How to Use an Abbe Refractometer
The angle of a ray of light tends to deflect slightly as it travels from one medium to another. Scientists refer to this as refraction. Chemists take advantage of this phenomenon to characterize liquids in terms of their refractive index, using an instrument known as an Abbe refractometer. This instrument passes light through a thin film of liquid sandwiched between two prisms, and then measures the liquid's refractive index, which represents the ratio of the speed of light in a vacuum to the speed of light traveling through the sample liquid.
A liquid's refractive index represents a physical property, similar to boiling point or freezing point; measuring it is a common way to identify compounds and verify their purity.
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Instructions
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Open the instrument's prism case. The two prisms comprising the case should contain a hinge, and the top prism should swing open. Many instruments' prism assemblies are equipped with latches that should open with minimal force.
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Place three or four drops of liquid on the bottom prism---enough to coat the entire prism with a thin film---and then close the prism assembly. Use a plastic pipette to deliver the liquid, so that you avoid scratching the prism's surface.
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Look through the instrument's eyepiece and turn the prism adjustment knob until a distinct light/dark border becomes visible.
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Adjust the compensator dial to remove as much color as possible from both the light and dark areas.
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Adjust the eyepiece to bring the light/dark border into sharp focus, and then readjust the prism knob to place this border in the center of the crosshairs.
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Read the refractive index from the scale visible through the eyepiece. (Some older instruments require the operator to depress a button to display the scale.)
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Note the temperature on the device's thermometer. Most refractive-index values in reference books were recorded at 20 degrees Celsius. If the temperature of the instrument is higher or lower, you will have to make the following temperature adjustment to the reading:
n(20) = n(T) + (T -- 20) * 0.00045,
where n(20) represents the refractive index at 20 degrees C and n(T) represents the refractive index taken at some temperature T. For example, a refractive index of 1.3336 taken at 24 degrees C would correct to:
n(20) = 1.3336 + (24 - 20) * 0.00045 = 1.3354.
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Tips & Warnings
With practice, a precision of 0.0001 refractive-index units is achievable with a high-quality refractometer. The last digit is usually an estimate.
Never use strongly acidic or alkaline solutions on a refractometer; they will permanently damage the optics.
