1.3 Test for reducing and non-reducing sugars

Question 1

method for testing reducing sugars:

Answer 1

• add equal volumes of sample + benedict’s reagent
• heat it gently for 5 minutes

-ve result: stays blue (no reducing sugars)
+ve result: sample changes colour (green, yellow, orange, red, brick red)

Question 2

more accurate method of finding concentration of reducing sugars of a unknown solution:

Answer 2

• use a colourimeter- to see how much light passes through the sample
• use a calibration curve to find concentration of a unknown solution

Question 3

alternative method of finding amount of reducing sugars apart from colourimeter;

Answer 3

• filter and dry precipitate
• find mass, more mass = more reducing sugars

Question 4

method for testing for non-reducing sugars:

Answer 4

• add sample and HCl at same volumes and boil for 5 minutes
• add NaHCO3 to the test tube
• add benedict’s reagent and heat it gently for 5 minutes

-ve result: stays blue, no reducing sugars
+ve result: colour change (green, yellow, orange, red, brick red)

Question 5

why add HCl to the sample when testing for non-reducing sugars?

Answer 5

it hydrolyses the glycosidic bonds, producing reducing sugars

Question 6

why add NaHCO3 to the sample when testing for non-reducing sugars?

Answer 6

it neutralises the solution