Our objective is to detect the presence of sugar in the given sample of urine.
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.
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?
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.
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.
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.
|Colour of precipitate||Percentage of sugar present|
|Green||0.5 to 1%|
|Yellow||1 to 2 % sugar|
|Brick Red||2 % or more sugar|