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Investigation to compare quantitatively the concentration of reducing sugars in samples of fresh orange, lemon and grapefruit juice

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Investigation to compare quantitatively the concentration of reducing sugars in samples of fresh orange, lemon and grapefruit juice Introduction - Background Knowledge This experiment involves the test for reducing and non-reducing sugars, using the Benedict's test. When a reducing sugar is heated with an alkaline solution of copper II sulphate (Benedict's solution), it forms an insoluble precipitate of copper I oxide. The colour of the precipitate then changes from green through yellow, orange and brown to deep red, depending on the quantity of reducing sugar present. Any substance providing electrons to carry out reduction is called a reducing agent, which reduces other substances. Benedict's reagent contains copper (II) sulphate. The copper two plus ions from the copper sulphate are reduced by electrons from the (C double bond O or a carbonyl group) found in certain sugars to form copper one plus ions. A resulting changes to copper (I) oxide to form a red precipitate. Therefore the sugars that contribute the electrons are called the reducing sugars. Examples of reducing sugars include glucose, glyceraldehyde, lactose, arabinose and maltose. In general a reducing sugar is any sugar that, in basic solution, forms some aldehyde or ketone. The idea of a higher concentration of reducing sugar producing a red solution and precipitate, in contrast to the green/yellow solution and precipitate produced from a much lower concentration of reducing sugar is experienced during the Benedict's test. The differences in colour mean that the Benedict's test is semi-quantitative, i.e. ...read more.


* Using a 1cm� syringe, I took 1cm� of this solution from test tube 1 and placed it in test tube 2. Then using a 10cm� syringe, I added 9cm� water to test tube 2 and inverted it to mix the contents. So the 1cm� of 10% glucose solution had been diluted ten times to make a 1% solution. * I then transferred 1cm of solution from test tube 3 to test tube 4 and diluted it with 9cm� water to make a 0.01% solution. * I repeated this for test tube 5. * Test tubes 1-4 all had 9cm� of solution in them, but test tube 5 had 10cm�, so I removed 1cm� of solution from test tube 5 so that the subsequent test was a fair one. * Then using a 5cm� syringe, I placed 5cm� Benedict's solution in each test tube and placed all 5 test tubes in a thermostatically controlled water bath at 75�C for 9 minutes. * Removing all the test tubes from the water bath, I returned them to the test tube rack to compare the colours. Risk assessment: * Make sure a lab coat is worn, so that if any chemicals spill on you, they will not ruin your clothes and even more importantly will not harm you, or irritate your skin. * If a substance touches your hands or skin, wash it immediately as it may be irritant or corrosive in turn harming your skin. If it does irritate tell your teacher. ...read more.


created from this experiment, where I would then mark where the calibration curve and the horizontal line, marking the amount of light absorbed by the substances (for example orange) and then follow the mark down with a vertical line. From here you can see where this line crosses the x-axis (concentration mg cm3) and therefore be able to tell what the concentration is. The experiment could be made more accurate with the use of more precise instruments such as burettes to measure volumes; however enough of these were not available at the time. Also the fact that each of the juices has a different colouring makes the experiment even more inaccurate, due to the use of the colorimeter. There was the problem of the percentage transmissions on the colorimeter fluctuating, therefore there were two or three readings, and so a 100% accurate reading was not taken. Finally another problem was time keeping, because when we placed the boiling tubes in the boiling water bath it was impossible to put them all in at the same time and the same situation occurred when taking them out. Therefore, the time when each boiling tube went in and came out was different, maybe causing some glucose samples having more time to change colour. This test was quantitative as we measured the cloudiness (turbidity) of the solution using a colorimeter. To extend this investigation making it quantitative I could have also filtered and weighed the precipitate produced. This would have given me an indication of the measurement of the amount of sugar present in the sample. ANIL VAGHELA 12JFH6 ...read more.

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