Detection of unknown sugar sample solution
General Test:1
1.Benedict's Test
Experiment
3ml sample solution and 2-3 ml Benedict's qualitative reagent are mixed in a dry clean test tube and boiled for 5 minutes.
Observation
The mixture solution turns green, yellow and finally red.
Inference
The supplied sample is of reducing sugar.
The red ppt. is Cu2O (I).
Reactions:
Reducing sugar+alkali—>Enediols
Enediols+2 Cu2+---> 2Cu+ + acidic sugar
2Cu+ + 2OH– —-> 2CuOH
2CuOH +Heat —> Cu2O (red ppt) +H2 O
General Test
2.Barfoed's Test
Experiment
1 ml (5 drops) sample solution and 3 ml Barfoed's reagent ( a soln of cupric acetate and acetic acid) are mixed in a test tube and boiled in a boiling water bath for 3 minutes.
Observation
A reddish ppt.is formed.
Inference
The reducing sugar is a monosaccharide since ppt is formed within 3 minutes.
Reaction:
R–CHO + 2 Cu2+ +2H2O—-> R–COOH + Cu2O + 4H+
Confirmatory Tests:
1.Moore's Test
Experiment
2 ml sample solution and 2 ml 5% NaOH are mixed in a test tube ( it is a transparent mixture solution), then boiled in a water bath for 2-5 minutes.
Observation
The transparent mixture turns first yellow. Then the colour turns dark and ultimately turns reddish dark brown.
Inference
The reducing sugar may be glucose.
2.Cobalt chloride Test
Experiment
3 ml sample solution and 1-2 ml cobalt chloride solution are mixed and boiled.
After cooling, a few drops of NaOH solution is added.
Observation
The solution turns greenish blue.
Inference
The sample solution is glucose (confirmed).
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