Sucrose Test with Benedicts

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bigmack
Posts: 816
Joined: 15 Dec 2015, 10:49
Job Title: Lab Technician
School: FCAC
State/Location: QLD

Sucrose Test with Benedicts

Post by bigmack »

This is a bit miss leading but involves Inverting or Cleaving the Sucrose and testing for the Glucose .

My Chem teacher has been trying to do this for a few years now and it was always hit and miss . He blamed the Sugar .Also he was adding the Benedicts before heating and acidification .

The trick is to add the acid then bring to the boil .....this process inverts the Sucrose Cleaving it into Glucose and Fructose . Then let it cool to about 65 degC then add the Benedicts .

I tried adding varying amounts of citric acid .
On the left is no acid . You can see the approximate amounts I added and the effect on the test . If you add too much , it won't work . You can see from the pH test that the Sucrose has to be acidic to invert .
Citric acid.jpg
I tried a few amounts of Citric Acid from a few crystals on the left to to a small amount similar to the above test.
It showed again that there had to be a pH fairly low to start to invert the sucrose .
Varying amounts of Citric Acid.jpg


I decided that adding Citric acid was too hit and miss amount wise .
I tried HCl as it could be applied with a dropper in a more controlled way

You can clearly see again that there is a fairly critical amount of acid to get a positive test . ....again , add too much and the test does not show a positive . I believe that the excessive amount of acid is neutralizing the sodium hydroxide in the Benedicts .throwing the test .
( The numbers on the beakers shows how many drops of 1M HCL was added ) interestingly the one with 50 drops had no colour at all
Drops of 1M HCl.jpg
So here's the Method

Prepare a 1% Sucrose solution .
Add approx. 2-3 drops of 1M HCL to 25mls of solution .
Bring to the boil
Let cool to around 65 degC and add the Benedicts .

Hope this helps someone .
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linotas
Posts: 659
Joined: 21 Mar 2011, 22:39
State/Location: TAS

Re: Sucrose Test with Benedicts

Post by linotas »

Thank you! Very interesting Our TCE Bio kids do a banana prac every year (leave green bananas for a week and test with Benedict's and iodine throughout) and we never get very good results. It basically goes from negative to a wishy washy positive with no graduation of colour. Did you manage to get it to go brick red? I am wondering if we need to look at the method more
bigmack
Posts: 816
Joined: 15 Dec 2015, 10:49
Job Title: Lab Technician
School: FCAC
State/Location: QLD

Re: Sucrose Test with Benedicts

Post by bigmack »

linotas , Benedicts tests positive for Glucose . I've not done the Banana prac but perhaps the sugars that develop in the banana are other than Glucose :unsure:

Certainly if I heat a about 25 mls of 1% Glucose solution with a good squirt ( A full dropper full ) , it will turn brick red .

Actually that's a good point , you do need to add a good amount of Benedicts to the solution you are testing . I've read somewhere that it should be almost 20% but I don't find I need that much . As long as the solution is bluish , it will work if its going to .
bigmack
Posts: 816
Joined: 15 Dec 2015, 10:49
Job Title: Lab Technician
School: FCAC
State/Location: QLD

Re: Sucrose Test with Benedicts

Post by bigmack »

Been doing some Banana research
As a banana begins to ripen , Sucrose is formed .
As the banana ripens further , the levels of sucrose falls and Glucose and Fructose rises .
A fully ripen banana ( after 28 days) typically contains just under 6g of both Glucose and Fructose .
Our Banana trees are pretty loaded at home ATM ...time for some experimenting .
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