Sucrose Concentrations

Plan a procedure that will allow you to compare quantitavely the glucose concentration in fresh samples of orange, lemon and grapefruit juice.

Aim:

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.

Research:

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 128 90 66 0.73

Lemon 51 96 30 0.30

Orange 198 87 106 1.22

*Values are given per 100g of fruit

(T1: Sugar Content in Grapefruit, Lemon and Orange; Taken from www.thefruitpages.com/contents.shtml)

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