Science Buddies: "Ask an Expert"

Hi, I am doing a project relating to post harvest storage and ripening biochemistry that involves measuring reducing sugar content. I wanted to use Benedict's solution for a quantitative assay with a colorimeter. I already have the regular qualitative Benedict's solution and was wondering if it can be used quantitatively. I understand that the qualitative solution produces a precipitate of red copper oxide which produces a range of colors depending on concentration. Can this still be used fairly quantitatively within the types of sugar variation exhibited in fruit? The quantitative reagent is instead used by measuring the loss of blue color of copper sulphate when a white precipitate is formed that settles out. I just wanted to make sure before I buy or make the other solution. Thanks

Hello lxPowers,

I think that "quantitative" Benedict's solution gives a more accurate result for the amount of sugar not only because of the difference in composition, but mainly due to the difference in how the assay is done. In order to measure the amount of precipitate, one has to filter it out, dry it, and weigh it. Not only is this a multistep assay, but you will need quite a bit of precipitate to weigh it accurately. I don't know how accurately one can measure using the color of the precipitate.
When doing the quantitative assay, you can use just a small quantity of sugar solution and Benedict's solution and measure the amount of copper sulfate remaining in solution quite accurately with a spectrophotometer. Do you have access to a spectrophotometer?
If not, building your own is not out of the question, but it will make your project much more difficult and time-consuming. See http://www.rsc.org/Education/EiC/issues ... ometer.asp. Perhaps a better idea is to look for a university or commercial lab near you that would allow you to use one of theirs. Most bioscience or chemical labs have one.
Another issue that you raise is what kind of sugars are measured by the assay. The simple answer is "reducing sugars." See http://en.wikipedia.org/wiki/Benedict's_reagent. Sucrose is one example of a nonreducing sugar. It can be detected, however, if it is first broken into fructose and glucose.

I hope this helps. Good luck, WW