Tests for Carbohydrates Flashcards
The chemical tests employed to detect carbohydrates and to distinguish among them may be divided into two broad categories:
- tests based on production of furfural or a substituted furfural
- tests based on the reducing property of sugars
is a general test for carbohydrates
Molisch’s Test
In Molisch’s Test, compounds that are dehydrated by __ to form __ or __ will react with __ (in the Molisch’s reagent) to yield a __ condensation product. Although not a specific test for carbohydrates, a negative result is good evidence of the absence of carbohydrates.
- concentrated H₂SO₄ (sulfuric acid)
- furfural; hydroxymethyl furfural
- alpha-naphthol
- purple
a specific test for reducing sugars or the presence of free or potential aldehydes.
Benedict’s Test
In Benedict’s Test, a positive test is the appearance of a __ of __. However, the size of the particles may make it appear too __, __, or even __ if there is much deep blue cupric ion unreacted.
- red-orange precipitate
- Cu₂O (Copper(I) oxide)
- orange; yellow; green
a test used to distinguish between monosaccharides and disaccharides
Barfoed’s Test
In Barfoed’s Test, a positive reaction may be only a little __ in the bottom of the test tube. If this appears within 2-3 minutes, it indicates a __; if it does not appear or show after 10 minutes, a __ is indicated.
- dark red precipitate
- monosaccharide
- disaccharide
a specific test for ketoses
Selivanoff’s Test
This test is useful for distinguishing fructose (a ketohexose) from glucose, mannose, and galactose.
Seliwanoff’s Test
In Seliwanoff’s Test, the dehydration of __ with hot __ occurs much faster than the dehydration of the corresponding __. During the same time interval in which the dehydrated __ reacts with the __ (in Selivanoff’s reagent) to form a condensation product, the __ yields only a pale pink coloration.
- ketohexoses
- HCl
- aldohexoses
- ketohexose
- resorcinol
- aldohexose
a test for pentoses and nucleotides that contain the pentose sugar.
Bial’s Test
In Bial’s Test, it is based on the observation that __, which is formed from pentoses, yields a __ compound when treated with __ in the presence of __ (in the Bial’s reagent).
- furfural
- blue-green
- orcinol
- ferric ions
Bial’s Test is not specific for pentoses since other compounds such as __, __, and certain __ will produce blue or green products. __ (formed from hexoses) will give a yellow-brown coloration product.
- hexoses
- uronic acids
- heptoses
- Hydroxymethyl furfural
This test is positive with glucose and sucrose and negative for galactose and lactose. The most obvious sign of fermentation is the production of carbon dioxide.
Fermentation by Yeast
The most obvious sign of fermentation is the production of __.
carbon dioxide
Starch gives a blue color with iodine, and glycogen gives a reddish color with iodine. What test?
Iodine Test for Starch and Glycogen
In the Iodine Test for Starch and Glycogen, starch gives a __ color with iodine, and glycogen gives a __ color with iodine.
- blue
- reddish
Test solutions for tests for carbohydrates
1% glucose (dextrose)
1% sucrose
1% fructose
1% galactose
1% lactose
1% ribose
1%starch solution
Test reagents for tests for carbohydrates
- Molisch’s Reagent
- Benidict’s Reagentt
- Barfoed’s Reagent
- Seliwanoff’s Reagent
- Bial’s Reagent
- Yeast
- I2 and KI solution
- Pentanol
Procedure: Molisch’s Test
a. Place 2 mL portions of test solutions in separate test tubes.
b. Add 2 drops of __ into each and mix thoroughly.
c. Incline the tube and carefully add down the side of the tube 1 mL of __. A __ color should develop at the interface if carbohydrate-containing material is present.
- Molisch’s reagent
- concentrated H₂SO₄
- purple
Procedure: Benedict’s Test
a. Place 1 mL portions of the test solutions in separate test tubes.
b. Add 3 drops of __ into each and mix by swirling
c. Put all the test tubes simultaneously into a beaker half full of boiling water. Note which solution reacts __.
- Benedict’s solution
- first
Procedure: Barfoed’s Test
a. Place 1 mL portions of test solutions in separate test tubes.
b. Add 3 mL __, mix, and place in a __ bath simultaneously for up to 10 minutes. Note which gives a __ first and record the time taken for the precipitate to form.
- Barfoed’s reagent
- boiling water
- dark red precipitate
Procedure: Saliwanoff’s Test
a. Place 2 mL portions of test solutions in separate test tubes.
b. Add 3 mL __ and place in a __ bath at the same time.
c. Record the time when a __ color appears in the clear solution (no precipitate). Compare colors at the same time. A __ or __ color is not a positive test.
- Seliwanoff’s reagent
- boiling water
- dark red-brown
- yellowish; apricot
Procedure: Bial’s Test
a. Place 1 mL portions of test solutions in separate test tubes and add 3 mL of __.
b. Carefully heat the mixture until it boils, then let it cool. A __ color is a positive test for a pentose. If not easily recognized, add 1 mL of __ and shake. The green color will be extracted by the alcohol if a pentose is present. Note: Fructose and sucrose interfere in the test tube since they produce a __.
- Bial’s reagent
- bright
- pentanol
- brown color
Procedure: Fermentation by Yeast
a. Weigh 0.2 g of __ and place it in a small test tube.
b. Half fill with test solution, close the end with a finger, and shake well.
c. When the solution and the yeast are well mixed, fill the test tube to the brim with test solution and cap it with a paper circle. It will be held by the pressure. Place the beaker on a water bath or in a warm place.
- dry yeast
Procedure: Iodine Test for start and glycogen
a. Place 1 mL of 1N __ and __ in separate test tubes.
b. Add a drop or two of __ in KI. Note the color produced. For better results with glycogen, place test tubes in __.
- starch solution; glycogen
- iodine
- ice water