Principle for Estimation of Percentage of Reducing and Total Sugar by Fehling Soxhlet Method?

Reducing sugars are those which have free sugar groups and hence may be estimated directly by titrating the solution of the sample with Fehling's solution. Total sugars include both reducing and non-reducing sugars. Non-reducing sugars do not contain free aldehyde or ketone groups and cannot reduce Fehling's solution, e.g. sucrose and starch.

Hence, non-reducing sugars must be hydrolysed to monosaccharides by heating with acid before titration. The reducing sugars formed are acted upon by the alkali of Fehling's solution to form enediols. These enediols are very unstable and reactive and they reduce Cu²⁺ ions to Cu⁺ ions.These Cu⁺ ions combine with hydroxyl groups to form cuprous hydroxide which on heating gives red precipitate of cuprous oxide. To get sharp end point, methylene blue is added which is reduced by sugar solutions to a leuco compound restoring the red colour of the solution.