Carbohydrates chemical tests
summary notes
16 September 2022 09:34
Starch
- 3 drops of iodine
- Positive test goes from brown to blue/black
- For dry samples can mix with water
Iodide ions get stuck in the middle of the
amylose helix which makes it blue/black
Reducing sugar
- Equal volume of benedict's as sample
- Heat in hot water bath - 95 degrees Celsius- 5
mins
- Goes from blue to brick red
Cu2+, which is blue, gains electrons, is reduced, to
form Cu+ which is brick red
Non reducing sugar
- Test with benedict's solution to ensure it is not a
reducing sugar
- Heat the sample with acid (HCl)
This breaks the disaccharide into monosaccharides (ie
sucrose into glucose and fructose)
- Neutralise the acid with a hydrogen carbonate
(sodium hydrogen carbonate)
This is because the Benedict's solution cannot be
reduced when there are H+ ions around as the H+ will
gain electrons rather than the Cu2+ so Cu+ will not be
formed
- Benedict's test again- will go positive- brick red
Quantitative
summary notes
16 September 2022 09:34
Starch
- 3 drops of iodine
- Positive test goes from brown to blue/black
- For dry samples can mix with water
Iodide ions get stuck in the middle of the
amylose helix which makes it blue/black
Reducing sugar
- Equal volume of benedict's as sample
- Heat in hot water bath - 95 degrees Celsius- 5
mins
- Goes from blue to brick red
Cu2+, which is blue, gains electrons, is reduced, to
form Cu+ which is brick red
Non reducing sugar
- Test with benedict's solution to ensure it is not a
reducing sugar
- Heat the sample with acid (HCl)
This breaks the disaccharide into monosaccharides (ie
sucrose into glucose and fructose)
- Neutralise the acid with a hydrogen carbonate
(sodium hydrogen carbonate)
This is because the Benedict's solution cannot be
reduced when there are H+ ions around as the H+ will
gain electrons rather than the Cu2+ so Cu+ will not be
formed
- Benedict's test again- will go positive- brick red
Quantitative
