Lab Task 4 General Tests for Carbohydrates PDF

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LAB TASK 4: GENERAL TESTS FOR CARBOHYDRATES(Group 2)I. MOLISCH’S TESTMaterials Test Tubes droper Testtube rack Marker and tapeReagents Used Molisch reagent Conc. H2SOSubstance Tested Using the simulatorDescription of Visible Result Solubility Existence of ringRing ColorControl miscible No existence ...

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LAB TASK 4: GENERAL TESTS FOR CARBOHYDRATES (Group 2)

I. MOLISCH’S TEST Materials Test Tubes droper Testtube rack Marker and tape Reagents Used Molisch reagent Conc. H2SO4 Substance Tested

Using the simulator

Control Glucose Fructose Lactose Sucrose

Estimated: 5ml samples + 3 drops of Molisch reagent + 2 drops of conc. H2SO4

Description of Visible Result Solubility Existence of ring miscible No existence of ring immiscible visible immiscible visible immiscible visible immiscible visible

Ring Color No appearance Reddish- violet Reddish -violet Reddish- violet Reddish -violet

Interpretation By adding Moliche reagents, it enable the sample to be check for a purple ring which is formed by the concentrated sulphuric acid acting on the sugar. It happens when a carbohydrate undergo dehydration by sulphuric acid (H2SO4) to form furfural (furfuraldehyde) that reacts with α-naphthol.

Based on the results, which carbohydrate gave a positive result with Molisch’s test? All carbohydrates (monosaccharides, disaccharides and polysaccharides) give a positive reaction for Molisch test. Based on the result above, glucose, fructose, lactose and sucrose gives a postive result. However, the control sample (distilled water) shows no changes in color which explains the absence of carbohydrates and gives a negative Molisch’s test.

II. BIAL’S ORCINOL TEST Materials Hot plate Beaker Pipet Test tubes Test tube rack Beaker tongs Reagent Used Bial’s Reagent Substance Tested

Description of Visible Results Solubility

Xylose Glucose Tube #5 Tube #6 Tube #9 Tube #11 Tube #12

Estimated: @drops of sample + 5 ml of Bial’s Reagent

Miscible Miscible

Precipitate Initial Color Black ppt colorless Brown ppt colorless

Miscible Miscible Miscible Miscible

No ppt Black ppt Red ppt No ppt

colorless colorless colorless colorless

Miscible

No ppt

colorless

Final Color Dark-blue Yellowishbrown brown Dark-blue Muddy red Darkgreen Yelloworange

Interpretation The samples were put into the test tubes then mixing it with the Bial’s reagent, afterward water bath was used as a medium to start the reaction (this is when the dehydration of pentoses occur to produce furfural then it further react with other substance like iron ion present in the reagent). Thus, producing a bluish color. A positive Bial’s test is the formation of blue-green color which are pentoses while a negative Bial’s test is the formation of any other color. Hexoses sugar generally react to form green, red or brown products III. SELIWANOFF’S TEST Materials Used Hot plate Beaker Pipet Test tubes Test tube rack Beaker tongs Reagents Used Seliwanoffs Reagent

Substance Tested

Glucose Fructose Sucrose Maltose Starch

Result Solubility

Initial Color

Final Color

1ml of samples + 1ml of Seliwanoffs Reagent

Miscible

colorless

Colorless-Pale yellow

Miscible

colorless

Dark red solution very quickly

5 minutes water bath

Miscible

colorless

Red solution quickly

Miscible Miscible

colorless cloudy

Colorless-Pale yellow Colorless-Pale yellow

Interpretation Formation of red colored complex indicates positive result which means that fructose and sucrose contains ketoses. However, sucrose is a disaccharide made of aldose (glucose) and ketose (fructose). Hence, the red coloration will occur because there is a presence of fructose as seliwanoff’s reagent reacted quickly. The the glucose, maltose and starch are aldoses that reacted

slowly or weakly to the seliwanoff’s reagent. But if the boiling is prolonged, all samples will obtain red color but due to the water bath taking only 5 minutes, the difference in reaction can easily be seen.

GUIDE QUESTIONS: 1.Will disaccharides and polysaccharides give a positive result for Molisch test? The Molisch’s test gives out a positive result for all carbohydrates with tetrose and triose as exceptions which are monosaccharides with four and three carbon base units respectively. Since disaccharides and polysaccharides are classifications of carbohydrates where the former is composed of two monosaccharides bonded by a glycosidic linkage and the latter contains more than ten monosaccharide units, then both will yield a positive result for this test. However, unlike monosaccharides that gives the result rapidly, disaccharides and polysaccharides react slowly and gives a delay in the result, considering that they are the type of sugars that are more complex in structure. 2. Which of the different carbohydrate tests would give a positive result for maltose? Describe the color change. Molisch’s Test would give a positive result for maltose since it is a disaccharide with two Dglucose units bonded by an α (1-4) linkage. The reaction will occur slowly turning a clear sugar solution mixed with α -naphtholand concentrated sulfuric acid into a purple color at the junction of two layers, indicating the presence of carbohydrate.

Benedict’s Test would yield a positive result for maltose since this disaccharide has free functional group, specifically aldehyde, that are not involved in a glycosidic linkage. This test is for reducing sugars wherein a color change will occur from blue to red. Maltose would reduce the blue copper (II) ions in the Benedict’s reagent to red precipitate of cuprous oxide.

Fehling’s Test would give a positive result to maltose as it is used for detection of reducing and non-reducing sugars. This involves the solution of Fehling’s A and Fehling’s B which are copper (II) sulphate and potassium sodium tartrate (Rochelle salt) with a strong alkali respectively. The combination of these two produces a deep blue color and causes formation of orange to red precipitate as its positive result, indicating a presence of reducing sugar with functional group of aldehyde.

Tollen’s Test detect reducing sugars as well that would yield a positive result with maltose. This carbohydrate would react with tollen’s reagent to form an appearance of silver mirror to indicate

a presence of reducing sugar. The initial color would be a clear solution that would turn into silver as the sugar reduce silver ions in the test reagent to elemental silver after they are submerged in a boiling water bath for 10 minutes. This elemental silver accumulate on the inner surface of reaction tube and produce silver mirror appearance.

Reference: AMRITA VISHWA VIDYAPEETHAM & CDAC MUMBAI. FUNDED BY MEITY (MINISTRY OF ELECTRONICS & INFORMATION TECHNOLOGY)

http://amrita.olabs.edu.in/?sub=73&brch=8&sim=209&cnt=1

EXPERIMENT 1- QUALITATIVE ANALYSIS OF CARBOHYDRATE http://www.chem.boun.edu.tr/wp-content/uploads/2014/04/Chem-415-Experiment-1.pdf

BIALS TEST http://dept.harpercollege.edu/chemistry/chm/100/dgodambe/thedisk/carbo/bial/bials.htm

QUALITATIVE AND QUANTITATIVE TESTS FOR CARBOHYDRATES https://www.biologydiscussion.com/carbohydrates/test/qualitative-and-quantitative-tests-forcarbohydrates/13042

Pajarito, Dexter. (2020, September 20). Carbohydrates Color Reaction: Molisch Test [Video]. Yotube. https://www.youtube.com/watch?v=zDN8Ck7ETlg Prof. Collop, Drew. (2020, June 28). Bial’s Test for distinguishing Pentose vs Hexose [Video]. Youtube. https://youtu.be/kfFkrBonxA0 Question 1: http://dept.harpercollege.edu/chemistry/chm/100/dgodambe/thedisk/carbo/molisch/molisch.htm https://www.vedantu.com/chemistry/molisch-test Question 2: https://alevelbiology.co.uk/notes/tests-for-carbohydrates/

https://byjus.com/chemistry/tests-of-carbohydrates/ https://www.youtube.com/watch?v=zDN8Ck7ETlg https://www.youtube.com/watch?v=bVvy2UTfcl4 https://www.youtube.com/watch?v=yTyP8cBpGX4

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