Determination of hydroxyl groups in carbohydrates

Discussion. Hydroxyl groups present in carbohydrates can be readily acetylated by acetic (ethanoic) anhydride in ethyl acetate containing some perchloric acid. This reaction can be used as a basis for determining the number of hydroxyl groups in the carbohydrate molecule by carrying out the reaction with excess acetic anhydride followed by titration of the excess using sodium hydroxide in methyl cellosolve. 

Solutions required. Acetic anhydride. Prepare 250 mL of a 2.0M solution in ethyl acetate containing 4.0 g of 72 per cent perchloric acid. The solution is made by adding 4.0 g (2.35 mL) of 72 per cent perchloric acid to 150 mL of ethyl acetate in a 250 mL graduated flask. Pipette 8.0 mL of acetic anhydride 

Sodium hydroxide. Prepare a solution of approximately 0.5M sodium hydroxide in methylcellosolve. This should be standardised by titration with potassium hydrogenphthalate using the mixed indicator given below. 

Pyridine/water. Make up 100 mL of a mixture formed from pyridine and water in the ratio of 3 parts to 1 part by volume. 

Mixed indicator. This should be prepared from 1 part of 0.1 per cent neutralised aqueous cresol red and 3 parts of 0.1 per cent neutralised thymol blue. 

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