The measurement of the methanol concentration of liquid solution based on laser interferometer

This study develops a high-precision, nondestructive measurement technique based on a laser interferometer for determining the methanol concentration of a liquid solution from its refractive index. The optical metrology system is employed to measure the refractive indexes of samples with known methanol concentrations ranging from 0 ~ 90%. The error between the experimental refractive index and the literature refractive index is small than 0.00062. The error between the measured value of the refractive index and the analytically derived value is reduced form 0.00064 to 0.00007. The decreased error ratio is about 89%. When the relative function between the refractive index and methyl alcohol concentration is cubic, this can reduce the regressional analysis error. By applying regressional analysis to the experimental results, an analytical expression is derived to describe the cubic relationship between the refractive index and the methanol concentration. An excellent agreement is observed between the experimentally determined values of the methanol concentration and the analytically predicted results.