Estimation of monosaccharides by the orcinol–sulphuric acid reaction

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Abstract
Optimum conditions for the reaction were established. One ml of carbohydrate solution and 1 ml of 4% orcinol solution are mixed in a round-bottom flask and 8.0 ml of 31.2 N H2SO4 added over 1.5 minutes with cooling and then 5.6 ml are placed in a thin-walled test tube and immersed exactly 50 seconds in vigorously boiling water, cooled for 3 minutes and measured in a spectrophotometer. Fructose and sorbose showed maximum absorption of 475 mu, galactose, 525 mu, glucose, a broad band at 400-500 mu, the aldopentoses, manhose and arabinose show 2 peaks in the 400-430 and 525-575 mu regions. From the shape of the absorption curves, many of monosaccharides can be characterized and quantitatively estimated. Methods are also given for calculating the quantities of 2 monosaccharides in a mixture.