Benedicts test...

So I have tried 0%, 0.1, 0.25, 1, 2 & 5% glucose concentrations and I am not getting the below:
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All I am getting are the orange/red colours and I am curious as to why this might be?

I am using the benedicts qualitative solution.
 
Just did a bit more reading and I was quickly testing this, so I wasn't as accurate regarding the amount of Benedicts I was adding (I now know it is 10 drops). Will a change in this quantity influence the results much?
 
Just did a bit more reading and I was quickly testing this, so I wasn't as accurate regarding the amount of Benedicts I was adding (I now know it is 10 drops). Will a change in this quantity influence the results much?
CLEAPSS say to use equal quantities of test solution and Benedict's.
 
In my experience, the green and yellow do not exist :laughing: I assume you're getting no change for the 0%.
Yup, no change for 0%. Out of curiosity, what would using quantitative benedicts look like? I do notice differing quantities of orange precipitate with quantitative solution...
 

labsleuth

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Yup, no change for 0%. Out of curiosity, what would using quantitative benedicts look like? I do notice differing quantities of orange precipitate with quantitative solution...
Quantitative should turn glucose solutions white.
 
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Ah - I kept the test solution the same, but didn't measure the quantity of benedicts... has anyone managed to get the full colours?
Yes and no. Have only managed a yellow colour when testing skimmed milk solution. I don't see how it would be possible to get a yellow solution using glucose and Benedict's only. The point is at low reducing sugar conc you get a small amount of Cu+ ions but you still have a lot of blue Benedicts so low concentrations give you different shades of green until there's lots of Cu+ ions and little Benedicts when you see the orange/red colours. Mind you the diagram does say precipitate so perhaps if you spun them that's the colour you'd see at the bottom of the tube. The qualitative version produces a white precipitate and you need to find the mass of this by whatever means you fancy. The mass is directly related then to the concentration of reducing sugar in the solution. We don't use it as basically the spec talks about brick red precipitates of Cu+ and it's too confusing for students and some teachers to to have a different reaction/method. When testing I commonly use 1 ml Benedicts to 3 of sample and pop in freshly boiled water for 5 mins. We do use colorimeters and you can get a reasonable calibration curve at low concs of reducing sugar.
 
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