The angle of refraction is related to an index value called the index of refraction. Each compound has a specific index of refraction. The angle of refraction is dependent on the composition of the media and on the temperature. This composition dependency is what makes refractometers so useful. As the concentration of a particular compound in a solution increases, so does the degree to which the light is bent. Also, it is important to determine the temperature of the testing environment since temperature affects the angle of refraction.
As an example, refractive light can be used to determine the sodium chloride (NaCl) concentration/salinity in a brine solution. For each percent salinity value, there is a corresponding angle of refraction. That angle of refraction is converted to percent salinity. This percentage is the concentration of NaCl in the brine solution. To make the conversion easier, refractometers are available with scales that are calibrated to read the desired value, in this case, percent salinity.
Types of Refractometers
Handheld Analog Refractometer With an analog refractometer, the sample is place on a cover plate and a prism and then held to the light to view the scale inside the meter.
Handheld Digital Refractometer Digital refractometers require a drop of the tested solution to be placed in a well. That well is illuminated by a light source, usually an LED light and the meter interprets the light transmission into refractive index or whatever unit of measure the instrument is programmed to read.
Abbe (Laboratory) Refractometer Abbe Refractometers are bench-top meters that look similar to a microscope which provide highly precise measurements of refractive index.
Refractometers are available with a variety of scales:
- Salinity Measures sodium chloride solutions
- Brix Measures percent sucrose. Used in the food and beverage industry for quality control
- Coolant Freezing Point Determines the effectiveness of ethylene glycol and propylene glycol coolants
- Clinical: Measures Serum albumen and urine-specific gravity (e.g. to test for urine drug sample tampering)
- Specific Gravity Measures density of a liquid in relation to the density of water, which has a specific gravity of 1.
Calibration and Use of Analog Refractometers
- Calibrate the refractometer with a standard solution before use. Since the reading will be affected by temperature changes, it is best to calibrate at the temperature of the test environment. If this is not possible, correction charts may be used. Some refractometers have automatic temperature correction (ATC), a feature that allows the instrument to automatically correct for temperature differences.
- Place a small amount of liquid (usually 2–5 drops) on the prism and secure the cover plate. This will evenly distribute the liquid on the prism.
- Point the prism end of the refractometer toward a light source and focus the eyepiece until the scale is clearly visible.
- Read the scale value at the point where the dark and light portions meet. Below is an example of a salinity scale as seen through the eyepiece: