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Non-aqueous redox determination of ascorbic acid with copper (II)
694
SHORT COMMUNICATIONS
2. Y. Olliver, J. M. Liagre and I. A. Voinovitch, Paper presented at 32&me Congr&s du G.A.M.S., Paris, 1974. 3. I. A. Voinovitch. J. Debras-GuCdon and J. Louvrier. The Analysis of Silicates, Davey, New York, 1967. 4. I. A. Voinovitch, G. Legrand, G. Hameau and J. Louvrier, Me’thodes Physiques d’dnalyse, 1966, No. 3, 213. 5. I. A. Voinovitch, G. Legrand and J. Louvrier, ibid, 1967, No. 4, 200. 6. F. J. Feldman, Anal. Chew., 1970. 42, 719.
7. S. B. Smith, J. A. Blasi and F. J. Feldman, &Id., 1968, 40, 1525. 8. F J Feldman. J. A. Blasi and S. B. Smith, ibid., 1969, 41, 1095. 9. G. N. Bowers and F. J. Feldman, 20th National Meeting, Am. Assoc. Clin. Chemists, Washington, D.C., Aug. 1968, unpublished data. 10. I. A. Voinovitch, G. Legrand and J. Louvrier, Paper presented at 5th Intern. Conf. Atomic Spectroscopy, Melbourne, 1975.
Talanta,Vol 24,~~696695 PergamonPress. 1977Prmted,nGreatBr~tmn
NON-AQUEOUS
REDOX DETERMINATION WITH COPPER (II)
OF ASCORBIC ACID
BALBIR CHAND VERMA* and SWATANTAR KUMAR Chemistry Department, Himachal Pradesh University, Simla-171005 (India)
(Received 22 October 1976, Revised 7 April 1976. Accepted 19 May 1977)
Summary-Hydrated copper (II) perchlorate (in acetonitrile) has been used for the direct visual and potentiometric determination of ascorbic acid in acetic acid-acetonitrile media. Diphenylamine and diphenylbenzidine are suitable indicators. A bright platinum wire is used as indicator electrode and a modified calomel or an antimony electrode as reference electrode for the potentiometric titration. Ascorbic acid is oxidized to dehydroascorbic acid. The proposed method is simple, accurate and reliable. The reverse titration also works well. Copper(I1) in acetonitrile has been found to be a powerful oxidizing agent possessing good stability. The formal reduction potential for the copper(IIt(I) couple is +0.798 V vs. a silvera.OlM silver nitrate reference electrode.’ This high value is caused by the large solvation energy for copper(1) in acetonitrile, which raises the potential of the copper(H)-(I) couple and lowers that of the copper(IH0) couple such that a difference of more than 1.5 V exists between the two. Kratochvil et al. 2--5 used acetonitrile solutions of hydrated and anhydrous copper(I1) perchlorate for the oxidimetnc determination of tetrabutylammonium iodide, thioureas, arylamines and ferrocenes by potentiometric titration, using a platmum wire indicator electrode and a silve&.OlM silver nitrate (in acetonitrile) reference electrode. MruthyunJaya and Murthy6 have employed copper(H) nitrate and perchlorate solutions for the potentiometric titration of a copper(I)-thiourea complex, potassium ethyl xanthate, thiourea, copper(I) iodide, sodium iodide and tetra-alkylammonium iodide, using a glass and platinum electrode assembly. The same authors’ have studied anion effects on redox determination with copper(I1) in acetonitrile medium. Verma and Kumar’ have described the use of copper(H) nitrate for the visual and potentiometric determination of xanthates and organotrithiocarbonates in acetomtrile medium. The determination of copper(I1) with ascorbic acid in aqueous solution was studied by Erdey and Siposs.’ The instability of copper(I) in aqueous solutions and its susceptibility to aerial oxidation were the main difficulties described in the analysis. It was recommended that they should be avolded by the addition of a complexmg agent (KSCN, KBr or NaCl) before the titration, and the passage of carbon dioxide during the titration. The titration was medium with aqueous done in acetate-buffered 2,6-dlchlorophenolindophenol as indicator, which worked only when added near the end-pomt. An indicator correction was applied. * Address for correspondence.
The determination of ascorbic acid with copper(I1) in non-aqueous media does not appear to have been attempted so far. The present communication reports the use of hydrated copper(I1) perchlorate (in acetonitrile) for the direct visual or potentiometric determination of ascorbic acid in acetic acid-acetonitrile media. The use of an acetonitrile solution of copper (II) as a redox reagent has the advantage that the resulting copper(I) is stabilized by solvation with the acetonitrile” and that there is therefore little risk of aerial oxidation. Diphenylamine and diphenylbenzidine have been found to be excellent indicators. The potentiometric titrations have been performed with a bright platinum wire as indicator electrode and a modified calomel or an antimony electrode as reference electrode. Ascorbic acid is oxidized to dehydroascorbic acid under these conditions. The reverse titration is equally feasible. The proposed method for the determination of ascorbic acid with copper(I1) and vice uersu in acetic acid-acetonitrile media is simple. accurate and reliable. It is free from the difficulties which are encountered in aqueous medium. EXPERIMENTAL
Apparatus
Potentiometric titrations were performed with a Toshniwal-CL06A potentiometer, and a bright platinum wire as indicator electrode and modified calomel (saturated methanolic potassium chloride solution instead of aqueous) or antimony electrode as reference electrode. The solutions were stirred with a magnetic stirrer. Reagents Acetonitrile. Distilled twice from phosphorus
pentoxide
(5 g/l.).
Acetic acid. Distilled twice from chromium trioxide. Hydrated copper(l1) perchlorate, 0.05N in acetonitrile.
The solid compound was prepared“ by adding a slight excess of 60% perchlonc acid to a suspension of copper carbonate in water. The solution was boiled to eliminate
SHORT
carbon dioxide. The crystalline compound separated on cooling, and was dried under vacuum. A standard solution was prepared by dissolving a little more than the calculated amount of the dry solid in acetonitrile and standardizing it iodometrically in aqueous acidic medium. Ascot&c acid. A known weight was dissolved in the minimum quantity of warm acetic acid. The solution was left to attam room temperature, then diluted with acetonitrile to a known volume and aliquots were taken for analysis. Several aliquots were checked for ascorbic acid content by a published method. I1 The standard (0.05N) ascorbic acid solution in acetic acid-acetonitrile for the determination of copper(I1) was prepared in a similar manner. Indicator solutions. Diphenylamine and diphenylbenzidme solutions in acetonitrile. 0.1% and 0.05% respectively. Procedure
Aliquots of ascorbic acid in acetic acid-acetonitrile medium were transferred to dry titration vessels and diluted with 25-30 ml of acetonitrile. For visual titrations. 2-3 drops of indicator solution were added, the solution was cooled to room temperature (- 20”) and titrated with standard (0.05N) hydrated copper(I1) perchlorate solution (in acetonitrile) from a microburette provided with a guard tube to protect the titrant from atmospheric moisture. The end-point was signalled by appearance of a blue colour. Aliquots of copper(I1) in acetonitriie were titrated with standard (0.05N) ascorbic acid solution (in acetic acid-acetonitrile), the same indicators being used. The end-point was marked by disappearance of the blue colour.
Table 1. Determination of ascorhc acid with hydrated copper(I1) perchlorate (0.05N) and vice versa in acetic acidacetomtrile media
Taken, mg Ascorbic acid 9.00 43.00 Copper(I1) 6.40 25.60
Found*, mg Potentiometric Visual method method mean s.d. mean s.d.
8.95 42.84
0.05 0.10
9.02 42.88
0.04 0.08
6.42 25.70
0.02 0.04
6.41 25.66
0.02 0.04
* Mean of ten determinations.
695
COMMUNICATIONS
RESULTS
AND
DISCUSSION
Some results are recorded in Table 1. Since the results obtained by potentiometric titration were essentially the same for both types of reference electrode, but only those for the calomel electrode system are recorded in the table. Diphenylamine and diphenylbenzidine have been found excellent indicators for the titration, done in either direction. When ascorbic acid is titrated with copper( the solution is colourless to start with, becomes light yellow, and changes sharply to blue at the end-point. In the reverse titration, the initial blue colour changes sharply to light yellow at the end-point. In the potentiometric titrations, the potentials attamed stable values immediately on addition of each portion of oxidant. A sharp change in potential of 25&400 mV (antimony reference electrode) and 20&250 mV (modified calome1 reference electrode) per 0.05 ml of 0.05N titrant was observed at the equivalence point. In the titrations of ascorbic acid with c&pper(II) thk potentials of the inflection points were 8@200 mV (antimonv electrode) and 45G550 mV (calomel), and for copper(I1) titrated with ascorbic acid they were 5C&600 mV and 48&560 mV respectively. Glucose. fructose, maltose, formic acid and benzoic acid do not interfere when present in amounts up to 25 mg. Oxalic acid, phenylhydmzine, N,N-dimethylhydrazine, thiourea, xanthates, dithiocarbamates and organotrithiocarbonates, however, do interfere. Acknowledgement-The authors thank the Council of Scientific and Industrial Research (India) for the award of a research fellowship to one of them (S.K.). REFERENCES I. B. Kratochvil, Rec. Chem. Prog., 1966, 27, 262. 2. B. Kratochvil, D. A. Zatko and R. Markuszewski, Anal. Chem., 1966, 38, 770. 3. B. Kratochvil and D. A. Zatko, lbld., 1968, 40, 442. 4. P. F. Quirk and B. Kratochvil, ibid., 1970 42, 492, 535. 5. D. A. Zatko and B. Kratochvil. ibid., 1968, 40, 2120. 6. H. C. Mruthyunjaya and A. R. V. Murthy, Indian J. Chem., 1969, 7, 403. 7. Idem, ibid., 1973, 11, 481. 8. B. C. Verma and S. Kumar, Microchem. J., in the press. 9. L. Erdey and G. Siposs, Z. Anal. Chem., 1957, 157, 166. 10. I. M. Koltoff and J. F. Coetzee, J. Am. Chem. sot., 1957, 79. 1852. 11. L. Erdey and G. RBdy, Acta Chim. Acad. Sci. Hung., 1958, 15, 81.
SHORT COMMUNICATIONS
2. Y. Olliver, J. M. Liagre and I. A. Voinovitch, Paper presented at 32&me Congr&s du G.A.M.S., Paris, 1974. 3. I. A. Voinovitch. J. Debras-GuCdon and J. Louvrier. The Analysis of Silicates, Davey, New York, 1967. 4. I. A. Voinovitch, G. Legrand, G. Hameau and J. Louvrier, Me’thodes Physiques d’dnalyse, 1966, No. 3, 213. 5. I. A. Voinovitch, G. Legrand and J. Louvrier, ibid, 1967, No. 4, 200. 6. F. J. Feldman, Anal. Chew., 1970. 42, 719.
7. S. B. Smith, J. A. Blasi and F. J. Feldman, &Id., 1968, 40, 1525. 8. F J Feldman. J. A. Blasi and S. B. Smith, ibid., 1969, 41, 1095. 9. G. N. Bowers and F. J. Feldman, 20th National Meeting, Am. Assoc. Clin. Chemists, Washington, D.C., Aug. 1968, unpublished data. 10. I. A. Voinovitch, G. Legrand and J. Louvrier, Paper presented at 5th Intern. Conf. Atomic Spectroscopy, Melbourne, 1975.
Talanta,Vol 24,~~696695 PergamonPress. 1977Prmted,nGreatBr~tmn
NON-AQUEOUS
REDOX DETERMINATION WITH COPPER (II)
OF ASCORBIC ACID
BALBIR CHAND VERMA* and SWATANTAR KUMAR Chemistry Department, Himachal Pradesh University, Simla-171005 (India)
(Received 22 October 1976, Revised 7 April 1976. Accepted 19 May 1977)
Summary-Hydrated copper (II) perchlorate (in acetonitrile) has been used for the direct visual and potentiometric determination of ascorbic acid in acetic acid-acetonitrile media. Diphenylamine and diphenylbenzidine are suitable indicators. A bright platinum wire is used as indicator electrode and a modified calomel or an antimony electrode as reference electrode for the potentiometric titration. Ascorbic acid is oxidized to dehydroascorbic acid. The proposed method is simple, accurate and reliable. The reverse titration also works well. Copper(I1) in acetonitrile has been found to be a powerful oxidizing agent possessing good stability. The formal reduction potential for the copper(IIt(I) couple is +0.798 V vs. a silvera.OlM silver nitrate reference electrode.’ This high value is caused by the large solvation energy for copper(1) in acetonitrile, which raises the potential of the copper(H)-(I) couple and lowers that of the copper(IH0) couple such that a difference of more than 1.5 V exists between the two. Kratochvil et al. 2--5 used acetonitrile solutions of hydrated and anhydrous copper(I1) perchlorate for the oxidimetnc determination of tetrabutylammonium iodide, thioureas, arylamines and ferrocenes by potentiometric titration, using a platmum wire indicator electrode and a silve&.OlM silver nitrate (in acetonitrile) reference electrode. MruthyunJaya and Murthy6 have employed copper(H) nitrate and perchlorate solutions for the potentiometric titration of a copper(I)-thiourea complex, potassium ethyl xanthate, thiourea, copper(I) iodide, sodium iodide and tetra-alkylammonium iodide, using a glass and platinum electrode assembly. The same authors’ have studied anion effects on redox determination with copper(I1) in acetonitrile medium. Verma and Kumar’ have described the use of copper(H) nitrate for the visual and potentiometric determination of xanthates and organotrithiocarbonates in acetomtrile medium. The determination of copper(I1) with ascorbic acid in aqueous solution was studied by Erdey and Siposs.’ The instability of copper(I) in aqueous solutions and its susceptibility to aerial oxidation were the main difficulties described in the analysis. It was recommended that they should be avolded by the addition of a complexmg agent (KSCN, KBr or NaCl) before the titration, and the passage of carbon dioxide during the titration. The titration was medium with aqueous done in acetate-buffered 2,6-dlchlorophenolindophenol as indicator, which worked only when added near the end-pomt. An indicator correction was applied. * Address for correspondence.
The determination of ascorbic acid with copper(I1) in non-aqueous media does not appear to have been attempted so far. The present communication reports the use of hydrated copper(I1) perchlorate (in acetonitrile) for the direct visual or potentiometric determination of ascorbic acid in acetic acid-acetonitrile media. The use of an acetonitrile solution of copper (II) as a redox reagent has the advantage that the resulting copper(I) is stabilized by solvation with the acetonitrile” and that there is therefore little risk of aerial oxidation. Diphenylamine and diphenylbenzidine have been found to be excellent indicators. The potentiometric titrations have been performed with a bright platinum wire as indicator electrode and a modified calomel or an antimony electrode as reference electrode. Ascorbic acid is oxidized to dehydroascorbic acid under these conditions. The reverse titration is equally feasible. The proposed method for the determination of ascorbic acid with copper(I1) and vice uersu in acetic acid-acetonitrile media is simple. accurate and reliable. It is free from the difficulties which are encountered in aqueous medium. EXPERIMENTAL
Apparatus
Potentiometric titrations were performed with a Toshniwal-CL06A potentiometer, and a bright platinum wire as indicator electrode and modified calomel (saturated methanolic potassium chloride solution instead of aqueous) or antimony electrode as reference electrode. The solutions were stirred with a magnetic stirrer. Reagents Acetonitrile. Distilled twice from phosphorus
pentoxide
(5 g/l.).
Acetic acid. Distilled twice from chromium trioxide. Hydrated copper(l1) perchlorate, 0.05N in acetonitrile.
The solid compound was prepared“ by adding a slight excess of 60% perchlonc acid to a suspension of copper carbonate in water. The solution was boiled to eliminate
SHORT
carbon dioxide. The crystalline compound separated on cooling, and was dried under vacuum. A standard solution was prepared by dissolving a little more than the calculated amount of the dry solid in acetonitrile and standardizing it iodometrically in aqueous acidic medium. Ascot&c acid. A known weight was dissolved in the minimum quantity of warm acetic acid. The solution was left to attam room temperature, then diluted with acetonitrile to a known volume and aliquots were taken for analysis. Several aliquots were checked for ascorbic acid content by a published method. I1 The standard (0.05N) ascorbic acid solution in acetic acid-acetonitrile for the determination of copper(I1) was prepared in a similar manner. Indicator solutions. Diphenylamine and diphenylbenzidme solutions in acetonitrile. 0.1% and 0.05% respectively. Procedure
Aliquots of ascorbic acid in acetic acid-acetonitrile medium were transferred to dry titration vessels and diluted with 25-30 ml of acetonitrile. For visual titrations. 2-3 drops of indicator solution were added, the solution was cooled to room temperature (- 20”) and titrated with standard (0.05N) hydrated copper(I1) perchlorate solution (in acetonitrile) from a microburette provided with a guard tube to protect the titrant from atmospheric moisture. The end-point was signalled by appearance of a blue colour. Aliquots of copper(I1) in acetonitriie were titrated with standard (0.05N) ascorbic acid solution (in acetic acid-acetonitrile), the same indicators being used. The end-point was marked by disappearance of the blue colour.
Table 1. Determination of ascorhc acid with hydrated copper(I1) perchlorate (0.05N) and vice versa in acetic acidacetomtrile media
Taken, mg Ascorbic acid 9.00 43.00 Copper(I1) 6.40 25.60
Found*, mg Potentiometric Visual method method mean s.d. mean s.d.
8.95 42.84
0.05 0.10
9.02 42.88
0.04 0.08
6.42 25.70
0.02 0.04
6.41 25.66
0.02 0.04
* Mean of ten determinations.
695
COMMUNICATIONS
RESULTS
AND
DISCUSSION
Some results are recorded in Table 1. Since the results obtained by potentiometric titration were essentially the same for both types of reference electrode, but only those for the calomel electrode system are recorded in the table. Diphenylamine and diphenylbenzidine have been found excellent indicators for the titration, done in either direction. When ascorbic acid is titrated with copper( the solution is colourless to start with, becomes light yellow, and changes sharply to blue at the end-point. In the reverse titration, the initial blue colour changes sharply to light yellow at the end-point. In the potentiometric titrations, the potentials attamed stable values immediately on addition of each portion of oxidant. A sharp change in potential of 25&400 mV (antimony reference electrode) and 20&250 mV (modified calome1 reference electrode) per 0.05 ml of 0.05N titrant was observed at the equivalence point. In the titrations of ascorbic acid with c&pper(II) thk potentials of the inflection points were 8@200 mV (antimonv electrode) and 45G550 mV (calomel), and for copper(I1) titrated with ascorbic acid they were 5C&600 mV and 48&560 mV respectively. Glucose. fructose, maltose, formic acid and benzoic acid do not interfere when present in amounts up to 25 mg. Oxalic acid, phenylhydmzine, N,N-dimethylhydrazine, thiourea, xanthates, dithiocarbamates and organotrithiocarbonates, however, do interfere. Acknowledgement-The authors thank the Council of Scientific and Industrial Research (India) for the award of a research fellowship to one of them (S.K.). REFERENCES I. B. Kratochvil, Rec. Chem. Prog., 1966, 27, 262. 2. B. Kratochvil, D. A. Zatko and R. Markuszewski, Anal. Chem., 1966, 38, 770. 3. B. Kratochvil and D. A. Zatko, lbld., 1968, 40, 442. 4. P. F. Quirk and B. Kratochvil, ibid., 1970 42, 492, 535. 5. D. A. Zatko and B. Kratochvil. ibid., 1968, 40, 2120. 6. H. C. Mruthyunjaya and A. R. V. Murthy, Indian J. Chem., 1969, 7, 403. 7. Idem, ibid., 1973, 11, 481. 8. B. C. Verma and S. Kumar, Microchem. J., in the press. 9. L. Erdey and G. Siposs, Z. Anal. Chem., 1957, 157, 166. 10. I. M. Koltoff and J. F. Coetzee, J. Am. Chem. sot., 1957, 79. 1852. 11. L. Erdey and G. RBdy, Acta Chim. Acad. Sci. Hung., 1958, 15, 81.