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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