Tuesday, September 1, 2015

Benedict's Test for Monosaccharides and Reducing Sugars

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Benedict’s test used to indicate the presence of monosaccharides and reducing sugars. Copper sulphate in the benedict’s reagent will react with monosaccharides and reducing sugars form brick red precipitate. Monosaccharides and reducing sugars can react with a benedict’s reagent because contain free aldehyde or ketone. The positive result is indicated by the color of the solution changes to green, yellow, orange, or brick red and appears green, yellow, orange or brick red sediment.

Benedict’s test was first discovered by an American chemist Stanley Rossiter Benedict. All kinds of monosaccharides will show a positive result with the test, reducing disaccharide such as maltose and lactose also showed positive results. Non-reducing disaccharide such as sucrose and other types of polysaccharides not react with copper sulphate so will show negative result.

benedict's test reaction
benedict's test reaction

Benedict test can be used to detect the presence of sugar in the urine. If the urine is tested with benedict showed positive results may be a sign of diabetes mellitus. Urine that used in benedict test should 24 hours urine, when we wake up, first urine waste while the second urine until the first urine the next day used to test.

Manufacture of reagents benedict:

Solution A:
  • Na. citrate 86.5 g
  • Na2CO3 50 g
  • Distilled water 400 ml
  • Dissolve Na. citrate and Na2CO3 into water (assisted by heating), the results are filtered with filter paper and diluted with distilled water up to 425 ml volume.
Solution B
  • CuSO4.5H2O 8.65 g
  • Distilled water 50 ml
  • Dissolve CuSO4.5H2O into distilled water until dissolved completely.
  • Pour the solution B into solution A while stirring slowly, add distilled water up to 500 ml volume.

Summary reaction:

Monosaccharides / reducing sugar + copper ions from the reagent benedict = carboxylic + copper (I) oxide (red brick)

Material and reagents:

  • Benedict’s eagents
  • Material to be tested

Work steps:

  • Enter 5 ml of benedict’s reagent into the test tube
  • Add to 0.5 ml of material to be tested
  • Heat in boiling water (water bath) for 5 minutes or over a direct flame for 2 minutes.
  • Note the color change and the appearance of a precipitate.

Note:

The more the concentration of monosaccharide or reducing sugar in a solution, the solution will show more brick red color. If the result shows green color, it means the concentration of monosaccharides or reducing sugar is slightly. If the result is yellow the concentration is more, and if the result is brick red the concentration more than yellow. However, if the solution remains blue, it indicates that there is no monosaccharide or reducing sugar in the solution. Benedict reagent can be stored in a sealed container in a very long time.

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