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  • Created by: Lucy
  • Created on: 01-05-10 10:10

Chemical tests


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To show the presence of starch, you can add a solution of iodine (in To show the presence of starch, you can add a solution of iodine (in potassium iodide) to the sample. If starch is present, the iodide solution changes colour from yellow-brown to blue-black.

All monosaccharide and many disaccharide sugars are known as reducing sugars. This means that a molecule can react with other molecules by giving electrons to them. When a reducing sugar is heated with Benedict’s solution (alkaline copper sulphate), the solution changes colour from blue to orange-red. The orange-red is described as a precipitate because the orange-red substance comes out of solution and forms solid particles determined in the solution. Some sugars do not react with Benedict’s solution at all, so a reducing sugar test would show up as negative (no colour change). Sucrose is a non-reducing sugar, and is formed by a condensation reaction forming a glycosidic bond between a glucose molecule and a fructose molecule. Fructose and glucose are monosaccharides, and sucrose is a disaccharide. The formation of the glycosidic bond in sucrose is slightly different from that in maltose (which is a reducing sugar). This difference prevents the sucrose from reacting with Benedict’s solution. If the substance does not react with Benedict’s solution, you have to use a different test.First make sure there are no reducing sugars in the sample. Boil the sample with hydrochloric acid.This hydrolyses any sucrose present, splitting sucrose molecules to give glucose and fructose monosaccharides. Then cool the solution and neutralise it by adding sodium hydrogencarbonate solution or sodium carbonate solution (both are alkalis). Carry out the reducing sugar test again. If sucrose is present in the original sample, the test will now give a positive result because the monosaccharides glucose and fructose are present.

You can test for the presence of proteins by adding biuret reagent to a sample. This is called the biuret test. Biuret reagent is a pale blue colour, and contains sodium hydroxide and copper sulphate. These chemicals react with the peptide bonds found in protein, which results in a colour change to lilac. This test does not require any heating.

You can test for the presence of lipid by using the ethanol emulsion test. Mix the sample with ethanol (an alcohol). This dissolves any lipid present (lipids are soluble in alcohols). Then pour the liquid (alcohol with dissolved fat) into water contained in another clean test tube. If lipid is present, a cloudy white emulsion will form near the top of the water- the lipid comes out of solution and becomes dispersed as tiny droplets in the water.potassium iodide) to the sample. If starch is present, the iodide solution changes colour from yellow-brown to blue-black.

All monosaccharide and many disaccharide sugars are known as reducing sugars. This means that a molecule can react with other molecules by giving electrons to them. When a reducing sugar is heated with Benedict’s solution (alkaline copper sulphate), the solution changes colour from blue to orange-red. The orange-red is described as a precipitate because the orange-red substance comes out of solution and forms solid particles determined in the solution. Some sugars do not react with Benedict’s solution at all, so a reducing sugar test would show up as negative (no colour change). Sucrose is a non-reducing sugar, and is formed by a condensation reaction forming a glycosidic bond between a glucose molecule and a fructose molecule. Fructose and glucose are monosaccharides, and sucrose is a disaccharide. The formation of the glycosidic bond in sucrose is slightly different from that in maltose (which is a reducing sugar). This difference prevents the sucrose from reacting with Benedict’s solution. If the substance does not react with Benedict’s solution, you have to use a different test.First make sure there are no reducing sugars in the sample. Boil the sample with hydrochloric acid.This hydrolyses any sucrose present, splitting sucrose molecules to give glucose and fructose monosaccharides. Then cool the solution and neutralise it by adding sodium hydrogencarbonate solution or sodium carbonate solution (both are alkalis). Carry out the reducing sugar test again. If sucrose is present in the original sample, the test will now give a positive result because the monosaccharides glucose and fructose are present.

You can test for the presence of proteins by adding biuret reagent to a sample. This is called the biuret test. Biuret reagent is a pale blue colour, and contains sodium hydroxide and copper sulphate. These chemicals react with the peptide bonds found in protein, which results in a colour change to lilac. This test does not require any heating.

You can test for the presence of lipid by using the ethanol emulsion test. Mix the sample with ethanol (an alcohol). This dissolves any lipid present (lipids are soluble in alcohols). Then pour the liquid (alcohol with dissolved fat) into water contained in another clean test tube. If lipid is present, a cloudy white emulsion will form near the top of the water- the lipid comes out of solution and becomes dispersed as tiny droplets in the water.

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Colorimetry

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The simple tests just described detect only the presence of the various molecules. If you want to determine how much is present, you need to carry out a quantitative test (a test that gives a value for the amount present). Benedict’s test reveals the presence of reducing sugars, and results in an orange-red precipitate. The more reducing sugar there is present, the more precipitate will be formed, and the more Benedict’s solution (copper sulphate) will be ‘used up’. If the precipitate is filtered out, then the concentration of the remaining solution can be measured. This will tell you how much Benedict’s solution has been used up, so you can then estimate the concentration of reducing sugar in the original sample.A colorimeter is a device that shines a beam of light through a sample. A photoelectric cell picks up the light that has bee passed through the sample, and it will give you a reading showing how much light has passed through. Place the solution is a sample chamber between the light and the photoelectric cell in a cuvette or in a special test tube. The more copper sulphate that is used up in a Benedict’s test in a sample, the less light will be blocked out and the more light will be transmitted. This means the readings taken give a measure of the Benedict’s reaction. Using a colorimeter to take readings of different samples that contain reducing sugar does not tell us how much reducing sugar is present- it simply tells us which sample contains more. In order to quantify (measure) the amount, a calibration curve must be made. Prepare a calibration curve by taking a range of known concentrations of reducing sugar. Carry out Benedict’s test on each one then filter the precipitate out of the solution. Use a colorimeter to give readings of the amount of light passing through the solutions. Plot the readings in a graph to show light getting through (transmission) against reducing sugar concentration. You are now ready to take a reading from an unknown sample. From the graph, you can read off the equivalent reducing sugar concentration for the new reading.

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