Immersed diffraction grating refractometers of liquids

In this work, we report the development, construction, and the performance of two liquid refractometers that use a reflective diffraction grating immersed in a test liquid. The liquid is contained in a transparent glass cell with a rectangular cross section. The grating is oriented in such a way that the propagation directions of the incident beam and the beam diffracted by the lower part of the grating immersed in the liquid are perpendicular. In this configuration, the refractive index is determined by measuring the angle of the zeroth-order diffraction beam coming from the upper part of the grating, which is in contact with air. The diffractive refractometers (DR-1 and DR-2) have different angle measurement procedures and different light detection systems, and their advantages and drawbacks are pointed out. In the experiments, precisions of the order of 10^-5 and 10^-4 for DR-1 and DR-2 are achieved, respectively. The performances of both systems are compared with the performance of a commercial Abbe refractometer in the measurement of sugar and NaCl aqueous solutions.