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Qualitative tests for carbohydrates

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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. a) 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 (a-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. ...read more.


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:- a) The Molisch test method for Carbohydrates used in this experiment - To 1 cm of each solution add 3 drops of a-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. b) 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. c) 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. d) 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. ...read more.


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. ...read more.

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