Theory & Procedure, Detection of Sugar in Urine | Additional Study Material for NEET PDF Download

Objective

Our objective is to detect the presence of sugar in the given sample of urine.

Theory

A large volume of volatile and non-volatile waste products is produced by different metabolic processes in the body. Urine is a liquid byproduct produced in animal and human bodies.  It is produced in the kidneys through a process called urination and is excreted through the urethra. Urination is the primary method of excreting water-soluble chemicals from the body.

Let’s discuss some characteristics of urine.

Normal urine usually has a light yellow colour, due to the presence of a yellow pigment called Urochrome. Average production of urine in adult humans is 2 liters per day, depending upon various conditions. The pH of urine varies between 4.6-8 and the specific gravity of urine varies between 1.010-1.40.
Now let’s see, what are the constituents of urine?

Normal Constituents of Urine

Actually, normal urine is a highly complex aqueous solution of organic and inorganic substances. Urine consists of about 95-96% water. The most important nitrogenous organic substances present in urine are urea, uric acid and creatine. The other organic substances are oxalic acid and lactic acid. The principle inorganic constituents of urine are sodium chloride, potassium chloride, sulphates and phosphates.

Abnormal Constituents of Urine

The abnormal constituents of urine are sugar (glucose), ketone bodies, blood, protein and bile.  Ordinarily, glucose (sugar) is absent in normal urine. But when the glucose level in blood exceeds the renal threshold of glucose (160 – 180 mg /dl), glucose starts to appear in urine. The presence of glucose in urine is called glucosuria and is usually an indication of diabetes mellitus.

Generally the following two tests are used to test the presence of sugar in urine sample.

• Benedict’s Test
• Fehling’s Test

In Benedict’s test, Benedict’s solution is used as the reagent.  Benedict’s reagent is a combination of sodium carbonate, sodium citrate and copper (II) sulphate pentahydrate (CuSO₄.5H₂O). In Fehling’s test, Fehling’s solution-A and Fehling’s solution-B are used as the reagents. Fehling’s solution-A is an aqueous solution of copper (II) sulphate, having blue colour, while Fehling’s solution-B is clear colourless aqueous solution of sodium potassium tartrate.
On boiling the urine sample with the reagents, the copper (II) sulphate (CuSO₄) present in the Benedict’s solution and Fehling’s solution is reduced by the reducing agent, glucose (sugar), to form a coloured precipitate of cuprous oxide.

Depending upon the concentration of glucose, green, yellow and brick red precipitates of cuprous oxide are formed. Below is the table showing the color sequence depending upon the concentration of glucose level.

Learning outcomes

• Students understand the characteristics of urine.
• Students understand the normal and abnormal constituents of urine.
• Students understand the test to detect the presence of sugar in urine sample.
• Students will be able to do the experiment more accurately in the real lab once they understand the steps through the animation and simulation.

Benedict's Test

Materials required:

Test tube, test tube holder, urine sample, measuring cylinders, Benedict’s solution and burner.

Procedure:

• Take 2 ml urine sample in a measuring cylinder from the urine sample bottle.
• Take a test tube and pour the urine sample in it.
• Take 5 ml Benedict’s reagent in a measuring cylinder.
• Add Benedict’s reagent to the test tube that contains urine sample.
• Using a test tube holder, hold the test tube firmly and heat it for 2 minutes on the burner.
• Keep shaking the test tube while heating.
• A yellow precipitate appears which indicates the presence of sugar in urine.
• Depending upon the concentration of sugar in the urine, either green, yellow, or brick red precipitates are formed.

Fehling's test

Materials required

Test tube, test tube holder, urine sample, measuring cylinders, Fehling’s solution A, Fehling’s solution B and burner.

Procedure

• Take 2 ml urine sample in a measuring cylinder from the urine sample bottle.
• Take a test tube and pour the urine sample in it.
• Take 2 ml Fehling’s solution A in a measuring cylinder.
• Add Fehling’s solution A to the test tube that contains urine sample.
• Take 2 ml Fehling’s solution B in a measuring cylinder.
• Add Fehling’s solution B to the test tube that contains urine sample.
• Using a test tube holder, hold the test tube firmly and heat it gently for 2 minutes on the burner.
• Keep shaking the test tube while heating.
• A green precipitate appears which indicates the presence of traces of sugar in urine.
• Depending upon the concentration of sugar in the urine, either green, yellow or brick red precipitates are formed.

Simulator Procedure (as performed through the Online Labs)

You can select the test from the ‘Select type of test’ drop down list.

Benedict's Test

• Drag the dropper containing Benedict’s reagent towards the test tube to pour the reagent into it.
• Click on the knob of the burner to turn it on.
• Drag the test tube towards the burner to heat it.
• Click on the information icon to see the inference.
• You can redo the experiment by clicking on the ‘Reset’ button.

Fehling’s Test

• Drag the dropper containing the Fehling’s reagent A towards the test tube to pour the reagent into it.
• Drag the dropper containing the Fehling’s reagent B towards the test tube to pour the reagent into it.
• Click on the knob of the burner to turn it on.
• Drag the test tube towards the burner to heat it.
• Click on the information icon to see the inference.
• You can redo the experiment by clicking on the ‘Reset’ button.