6.4.7

# Measuring Glucose Concentration

Test yourself

## Colorimetry

Colorimetry is a technique used to identify the concentration of glucose in an unknown 'urine' sample. The steps involved are:

### 1) Prepare urine samples

• Take three test tubes and label each with the name of the patient (urine sample).
• Add three cm3 urine samples from each patient.
• To each test tube add 2 cm3 of Benedict’s reagent and mix the contents of the tube.
• Benedict's reagent is used to detect the presence of glucose.
• The reagent is blue and turns from blue to red when glucose concentration is greater. The solution also becomes more opaque.

### 2) Prepare the calibration curve

• Label six test tubes 0 to 10 mmol dm−3.
• A glucose solution of 10 mmol dm−3 is provided. Dilute this standard glucose solution using distilled water to produce 5 additional concentrations which will be used to make the calibration curve.
• The concentrations should be 0 mmol dm−3, 2mmol dm−3, 4mmol dm−3, 6mmol dm−3, 8mmol dm−3, and 10mmol dm−3.

### 3) Incubate test tubes

• Add 2 cm3 of Benedict's reagent to each tube and mix the contents.
• Place all the test tubes into the water bath together (including the tubes with the urine samples) and time for four minutes.
• Allow to cool before taking readings from the colorimeter.

### 4) Use the colorimeter

• Use the contents of the 0.0 mmol dm−3 glucose solution tube, which you have heated with Benedict's, as a blank to calibrate the colorimeter to zero absorbance.
• Place the remaining samples in cuvettes into the colorimeter and read the absorbance.

### 5) Record the results

• Record your results in a table and plot a graph of the absorbance of the known concentrations of glucose. Plot concentration of glucose on the x-axis and absorbance on the y-axis.
• Using the graph and the absorbance values obtained for the urine samples read off from the graph the concentration of glucose in the urine samples.
• Record your results in a suitable table.