Question

Brick red color in Benedict’s test indicates _______ percentage of sugar in the urine.
A. Nil
B. 0.5
C. 1.0
D. 2.0

Answer 1

Hint: The percentage of sugar in urine that is indicated by the Brick red color in Benedict’s test. Benedict’s test is not used for the diagnosis of Glycosuria or Diabetes mellitus.

Step by step answer:This test was devised by the American chemist Stanley Rossiter Benedict, and thus it is named after him. Benedict’s test is a way of chemical analysis that is used to test the presence of reducing sugars in the given sample. To do this, Benedict’s reagent is needed.
Benedict's solution is an aqueous alkaline mixture of \(CuS{O_4}\) and Sodium citrate, and Sodium carbonate. The Sodium citrate acts as a complexing agent that keeps the \(C{u^{2 + }}\)ions in solution and prevents them from precipitating as cupric carbonate. The sodium carbonate keeps the solution alkaline. Benedict’s solution is deep-blue in color. When the test is positive then it is identified by a color change to Brick red with precipitate. This is due to the oxidation of the reducing sugars by the cupric \(C{u^{2 + }}\)complex of the reagent to the cuprous \(C{u^ + }\) ions. These ions then precipitate as insoluble red Copper (I) oxide. Due to this property of the test, it is widely used to test Glycosuria, which is indicative of Diabetes mellitus. Glycosuria is an elevated level of glucose in urine. When the benedict’s reagent is added to the urine sample then based on the color of the precipitate the quantity of the sugar in the urine can be determined. The following colors determine the following percentages of sugar in urine –
Bluish precipitate – Nil
Greenish precipitate – \(0 \cdot 5\% \)
Yellowish precipitate – \(1 \cdot 0\% \)
Orangish precipitate – \(1 \cdot 5\% \)
Brick red precipitate – \(2 \cdot 0\% \)
Thus, the Brick red color in Benedict’s test indicates \(2 \cdot 0\% \) the percentage of sugar in the urine.

Hence, the option (D) is correct.

Note: The Benedict's test also detects the presence of alpha-hydroxy-ketones, aldehydes, and hemiacetals, including those that occur in certain ketoses. Due to this the ketose fructose, which is not strictly a reducing sugar, but an alpha-hydroxy-ketone also gives a positive test because it is converted to glucose, aldoses, and mannose by the base in the reagent. Thus, Benedict’s test is not used for the diagnosis of Glycosuria or Diabetes mellitus. This is because the reagent reacts with other reducing substances also, creating a false positive.