Q. Can you explain Lane and Eynon method?
Reducing sugars are those, which have free sugar groups (e.g. glucose, fructose etc.) 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 (e.g. maltose, lactose, sucrose etc.) do not contain free sugar groups and cannot reduce Fehling solution. Hence, non-reducing sugars must be hydrolyzed to monosaccharides by heating with acid before titration.
Reducing sugars are acted upon by the alkali of the Fehling 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 a sharp end point, methylene blue is added which is reduced to a colourless leuco compound restoring the red colour of the solution. Sodium potassium tartarate keeps the Cu²+ ions in the solution, thus ensuring a continuous supply of Cu²+ ions for reduction.