I tried to prepare different concentration of glucose solution. It does not gives me green when the glucose level is low, the result it gives is like lesser brick red ppt in blue solution (as shown in the picture below).
The concentration of the glucose solution is (from left to right) 10%, 5%, 1%, 0.2%, 0% (control).
At first, I thought is my Benedict's solution is qualitative type, therefore it does not gives me a range of colour. However when I checked online and prepared my own quantitative Benedict's solution and tested it out, it does not give me a range of colour too, instead it gives a white ppt.
Below is the recipe I used to make my own Benedict's solution:
For quantitative Benedict's solution:
- Sodium Citrate (Dihydrate) - 17g
- Potassium Thiocyanate - 11g
- Sodium Carbonate (Anhydrous) - 10g
* Dissolved the above in 70ml of deionised water and warm it to let the solute fully dissolved, name this Solution A.
- Prepare 1.4g of Copper Sulfate (pentahydrate) and dissolved in 20ml of deionised water
* Add this copper sulfate solution to Solution A. Stir vigorously until a deep blue solution is formed. This will be your quantitative Benedict's Solution.
For qualitative Benedict's solution (normally I buy ready made Benedict's solution from Comak)
(I tried to make my own Benedict's solution just in case the ready-made solution I have has a different recipe which do not give me the result I want.)
Basically is the same as quantitative solution without potassium thiocyanate.
- Sodium Citrate (Dihydrate) - 17g
- Sodium Carbonate (Anhydrous) - 10g
* Dissolved the above in 70ml of deionised water and warm it to let the solute fully dissolved, name this Solution A.
- Prepare 1.4g of Copper Sulfate (pentahydrate) and dissolved in 20ml of deionised water
* Add this copper sulfate solution to Solution A. Stir vigorously until a deep blue solution is formed. This will be your quantitative Benedict's Solution.
Anyway the Benedict's solution I made gave me the same result like the ready-made Benedict's solution I buy from supplier. So I still can't solve my problem.
So I tried to change my solution. Instead of lab grade glucose, I bought Glucolin (blue container) and mix it in water to test for glucose. ;( Still give me the same result like the lab grade glucose.
Then I tried maltose (similar to the one in the picture below, but mine is not this brand)
 |
I use 5% maltose solution, which is 5g of this gluey stuff in 100ml of warm water.
I used this solution to test for glucose. It does gives me an orange ppt.
So in the end I tried using 5%, 1% & 0.5% maltose solution to give me orange, lighter orange (still not yellow based on theory) and green/light brown.
Then I used 10% lab grade glucose to get brick red ppt. Below is picture of the result I got. From left to right, 10% glucose, 5%, 1% & 0.5% maltose solution.
This is the best I can get. If you also encounter this problem, can try using maltose solution. Or if you have successfully done this experiment before, please share with me the glucose solution & the Benedict's solution you used.
9 comments:
try doing a serial dilution to prepare glucose concentration between 1
%- 0.0001%
or try testing with fruits such as tomatoes , orange , lemon and grape and see the colours you will get
Actually, fruits do works to give a yellow colour. Another way to get orange ppt, I add some albumin to the glucose solution.
Hello
I have been trying to get the test to work and I have been using two different batches of Benedict's solution, one from Breckland which is probably too old, but the second should be fresh. They are Benedict's Solution Quantitative But They Don't work.
Can anyone suggest why??
Maria
May I know the solution you are going to test is it glucose? Do note Benedict's solution only work on monosaccaride like glucose. It will not give a positive result if you are testing normal sugar solution. If you are testing sugar solution, you need to add some hydrochloric acid and heat it to break it down into monosaccaride before testing it with Benedict's solution.
Do note, after adding Benedict's solution, you need to heat the mxiture in a hot water bath for about 2 mins. It will not give a immediate result by just adding Benedict's solution into your test solution.
Thank you so much, you are right, I have been testing diluted sugar.
I will try to add Hydrochloric acid before. Any particular concentration? And heat it to what temperature?
I just add 1 - 2 drops of 1M HCl. Gentle heating until it boils then you can remove it from the flame and proceed with Benedict's test as usual.
Thanks a million!!!
I am going to bookmark your page in order to ask question in future if that's ok :)
Maria
No problem. Let me know again if you encounter any problem.
Read full Pressure Class 10| SEE Physics Notes
Post a Comment