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Refractometry
Refractometry
is a technique that measures how light is refracted when it passes
through a given substance, in this case, an unknown compound. The amount
by which the light is refracted determines the refractive index. Refractive
index can be used to identify an unknown liquid compound, or it can
be used as a means of measuring the purity of a liquid compound by
comparing it to literature values. The closer the refractive index
is to the literature values, the purer the sample. Refractive index
is defined as the ratio of the velocity of light in air to the velocity
of light in the medium being measured:
nD = [V air]/(V liquid)
Refractive index is temperature
dependent. To adjust for temperatures greater than 20oC, please use
the following formula:
nD20 = nDT + 0.00045(T - 20oC)
Apparatus
This is a view of the front of a refractometer.
Procedure
- Lift prism and clean both top and bottom surfaces with 95% ethanol.
- Once both surfaces are dry, place just enough of your neat (undiluted) sample onto the bottom prism until the entire surface is covered using a disposable pipette.
- Close the prism and raise the light as far as it will go.
- Turn the knob on the front of the instrument to nD. You will see that the light turns on.
- Look into the eyepiece. Turn the knob on the right side of the instrument until you see a black line.
- Use the dial directly under the eyepiece to focus the line so it is a clear as you can get it.
- Again, using the knob on the right side of the instrument, adjust the line so that it intersects with the cross hairs you see in the eyepiece.
- Press the READ button and record the value that shows on the digital readout. This is nDT.
- Press the TEMP button and record the value(T) that shows on the digital readout.
- Use the equation above to calculate nD20.
- Clean both top and bottom surfaces of the prism with 95% ethanol. Place a clean folded piece of Kimwipe on the bottom prism and close the prism lid.
Click below for a two-minute, silent movie on how to use the refractometer.