Using refractometers
Refractometry

In teaching and industrial control laboratories in the fields of physics and chemistry, various models of deflection or limit angle refractometers are used to measure the indices of solid materials (mineral or organic glasses, crystals) and liquid or gelled substances. Current accuracy is of the order of the fourth decimal place, but it is also possible to reach the fifth decimal place, provided that the temperature of the samples is measured and stabilized.

For pure gases or gas mixtures, interferometric or strioscopic methods are more commonly...