ข้อความ 8
วิจารณ์ผลการทดลองคาร์โบไฮเดรต
The Molisch Test
Shows positive test for:
All carbohydrates. Monosaccharides give a rapid positive test. Disaccharides and polysaccharides react slower.
Reactions:
The test reagent dehydrates pentoses to form furfural (top reaction) and dehydrates hexoses to form 5-hydroxymethyl furfural (bottom reaction). The furfurals further react with -naphthol present in the test reagent to produce a purple product (reaction not shown).
How to perform the test:
Two ml of a sample solution is placed in a test tube. Two drops of the Molisch reagent (a solution of -napthol in 95% ethanol) is added. The solution is then poured slowly into a tube containing two ml of concentrated sulfuric acid so that two layers form.
A positive test is indicated by:
The formation of a purple product at the interface of the two layers.
a negative test (left) and a positive test (right)
Barfoed's Test
Shows positive test for:
Reducing monosaccharides
Reactions:
Reducing monosaccharides are oxidized by the copper ion in solution to form a carboxylic acid and a reddish precipitate of copper (I) oxide within three minutes. Reducing disaccharides undergo the same reaction, but do so at a slower rate.
How to perform the test:
One ml of a sample solution is placed in a test tube. Three ml of Barfoed's reagent (a solution of cupric acetate and acetic acid) is added. The solution is then heated in a boiling water bath for three minutes.
A positive test is indicated by:
The formation of a reddish precipitate within three minutes.
a negative test (left) and a positive test (right)
Iodine/Potassium Iodide Test
Shows positive test for:
Starch
Reactions:
Iodine complexes with starch to form a blue-black product. Note that other polysaccharides may give other colors including blue or red.
How to perform the test:
Two ml of a sample solution is placed in a test tube. Two drops of iodine/potassium iodide solution and one ml of water are added.
A positive test is indicated by:
The formation of a blue-black complex.
a negative test (left) and a positive test (right)
Benedict's Test
Shows positive test for:
Reducing sugars
Reactions:
Reducing sugars are oxidized by the copper ion in solution to form a carboxylic acid and a reddish precipitate of copper (I) oxide.
How to perform the test:
One ml of a sample solution is placed in a test tube. Two ml of Benedict's reagent (a solution of sodium citrate and sodium carbonate mixed with a solution of copper sulfate) is added. The solution is then heated in a boiling water bath for three minutes.
A positive test is indicated by:
The formation of a reddish precipitate within three minutes.
A negative test (left) and a positive test (right)
Seliwanoff's Test
Shows positive test for:
Ketoses
Reactions:
The test reagent dehydrates ketohexoses to form 5-hydroxymethylfurfural. 5-hydroxymethylfurfural further reacts with resorcinol present in the test reagent to produce a red product within two minutes (reaction not shown). Aldohexoses react to form the same product, but do so more slowly.
How to perform the test:
One half ml of a sample solution is placed in a test tube. Two ml of Seliwanoff's reagent (a solution of resorcinol and HCl) is added. The solution is then heated in a boiling water bath for two minutes.
A positive test is indicated by:
The formation of a red product.
A negative test (left) and a positive test (right)
Bial's Test
Shows positive test for:
Pentoses
Reactions:
The test reagent dehydrates pentoses to form furfural. Furfural further reacts with orcinol and the iron ion present in the test reagent to produce a bluish product (reaction not shown).
How to perform the test:
Two ml of a sample solution is placed in a test tube. Two ml of Bial's reagent (a solution of orcinol, HCl and ferric chloride) is added. The solution is then heated gently in a Bunsen Burner or hot water bath. If the color is not obvious, more water can be added to the tube.
A positive test is indicated by:
The formation of a bluish product. All other colors indicate a negative result for pentoses. Note that hexoses generally react to form green, red, or brown products.