Wednesday, May 9, 2012

Chemical test

Chemical test

During a water bath, which is usually 4-10 minutes, the solution should progress in the colors of blue (with no glucose present), green, yellow, orange, red, and then brick red or brown (with high glucose present).[4] A color change would signify the presence of glucose. The common disacharrides lactose and maltose are directly detected by Benedict's reagent, because each contains a glucose with a free reducing aldehyde moiety, after isomerization.

Sucrose (table sugar) contains two sugars (fructose and glucose) joined with by their glycosidic bond in such a way as to prevent the glucose isomerizing to aldehyde, or the fructose to alpha-hydroxy-ketone form. Sucrose is thus a non-reducing sugar which does not react with Benedict's reagent. Sucrose indirectly produces a positive result with Benedict's reagent if heated with dilute hydrochloric acid prior to the test, although after this treatment it is no longer sucrose. Starches do not react or react very poorly with Benedict's reagent, due to the relatively small number of reducing sugar moieties, which occur only at the ends of carbohydrate chains. Inositol (myo-inositol) is another carbohydrate which produces a negative test.

Benedict's reagent can be used to test for the presence of glucose in urine. Once a reducing sugar is detected in urine, further tests have to be undergone in order to ascertain which sugar is present. Only glucose is indicative of diabetes.

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