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The key points covered in this video include:
1. Reducing Sugars - Benedict's Test
2. Reducing Sugars - Reagent Test Strips
3. Non-reducing Sugars - Benedict's Test
4. Starch -Iodine Test
Reducing Sugars - Benedict's Test
The Benedict's test can be used to detect the presence of reducing sugars. Reducing sugars are sugars that can give electrons to (reduce) other molecules. Reducing sugars include all monosaccharides and most disaccharides such as maltose and lactose. The sample to be tested is placed in a clean test tube. An equal volume of Benedict's reagent (copper (II) sulfate) is added. The solution is then heated in a boiling water bath for 3-5 minutes. If reducing sugars are present, they will reduce the Cu2+ ions in copper (II) sulfate to Cu+ ions, forming the orange-red precipitate copper (I) oxide. The formation of red-orange precipitate results in the mixture changing colour from blue to green to yellow to orange to red. When only a small amount of precipitate is formed, the colour of the reaction mixture may appear green or yellow. The colour of the reaction mixture can therefore be used to estimate the initial concentration of reducing sugar.
Reducing Sugars - Reagent Test Strips
The presence of reducing sugars can also be tested for using commercial test strips. The strip is simply dipped into the test sample. The colour that the test strip turns is then compared to a calibration card to estimate the concentration of reducing sugars. Test strips are often used to test urine samples for diabetic patients.
Non-reducing Sugars - Benedict's Test
Some disaccharides, such as sucrose, are non-reducing sugars and do not change the colour of Benedict's reagent when tested. Non-reducing sugars can be tested for by first adding hydrochloric acid to the sample. The test tube is then heated in a boiling water bath for 3-5 minutes. This results in the hydrolysis of the non-reducing disaccharide into two monosaccharides - these are reducing sugars. The sample is then neutralised with sodium hydrogencarbonate solution. The sample can then be tested using the usual method for Benedict's test. If a colour change now occurs, this means that non-reducing sugars were present in the original sample.
Starch - Iodine Test
Iodine can be used to test for the presence of starch. The sample is placed in a clean test tube. Potassium iodide solution is added to the test tube and then the tube is gently shaken. If starch is present, the solution changes colour from yellow-brown to blue-black.
Summary
Benedict's test can be used to test for the presence of reducing sugars and estimate concentrations
Reagent test strips can also be used to test for reducing sugars
Non-reducing sugars must first be hydrolysed to reducing sugars before using Benedict's test
Iodine can be used to test for the presence of starch