Precision Laser-Based Concentration and Refractive Index Measurement of Liquids

Measurement of liquid concentration is important in fields such as chemical analysis and processing, diagnostics, and semiconductor manufacturing. This work presents a highprecision, noncontact, laser-based technique to measure the refractive index of liquid mixtures and solutions, which can be used to determine the concentration of a mixture. The technique can resolve extremely small changes in concentration, on the order of 0.01%. The essential components in the system include a HeNe laser as the light source, a precision glass cuvette as the liquid container and optical cell, and a very-high-resolution position sensor and high-precision digital voltmeter. The experimental configuration is small, reliable, and inexpensive. Results are presented for ethanol - water and methanol - water mixtures and two water - salt solutions, with very good agreement found between measured values and those reported in the literature.