Refractometer

Theory of the method:

Refractometry

Protocol for measuring the concentration of a solution

using the refractometer

Materials:

  • Abbe refractometer
  • Distilled water (refractive index for water: n0 = 1.333)
  • Glycerin solutions: 5%, 10%, 20% and x%
  • Cotton, filter paper

Procedure:

A. Measuring the refractive indices:

1. open the prism block and keep the lower prism horizontally

2. add two-three drops of distilled water on the surface of the lower prism so that a thin film of solution is formed. Attention! Please add the solution carefully to not scratch the surface of the prism

3. close the prism block

4. start the source of light and adjust the mirror so that will be obtained a good illumination of the visual field

5. rotate the prism block (using the corresponding knob) until will be obtained a dark region (down) and a bright region (upper) in the visual field

6. adjust the dispersion using the compensator knob (no rainbow/ colored line should be visible between two regions in the visual field)

7. bring the clear delimitation line localized between the bright and dark regions to the center of the visual field (at the intersection of the rectangular wires)

8. read the refractive index with three decimals

9. repeat the steps 1 to 8 for all glycerin solutions: 5%, 10%, 20% and x%

10. perform nine measurements for each glycerin solution and write the results in a table:

Observation:

Clean carefully with distilled water and cotton the surfaces of the prisms between the measurements

11. reorganize the working place

B. Calculating the unknown concentration:

1. calculate the average, standard deviation, standard error mean for each analyzed solution

2. draw a graph n = f(c) – refractive index function of concentration

Attention!

  • Start the refractive index scale with the value of refractive index of water (not from zero)
  • Select a larger unit for the refractive index scale, so that you will have enough space to represent values with three decimals

3. draw the regression line for the experimental points

4. identify the concentration for x% glycerin solution by interpolation.

5. state one-two conclusions according to your results