Testing for Sugars

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Test for Reducing Sugars

  • Benedicts reagent (mixture of blue copper sulphate in a strong alkaline solution) - reducing sugars will reduce the Cu2+ ions from the copper sulphate through a number of oxidation states 
  • At each oxidation state the ions appear a different colour- this can be used to estimate the amount of reducing sugars present 

1. 2cm3 of sample to be tested is placed in a boiling tube

2. Add 10 drops of Benedict's reagent

3. Heat the mixture in a water bath for 3 minutes 

Scale:
Blue- none
Green- very low
Yellow- low
Orange- medium
Red- high

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Test for Non-Reducing Sugars

1. Add 2cm3 of solution to be tested e.g. sucrose

2. Add 2 drops of 1mol hydrochloric acid (for the H+ ions, acid hydrolysis- separates bonds)

3. Add an excess of sodium hydrogen carbonate to neutralise the solution

4. Heat in a water bath for 5 minutes

5. Add 10 drops of Benedict's reagent 

6. Heat again in a water bath for 2 minutes 

Scale:
Blue- none
Green- very low
Yellow- low
Orange- medium
Red- high

2 of 2

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