The following procedure was taken from Practical Physiological Chemistry, by P. B. Hawk, and O. Bergham (7th edition, Blakiston, Philadelphia, 1943.) To make the indicator solution, dissolve 0.05 g of reagent-grade resorcinol in 100 mL of HCl solution (dilute commercial HCl 1:2 to obtain a solution of density 1.18-1.19 g/mL). Add about 5 drops of a 1% solution of the unknown carbohydrate to a test tube containing 1 mL of the freshly prepared resorcinol solution and heat the mixture to boiling. A cherry red color develops within a few minutes if fructose or sucrose is present. A yellow to salmon color has no significance. If the boiling is prolonged a red color can be obtained for other sugars.
Most analytical procedures for disaccharides exploit the optical activity of sugars (their ability to rotate plane-polarized light).
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