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Determination of ascorbic acid with ferricyanide
J,rf,tnr<,. Vol. 2X. pp. 232 k, 332. IYHI Prmted m Great arltain All rlghrs rcwrved
@XL)-9140 XI 050332-02s02.00 0 Copyright 0 19x1 Pcrgamon Press Ltd
DETERMINATION OF ASCORBIC ACID WITH FERRICYANIDE N. RUKMINI, V. S. N. P. KAVI-IFIA and K. DEVENDRA VIJAYA Department of Chemistry. Andhra University. Waltair 530003. India
(Receiaed 27 May 1980. Revised 11 September 1980. Accepted 18 November 1980)
Summary-Ascorbic acid can be determined by titration with potassium ferricyanide in sulphuric. phosphoric or hydrochloric acid media. Twelve indicators. including ferroin and some triphenylmethane and thiazine dyes, have been found suitable. The method has been successfully applied to assay of commercial products for vitamin C. Several organic substances likely to be associated with ascorbic acid in preparations and plant tissues do not interfere.
Potassium ferricyanide is available in high purity and can easily be recrystallized from water.’ It has a high equivalent weight and can be used as a primary standard after repeated recrystallization and drying at loo”. Its aqueous solutions are very stable when prepared and preserved properly. It has been used for titration of ascorbic acid in approximately neutral medium,‘-” and also in 4M sulphuric acid with barium diphenylamine sulphonate (BDS) as indicator,6 but it was observed that though BDS is rapidly oxidized by ferricyanide, the oxidized form is not readily reduced by ascorbic acid. In our experience, however. the end-point is not very satisfactory, so we have examined the use of other indicators. We have already reported that the redox potential of ferricyanide is increased remarkably at high acidity and that ferroin can be used as the indicator.’ Further work has revealed that some triphenylmethane dyes and thiazine derivatives are also suitable indicators under these conditions and can be used in the titration of ascorbic acid with ferricyanide. Thiazine dyes are normally used as indicators in titrations of strong reductants, the unstable leuco dye being oxidized to the blue or bluish green form at the end-point, but the further oxidation of the blue dye to its pink product is seldom utilized.8.9 The proposed method has been applied successfully for the assay of vitamin C preparations.
EXPERIMENTAL Reagents Potassium ferricyanide, 0.1 M. Prepared by dissolving an accurately weighed amount of analytical-grade reagent
which has been recrystallized from water and dried at loo”. Ascorbic acid, 0.05M. Prepared from analytical-reagent grade material, stabilized with EDTA and formic acid and standardized with iodate.‘O Indicators. Ferroin (O.OZSM), 0.1% solutions of Erioglau-
tine (C.I.671). Eriogreen (C.I.735), Setaglaucine (C.I.658) 332
and Xylenecyanol FF (C.I.715) and 0.05°/0 solutions of Methylene Blue (C.I.52015), Toluidine Blue (C.1.52040). New Methylene Bhie (C.I. 52030), Methylene Green, Azure A. Azure B and Azure C, prepared in demineralized water. Analytical-reagent grade chemicals were used whenever possible. Procedure
Take enough sulphuric acid (IOM) or concentrated hydrochloric or phosphoric acid to give the desired acidity (Table 1) at the end-point. Dilute with distilled water if necessary and cool. Add the ascorbic acid solution and one drop of ferroin or 0.2 ml of triphenylmethane dye solution or 0.1 ml of thiazine dye solution, and titrate with ferricyanide solution to the sharp colour change at the end-point. For analysis of commercial preparations (vitamin C tablets, injection solutions and syrups) make a solution of the sample in demineralized water such that the ascorbic acid concentration is 3-5 mg/ml, and filter if the solution is very turbid. Acidify and titrate as above.
RESULTS AND DISCUSSION
From preliminary experiments we found that ferricyanide oxidizes ferroin in SlOM sulphuric acid, 7-10M hydrochloric acid or 9-13M phosphoric acid and the triphenyhnethane dyes in 5-1OM sulphuric acid or 9-13M phosphoric acid. The dyes can also be oxidized in hydrochloric acid media but the colour change is too poor to be useful. However, narrower acidity ranges are necessary for the determination of ascorbic acid (Table 1). If the acidity is too high, premature end-points are obtained because the reduction of the oxidized form of the indicator by ascorbic acid is slow. If the acidity is too low, neither the indicator nor the ascorbic acid is oxidized. In hydrochloric acid medium only ferroin functions satisfactorily as the indicator. All the indicators give sharp colour changes. With 0.1N solutions an inert atmosphere is not needed, but with more dilute solutions (<0.025N) it is necessary
333
SHORT COMMUNICATIONS
malic acids, or glucose., fructose and sucrose, which are likely to be present with ascorbic acid in commercial preparations and plant tissues, do not interfere. The non-interference of oxalic acid is particularly important, as it can be used as a potential extractant” for ascorbic acid from fruit juices and multivitamin preparations.
Table 1. Conditions for the determination of ascorbic acid with ferricyanide in various acid media Indicator Ferroin Erioglaucine Eriogreen Setoglaucine Xylene Cyanol FF Thiazine dyes
H$O,,
A4 HCI, M
5-7.5 6.5-8 7-8 7-8 6.5-8 7-9
H,PO,,
7.5-10 -
M
9-11 12-13 12-13 12-13 12-13 12-13
for titrations in sulphuric and hydrochloric media but not in phosphoric acid media.
Acknowledgements-One of us (V.S.N.P.K.) thanks the Council of Scientific and Industrial Research (India) for the award of a research fellowship.
acid
REFERENCES
I. 1. M. Kolthoff, R. Belcher, V. A. Stenger and G. Mat-
Tables 2 and 3 show some results for pure ascorbic acid solutions and some commercial preparations, and comparative results by the BP method.”
suyama, Volumetric Analysis, Vol. III, pp. 236, 645. Interscience, New York, 1957. 2. K. Hubicka, Ann. Univ. Mariae Curie-Sklodowsku, Lub/in-Poloniu,
Interferences
Strong oxidants and reductants interfere. Coloured ions such as copper(H), nickel and cobalt(H) interfere. Cerium(III), arsenic(III), hydroxylamine hydrochloride and hydrazine sulphate do not interfere, but antimony(III), iron and manganese(I1) do. Vanadium(IV) gives results that are only 0.5-l% too high in phosphoric acid media, but are very high in the other media. An advantage of tlie method is that organic compounds such as oxalic, tartaric, citric and Table 2. Determination Indicator
Acid
Ferroin
Sect. AA,
1957, 10, 35.
3. J. A. C. van Pinxteren and E. Verloop, Pharm. Weekblad, 1958,23,203.982. 4. L. Erdey and G. Svehla, Z. Anal. Chem., 1956,150,407. 5. B. R. Sant, Chemist-Analyst, 1958, 47, 65. 6. G. S. Sastry and G. G. Rao, Talanta, 1972, 19. 212. 7. N. Rukmini, V. S. N. P. Kavitha and K. R. Rao. ibid.. 1979.26, 579. 8. N. Rukmini and V. S. N. P. Kavitha, Acta Cinecia Indica,
1979, 5, 44.
U. Muralikrishna, Ind. J. Chem., 1972, 10, 559. 10. R. Erdey and E. Bodor, Z. Anal. Chem., 1952,136, 109: Magy. Kern. Foly. 1952, 58, 295. I 1. British Pharmacepoeia, p. 36. HMSO, London, 1973. 12. J. D. Panting, Ind. Eng. Chem., Anal. Ed. 1943, 15, 389. 9.
of ascorbic acid with ferricyanide Range determined,
mg
Error range, %
Hz% HCI H,PG,
13.24-88.24 13.24-88.24 13.24-88.24
0.1-0.4 0.2-0.4 0.2-0.4
Hz=4
Triphenylmethane dyes
H3PO4
13.24-88.24 13.24-48.53
0.2-0.4 0.2-0.4
Thiazine dyes
H,SO, H,l%
13.24-88.24 13.24-48.53
0.24.4 0.24.5
Table 3. Assay of commercial vitamin C preparations: amount (g) present in 5 tablets or 30 ml of syrup or 5 ampoules of injection solution Ferricyanide method Sample
BP method*
Ferroin
Erioglaucine
Methylene Blue
2.52 2.48
2.52 2.49
2.51 2.48
2.52 2.48
2.50 2.52
2.50 2.52
2.50 2.52
3.62 3.78
3.64 3.75
3.64 3.76
Tablets
Ampoules 2.50 2.52
*
Syrups 3.62 3.78
@XL)-9140 XI 050332-02s02.00 0 Copyright 0 19x1 Pcrgamon Press Ltd
DETERMINATION OF ASCORBIC ACID WITH FERRICYANIDE N. RUKMINI, V. S. N. P. KAVI-IFIA and K. DEVENDRA VIJAYA Department of Chemistry. Andhra University. Waltair 530003. India
(Receiaed 27 May 1980. Revised 11 September 1980. Accepted 18 November 1980)
Summary-Ascorbic acid can be determined by titration with potassium ferricyanide in sulphuric. phosphoric or hydrochloric acid media. Twelve indicators. including ferroin and some triphenylmethane and thiazine dyes, have been found suitable. The method has been successfully applied to assay of commercial products for vitamin C. Several organic substances likely to be associated with ascorbic acid in preparations and plant tissues do not interfere.
Potassium ferricyanide is available in high purity and can easily be recrystallized from water.’ It has a high equivalent weight and can be used as a primary standard after repeated recrystallization and drying at loo”. Its aqueous solutions are very stable when prepared and preserved properly. It has been used for titration of ascorbic acid in approximately neutral medium,‘-” and also in 4M sulphuric acid with barium diphenylamine sulphonate (BDS) as indicator,6 but it was observed that though BDS is rapidly oxidized by ferricyanide, the oxidized form is not readily reduced by ascorbic acid. In our experience, however. the end-point is not very satisfactory, so we have examined the use of other indicators. We have already reported that the redox potential of ferricyanide is increased remarkably at high acidity and that ferroin can be used as the indicator.’ Further work has revealed that some triphenylmethane dyes and thiazine derivatives are also suitable indicators under these conditions and can be used in the titration of ascorbic acid with ferricyanide. Thiazine dyes are normally used as indicators in titrations of strong reductants, the unstable leuco dye being oxidized to the blue or bluish green form at the end-point, but the further oxidation of the blue dye to its pink product is seldom utilized.8.9 The proposed method has been applied successfully for the assay of vitamin C preparations.
EXPERIMENTAL Reagents Potassium ferricyanide, 0.1 M. Prepared by dissolving an accurately weighed amount of analytical-grade reagent
which has been recrystallized from water and dried at loo”. Ascorbic acid, 0.05M. Prepared from analytical-reagent grade material, stabilized with EDTA and formic acid and standardized with iodate.‘O Indicators. Ferroin (O.OZSM), 0.1% solutions of Erioglau-
tine (C.I.671). Eriogreen (C.I.735), Setaglaucine (C.I.658) 332
and Xylenecyanol FF (C.I.715) and 0.05°/0 solutions of Methylene Blue (C.I.52015), Toluidine Blue (C.1.52040). New Methylene Bhie (C.I. 52030), Methylene Green, Azure A. Azure B and Azure C, prepared in demineralized water. Analytical-reagent grade chemicals were used whenever possible. Procedure
Take enough sulphuric acid (IOM) or concentrated hydrochloric or phosphoric acid to give the desired acidity (Table 1) at the end-point. Dilute with distilled water if necessary and cool. Add the ascorbic acid solution and one drop of ferroin or 0.2 ml of triphenylmethane dye solution or 0.1 ml of thiazine dye solution, and titrate with ferricyanide solution to the sharp colour change at the end-point. For analysis of commercial preparations (vitamin C tablets, injection solutions and syrups) make a solution of the sample in demineralized water such that the ascorbic acid concentration is 3-5 mg/ml, and filter if the solution is very turbid. Acidify and titrate as above.
RESULTS AND DISCUSSION
From preliminary experiments we found that ferricyanide oxidizes ferroin in SlOM sulphuric acid, 7-10M hydrochloric acid or 9-13M phosphoric acid and the triphenyhnethane dyes in 5-1OM sulphuric acid or 9-13M phosphoric acid. The dyes can also be oxidized in hydrochloric acid media but the colour change is too poor to be useful. However, narrower acidity ranges are necessary for the determination of ascorbic acid (Table 1). If the acidity is too high, premature end-points are obtained because the reduction of the oxidized form of the indicator by ascorbic acid is slow. If the acidity is too low, neither the indicator nor the ascorbic acid is oxidized. In hydrochloric acid medium only ferroin functions satisfactorily as the indicator. All the indicators give sharp colour changes. With 0.1N solutions an inert atmosphere is not needed, but with more dilute solutions (<0.025N) it is necessary
333
SHORT COMMUNICATIONS
malic acids, or glucose., fructose and sucrose, which are likely to be present with ascorbic acid in commercial preparations and plant tissues, do not interfere. The non-interference of oxalic acid is particularly important, as it can be used as a potential extractant” for ascorbic acid from fruit juices and multivitamin preparations.
Table 1. Conditions for the determination of ascorbic acid with ferricyanide in various acid media Indicator Ferroin Erioglaucine Eriogreen Setoglaucine Xylene Cyanol FF Thiazine dyes
H$O,,
A4 HCI, M
5-7.5 6.5-8 7-8 7-8 6.5-8 7-9
H,PO,,
7.5-10 -
M
9-11 12-13 12-13 12-13 12-13 12-13
for titrations in sulphuric and hydrochloric media but not in phosphoric acid media.
Acknowledgements-One of us (V.S.N.P.K.) thanks the Council of Scientific and Industrial Research (India) for the award of a research fellowship.
acid
REFERENCES
I. 1. M. Kolthoff, R. Belcher, V. A. Stenger and G. Mat-
Tables 2 and 3 show some results for pure ascorbic acid solutions and some commercial preparations, and comparative results by the BP method.”
suyama, Volumetric Analysis, Vol. III, pp. 236, 645. Interscience, New York, 1957. 2. K. Hubicka, Ann. Univ. Mariae Curie-Sklodowsku, Lub/in-Poloniu,
Interferences
Strong oxidants and reductants interfere. Coloured ions such as copper(H), nickel and cobalt(H) interfere. Cerium(III), arsenic(III), hydroxylamine hydrochloride and hydrazine sulphate do not interfere, but antimony(III), iron and manganese(I1) do. Vanadium(IV) gives results that are only 0.5-l% too high in phosphoric acid media, but are very high in the other media. An advantage of tlie method is that organic compounds such as oxalic, tartaric, citric and Table 2. Determination Indicator
Acid
Ferroin
Sect. AA,
1957, 10, 35.
3. J. A. C. van Pinxteren and E. Verloop, Pharm. Weekblad, 1958,23,203.982. 4. L. Erdey and G. Svehla, Z. Anal. Chem., 1956,150,407. 5. B. R. Sant, Chemist-Analyst, 1958, 47, 65. 6. G. S. Sastry and G. G. Rao, Talanta, 1972, 19. 212. 7. N. Rukmini, V. S. N. P. Kavitha and K. R. Rao. ibid.. 1979.26, 579. 8. N. Rukmini and V. S. N. P. Kavitha, Acta Cinecia Indica,
1979, 5, 44.
U. Muralikrishna, Ind. J. Chem., 1972, 10, 559. 10. R. Erdey and E. Bodor, Z. Anal. Chem., 1952,136, 109: Magy. Kern. Foly. 1952, 58, 295. I 1. British Pharmacepoeia, p. 36. HMSO, London, 1973. 12. J. D. Panting, Ind. Eng. Chem., Anal. Ed. 1943, 15, 389. 9.
of ascorbic acid with ferricyanide Range determined,
mg
Error range, %
Hz% HCI H,PG,
13.24-88.24 13.24-88.24 13.24-88.24
0.1-0.4 0.2-0.4 0.2-0.4
Hz=4
Triphenylmethane dyes
H3PO4
13.24-88.24 13.24-48.53
0.2-0.4 0.2-0.4
Thiazine dyes
H,SO, H,l%
13.24-88.24 13.24-48.53
0.24.4 0.24.5
Table 3. Assay of commercial vitamin C preparations: amount (g) present in 5 tablets or 30 ml of syrup or 5 ampoules of injection solution Ferricyanide method Sample
BP method*
Ferroin
Erioglaucine
Methylene Blue
2.52 2.48
2.52 2.49
2.51 2.48
2.52 2.48
2.50 2.52
2.50 2.52
2.50 2.52
3.62 3.78
3.64 3.75
3.64 3.76
Tablets
Ampoules 2.50 2.52
*
Syrups 3.62 3.78