A REFRACTOMETER FOR REACTIVE LIQUIDS BY WALTER CULE DAVIES A S D WlLLIAM J. JONES
The determination of the refractive indices of some liquids is rendered difficult by their reactive and corrosive nature. For instance, certain organometallic compounds are so quickly decomposed by oxidation on exposure to the air, and most acid halides are so rapidly hydrolysed by the atmospheric moisture, that the measurement of their indices by means of ordinary refractometers, such as the Abbe and the Pulfrich, in which contact with the air is involved, is out of the question. Even if special, air-tight, cells are used with such instruments, troubles are encountered where such liquids as the chlorides of phosphorus and arsenic are placed in contact with the optical glass, which they and their products of decomposition rapidly pit and corrode. For these reasons, where reactive liquids have to be dealt with, a refractometer with an air-tight cell of readily replaceable optical parts is desirable, and such an instrument, based on the principle of the spectrometer with liquid-filled prism, is illustrated in Fig. I . The prism, P, is of glass, through which a cylindrical hollow has been bored parallel to the base. The hollow is closed by a thin plane glass front and a back, plane, glass covered, mirror. These parts are kept in position by means of rubber bands, or in certain cases it may be preferable to cement the plates to the prism with Canada balsam. Communication to the hollow is afforded by two tubes carrying taps which are ground into the base and top of the prism, respectively. The assembled empty prism is placed on the prism table, where it is filled, and is held in position by means of the spring clip, C,.’ The angle of the prism and parts is determined by the use of a standard liquid. This determination was found to be necessary; although the prism used in the present work had itself been accurately cut to 30°, owing to adjustments of the plates, involving the use of bands or of Canada balsam, the composite prism did not actually function with exactly t h a t angle. The standard benzene used for this purpose had the following refractive indices: niooI . j I I I , n g ” 1,49989, n?” 1.49527. To take the readings, the source is placed so that the light falls on the rhomb on the left-hand side of the eyepiece. The zero position, in which the ray is reflected back from the front plate along its original path, is obtained by narrowing the slit, controlled by the milled head, S, and bringing the image of the slit into coincidence with the intersection of the cross wires. The prism with its table is then rotated by means of the arm, A, until the refracted image of the slit is brought into coincidence TTith the intersection of the cross wires. The refractive index is given by the ratio of the sine of the angle of rotation to that of the angle of the prism, and, the value of the index being known for the standard liquid, benzene, the angle of
-4 REFRACTOXETER FOR REACTIVE LIQUIDS
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FIG.I
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WALTER CCLE DllVIES AND WILLIAM J. JONES
the prism and parts is found. For instance, the angle of the prism and parts thus determined in one of the present settings was 30' 2 ' . The benzene is then removed, without disturbing the plates, from the prism, which is thoroughly dried by a rapid stream of air. The liquid under investigation, already purified, is now distilled in an allglass apparatus, and is condensed and collected in the two-way receiver, shown in Fig. 2 , with guard tube, G. Air is swept out of the apparatus by the admission of an indifferent gas, and, if necessary, the apparatus is then evacuated. Half of the liquid is collected in the bulb receiver, B, and then the receiver, R, originally horizontal, is turned into the vertical position shown in the figure, and the collection of a specimen is effected in R. The tap, T, is then closed, and the receiver, R, is attached by its ground end, E, to the corresponding ground end of the horizontal tube of the prism, which is now swept out with the inert gas, a guard tube having been affixed to the vertical tube on the top of the prism. The liquid is now allowed to flow into the prism. It is an easy matter to maintain the temperature a t the standard, uiz.,zo°C. As an example we may quote the following measurements of the refractive index of acetyl chloride: ni0" 1.3933j, n g " 1.38708, ntoo 1.38548. Investigation of the refractivities of highly reactive liquids such as acid chlorides by means of this instrument is now in progress. I n conclusior we have to thank Messrs. Bellingham and Stanley, London, for their help and advice in the construction of the instrument. Tatem Laboratones, University College, Card@. October 68,1960.
