Benedict's test for Reducing Sugars 

Having a tons of them is sometimes confusing tho😅

quantitative is using the calibration curve. semi-quantitative is comparing the colours to say which is 'more' or 'less' concentrated with reducing sugar.

+Amy Whitaker you're welcome.

You're very welcome.

pleasee reply asap

+Najiah HMG without the calibration curve, it is not quantitative because we don't know what concentration of reducing sugar the colour corresponds to. Put simply: comparing colours by eye is qualitative. Comparing colours by colorimeter is semi-quantitative. Comparing colours with colorimeter AND using a calibration of known concentrations (so we know what concentration each colour represents) is quantitative. Water would be used only if the solutions are too intense for the colorimeter. In that the volume added should be kept the same.

Chaiyasit Phawa yes. Jus make a serial dilution.

+tony Tran yes. Sucrose is a disaccharide and it is not a reducing sugar. The chemical group needed to react with Benedicts reagent is not available in sucrose. So sucrose does not cause the appearance of red precipitate when heated with Benedicts reagent.

+LSC Biology so it would not have any color difference? Thank you

+tony Tran sucrose is not a reducing sugar, so no, it would not make a colour change.

It is a non-reducing sugar so you would convert it into a reducing sugar. This can be done by adding hydrochloric acid, heating in the water bath, then adding sodium hydrogencarbonate (neutralises acidity), then proceeding with the normal Benedict's test.

***** Good question. It all depends what you are measuring. If you are removing the precipitate, then the blue colour (of copper ions) should decrease with increasing glucose concentration. In that case absorbance of red light should decrease (and transmission increases, if that's what you're measuring). BUT, if you're leaving the precipitate (red, CuO) in the tubes, and looking at absorbance of blue light, in that case, your absorbance will increase as glucose concentration increases (and transmission will therefore go down). If this doesn't resolve your question, maybe you could give me details of your experiment and the colour of the solutions you are testing. thanks!

i think it can, since all it's really testing for is a free aldehyde or ketone group.

thank you!

Glad it helped

Yes spectrophotometer would work as long as you are detecting the right wavelength. Colorimeters are cheaper and will also work but you must make sure to use the right filter.

+LSC Biology Can we use this method to measure quiet low glucose concentration like mmol/l?

oanh dao thi Honestly, I haven't tested this method at low levels and so I couldn't say. The Method would work, but it would depend on how sensitive the colorimeter is. I would test the method with standard concentrations of glucose that span the range you are interested in. Good luck!

thanks alot :)

Dr Bhavsar Biology has Testing for Reducing SugarsTesting for Reducing SugarsTesting for Reducing SugarsTesting for Reducing Sugars

+That Chickpea Chick / Summer Lo set to reading absorbance, with blue/green filter (you may need to check which wavelengths that corresponds to). Good luck.

+ROMAN HANG LIMBU blue solutions absorb most strongly in red part of visible light spectrum. So for colorimetric analysis of blue solutions, light is passed through a red filter to get more accurate readings of transmission/absorbance.

Dr Bhavsar But what exactly is a Red filter? Is it something on the actual colorimeter? Thanks for the response.

+ROMAN HANG LIMBU yes. It's something that visible light can go through, some wavelengths are absorbed and others can pass through. A red filter allows red light to pass through, not others.

Thankyou very much.

N

The reference should be solutions of reducing sugar with known concentration.

That's quite unexpected. Tell me the details of your experiment.

I combined 1 ml of a 1% Glucose Standard Solution and 4 ml of benedict's quantitative solution. After combining the two things in a test tube, I put the test tube in beaker full of hot water (80 degrees celcius). After 5 minutes, I noticed that the solution had changed from the blue green color to a light blue color. At the bottom of the test-tube, there was a white powder. Image of White Powder i.imgur.com/TSzMiFD.jpg?1

+Jaeho Lee I wish I could say something more helpful than, that is really quite inexplicable. We usually have the glucose solution and Benedict's in a 1:1 ratio, and usually have glucose solution in excess over Benedict's rather than the other way round. We also use a waterbath that maintains the 80C temperature and even then sometimes it can take up to 15 minutes. Without a waterbath it's likely your temp dropped and possibly the reaction didn't go to completion. I have a theory also, that perhaps if your glucose solution was more than 1% (significantly more), adding the benedicts may have caused the glucose to precipitate out of the solution and so not take part in the reaction AND explaining the white crystals. But that's a bit out there!

I redid the experiment with a 1:1 ratio. I also used a water bath that maintained an 80 C for 15 minutes. After I did the experiment, the solution turned yellow, but the precipitate was white instead of red.

+Jaeho Lee It seems to me that you used the quantitative Benedict's reagent, and +LSC Biology used the qualitative Benedict's reagent. LSC Biology's method gives an estimation of glucose concentration, while your quantitative solution is used to accurately determine the glucose concentration (either by I2/thiosulphate redox titration or by colourimetric measurements (absorbance)). Both solutions give different precipitates. Check this link for the difference in composition between the two: www.mystrica.com/Applications/Benedicts

en.m.wikipedia.org/wiki/Reducing_sugar If a sugar molecule has an aldehyde or ketone group (ie a c=o somewhere) it can act as a reducing agent. So all monosaccharides can be reducing when they are in straight chain form. But when monosaccharides are joined by glycosidic bonds, they can’t go into the straight chain form, and so don’t have the c=o group, so are not reducing.

@@LSCBiology Thank you

sorry to disappoint!

+Trillian Times you need to watch my video on carbohydrates!

Trillian Times sugar

Dr Bhavsar can you please arrange all the sugars in order of their reactivity...

Do you happen to know the expected color change for coconut water? I mean, what shade should I expect when I perform the experiment?

I don’t know that. But you could look up the expected composition and see if it contains any reducing sugars.

Dr Bhavsar Ah, okay. Thank you! Both for the response, and the rate at which you replied. :)

Dr Bhavsar what should u do if the experiment is semi quantitative?

LOL

ouch. why?