Critical-angle refractometer

In Figure 18.3, the optical system of Abbe Refractometer has been shown based on the critical angle principle. 

A typical arrangement for an on-line process refractometer is shown in Fig. 6.45. Besides the prism, which is in contact with the process liquid, there is commonly a light source, a beam collimator and a photo-conductive sensor. As the nature or concentration of the process liquid varies, the critical angle changes, so moving the internally reflected collimated beam of light across the photo-detector. The latter is divided into two sections, one of which is arranged to receive a constant amount of... 

Refractometry can be used to determine the composition of a copolymer. In addition, differential refractometry has been used to study micellization in dilute block copolymer solutions (Tfizar and Kratochvfl 1972). The refractive index (n) is obtained in an Abbe refractometer via measurements of the critical angle for external reflection. The refractive index increment dn/dc, where c is the polymer concentration, can be related to the molecular weight of particles in solution. Further details of the method are provided by Pepper and Samuels (1989). 

Most refractometers operate on the concept of the critical angle 4> it this is the angle for which (or 4> ) is exactly 90° (see Fig. 9b). A ray in the medium with any greater angle will be totally reflected at an equal angle as shown in Fig. 9c. The index of refraction is given in terms of the critical angle by... 

FIGURE 36.6 (a) Portable jeweler s refractometer tor measuring the critical angle (b) the optics of determining the critical angle using the refractometer. 

Refractive index can be measured with a high precision to five decimal places using critical angle method with Bausch Lomb Precision Refractometer. This method can be used for the transparent and light colored liquids measured at temperatures from 20 to 30°C. Several light sources and filters are specified in order to provide monochromatic light. Also, calibration and calibration liquids are described to assure high precision of measnrement. 

The refractive index (RI) of a mixture is a function of the composition of the mixture and the respective refractive indices of the constituents [8]. The mixture refractive index follows mixture laws such as the Lorentz-Lorenz law. Operational measuring instruments are usually differential refractometers or critical-angle re-fractometers [4]. A large disadvantage in the method is that it only provides meaningful results when a two-component system is considered. However, a differential refractometer is commonly used as a concentration detector in the effluent of a gel permeation chromatography (GPC) column for molecular weight determination. 

More precise measurements can be obtained with commercial refractometers, such as an Abbe or Pulfrich. Both refractometers are based on measuring the critical angle of incidence between the specimen and a standard prism. The sample consists of a flat plate with one surface and an adjacent vertical edge well pohshed. The contact between the polished sample and the prism is enhanced with an approximately matching index oU. A split image is adjusted to coincide, and the refractive index is read directly. The precision here is generally of the order 10". ... 

In Abbe s double prism method a thin ho-meotropically oriented nematic film is placed between the hypotenuse faces of two rectangular prisms. Using light polarized parallel and perpendicular to the optical axis, respectively, the principal refractive indices and can be obtained directly from the critical angle of total reflection, provided that the refractive index of the prism is greater than that of the liquid crystal [2, 8-10]. In some cases the director of the nematic phase was oriented uniformly parallel to the prism planes [11-14]. Special variants of the total reflection method have been described [3, 4, 15, 16]. A total reflection refractometer of the Abbe type has also been used to determine n, n and An in the infrared region between 2.5 and 14 p.m [17]. 

The refractive index is measured by the critical angle method with a Bausch Lomb Precision Refractometer using monochromatic light. The instrument is previously adjusted by means of a solid reference standard and the observed values are corrected, when necessary, by a calibration obtained with certified liquid standards. 

Now, based on the two constants, viz., A and N, for a specific prism and a measurable angle a it is convenient to determine the refractive index of the liquid n relative to air from Eq. (/ ). With the help of the Abbe refractometer the angle a lying between the normal and the critical ray emerging from the upper surface of the prism may be measured. By the aid of the two constants A and N (for a particular prism) the angle a has been converted into the refractive index directly and the scale of the instrument has been duly calibrated and printed accordingly. 

The most commonly used form of refractometer is the Abbe refractometer, shown schematically in Fig. 11. This differs from the immersion refractometer in two important respects. First, instead of dipping into the liquid, the refractometer contains only a few drops of the liquid held by capillary action in a thin space between the refracting prism and an illuminating prism. Second, instead of reading the position of the critical-ray boundary on a scale, one adjusts this boundary so that it is at the intersection of a pair of cross-hairs by rotating the refracting prism until the telescope axis makes the required angle 8 with the normal to the air interface of the prism. The index of refraction is then read directly from a scale associated with the prism rotation. 

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