Plan a procedure that will allow you to compare quantitavely the glucose concentration in fresh samples of orange, lemon and grapefruit juice.
The aim of this investigation is to produce a set of data which will enable comparison of the glucose concentrations of orange, lemon and grapefruit juice.
Benedict's Reagent is a mixture of sodium compounds and copper sulphate. The copper ions are reduced by fructose (a reducing sugar) to form copper oxide, thus turning it from blue to a brick red/orange precipitate. The more red the precipitate is, the higher the concentration of reducing sugar in the solution tested. If more precipitate has formed, there are fewer copper ions remaining in the Benedict's Reagent, therefore, the solution will appear less blue. This can be measured using a colorimeter, as it is known that the concentration of a substance is proportional to its absorbance of light. Fructose is the sugar found in fruit. It is a monosaccaride, and is a reducing sugar because it acts as a reductant in chemical reactions. This can be seen in the Benedict's test where the fructose in the fruit juice will reduce CuSO4 to CuO when heated. Cu 2+ → Cu +
The following table indicates approximate nutritional values for each of the three fruits to be tested:
Energy / kJ
Water / %
Sugar / mg*
Sugar Concentration / mg cm3 -1 Red Grapefruit
*Values are given per 100g of fruit
(T1: Sugar Content in Grapefruit, Lemon and Orange; Taken from www.thefruitpages.com/contents.shtml)
The table above suggests that orange juice will have the highest concentration of fructose. This will result in the Benedict's Test with oranges becoming most brick-red/orange in colour and the most precipitate forming during this test. This is because more copper sulphate ions have been reduced by the fructose to form copper oxide ions. The colorimeter will indicate this solution having more absorbance of light.
The table also suggests that Lemon juice has the lowest concentration of fructose. This will result in the Benedict's Test with lemons to become the least brick-red/orange in colour, and the least precipitate forming during this test. This is because fewer copper sulphate ions have been reduced by the fructose (as there is less fructose), to form copper oxide ions. The colorimeter will indicate this solution to have less absorbance of light.
Justification of Use
Heating equipment (tripod, gauze, Bunsen Burner etc..)
The Benedict's solution requires heating before any change in the colour of solution, or formation of precipitate occurs. 4% [40mg cm3 1] glucose solution
This will be diluted to different concentrations (at intervals) and will act as a control throughout the experiment. Distilled water
Will be used to dilute the glucose solution
The Benedict's test will be carried out on the different solutions in test tubes. Many can be fitted into a water bath (for efficiency), and they have a large-enough capacity to hold the 6cm3 of solution needed for the experiment. Colorimeter
This is a device that measures the ability of particular solutions to absorb light and give quantitative results. Measurement using a colorimeter will prevent the subjective assessment of the "blueness of solution" by the experimenter. Digital Pan-Top Balance
This will be necessary to accurately measure the mass of the precipitate formed from the Benedict's Test. Filter Paper
This will be required to filter the precipitate from the solution.The precipitate will be allowed to dry on the filter paper, and will be carefully scrapped off using a spatula into a weighing boat on a pan-top balance. Thermometer
This will enable the experimenter to ensure that the temperature of the water bath remains around 95 ºC throughout the Benedict's Test Stopwatch
This is to measure the amount of time the Benedict's Test runs...
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