Determining the refractive index of liquids using a cylindrical cuvette

The refractive index of a liquid carries important information about its physical properties, including concentration and density, thus making it possible to determine and monitor the composition of the solution. This is important in fundamental research, chemical analysis and medical diagnostics, as well as in the processing and manufacturing of various substances. The authors have developed a compact and adaptable device of high sensitivity for measuring the refractive indices of both stationary liquids and continuous liquid flows. This device can be used in various technological processes requiring real-time analysis of flowing liquid substances, including aggressive compounds. The refractive index is determined by measuring the deviation of a laser beam passed through a cylindrical cuvette containing the test liquid. The magnitude of the deviation, which depends on the RI, is measured as the displacement of the transmitted beam's projection on a linear measuring element, such as a linear CMOS or CCD image sensor. In order to significantly improve the resolution and stability of RI measurements, an efficient solution has been developed, based on repeated reflection and refraction of the light beam travelling through the cylindrical cuvette with liquid. By this, deviation of the rays exiting the cuvette increases several times in respect to refractive index of the liquid. Additionally, a new method for detecting the position of the projected laser beam on a linear optical sensor is employed. Also, an increase in the intensity of the exiting rays has been achieved. By applying the techniques developed, it is possible to achieve high resolution and stability of refractive index measurements even when the distance between the image sensor and the cuvette is short. Hence, a basis for the construction of accurate and compact devices for determining the refractive indices of liquids is provided, suitable for a broad spectrum of applications.