Sucrose Test with Benedicts
Posted: 20 Mar 2018, 11:33
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 . 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 .
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 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 .
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 . 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 .
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 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 .