To Perform Qualitative Test for Reducing
Substances in Urine^11
11.1 Theory
Sugars may be called as reducing or nonreducing based on their ability to reduce
copper during the Benedict’s test. The reducing property of sugar is based on the
presence of free aldehyde or ketone group in them. Most of monosaccharides and
disaccharides are reducing sugars, while sucrose is nonreducing sugar. Reducing
sugars are capable of reducing Cu2+(cupric ions) to Cu+(cuprous ions) in alkaline
medium which produces red precipitate of cuprous oxide or yellow precipitate of
cuprous hydroxide. The free carbonyl carbon of aldehyde or keto sugars shows the
reducing property. The urine of normal individuals contains small amount of
reducing substances which are not sufficient to give positive test with Benedict’s
test or Fehling’s test. Various reducing sugars present in the urine are glucose,
galactose, fructose, and lactose.
11.2 Qualitative Test for Reducing Sugars...................
11.2.1 Benedict’s Test
Principle
Benedict’s qualitative reagent contains Cu2+ions complexed with citrate in alkaline
medium. The free aldehyde or keto group of the reducing carbohydrates reduces
cupric ions to cuprous ion with the resultant formation of yellow or red precipitates
of cuprous oxide. Sodium citrate prevents the spontaneous reduction of CuSO 4
while Na 2 CO 3 is used to provide alkaline medium.
#Springer Nature Singapore Pte Ltd. 2018
V. Kumar, K. D. Gill,Basic Concepts in Clinical Biochemistry: A Practical Guide,
https://doi.org/10.1007/978-981-10-8186-6_11
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