Refractometer

A refractometer is a simple instrument used for measuring concentrations of aqueous solutions. It requires only a few drops of liquid and is used throughout the food, agricultural, chemical, and manufacturing industries.

When light enters a liquid it changes direction; this is called refraction. Refractometers measure the degree to which the light changes direction, called the angle of refraction. A refractometer takes the refraction angles and correlates them to refractive index (nD) values that have been established. 

Using these values, you can determine the concentrations of solutions. For example, solutions have different refractive indexes depending on their concentration in water.