Re: What is the chemistry of quantitative Benedict's solution

I finally found information regarding the oxidation of sugars (esp. glucose) with Benedict's solution and the role of methylene blue in the reaction.

This reaction is the basis for the well-known "Blue Bottle" demonstration. Two excellent and readily-available references are:

Shakashiri, Bassam Z. Chemical Demonstrations, Vol. 2. University of Wisconsin Press, 1985. pp. 142-146.

Ealy, Julie B., and Ealy, James L., Jr. Visualizing Chemistry. American Chemicial Society, 1995. pp. 269-272, 385-392.

The overall reaction for the oxidation of glucose (represented as RCHO here) with Benedict's solution (sodium citrate, sodium carbonate, copper (II) sulfate) is:

RCHO (aq) + 2 Cu²⁺ (aq) + 5 OH^- (aq) ---> RCOO^- (aq) + Cu₂O (s) + 3 H₂O (l)

The citrate ion in the Benedict's solution complexes with the copper (II) ion to prevent precipitation of Cu(OH)₂ or CuO.

Your solution apparently also contains the thiocyanate ion (SCN^- ) to create a white CuSCN precipitate rather than the brick red Cu ₂O in the equation above.

Methylene blue is an indicator that is colorless in its reduced state and blue in its oxidized state. (See the figure below, which is taken from p. 391 in Ealy and Ealy.) Methylene blue acts as a catalyst in the oxidation of sugars.

I hope this helps! While I had seen the "Blue Bottle" demonstration several times, this was the first time I actually spent some time studying the chemistry behind it!