Benedicts soultion (test for sugars)

Discussion in 'Supporting Biology' started by Shannon, Sep 14, 2018.

  1. Shannon

    Shannon Shannon

    A teacher wanted to trial this last night as its been a while (It's doing it with visking tubing)

    For some reason, with the glucose and a sugar solution... it would not change.. Does anyone know what the issue could be?

    Many thanks,
  2. Hi Shannon,
    Did you just do it at room temperature? Once you've added the Benedict's to the solution you're testing, you need to stand the test tube in a water bath of very hot water for the reaction to take place. For a class prac, I usually put out a water bath set to 80C. Or the students can make individual water baths with a beaker over a bunsen. If it's just a demo (or if I'm trialling in the prep room), I would put the test tube in a beaker, then fill the beaker with boiling water from a kettle. After a couple of minutes, you will then get a brick red precipitate proving the presence of glucose.

    If you are using sucrose, you won't get a precipitate straight away as it's a non-reducing sugar. First of all you need to add some 1M hydrochloric acid, heat for a few minutes, then some sodium hydrogencarbonate to neutralise. NOW add the Benedicts and heat as before, and you will get the red precipitate. It's a good test to distinguish between glucose and sucrose!

    Oh.....and make sure you've got QUALITATIVE Benedicts! (if you use Quantitative by mistake, the precipitate is white).

    Hope this helps :)
  3. Hi Shannon. As Stokes as said you do need heat to get the reaction going to form the Copper (I) precipitate. I usually just add the tube to a Beaker of hot water and let the Students watch the colour change from Blue to Green then Orange( depending on how much Reducing Sugar is there. I've found that hydrolysing a Non-reducing sugar like Sucrose needs a good 30 minutes in a water bath at 70C to get some reducing sugar (glucose and Fructose). If you were to quantify the amount of sugar with Quantitative Benedict's you would get the white precipitate of Copper Thiocyanate.:);)
  4. I have had Benidicts "go off" in the past and give no result.
    GeorgetheScienceTech likes this.
  5. Yeah you are perfectly right Allan. I've used bought in solution in the past and it didn't work which is why I always make it myself. Others should be aware of this.:);)
  6. this is for diffusion/digestion demo i presume, it takes time for enough glucose to pass out of the visking tubing, putting it in a 30C water bath helps or leave it until next lesson.

    or cheat and use the technicians "special" water outside the visking tube :eek:
    ClaireS likes this.
  7. I'm glad I am not the only one who uses "special water", works beautifully every time :)
    ClaireS and GeorgetheScienceTech like this.
  8. I'm not a biologist so forgive me if I'm being dense but should it have been a glucose solution NOT sucrose?
  9. OOps I apologise, just read the thread again and realised that glucose had also been used. :confused::oops:
    Lesley Newcombe likes this.
  12. I mega cheat (mainly because we use starch + amylase in the visking tubing and it takes ages for any reducing sugars to pass through) so I give "special distilled water" out in beakers that has just a hint of glucose :)
    Last edited: Sep 17, 2018
  13. Hi Lesley. So Tech Cheats do exist and are not an actual Urban Myth.:D:D:D:D
    Lesley Newcombe likes this.
  14. Desperate times call for desperate measures :)
    ClaireS likes this.
  15. starch + alpha amylase makes maltose anyway (not glucose as bitesize says) and although a reducing sugar it will be slower to diffuse , don't think it changes clinistix either.

    Lesley Newcombe likes this.
  16. You'd be right in thinking that Maltose wouldn't give a colour change with Clinistix Paul.:);)
  17. Realise that Paul, which is why I only said reducing sugars (not glucose) and we don't use the test strips as they are too expensive :)

    P.S One of the suggestions is to use milk and lactase as an alternative as it is meant to work reasonably fast. Never tried it mind :)
    Last edited: Sep 17, 2018