Science Buddies: "Ask an Expert"

Hi, I'm doing an experiment finding the amount of sugar in two types of onions. I have Benedict's Solution quantitative to use as my sugar indicator. I know that to find the concentration of the sugar in the two types of onions, I need to use titration for my onion glucose solution and the Benedict's Solution. My first question is do I drip the Benedict's Solution into the onion glucose solution or the other way around? Also, I can't find the molarity of the Benedict's Solution, which I need after the titration for the stoichiometry to find the concentration of sugar. Is there a regular molarity for Benedict's Solution quantitative? Or do I have to calibrate my own?
Thanks so much!

Well, I am not a chemist. I'll try to contact one to give you a better answer. But I did find
the following web page

http://neohumanism.org/b/be/benedict_s_reagent.html

which gives a rather detailed (to my eyes) procedure for using Benedict's quantitative reagent (after the first part about the qualitative test). The use of a known sugar solution, 1% glucose, seems to be required to calibrate the titration.

I hope this helps. Let us know how it goes.

Best of luck with your project!

chillinwcello,

As John Dreher disclaimed, I also am not a quantitative expert. I found this reference that gives the contents of Benedict's standard quantitative reagent. Your reagent, however, may not be the same strength. As John said, to calibrate your reagent, mix up a 1% solution of glucose and run a test. If you don't get 1%, then you'll know that your reagent is of a different concentration.

Browsing references for the test procedure, I find both reagent added to sample and vice versa. Either can work. The key measurement you will make is how much sugar solution exactly titrates how much reagent. You don't have to know the reagent concentration after the test, just a record of how much you added. Of course, if you know how much of each solution you used, the final concentration is pretty easy to calculate.

Maybe a better expert will show up.

WW

Hi, chillinwcello,

The above two experts provided you with some great advice and links. When you calibrate your regent, you might not end up with the concentration you expected, so be extra careful with recording what was mixed with what. Titration by itself is a process that can be rather tricky if you don't remember how much of a solution was used! As for your question about adding the sample to the reagent or the reagent to the sample, I agree with Wendell. In the lab, I don't recall coming across a priority or technique that required a certain way, so just keep track of all the materials you use.

Have fun with your project!

-RM