Determining the refractive index of liquids using a cylindrical cuvette

Abstract

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.

Measurement of Low Concentration of Nanosized Objects Suspended in a Liquid Medium

“…The refractometer proposed by the authors is free from these unsatisfactory features. Waveform of the images shows that the interference of laser light after it is reflected from different regions in the cuvette generates an interference pattern, as described in more detail in [8], [9]. The image position is determined by the position of the first or any subsequent minimum following the position of the maximum intensity of distribution.…”

“…As the outside surface of the cylinder is coated with a reflective film, the rays falling on this unit area of the boundary surface are reflected and intersect the cell wall and the liquid several times in a similar way. The analytical calculation of the RI of liquids contained in the system described above is given in [8], [9], where applying the laws of light refraction and reflection and using simple trigonometric formulae, the central angle η m (see Fig. 1 for the case when m = 5 -the number of reflections of the laser beam (inside the cuvette) was calculated as follows:…”

Refractive index measurement using virtual—apparent image coincidence approach

“…Additionally, Ochoa et al transmitted beams of a red laser diode into a long rectangular glass box to obtain RI values based on the measurement of the beam flight time [27]. Vilitis et al formulated the RI based on the reflection-refraction principle of a laser beam using a cylindrical cuvette container [28].…”

Theoretical and experimental study of liquid refractive index measurement method based on the double external-cavity-laser feedback effect

“…Although there is no need to know the RI of the cell, the error of internal thickness and the parallelism can affect the result significantly. Victor et al [9], Vilitis et al [10] and Hattori et al [11] measured the liquid RI by measuring the deviation of the laser beam passed through cylindrical cell. This method requires several intermediate quantities: the air RI, container RI, container thickness, angle of incidence, offset displacement of emitting light, and the uncertainty of any intermediate quantity will cause the measurement error of final result.…”