Benedict’s Test: Estimation Glucose in urine

The amount of glucose present in the urine of a normal subject is about 100mg/day, which is not detectable by the commonly employed qualitative methods. However,

in some persons glucose may appear in the urine as in benign glucosuria which is not uncommon.Benedict’s Test

METHOD: Benedict’s Method:

PRINCIPLE:

Glucose or reducing substances in the urine reduces the cupric ions, on boiling in an alkaline solution containing potassium thiocyanate, to cuprous ions. Potassium thiocyanate, on forming a white precipitate of cuprous thiocyanate prevents the formation of a red precipitate of cuprous oxide. While, Potassium Ferrocyanide, though present in small amount, keeps the cuprous oxide in solution.

REAGENTS:

  • Benedict’s Quantitative Reagent (BQR): Contains:
  • a Copper sulphate,
  • b Sodium carbonate,
  • c Sodium citrate,
  • d Potassium thiocyanate
  • e Potassium ferrocyanide.
  • Sodium carbonate anhydrous.

PROCEDURE:

  • To 25ml Benedict’s Quantitative Reagent (BQR) in a white porcelain dish, add 10gm anhydrous sodium carbonate and a few glass beads. Heat to boil, with continuous stirring, till sodium carbonate has dissolved completely.
  • Titrate this against urine in a burette, with continuous boiling and stirring of BQR, until a chalk-white precipitate is formed and the last trace of blue colour of BQR disappears, which marks the end point. However, distilled water may be added to the porcelain dish, from time to time, during the boiling process, if the mixture becomes too much concentrated.
  • The final color  of the end point may be yellow or brown, due to the presence of urinary pigments, but there should be no trace of blue or green colour.

CALCULATIONS:

1ml BQR is reduced by 2mg glucose.

25ml BQR = 50mg glucose or,

= 0.05gm glucose.

Therefore, the volume of urine required (from the burette) to reduce 25ml BQR contamed 50mg glucose.

Volume of urine used = ‘a’ ml

‘a’ ml of urine contained = 0.05 gm

glucose.

100ml of urine will contain = 0.05 X 100/’a’ ml

= ‘b’ gm glucose/100ml

2 ‘b’ gm glucose/100ml.

Interpretations:

Glucose, if present in sufficient amounts in the urine, i.e., 0.1-0.20o or more, when it becomes detectable by the commonly employed qualitative tests, 1ndlcates pathologlcal condition. 

Glucose starts appearing in the urine in: Diabetes mellitus,

Adrenal diabetes,

Pituitary diabetes,

Hyperthyroidism, Renal diabetes, and Glomerulonephritis.

 

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