Qualitative tests for carbohydrates

Qualitative Tests For Carbohydrates

Introduction

In this practical experiment there are given , five known carbohydrate solutions. Namely Glucose, Fructose, Xylose, Starch and Sucrose (common table sugar). The first four are what are known as “reducing sugars” which in solution form either an aldehyde or a ketone. The latter, sucrose, is known as a “non-reducing” sugar. In addition to these is an unknown carbohydrate solution. There are qualitative food tests that are regularly used to identify the presence of these carbohydrates in foodstuffs. The following are a brief description of these well proven tests for each named carbohydrate.

The Molisch test for Carbohydrates – the result of this test shows positive where all carbohydrates are present. Monosaccharides (simple one sugar units) give a rapid positive test, whereas Disaccharides (double sugar units) react more slowly. This is due to the Molisch reagent (α-naphthol 95% in ethanol) either dehydrating pentoses (sugars with 5 carbon atoms) to form furfural and dehydrates hexoses (sugars with 6 carbon atoms) to form 5-hydroxymethyl furfural. The reaction of the Molisch reagent with these furfurals produces the purple product. A positive result produces a purple furfural product. (see fig.1) ( – Nov. 2007)

(fig.1 – Molisch test positive result)

Benedict’s test for Reducing Sugars –

Benedict’s reagent is used as a test for the presence of reducing sugars. It can be prepared from sodium carbonate, sodium citrate and copper (II) sulphate. Benedict’s reagent contains blue copper (II) ions (Cu2+) which are reduced to copper(I) (Cu+). These are precipitated as red copper (I) oxide which is insoluble in water. A positive result produces a nice brick orange precipitate. (see fig.2) ( – Nov.2007)

(fig.2 – Benedict’s test positive result for reducing sugars - note Sucrose is a non-reducing sugar hence the blue hue)

Seliwanoff’s test for Fructose – this reagent consists of resorcinol with concentrated hydrochloric acid. Fructose and other Hexoses undergo dehydration when reacted with this reagent to form hydroxymethylfurfural that condenses with resorcinol to give a deep red colour. Ketohexoses such as Fructose and disaccharides (containing a ketohexose such as sucrose) form a cherry-red condensation product. Other sugars may produce yellow to faint pink colours. A positive result gives a cloudy deep reddish brown product. (see fig. 3) ( – Nov.2007)

(fig.3 Seliwanoff’s test positive result)

Iodine test for Starch - the iodine test is used ...

This is a preview of the whole essay

Seliwanoff’s test for Fructose – this reagent consists of resorcinol with concentrated hydrochloric acid. Fructose and other Hexoses undergo dehydration when reacted with this reagent to form hydroxymethylfurfural that condenses with resorcinol to give a deep red colour. Ketohexoses such as Fructose and disaccharides (containing a ketohexose such as sucrose) form a cherry-red condensation product. Other sugars may produce yellow to faint pink colours. A positive result gives a cloudy deep reddish brown product. (see fig. 3) ( – Nov.2007)

(fig.3 Seliwanoff’s test positive result)

Iodine test for Starch - the iodine test is used to test for the presence of starch. Iodine solution is iodine dissolved in an aqueous solution pf potassium iodide which reacts with starch to produce a deep blue-black colour. This reaction is the result of the formation of polyiodide chains from the reaction of starch and iodine. The amylase or straight chain portion of starch, forms helices where iodine molecules assemble, forming a dark blue-black colour. A positive result gives a dark blue-black product. (see fig.4 Iodine test positive result on the right) ( – Nov. 2007)

Bial’s test for Pentose - The test reagent consists of orcinol, hydrochloric acid and ferric chloride which dehydrates pentoses to form furfural. Furfural further reacts with orcinol and the iron ion present in the test reagent to produce a bluish green product which may precipitate. So a positive result gives a bluish green product. (see fig.5)( – Nov. 2007)

(fig. 5 Bial’s test positive result)

Aims

The purpose of this experiment is to prove that the tests work e.g. Molisch test on carbohydrates gives a purple result when reacted with glucose, fructose, xylose, and sucrose solutions but not starch. That the Benedict’s test produces a nice brick orange precipitate when reacting with glucose, fructose and Xylose solutions, but not the sucrose and starch. That Seliwanoff’s test for fructose gives a deep red cloudy result when reacted with fructose and sucrose but not glucose, xylose and starch. That the iodine test for starch gives a deep dark-blue result when reacted with the starch solution but not with the remaining solutions. Finally, that the Bial’s test for pentose gives a blue result when reacted with the Xylose solution but not the others. If these tests prove positive it should be able to determine what the “UNKNOWN” solution contains and possibly identify it.

The Test methods employed were:-

The Molisch test method for Carbohydrates used in this experiment – To 1 cm of each solution add 3 drops of α-naphthol 95% in ethanol solution and mix using a glass stirring rod. Tilt the tube and very carefully pour down the inside of the test tube 1cm of concentrated H2SO4 so that the sulphuric acid forms a separate layer under the sugar layer.
The Benedict’s test method for Reducing Sugars used in this experiment – To about 2cm3 of sugar solution add 2cm3 of Benedict’s reagent. Mix the reagents and heat in the water bath until boiling. Complete this test with each sugar solution.
The Seliwanoff’s test method for Fructose used in this experiment - To 3cm3 of Seliwanoff’s reagent (resorcinol in HCl), add a few drops of sugar solution and heat. The solution should go a nice brick orange colour in the presence of fructose and on cooling should go cloudy as well. Complete this with each sugar solution.
The Iodine test method for Starch used in this experiment – To about 3cm3 of starch solution add 3 drops of iodine reagent,( iodine dissolved in potassium iodide)and note the colour formed. Complete with each sugar solution.
The Bial’s test method for Pentose used in this experiment – Mix about 3cm3 of Bial’s reagent with the same volume of each sugar solution and heat until the liquid boils. Pentose gives a bluish green colour.

The apparatus and materials utilised included:-

6 boiling tubes
Water bath
Bunsen burner
Starch solution, glucose solution, fructose solution, sucrose solution, xylose solution and “UNKNOWN” solution
Bial’s reagent, Molisch reagent, Benedict’s reagent, Iodine solution, Seliwanoff’s reagent, conc. sulphuric acid, water
1 cm3 teat pipette
Test tube rack
Tripod and Gauze
Test tube tongs
Goggles

Table Of Results for the qualitative tests carried out on various sugar solutions

= a positive result = a negative result

Discussion

The results for the sugar solutions utilised are as follows:-

Glucose – gave a positive purple result under the Molisch test, a nice orange precipitate under the Benedict’s test, and no visual change under the Seliwanoff’s test, the iodine test or the Bial’s test. As a reducing sugar and a hexose not a pentose these results would be expected.
Fructose – gave a positive purple result under the Molisch test, a nice orange precipitate under the Benedict’s test, a deep reddish brown under the Seliwanoff’s test and no visual change under the iodone test or the Bial’s test. As a reducing sugar and a hexose not a pentose these results would be expected in addition to the Seliwanoff’s test which is for fructose.
Xylose – gave a positive purple result under the Molisch test, a nice orange precipitate under the Benedict’s test, and a nice bluish green result under the Bial’s test. There was no visual change under the Seliwanoff’s test or the iodone test. As a reducing sugar and a pentose (Bial’s test)not a hexose these results would be expected.
Sucrose – gave a positive purple result under the Molisch test, and a deep reddish brown under the Seliwanoff’s test. It went blue under the Benedict’s test which is negative, and no visual change under the Bial’s test and the iodine test. Sucrose is a non- reducing sugar so these results would be expected, especially under the Benedict’s which is a test for reducing sugars.
Starch – gave a positive dark blue-black result under the iodine test, which is for starch, and negatives under the other tests. It did give a blue result under Benedict’s as well which is not what was expected from a reducing sugar, as it would have been expected to produce an orange precipitate. This may have been down to contamination but with what remains unknown.

Conclusion

As can be seen from the table of results shown above, the “UNKNOWN” gave three positive results . It gave a positive result under the Molisch test indicating that it is a carbohydrate and it also gave a positive result under the Benedict’s test indicating that it is a reducing sugar. In addition to this it tested positive under the iodine test indicating the presence of starch. It gave negative results under the Seliwanoff’s test which indicates it is not Fructose. It also gave a negative result under the Bial’s test which indicates that it is a hexose sugar and not a pentose sugar.

As previously mentioned the starch gave a blue result under Benedict’s test which is not what was expected from a reducing sugar, as it would have been expected to produce an orange precipitate. This may have been down to contamination but with what remains unknown. In the main the errors could have been down to the inaccuracies in measurement e.g. it is difficult to be precise using a teat pipette and approximate quantities although it seems the results gained were on the whole correct. For more accurate results for future experiments the use of a burette could prove to be an improvement.

References