Note:  Do not rely on this information. It is very old.

Fehling's Solution

Fehling's Solution, a reagent which is largely employed for the volumetric estimation of glucose, or grape sugar. It is prepared by dissolving copper sulphate, Rochelle salt, and caustic soda in water, in definite proportions. The solution of the two latter is kept separate from that of the copper salt, and only mixed when required for use. When a solution of grape sugar is added to this reagent, heated to boiling, a red precipitate of cuprous oxide Cu20 is formed, and the blue colour which the reagent possesses is discharged. When sufficient sugar solution has been added to completely discharge the colour, the reaction is complete. The reagent is generally made of such a strength that 10 cc. are equivalent to -05 dram of glucose. If cane sugar has to be estimated, it must be first converted into glucose, this being performed by treatment with a dilute acid.