Which instrument would you use to determine the index of refraction of volatile oils?

• A. Spectrophotometer
• B. Pycnometer
• C. Refractometer
• D. Polarimeter

Answer: (C)

Determining how light bends when it enters a liquid gives its refractive index. A refractometer is designed for this, placing a tiny amount of the volatile oil on a prism and measuring the angle at which light changes direction to provide a direct readout of the oil’s refractive index. This method is ideal for volatile oils because it’s fast, requires only a small sample, and is sensitive to composition and purity. The other instruments don’t measure bending of light: a spectrophotometer looks at how much light is absorbed or transmitted at different wavelengths, a pycnometer assesses density, and a polarimeter gauges how much the plane of polarized light is rotated. So the instrument that determines the index of refraction is the refractometer.