The Iodine Test for Reducing Sugars – A Safe, Quick and Easy Alternative to Copper(II) and Silver(I) Based Reagents

Holger Fleischer^1,

¹Scheffold-Gymnasium, Schwaebisch Gmuend, Germany

Cite this paper:
Holger Fleischer. The Iodine Test for Reducing Sugars – A Safe, Quick and Easy Alternative to Copper(II) and Silver(I) Based Reagents. World Journal of Chemical Education. 2019; 7(2):45-52. doi: 10.12691/wjce-7-2-3

Abstract

The reaction of Fehling’s and Benedicts’s test solutions with solutions of reducing sugars, e.g. glucose, is frequently misinterpreted with respect to the products formed. The tests are by no means suitable to detect aldehyde groups in organic molecules. The course of the reaction is known since long to be rather complex and the corresponding carboxylate, often assumed to be the major reaction product, does not occur at all. Both tests, as well as Tollens’ test using alkaline silver(I) solution, have some drawbacks for chemistry lessons, especially in students’ lab courses. The “iodine test”, well established to quantitatively determine glucose, could be a suitable alternative in many cases. It allows a quick detection, even of small amounts or reducing sugars at room temperature by decolourisation of a weakly alkaline iodine–starch–solution. The test is based on several coupled equilibria, and hypoiodous acid is the oxidizing agent. The complexity of the molecular description of the reaction can easily be reduced. Hence, the iodine test is suited to set different levels of difficulty, in lab work as well as in exams. The “iodine test” can even be used in Primary School, since the necessary solutions do not represent a danger.

Keywords:
iodine test Fehling’s test Benedict’s test reducing sugars

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References:

[1]	James, G., Mastersson, N., McWhinnie, J.-A., Hickling, B., Healthy eating. Curriculum Kit for South Australian Primary Schools, Government of South Australia 2013.
[2]	Aston, T.-A., The really useful book of science experiments; Rootledge, New York, 2016, p. 20.
[3]	Elzagheid, M.I., Laboratory Activities to Introduce Carbohydrates Qualitative Analysis to College Students. World Journal of Chemical Education, 6(2), 82-86, 2018.
[4]	Fleischer, H., False Interpretation of Fehling’s Test for Reducing Sugars – From Observation in the Lab to Evidence against an Aldehyde Oxidation. CHEMKON, 24(1), 27-30, 2017.
[5]	Stojanovska, M. Šoptrajanov, B., Petruševski, V. M., Misconceptions in the Chemistry Teaching in the Republic of Macedonia Regarding the Oxidation Reactions of Monosaccharides. Contributions, Sec. Nat. Math. Biotech. Sci., 34 (1-2), 27-32, 2013.
[6]	Amick, C. A., The Copper Number of Glucose, J. Phys. Chem., 31, 1441-1477, 1927.
[7]	Evans, W. L., Nicoll, W. D., Strous, G. C., Waring, E., The Mechanism of Carbohydrate Oxidation. IX. The Action of Copper Acetate Solution on Glucose, Fructose and Galactose. J. Am. Chem. Soc. 50, 2267-2285, 1928.
[8]	Scherz, H., On the mechanism of the Fehling Reaction. Z. Lebensm. Unters. Forsch., 168, 91-98, 1979.
[9]	Daniels, R, Rush, C.C., Bauer L., The Fehling and Benedict Test. J. Chem. Edu., 37(4), 205, 1960.
[10]	Willstädter, R., Schudel, G., Bestimmung von Traubenzucker mit Hypojodit. Ber. dtsch. chem. Ges. 51, 780-781, 1918.
[11]	Goebel, W. F., On the Oxidation of Glucose in Alkaline Solutions of Iodine. J. Biol. Chem. 72, 801-807, 1927.
[12]	Ingles, O. G. Israel, G. C., The Oxidation of Some Aldoses by Alkaline Solutions of Iodine. J. Chem. Soc. 810-814. 1948.
[13]	Ingles, O. G., Israel, G. C., The Oxidation of Aldoses by Hypoiodous Acid. J. Chem. Soc. 1213-1216, 1949.
[14]	Mehltretter, C. L., Some Landmarks in the Chemical Technology of Carbohydrate Oxidation. Die Stärke, 15(9), 313-319, 1963.
[15]	Evans, W. L. Some less Familiar Aspects of Carbohydrate Chemistry. Chem. Rev. 31, 537-560, (1942).
[16]	Wadsworth, B., J. Piaget's theory of cognitive and affective development: Foundations of constructivism (5th ed.). White Plains, NY, England: Longman Publishing. (1996).
[17]	Fleischer, H., Quantitative determination of glucose by the iodine-test. Manuscript in preparation.