Re: cAN YOU CONDUCT A QUANTITATIVE BENEDICTS TEST ON SOLID BANANA PULP?

Dear Katy,

There are two ways to conduct Benedict's test. Whether using banana pulp is a good idea depends on which method you are using. I'm not sure which one you mean, so I'll go into both of them--bear with me if you already know all this stuff.

The chemistry behind Benedict's test is an oxidation-reduction reaction (usually abbreviated to "redox" reaction). The reducing sugars in your banana pulp reduce the Cu++ ions in Benedict's solution. This just means the sugars give some of their electrons to the Cu++ (which is blue), changing it to Cu+ (which is red). So, the more sugars there are, the faster they can reduce Cu++ to Cu+, the faster the entire solution changes from blue to red.

There is a qualitative method and a quantitative method. The qualitative one will tell you if any reducing sugars are present at all, and give you a rough estimation of sugar concentration. The quantitative method will allow you to calculate the exact sugar concentration of your sample.

For the qualitative method, you put a bit of sample (in your case, the banana pulp) into a vial, add some water and a few drops of Benedict's solution, and heat it. After about five minutes, if there was reducing sugar in the sample, a colored precipitate will form (ranging between green and red). The more red the precipitate, the more sugar was in your sample.

As you can probably tell, this would only give you a rough idea of the sugar concentration, since it's hard to quantify how "green" or how "red" something is (especially since the reaction continues for a while even after you stop heating it). For this method, it would be perfectly fine to use banana pulp, since your result would not be exact anyway. Just make sure you thin down the pulp with some water first (and use the same amount of banana and water in each trial, to stay consistent).

The true quantitative Benedict's test involves an redox titration. You can read more about titrations here, but this is the general idea: you use Benedict's solution whose concentration you know, and you add to that your sugar solution, whose concentration you don't know. Once all of the Cu++ have been reduced, the Benedict's solution will no longer be blue--this is called the "end point" of the titration, and it is where the number of sugars you added balance out the number of Cu++ ions that was originally in the Benedict's solution.

(This is a side note, but you might find it interesting: quantitative Benedict's solution forms a white precipitate, not a red one. This makes it easier to tell when all of the blue color has disappeared).

Since you can measure how many milliliters of sugar sample you used, and since you know how many Cu++ ions you started out with originally, you can then use simple stoichiometry to calculate the concentration of your sugar sample.

Having banana pulp here is definitely bad. Not only will it be hard to see the color changes with bits of banana hanging around, the banana pulp has its own volume, so it will mess up your volume measurements. Not to mention, it would be difficult to add banana pulp drop by drop.

Fortunately, there's an easy way to get the sugar without the pulp. You can take a chunk of your banana (record its mass!) and smash it up. Then, add some distilled water and mix it really well. After that, pour the pulp-water mix through filter paper. Since sugar is soluble, it will pass through the filter paper with the water, and you will get a pulp-free sugar solution (record its volume!).

Then, using this sugar solution to do the titration, you can figure out its concentration. Once you get its concentration, you can then calculate how much sugar was in your banana chunk. Since you measured how much banana you had, you can then calculate moles of sugar per gram of banana, which is what you will ultimately be measuring, right?

It's important to use just the right amount of water. If you use too little, you might not get all the sugar out of the banana chunk. If you use too much, your sugar solution will be too dilute and you'll spend forever doing the titration. I roughly estimate about 500mL of water should be about right for half a banana. To get maximum amount of sugar out of the banana, you can divide the water into aliquots--for example, use 300mL to blend with the banana initially, and pass it through the filter. Then, rinse the leftover pulp with 50mL of fresh water 4 times. (By rinse, I mean you have the pulp still on the filter paper, and pour water on top of it. The water will dissolve any remaining sugar and drain into your solution).

Your experiment sounds really interesting, and I hope you have fun doing it. Good luck! (and remember, don't eat anything while you're in the lab!). If you have any more questions, feel free to follow up.

Kathy Xie

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You can see some nice pictures of the qualitative Benedict's test here: http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA5/MAIN/1ORGANIC/ORG18/TRAM18/B/MENU.HTM

And this is a link to a video about how to conduct a titration (the one they mention here is an acid-base titration, not a redox one, but the idea is the same). It is video #2. There is also a video describing filtration (video #7): http://ocw.mit.edu/OcwWeb/Chemistry/5-301January--IAP-2004/StudyMaterials/index.htm