12
ASCORBIC ACID
[2]
[2] V i t a m i n C ( A s c o r b i c A c i d ) By
N.
KRISHNA
MURTY
and K. RAMA RAO
Determination with Iodine, Potassium Iodate, Potassium Bromate and Iodine Monochloride The assay of ascorbic acid with iodine, 1 potassium iodate, 2 potassium bromate, s and iodine monochloride4 using starch as indicator is reported by different workers, but Erdey and Bodor 5 and Gopala Rao and Narayana Rao 6 were of the opinion that starch cannot be used as indicator because it decreases the reaction rate between ascorbic acid and iodine. Variamine blue is proposed as the indicator in the place of starch for the assay of ascorbic acid with iodine, 7 potassium iodate, 7 and iodine monochloride. 4 Carbon tetrachloride8 or chloroform in the presence of mercuric chloridea is also used for the detection of the equivalence point by extractive end point. Schulek, Kovaes, and Rozsa 1° used pethoxychrysoidine as indicator with potassium bromate in the presence of potassium bromide, but they stated that the endpoint is not sharp. We have developed a simple and rapid titrimetric procedure for the assay of ascorbic acid with iodine, potassium iodate, potassium bromate, or iodine monochloride using naphthol blue black, amaranth, or Brilliant Ponceau 5R as indicator, n
Reagents Iodine, 50 mM Potassium iodate, M/60 Potassium bromate, M/60 Iodine monochloride, 50 mM Ascorbic acid, 50 mM Potassium iodide, 1% 1 A. J. Lorenz, R. W. Reynolds, and S. J. W. Stevans, Symp. Vitamin., Am. Chem. Soc., 92nd Meeting, 1936. 2 R. Ballentine, Ind. Eng. Chem., Anal. Ed. 13, 89 (1941). a L. Szekeres, E. Sugar, and E. Pop, Z. Anal. Chem. 121, 17 (1951). 4 j. Cihalik, Doctoral thesis, University of Chemical Technology, Prague, 1957. 5 L. Erdey and E. Bodor, Anal. Chem. 24, 418 (1952); cf. Z. Anal. Chem. 137, 293 (1952/1953). e G. Gopala Rao and V. N a r a y a n a Rao, Z. Anal. Chem. 147, 338 (1955). r L. Erdey and L. Kaplar, Z. Anal. Chem. 162, 180 (1958). a G. S. D e s h m u k h and M. G. Bapat, Z. Anal. Chem. 145, 254 (1955). a B. Singh, G. P. K a s h y a p , and S. S. Sahota, Z. Anal. Chem. 162, 357 (1958). to E. Schulek, J. K o v a e s , and R. Rozsa, Z. Anal. Chem. 121, 17 (1951). tl N. K r i s h n a M u r t y and K. R a m a Rao, J. Indian Chem. Soc. 53, 532 (1976).
METHODS IN ENZYMOLOGY. VOL. 62
Copyright © 1979by AcademicPress. Inc. All rights of reproduction in any form reserved. ISBN 0-12-181962-0
[2]
TITRAMETRIC DETERMINATIONS OF ASCORBIC ACID
13
Naphthol blue black, Brilliant Ponceau 5R, and amaranth, aqueous (0.2% solutions) Procedure. An aliquot of ascorbic acid is taken in a 150-ml titration vessel, and the required amount of hydrochloric acid is added to give an overall acid concentration of 0.1-2.0 M for potassium iodate, 0.1-1.0 M for iodine, 0.2-0.3 M for potassium bromate, and 1.0-2.5 M for iodine monochloride titration. To the above mixture, about 0.1 ml of the indicator solution is added; the titration is carried out against a standard solution of the respective oxidant. In the assay of ascorbic acid with potassium iodate or with potassium bromate, 2-10 ml of 1% potassium iodide solution are added before the titration commences. The color changes are from blue to green for naphthol blue black, or from rose pink to brownish yellow for both amaranth and Brilliant Ponceau 5R. The color changes are fairly reversible, and the indicator correction is negligible. Determination with Potassium Dichromate The iodometric procedure of Lorenz, Reynolds, and Stevans 1 underwent numerous modifications to overcome interferences present along with ascorbic acid in the various vitamin sources. In view of the advantages of potassium dichromate as a primary standard oxidimetric reagent, the usual difficulty encountered in the assay of ascorbic acid in colored solutions by visual methods, and the problem of stability of iodine in very dilute solutions, the authors extended the use of potassium dichromate through a potentiometric end point. 12 A visual method is also proposed using naphthol blue black or amaranth or Brilliant Ponceau 5R as indicator. TM The principle involved in this procedure is that potassium dichromate, when added to the titration mixture, first reacts with potassium iodide, added in the beginning, liberating iodine, which in turn reacts with ascorbic acid that is reduced to iodide. Oxalic acid is used as the catalyst to accelerate the reaction between potassium dichromate and potassium iodide.
Materials Potassium dichromate, M/60 Ascorbic acid, 50 mM Oxalic acid, 1 M Potassium iodide, 1% Naphthol blue black, amaranth, and Brilliant Ponceau 5R Aqueous (0.2%) solutions ~2 K. Rama Rao, Ph.D. thesis, Andhra University, Waltair, 1977. This research was supported by the U.G.C. (India) Junior Research Fellowship.
14
ASCORBIC ACID
[2]
The potentiometric titration assembly consists of a saturated calomel reference electrode, a bright platinum rod as indicator electrode, and a salt bridge with porous plates filled with saturated potassium chloride solution. Procedure. A suitable volume of 50 mM ascorbic acid solution is taken in a 250-ml titration vessel and treated with enough dilute sulfuric acid (5-15 ml of 50 mM acid) to give an overall acid concentration of 0.05-0.15 M when diluted to 50 ml. About 5-15 ml of 1 M oxalic acid and 2-5 ml of I% potassium iodide solutions are added to the above solution, and the latter is diluted to 50 ml with distilled water. The resulting mixture is titrated with M/60 potassium dichromate either potentiometrically or visually, using 0.1 ml of the naphthol blue black, amaranth, or Brilliant Ponceau 5R as indicator. In the potentiometric titration, stable potentials can be recorded immediately after the addition of each portion of the titrant. A potential break of about 130-140 mV per 0.04 ml of M/60 potassium dichromate is observed. In the visual titrations, the color changes are from pinkish blue to pinkish green for naphthol blue black, or from rose pink to brownish yellow for amaranth or Brilliant Ponceau 5R. During the visual method of titration, the solution acquires a pink color due to the formation of chromium(III)-oxalate complex. Hence, the detection of the endpoint becomes difficult if the ascorbic acid content exceeds 80 mg where a potentiometric method can be used. This procedure is applied for the assay of ascorbic acid in juices of such fruits as citrus fruits (Citrus aurantifolia and Citrus sinensis), tomato (Lycoperiscon esculentum), and emblic myrobalam (Phyllanthus emblica or Emblica officinalis) and in pharmaceutical preparations containing vitamin C alone and in the presence of other vitamins. The extraction of vitamin C from fruit juices is carried out using oxalic acid as the extractant as recommended by Goldblith and Harris. 13 Pharmaceutical preparations containing vitamin C are directly assayed as given in the procedure either potentiometrically or visually. In the assay of pharmaceutical preparations containing vitamin C, compounds are separated using light petroleum ether (boiling range 40°-60°C) as recommended by Hashmi, 14 and then the determination is carried as described above. Potentiometric procedure is used for the pharmaceuticals containing coloring materials.
~3 S. A. Goldblith and R. S. Harris, Anal. Chem. 20, 649 (1948). ~4 M. H. Hashmi, "Assay of Vitamins in Pharmaceutical Preparations," p. 236. Wiley, New York, 1973.
ASCORBIC ACID
[2]
[2] V i t a m i n C ( A s c o r b i c A c i d ) By
N.
KRISHNA
MURTY
and K. RAMA RAO
Determination with Iodine, Potassium Iodate, Potassium Bromate and Iodine Monochloride The assay of ascorbic acid with iodine, 1 potassium iodate, 2 potassium bromate, s and iodine monochloride4 using starch as indicator is reported by different workers, but Erdey and Bodor 5 and Gopala Rao and Narayana Rao 6 were of the opinion that starch cannot be used as indicator because it decreases the reaction rate between ascorbic acid and iodine. Variamine blue is proposed as the indicator in the place of starch for the assay of ascorbic acid with iodine, 7 potassium iodate, 7 and iodine monochloride. 4 Carbon tetrachloride8 or chloroform in the presence of mercuric chloridea is also used for the detection of the equivalence point by extractive end point. Schulek, Kovaes, and Rozsa 1° used pethoxychrysoidine as indicator with potassium bromate in the presence of potassium bromide, but they stated that the endpoint is not sharp. We have developed a simple and rapid titrimetric procedure for the assay of ascorbic acid with iodine, potassium iodate, potassium bromate, or iodine monochloride using naphthol blue black, amaranth, or Brilliant Ponceau 5R as indicator, n
Reagents Iodine, 50 mM Potassium iodate, M/60 Potassium bromate, M/60 Iodine monochloride, 50 mM Ascorbic acid, 50 mM Potassium iodide, 1% 1 A. J. Lorenz, R. W. Reynolds, and S. J. W. Stevans, Symp. Vitamin., Am. Chem. Soc., 92nd Meeting, 1936. 2 R. Ballentine, Ind. Eng. Chem., Anal. Ed. 13, 89 (1941). a L. Szekeres, E. Sugar, and E. Pop, Z. Anal. Chem. 121, 17 (1951). 4 j. Cihalik, Doctoral thesis, University of Chemical Technology, Prague, 1957. 5 L. Erdey and E. Bodor, Anal. Chem. 24, 418 (1952); cf. Z. Anal. Chem. 137, 293 (1952/1953). e G. Gopala Rao and V. N a r a y a n a Rao, Z. Anal. Chem. 147, 338 (1955). r L. Erdey and L. Kaplar, Z. Anal. Chem. 162, 180 (1958). a G. S. D e s h m u k h and M. G. Bapat, Z. Anal. Chem. 145, 254 (1955). a B. Singh, G. P. K a s h y a p , and S. S. Sahota, Z. Anal. Chem. 162, 357 (1958). to E. Schulek, J. K o v a e s , and R. Rozsa, Z. Anal. Chem. 121, 17 (1951). tl N. K r i s h n a M u r t y and K. R a m a Rao, J. Indian Chem. Soc. 53, 532 (1976).
METHODS IN ENZYMOLOGY. VOL. 62
Copyright © 1979by AcademicPress. Inc. All rights of reproduction in any form reserved. ISBN 0-12-181962-0
[2]
TITRAMETRIC DETERMINATIONS OF ASCORBIC ACID
13
Naphthol blue black, Brilliant Ponceau 5R, and amaranth, aqueous (0.2% solutions) Procedure. An aliquot of ascorbic acid is taken in a 150-ml titration vessel, and the required amount of hydrochloric acid is added to give an overall acid concentration of 0.1-2.0 M for potassium iodate, 0.1-1.0 M for iodine, 0.2-0.3 M for potassium bromate, and 1.0-2.5 M for iodine monochloride titration. To the above mixture, about 0.1 ml of the indicator solution is added; the titration is carried out against a standard solution of the respective oxidant. In the assay of ascorbic acid with potassium iodate or with potassium bromate, 2-10 ml of 1% potassium iodide solution are added before the titration commences. The color changes are from blue to green for naphthol blue black, or from rose pink to brownish yellow for both amaranth and Brilliant Ponceau 5R. The color changes are fairly reversible, and the indicator correction is negligible. Determination with Potassium Dichromate The iodometric procedure of Lorenz, Reynolds, and Stevans 1 underwent numerous modifications to overcome interferences present along with ascorbic acid in the various vitamin sources. In view of the advantages of potassium dichromate as a primary standard oxidimetric reagent, the usual difficulty encountered in the assay of ascorbic acid in colored solutions by visual methods, and the problem of stability of iodine in very dilute solutions, the authors extended the use of potassium dichromate through a potentiometric end point. 12 A visual method is also proposed using naphthol blue black or amaranth or Brilliant Ponceau 5R as indicator. TM The principle involved in this procedure is that potassium dichromate, when added to the titration mixture, first reacts with potassium iodide, added in the beginning, liberating iodine, which in turn reacts with ascorbic acid that is reduced to iodide. Oxalic acid is used as the catalyst to accelerate the reaction between potassium dichromate and potassium iodide.
Materials Potassium dichromate, M/60 Ascorbic acid, 50 mM Oxalic acid, 1 M Potassium iodide, 1% Naphthol blue black, amaranth, and Brilliant Ponceau 5R Aqueous (0.2%) solutions ~2 K. Rama Rao, Ph.D. thesis, Andhra University, Waltair, 1977. This research was supported by the U.G.C. (India) Junior Research Fellowship.
14
ASCORBIC ACID
[2]
The potentiometric titration assembly consists of a saturated calomel reference electrode, a bright platinum rod as indicator electrode, and a salt bridge with porous plates filled with saturated potassium chloride solution. Procedure. A suitable volume of 50 mM ascorbic acid solution is taken in a 250-ml titration vessel and treated with enough dilute sulfuric acid (5-15 ml of 50 mM acid) to give an overall acid concentration of 0.05-0.15 M when diluted to 50 ml. About 5-15 ml of 1 M oxalic acid and 2-5 ml of I% potassium iodide solutions are added to the above solution, and the latter is diluted to 50 ml with distilled water. The resulting mixture is titrated with M/60 potassium dichromate either potentiometrically or visually, using 0.1 ml of the naphthol blue black, amaranth, or Brilliant Ponceau 5R as indicator. In the potentiometric titration, stable potentials can be recorded immediately after the addition of each portion of the titrant. A potential break of about 130-140 mV per 0.04 ml of M/60 potassium dichromate is observed. In the visual titrations, the color changes are from pinkish blue to pinkish green for naphthol blue black, or from rose pink to brownish yellow for amaranth or Brilliant Ponceau 5R. During the visual method of titration, the solution acquires a pink color due to the formation of chromium(III)-oxalate complex. Hence, the detection of the endpoint becomes difficult if the ascorbic acid content exceeds 80 mg where a potentiometric method can be used. This procedure is applied for the assay of ascorbic acid in juices of such fruits as citrus fruits (Citrus aurantifolia and Citrus sinensis), tomato (Lycoperiscon esculentum), and emblic myrobalam (Phyllanthus emblica or Emblica officinalis) and in pharmaceutical preparations containing vitamin C alone and in the presence of other vitamins. The extraction of vitamin C from fruit juices is carried out using oxalic acid as the extractant as recommended by Goldblith and Harris. 13 Pharmaceutical preparations containing vitamin C are directly assayed as given in the procedure either potentiometrically or visually. In the assay of pharmaceutical preparations containing vitamin C, compounds are separated using light petroleum ether (boiling range 40°-60°C) as recommended by Hashmi, 14 and then the determination is carried as described above. Potentiometric procedure is used for the pharmaceuticals containing coloring materials.
~3 S. A. Goldblith and R. S. Harris, Anal. Chem. 20, 649 (1948). ~4 M. H. Hashmi, "Assay of Vitamins in Pharmaceutical Preparations," p. 236. Wiley, New York, 1973.
