Testing for cabohydrates Flashcards
How do you test for starch?
Carbohydrates
Add iodine solution (in potassium iodide) to a sample.
If starch is present, will see a colour change of yellow-brown to blue-black. When dissolved in potassium iodine, the iodine (I2) forms a triiodide ion I3-, which slips into the middle of the amylose helix. This causes the colour change.
How do you test for reducing sugars?
Carbohydrates
These include all monosaccharides and some disaccharides. They are known as reducing sugars because they can reduce or give electrons to, other molecule. If you heat a reducing sugarwith Benedict’s solution (alkaline copper (II) sulfate), there is a colour change from from blue to green to yellow to orange-red.
Benedict’s solution contains Cu2+ions, which are reduced to Cu+ ions, forming orange -red copper(I) oxide (CU2O). This is callled a precipitate ecause it comes out of solution and forms a solid, suspended in th reaction mixture
What happens is you use Benedict’s solution in excess when testing for reducing sugars?
(Carbohydrate
The intensity of the re colour is proportioanl tot eh cincentration of sugar. The reaction mix will apprea grean if only precipitate is formed, and fully orange-red if a lot of precipitate if formed. See the next topic on quantitive testing for a reducing sugar.
It is also possible to use commercially manufactured test strips for testing for reducing sugars. Here you simply dip the strip into the test solution, compare the compare the colour with the calibration card supplied. This tells you whether reducing sugar is present or absent from your solution. These are often used to test for glucose in the urine of diabetic patients.
How do you test for reducing sugars?
Carbohydrate
To test for non-reducing sugars, we have to hydrolyse the bond first, to ‘free up’ these ‘reducing groups’, and then test for reducing sugars as normal:
First, test a sample for reducing sugars to check there are none there in the first place.
Take a separate sample and boil it with hydrochloric acid to hydrolyse the sucrose into glucose and fructose
Cool the solution and use sodium hydrogencarbonate solution to neutralize it
Test for reducing sugar again
A positive result (green-yellow-orange-red) indicates that non-reducing sugar (e.g. sucrose) was present in the original sample.
What do you do if the sample contains reducing and non-reducing sugars?
(Carbohydrates)
If you have a positive test for non-reducing sugars from your first sample, you can go on to test for non-reducing sugars in an equal-size second compound. If present, the precipitate from this second sample will have more mass than the precipitate from the first sample. You can extract the precipitate from the mixture by filtration
How do you test for lipids?
The emulsion test is used to test for the presence of lipids:
Take a sample and mix it thoroughly with ethanol. Any lipid will go into solution in the ethanol. Any lipid will go into solution in the ethanol
Filter
Pour the solution into water in a clean test tube
A cloudy white emulsion indicates the presence of lipids. this is made up of tiny lipid droplets that com out of solution when mixed with water
How do you test for proteins?
For this, you use the biuret test. If protein is present, the colour changes from light blue to lilac. You may find the reagents are supplied to you separately as biuret A (sodium hydroxide), which you add next. The colour is formed by a complex between the nitrogen atoms in a peptide chain and Cu2+ ions, which is why this test really detects the presence of peptide bonds
