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Qualitative identification of a substance is of significant importance in chemistry. Physical constants such as melting points have traditionally been used by organic chemistry for identification of unknown compounds. As for inorganic substances, the precipitation of a solid, results of a flame test, or the formation of a colored substance could all be keys to identifying a sample. Chromatography and spectra are amongst the newer techniques practiced today. A problem that arises in biochemistry when attempting to identify a particular compound is that the tests that are utilized to identify these compounds rely on the reaction of a functional group and therefore will yield positive reactions with more than just one compound. This means that more than one test has to be performed in order to deduce which compound is present by process of elimination. Carbohydrate chemistry allows us a great opportunity to better understand qualitative testing. The tests run within this experiment are usually carried out to identify simple carbohydrates. It is possible that by running these tests with a variety of different carbohydrates we determine the identity of an unknown sample. Tests
Tests for: Reducing Sugar (+), Nonreducing Sugar (–)
Positive result: Red precipitate Negative result: No precipitate Bial’s Test
Tests for: Pentoses (+), Other (–)
Positive result: Change in color to cloudy dark blue Negative result: Not cloudy dark blue Barfoerd’s Test
Tests for: Monosaccharides (+), Disaccharides (–)
Positive result: Red precipitate Negative result: No precipitate Seliwanoff’s Test
Tests for: Fructose (+), Other (–)
Positive result: Change in color to orange Negative result: Not orange Glucose Oxidase Test
Tests for: Glucose (+), Other (–)
Positive result: Changes color Negative result: Remains same color Starch Iodine Test
Tests for: Starch (+), Sucrose (–)
Positive result: Change in color to blue Negative result: Not blue
We determined our carbohydrate to be ribose considering it matched up perfectly with all the test results we had ran. Ribose was the only carbohydrate of the ten that had similar reactions to the tests performed. The three tests that ribose was subject to were Benedict’s test, Bial’s test, and Barfoerd’s test, all of which ribose tested positive for. Since the unknown also tested positive to these three tests and negative to all other tests that we ran, this means the unknown is a reducing sugar, a pentose, and a monosaccharide. We did have a bit of trouble trying to decide whether the unknown was arabinose or ribose because they both tested positive to the aforementioned tests. However, arabinose had a greenish tint and our unknown matched in color more closely to ribose’s dark blue.
Read more: Iodine Test for Starch Lab Report