Polarimeter

Theory of the method:

Polarimetry

Protocol for measuring the concentration of an optic active solution

using a polarimeter

Materials:

• Polarimeter; length of the polarimeter tube = 200 mm = 2 dm
• Distilled water (dH₂O)
• Glucose solutions: 5%, x%
• Filter paper

Procedure:

A. Measuring the rotation of plane polarized light for glucose solutions:

1. switch on the polarimeter (please, wait a few minutes until the sodium lamp will warm up)

2. fill the polarimeter tube with distilled water

• remove the black screw top and the glass cover
• fill the tube will distilled water without any air bubbles
• close the tube with the glass cover and the black screw top
• remove the excess of liquid using the filter paper
• place the tube back in the polarimeter

3. obtain a homogeneous visual field using the lever of the analyzer; check the visual field from ~ 10 cm distance to the eye slit; homogeneous visual field is localized between two marks:

• mark 1 – visual field shows two dark lateral regions and a bright central band
• mark 2 – visual field shows two bright lateral regions and a dark central band

4. read the rotation on vernier scale of the analyzer; the polarimeters are set so that the rotation for distilled water to be 0˚.

• read the integer value on the external scale
• read the decimal value on the internal scale – in the place where it can be detected the proper alignment of two line segments on both external and internal scales

Observation: in the case the rotation is different from 0˚, the rotation value will be written down for making the correction

5. repeat the steps from 2 to 4 for glucose solutions: 5% and x%

6. perform nine measurements for each glucose solution

7. write the data in a table:

8. reorganize the working place

9. calculate the average, standard deviation and standard error for each rotation

B. Calculation of the unknown concentration

1. calculate the specific rotation for glucose solution using the following formula:

[α _glc] = α _5% / l * c _5%

[α _glc] = specific rotation for glucose solution

α _5% = rotation for 5% glucose solution

l = length of the polarimeter tube

c _5% = concentration of 5% glucose solution

2. calculate the unknown concentration of glucose solution using the following formula:

c _x% = α _x% / [α _glc] * l

[α _glc] = specific rotation for glucose solution

α _x% = rotation for x% glucose solution

l = length of the polarimeter tube

c _x% = concentration of x% glucose solution

3. state one-two conclusions according to your results