on reduction of periodate to iodate by leucothionine generated in situ by photochemical reaction between EDTA and blue thionine. Biamperometric curves are used for evaluation, representing reduction time for the periodate versus the current generated in the oxidation of the Tred in excess after the end-point. A polarographic study of the process is presented. An applied potential of 100 mV is suitable for the biamperometric measurements with Pt-Pt electrodes. Tbe method is applicable for 0.4-45 ppm periodate.
IodaB does not interfere up to IO; : IO; ratios of 1OO:l.
In a seriesof generai studies of photochemical
reactions in analytical 113, a spectrophotometric determination of chromium as DATAcr(III) was developed f2’j. The complex was formed by reductio= of Cr(Vf) with Ieucotoluidine generated in the photochemical reaction between toluidint blue and 1,24iiaminocycIohexanetetraacetic acid (DCTA). The present paper reports a rather similar procedure for the determination chemistry
of periodate, both alone and in the presenceof iodate and other anions. The reductant is leucothionine which is formed in situ by photoreduction of the blue oxidized dyestuff with EDTA in the absence of oxygen. Many workers have studied the photochemical reduction of thionine and other thiazine, oxazine and phenazine based dyestuffs by EDTA; the mechanisms are complex and ill-defined [3-Y]. The overall reaction is always irreversible,the final products being the leuco dyestuff and variousoxidized species from the aminopofycarboxylate, dependingon the experimental conditions. At appropriatepH v&es, the ~eu~o~ion~e (Tma) formed in the photochemical reaction between T,, and EDTA rapidly and stoichiometrically reduces periodate to iodate, the dyestuff being E-oxidized to its blue form. The cycle keeps repeatingas long as periodate is present,The evolution of the process can be folIowed biamperometrically,by determiningthe current correspondingto oxidation of Ieucothionineafter all the periodate has been reduced. Thus the periodatereduction time is a&ualIy tieasured, and this obviously depends on the periodate concentration.
130
The conventional methods for the determination of period&e in the presence of iodate are usually based on selective titrations, periodate being reduced to iodate in neutral medium. The micro method presented here is accurate and precise and is advantageous for periodate concentrations of 0.4-45 ppm in the presence of iodate. E:U’ERMENTAL
Apparatus A Radiometer PO4 polarograph was coupled to the illumination device (Arrosu Electromedidas, Murcia) described previously 181. A Radiometer rotating platinum wire electrode was used to obtain We voltammograms. Amperometric titrations were followed with two platinum electrodes of about l-cm2 area. Reagerzts The chemicals used were of reagent grade (Merck). A 0.02 M solution of potassium metaperiodate was standardized with sodium arsenite in the conventional way; 10e3 and lo4 M solutions were prepared by suitabIe dilution. Acetate buffers (1 M) were used for pH 4-6 and 1 M borax buffers for pH 8-10. The pH values were always checked with a pH meter. Distilleddeionizcd water was used for all solutions. Procedure The sample must always be prepared under diffuse light. To the reaction cell, add 5 ml of acetate buffer pH 5,5 ml of 0.2 M EDTA (disodium salt), 2 ml of aqueous lo-’ M thionine solution and different volumes of standard potassium periodate solution (low3 M or 10e4 M); the final concentration of periodate should be belzveen 0.5 and 45 ppm. Dilute to exactly 30 ml with deionized water. Thermostat cell to 30 F 0.5%. Remove oxygen from the solution by bubbling pure nitrogen (99.997%) through it for 20 min. Apply an e-m-f. of 100 mV between the two platinum indicator electrodes. Then, switch on the halogen lamp of.the illumination device [S] and the polarograph recorder, simul~ku~eously, and record the i vs. t curve until the sample is decolorked. The shapes of the curves correspond to the amperometric titration of periodate with photogenerated Trcdr and permit evaluation of the time required for total reduction of periodate (see Fig. 2). In order to obtain reproducible results, the reagent concentrations and *he light intensity must be kept constant throughout the series of measurements. RZSULTS
AND DISCUSSION.
When a solution containing thionine and EDTA in a medium of pH > 4 and in the absence of oxygen is exposed to white light, the dyestuff is reduced,
131 and the blue solution becomes colourless [ 53 . The photoreduction process is very fast at pH 5-7 under intense light. ‘When perioda’& is added to a solution of EDTA and thionine, the photoreduced thionine is oxidized T red + IO, == T,,
+ 10;
T, is photoreduced again by the EDTA, and the cycle is repeated until no periodate remains. At the end of the process the dyestuff is in its leuco form. The main factors affecting both redos processes involved are pH, temperature and light intensity. The rate of the photochemical reaction between EDTA and thionine depends strongly on pH, reaching its maximum value at pH 6.5 151. The reduction of periodate by leucothionine was shown to be fast in the pH range 4-9. In order to meet both requirements and to achieve a periodate reduction time which was properly measurable under the experimental conditions, pH 5 was selected. Because of kinetic considerations, unwanted photo-oxidation of EDTA and of thionine by periodate is negligible at pH 5. The rate of the overall redox process is affected by temperature changes. This influence is stronger above 5O”C, when the periodate reduction time is substantially shorter (Table 1). A temperature of 30 -c 0.5”C was selected. The light intensity was controlled by using ;GIall cases a lamp of the same characteristics [ 81. Electrochemical behauiour of the periodate/T,, reaction To appraise the electrochemical behaviour of the systems involved and to enable the optimum conditions for biamperometric measurements to be selected, voltammetic curves were ,-ecorded (Fig. 1). The residual current of the acetate buffer is negligible (Curve 1). Curves 2 and 3 show voltammetric curves for a solution of the reactants, and for the same solution after it had been exposed to light for long enough to achieve complete reduction of periodate and produce similar concentrations of T,, and T,,. Under the iest conditions, the periodate-iodate redox system appears to be irreversible whereas the T,,J&, system is reversib!e. These current-voltage curves indicated that a potential of 100 mV is most suitable for biamperometric determinations with two platinum electrodes. TABLE
‘I
Effect of temperature on the photoreduction of periodate (Experimental conditions: 2 ml of lo-’ M periodate solution, 10 ml of 0.1 M EDTA, 5 ml of 1 M acetate buffer pK 5 and 2 ml of 0.001 ?tl thionine and dilute to exactly 30 ml with deionized water.) Temp.
(” C)
Time (sl
25 383
30 279
35 273
40 265
50 232
60 199
70 175
132
Fig. 1. Current-voltage curres at a rotating platinum microelectrode (vs.SCE). Curve 1, residual current of acetate buffer pH 5. Curve 2, voltammogramfor lo-’ M periodate, 5 x 1iF M SDTA and lo* M thionine in acetate buffer pH 5. Curve 3. voltammogram for the solution used for curve 2 after exposure ta light. Fig. 2. Determination
of the end-point
for the determination
of periodate.
F’igure 2 shows a biamperometric determination of periodate with photogenerated T,, The total periodate reduction time can be observed accurately. The ascending portion of the curve measures the current increase after the periodate equivalence point, when the T,/T,,, ratio increases. Calibration graphs are prepared by plotting periodate concentration versus the time t, required for complete reduction of periodate. Application of appmpriat. light intensities, which can be adjusted by changiig the distance between the lamp and the reaction vessel, gives t, values ranging from 15 to 600 s. Linear calibration graphs were obtained with 550 lux for periodate concentrations of 0.5-10 ppm (t, = 15-550 s); under 1500 Iux, linear graphs were a&o obtained for periodate concentrations of 6-45 ppm (t, = 60-1000 s). Sensitivity, reproducibility and interferences The sensitivity of the method is ca. 0.4 ppm of periodate. This limit is imposed by the sensitivity or’.the instruments available (chronometers, etc.). Reproducibility in the given range of periodate concentrations (0.4 to 45 ppm) is good (Table 2). Oxidizing anions such as MnO; ,.Cr,O2,-, VOj, Fe(C&-;.etc., interfere, and also affect Td at pH 5. Reducing agents such as iron(U) and tin@) must be previously oxidized. The presetice of metal ions tails for an intie& in thC EDTA concentration.
133 TABLE 2 Reproducibiiity
at various periodate concentrations
Periodate
sr (%I) (N = 10)
2.34 x lo-* M (0.448 ppm) 6.70 x lo- M (12.80 ppm) 2.34 x lo* M (44.80 ppm)
4.0 1.2 2.6
TABLE 3 Determinations
of periodate in presence of iodate
10; taken (mg)
10; taken (mg)
10; found (mg)
Error (SC)=
0.382 0.382 0.382 0.382 0.0764
8.745 17.49 34.98 43.73 34.98
0.387 0.389 0.391 0.393 0.0789
i-l.3 +1.8 i2.3 +2.8 +3.2
0.0764
43.73
0.0793
+ 3.8
aBaxd on 3 samples at each level.
Table 3 shows the errors obtained in determinations of periodate in the presence of iodate. REFERENCES 1 F. Sierra, C. Sanchez-Pedreno, T. Perez and C. Martinez, An. R. Sot. Esp. Fis. Quim.. 68 (1972) 1091; 70 (1974) 595; 72 (1976) 456 and 72 (1976) 538. 2 F. Sierra, C. Sanchez-Pedreno, T. Perez and C. Martinez, An. R. Sot. Esp. Fis. Quim., 71(1975) 382. 3 G. Oster .md N. Wotherspoon, J. Am. Chem Sot, 79 (1957) 4836;81 (1959) 5543. 4 J. Joussot-Dubien and J. Faure, J. Chim. Phys_, 60 (1963) 1214; Bull. Sot. Chim. Belg., 71 (1962) 877. 5 J. Faure and J. Joussot-Dubien, J. Chim. Phys., 63 (1966) 621. 6 K Sbimada and N. Masse, Sei Rep. Tohoku Univ. Ser. 1,52(3) (1969) 119. 7 M. Koizumi and Y. Usui. Mol.Photochem., 4 (1972) 57. 8 F. Sierra, C. Sanchez-Pedrenc, T. Perez, C. Martinez and M. Hernandez, Anal. Chim. Acta, 78 (1975) 277 and 78 (1975) 498.