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To compare quantitatively the concentrations of glucose and other reducing sugars in samples of fresh orange, lemon and grapefruit juice. The standard test for glucose (and other reducing sugars) is to use Benedict's reagent

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Introduction

Glucose concentrations in orange, lemon and grapefruit juice Introduction In this experiment, I am going to have to compare quantitatively the concentrations of glucose and other reducing sugars in samples of fresh orange, lemon and grapefruit juice. The standard test for glucose (and other reducing sugars) is to use Benedict's reagent. Benedict's reagent is copper (II) sulphate in an alkaline solution, and so has a blue colour to it. If it is added to a reducing agent, its Cu2+ ions will be reduced to Cu+, resulting in the precipitant changing colour to the red of copper (I) sulphate. Reducing sugars have this effect on Benedict's reagent because they have a -C=O group somewhere in their molecules which can contribute an electron to the copper, hence the name "reducing sugars" (1). If a reducing sugar is present when Benedict's solution is added, the solution will change colour through green, yellow and orange, to brick red as the copper (I) sulphate forms a precipitant. All fruit contains the sugars glucose and fructose. Fructose is an isomer of glucose, so both sugars have the molecular formula: C6H12O6. Both are monosaccharides, and are hexoses (they both have 6 carbon atoms in each molecule). ...read more.

Middle

5. Repeat this operation for tubes 3, 4, 5, 6 and 7. Once the solution has been mixed in test tube 7, I will remove 1 cm3 of the solution so that there is an equal volume of test solution in each test tube - this will make the test fair. 6. I will now have glucose concentrations of 4%, 2%, 1%, 0.5%, 0.25%, 0.125% and 0.0625%. These are known as serial concentrations. This is a good range of test solutions, as I would have a strong solution, a very weak solution and a lot of concentrations in the middle so that I can draw an accurate line of best fit, and so that my calibration curve is generally as accurate as possible. In previous work, I found out that Benedict's solution stops detecting glucose between 0.1% and 0.01% glucose concentration, so it should still detect glucose at my lowest test concentration of 0.0625%. 7. Add 10 cm3 of Benedict's solution to each test tube using a 10 cm3 syringe (I will use the same concentration of Benedict's with each test solution in order to keep the test fair). The reason for using excess Benedict's solution is so that all of the sugar present will have definitely reacted (1). ...read more.

Conclusion

5. Filter each fruit juice into clean test tubes. 6. Using a clean syringe, add equal volumes of each filtered juice into separate cuvettes so that the cuvettes are more than half full. 7. Check that filter no 7 (orange) is in place in the colorimeter, and take one colorimeter reading for each cuvette. 8. Repeat this part of the experiment three times as well in order to get a mean transmission of light for each fruit juice. 9. Estimate the concentration of each juice by reading off the concentration at each average transmission of light. I will record my results on a similar table to the one on which I will record my test solutions on, but instead of having a column with known concentrations in, I will have the column with the name of the fruit juice. Prediction I predict that the lemon juice will have the smallest concentration of glucose, and the orange juice will have the largest. In previous research involving orange and lemon flavoured drinks, I have found out that the orange drink had a higher concentration of glucose than the lemon drink. The only problem that may occur in using this method is the colour of the fruit juices may affect the colorimeter reading, but as there will be a very large volume of Benedict's solution used compared to fruit juice volume, these effects should be kept to a minimum. ...read more.

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