Qualitative Tests of Carbohydrates

Topics: Glucose, Disaccharide, Monosaccharide Pages: 5 (1231 words) Published: February 12, 2013
QUALITATIVE TESTS OF CARBOHYDRATES
Final Lab Report

Submitted by

Brittany Fitzgerald
545301

Prepared for

Nancy Cook
Chemistry 3501
Monday

November 26, 2012

RESULTS
PART A:
As a result of the Benedict’s test on various sugar solutions, it was found that galactose, mannose, arabinose, ribose, lactose, fructose, maltose, and cellobiose tested positively and therefore are considered reducing sugars. Glucose, starch, sucrose, and methyl-D-glucopyranoside on the other hand tested negatively and therefore are considered non-reducing sugars. Table 1. Results of Benedict’s test on various sugar solutions Sugar Solution| Color| Clarity|

Glucose| Bright blue with orange tint on top | Clear |
Galactose | Brown on top, blue-ish on bottom| Opaque |
Mannose | Brown on top and bottom, green/blue in middle | Opaque | Arabinose | Brown/copper on top, blue/green on bottom| Opaque| Ribose | Brown/green throughout | Opaque |
Lactose | Brown on top and bottom, blue-ish in middle | Opaque | Fructose | Brown/green throughout, red on bottom | Opaque | Maltose | Green/blue throughout, red on bottom | Opaque | Cellobiose | Green/blue through, red on bottom | Opaque | Starch | Bright blue | Clear |

Sucrose | Bright blue | Clear |
Methyl-D-glucopyranoside| Bright blue | Clear |

PART B:
As a result of the Seliwanoff’s test on various carbohydrates, it was found that only fructose and sucrose tested positively and therefore they are considered ketoses.

Table 2. Results of Seliwanoff’s test after 1 minute
Carbohydrate/Water | Color |
Glucose | Colorless |
Fructose | Colorless |
Sucrose | Colorless |
Water | Colorless |

Table 3. Results of Seliwanoff’s test after 2 minutes
Carbohydrate/Water | Color |
Glucose | Colorless |
Fructose | Light pink/red |
Sucrose | Colorless |
Water | Colorless |

Table 4. Results of Seliwanoff’s test after 3 minutes
Carbohydrate/Water | Color |
Glucose | Colorless |
Fructose | Brighter red/orange|
Sucrose | Peachy|
Water | Colorless |

Table 5. Results of Seliwanoff’s test after 4 minutes
Carbohydrate/Water | Color |
Glucose | Colorless |
Fructose | Deeper red |
Sucrose | Light orange|
Water | Colorless |

Table 6. Results of Seliwanoff’s test after 5 minutes
Carbohydrate/Water | Color |
Glucose | Colorless |
Fructose | Very deep red |
Sucrose | Orange red|
Water | Colorless |

PART C:
As a result of the Seliwanoff’s test on various sugar solutions, it was found that all the sugar solutions developed a change in color; however it was the solutions containing fructose that developed the biggest overall change. Table 7. Results of Seliwanoff’s test on various sugar solutions Sugar Solution| Color |

0.5ml glucose + 0.5ml Ba(OH)2| Beige/yellow|
0.5ml glucose + 0.5ml distilled H2O| Beige/yellow|
0.5ml fructose + 0.5ml Ba(OH)2| Peachy|
0.5ml fructose + 0.5ml distilled H2O| Peachy|

DISCUSSION
PART A:
The Benedict’s test is a mixture of copper (II) sulfate, sodium citrate, and sodium carbonate in a mildly basic solution that is used to detect reducing sugars (Schreck & Loffredo, 2012). If the carbohydrate is a reducing sugar, it will reduce the copper (II) ions to produce copper (I) oxide (red precipitate). Reducing sugars are all aldoses containing either a free aldehyde group or a cyclic hemiacetal (Schreck & Loffredo, 2012). However, ketoses may also yield a positive test for reducing sugar even though no aldehyde group is present. Monosaccharides and certain disaccharides that have a potential aldehyde group will reduce reagents such as Benedict’s solution to give a positive test, the appearance of a red precipitate (Mount Saint Vincent University). A precipitate that varies in color from red to brown or yellow is considered a positive test for Benedict’s. Prior to the addition of...

References: Eaton, David C. 1989. “Experiment 33: Carbohydrates.” Accessed November 17. http://moodlelive.msvu.ca/file.php/5622/Carbohydrates.pdf
Mount Saint Vincent University. 2012. “Chemistry 3501 Lab Manual”. Nova Scotia: Halifax.
Schreck, James O., Loffredo, William M. 2012. “Qualitative Testing for Carbohydrates.” Accessed November 16. http://kurdchemists.org/files/carbohydrates%20test%20new.pdf
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