Measurement of Refractive Indices of Resins and Plastics C. D. WEST The Land-Wheelwright Laboratories, Inc., Boston, Mass.
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S h PAPER on the ineasuieinent of refiactiJ e indices of
The light path in the Abbe m a j follow one of three courses: The light may be admitted through the front opening of the I efracting prism [reflection position) ; through the back or accessory prism (transmission po-ition) ; or through the edge of the specimen itself. Tlie second position is commonly used for liquids, the third for bolid specimens of appreciable thickness; both give a stronger contrast a t the critical edge than the fiist position. The following discussion is confined to the first or reflection position, because i t is more convenient and more generally applicable to the materials under consideration, especially when the specimen is left adhering t o a separatesupport.
resins, Bradley ( 1 ) mentioned the Abbe refractometer as being "sometimes desirable." A later writer ( 3 ) , while calling for a simple method of making these measurements. th,inissed this instrument v-ith the sentence, "The Abbe type of iefractometer is not suitable, because most resins melt between 70" and 200"." In this note, based on experience with a standard Bausch cLLomb instrument, the Abbe refractometer is shown to be in fact suitable for measuring refractive indices of a variety of resins and similar transparent plastic materials that fall in its range, including those that soften only above 70" C. The total reflection method requires t h a t the specimen have one smooth flat surface. It will only rarely be necessary to cut and polish this surface; generally i t will be simpler to form the surface because the materials in question are organic glasses that are plastic b y nature. For example, the material may be dissolved in a volatile solvent and the solution allowed to dry on a flat surface, or i t may be melted and cooled on a flat surface; a n oxidizing oil may be allowed to dry, or a heat-convertible resin may be baked on a flat surface, etc. The flat surface is sometimes the Abbe prism itself, but this is often inconvenient; for the material may dry or cure slowly, which would tie u p the instrument, the softening point or baking temperature may be high, which would put too great a strain on the expensive prism, or the material may tend to chip the prism. In such cases it is plainly preferable to form the material on a flat surface other than the Abbe prism, and one with which greater liberties may be taken. A smooth film may be cast from solution onto a n y flat surface, and after drying may be measured film side down on the Abbe without removing from the support. I n this m y , for example, the cleared gelatin layer of a photographic plate may be measured. It is not necessary, however, to go to the trouble of forming a smooth film. If the material is formed on a piece of glass of higher refractive index than the material, and having plane parallel faces, the free side of the flat may be placed in contact with the Abbe prism using a suitable liquid and the reading macle in the usual manner. ?\lieroseope slide glass is satisfactory for the few resins, such as vinyl acetate, of refractive index less than t h a t of the crown glass of which the slides are made ( n = 1.51 to 1.52). Most resins, however, have a higher refractive index than c r o r n glass; for these it is necessary to obtainflintglassflats. drectangular form about 2.5 X 1.25 X 0.075 cm. thick (1 X 0.5 X 0.030 inch) is satisfactory, and if the refractive index is about 1.63 it will take care of the majority of natural and artificial resins and plastics; to dispose of the whole range of the Abbe, the flats should, of course, have the same refractive index as the prism. Any specimen prior to measurement is best examined in a polariscope to determine whether it is isotropic or birefringent, and in the latter event to find the character of the birefringence with a suitable compensator plate. T o increase the accuracy of the measurement of a material not formed on the Abbe prism b u t left on a separate support two readings should be made; the second is made after rotating the specimen in the plane of its flat surface through 180" from the first position. This is easy if the support is rectangular in outline.
Reflection hlethod K i t h the reflection method the brighter portion of the field consists of totally reflected light, the weaker portion of ordinarily reflected light. With thicker specimens the contrast is ordinarily sufficient; b u t with thin films the contrast is often so weak t h a t a laborious search is necessary to locate the desired boundary. I n such a n event there are several ways of increasing the contrast. The common way is to vary the area of the illumination and the angle a t which the light strikes the front prism. As a source of diffuse light the writer uses an area about 5 em. (2 inches) square illuminated by a 15-watt 110-volt bulb. With this source and with the conventional design of Abbe refractometer, which has a horizontal axis about which the whole working part may be rotated as a unit, it is easy to find the angle where the contrast is a maximum. A second method is to suppress the internal reflection a t the film-air boundary, by increasing the film thickness, or if possible by adding a drop of inert liquid of higher refractive index than the film and closing the back prism. This cuts down the stray light which otherwise increases the intensity of the ordinarily reflected portion of the field a t the expense of the contrast. This method is useful at times, but it is not always possible (as when a film to be measured is left adhering to a separate support) or convenient. The third method to increase the contrast makes use of polarized light. A polarizing screen is placed between the front prism and the source with vibration direction a t 45" to the plane of incidence, and a cap analyzer over the eyepiece is crossed with it. With these crossed diagonal polarizers, if the film is thin enough and the angle of illumination is correct, a reversal of the fields is almost always noted-there is fairly complete extinction of the totally reflected portion of the field, and a less complete extinction of the ordinarily reflected portion. When the contrast is thus enhanced, the boundary is readily apparent in those cases where i t is difficult if not impossible t o locate by other methods. The effect is often striking. bince one would expect the relative intensities of the two portions of the field t o be unaffected by the polarizers, the ieversa1 of the fields is a t first qight paradoxical. But when it is considered that the sti a)- light resulting from internal reflection a t the film-air boundaiy is the cause of the trouble, the interpretation become- clearer. K h e n the cliagonal polarizer is used, this stray light becomes depolaiized in passing through the film. so that it can no longer be extinguished by the analyzer. This depoliiization i q not, in general, a phase 627
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shift a t internal reflection, since the intensity of the portion of the field in question is notfound to be reduced appreciablS, when a quarter-wave plate is inserted in various azimuths and the analyzer is rotated. It is observed, as would be expected, that the reversal of the fields is more distinct when the film is birefringent t,han it is optically isotropic; but qualitatively there is litt’le difference between films of Ti-eak and of stronger birefringence. The polarizing method is of considerable practical advantage in a laboratory where refractive index measurements are made on a large number of thin films and the time element is important. A Polaroid microscope cap analyzer, such as the one listed by the Bausch &I Lomb Optical Company and which fits over eyepieces up to 27 mm. in diameter, has been found to be suitable for
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t h e purpose described.
Such an analyzer is, of course, a necessity in identifying the double boundaries observed when birefringent films or crystals are examined on an Abbe or other total reflection refractometer(4). Bellingham and Stanley (London) are noT5- offering an Abbe refractometer Kith polarizing eyepiece ( 2 ) . The polarizing screen for the illuminator may be obtained from t h e Polaroid Corporation or other dealers. The combination of polarizing screen and cap analyzer may be used in a simple polari.cope f o r examination of specimens preliminary to their nieaiurement .
Literature Cited (1) Bradley, T. F., ISD.EXG.CHEW,Anal. Ed., 3, 304 (1931). (2) Guild, J.,J . Sci. Instrunzents, 15, 65 (1938). (3) Morrell, R. S.,ed., “Synthetic Resins and Allied Plastics,” p. 356, New York, Oxford University Press, 1937. (4) ITest, C, D., J . CILemn. sot,, S9, 7 4 2 (1937), RECEIVED June 17, 1938.
Effect of Ions on Mohr Method for Chloride Determination Hydrogen Peroxide Modification for Sulfite Elimination R. T. SHEEN
AND
H. L. KAHLER, W. H. &- L. D. B e t z , Philadelphia, Pa.
The Mohr method is found to be unaffected by ions, except sulfite, that are prevalent in steam condensate, raw and mixed boiler feed waters, and in boiler salines. A modified Mohr method is suggested for the elimination of sulfite and is proved to be satisfactory. pH between 7.4 and 10.8 has no effect on the method.
The work showed that sulfite interfered, giving high results. The effect of this ion is also presented in the diagram. I n the low range of chloride, ion concentrations of 328 p. p. m. of sulfate, 400 p. p. m. of total alkalinity as calcium carbonate, 600 p. p. m. of total hardness as calcium carbonate, 20 p. p. m. of phosphate, 40 p. p. m. of silicate expressed as SiOz, and 260 platinum units of color were found to have no influence. Sulfite also interfered in this range. I
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HE Rlohr method (1) and modifications of it for the
determination of chlorides are widely used in boiler feedwater studies and yet a fairly exhaustive search of the literature has failed to disclose complete studies of the effects of the ions prevalent in these waters. Although it was felt that the majority of the ions exert no influence, the present investigation was planned to ascertain just what ions affected the method and to what extent. Because of the nature of the maters involved in this work, the study of these effects lent itself to subdivision into low and high ranges of chlorides, the former ranging from 0 to 10 p. p. m. and being representative of what would be encountered in steam condensate and in raw and mixed boiler feed waters, and the latter ranging from 10 to 1000 p. p. m . of chloride and representing concentrations encountered in boiler salines. Owing to the absence of adequate samples of rarious chloride concentrations, synthetic solutions were prepared with known chloride concentration, and the particular ion or ions (chloride-free) to be studied were introduced prior to the titration. Table I presents the results of this work. Reference to Table I will reveal that the method under these conditions was unaffected in the high chloride range by 3000 p. p. m. of sulfate, 100 p. p. m. of total hardness as calcium carbonate, 400 p. p. m. of silicate expressed as Si02, 160 p. p. m. of phosphate, 40 p. p. m. of iron (ferric), 2000 p. p. m. of total alkalinity as calcium carbonate, and approximately 10,000 platinum units of color obtained from tannin.
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The diagram s h o w that considerable sulfite is required to give an appreciable interference. This being the case, solutions containing low sulfite and high chlorides will not be seriously affected by the presence of this ion. When, however, the sulfite content is high and the chloride concentration low, the error in the chloride determination can be serious. T o eliminate the effects of sulfite, the following procedure was found to be satisfactory: After neutralization of the sample to pH 4.3 (methyl orange end point), 2 cc. of hydrogen peroxide (3 per cent by volume) are introduced. The solution is stirred and reneutralized to the alkaline side of methyl orange. This reneutralization step is necessary because the sulfuric acid present in the hydrogen peroxide reduced the pH value, giving slightly high results due to the fugitive nature of the end point. The solution is then ready for titration with silver nitrate after the addition of the chromate indicator. The results using this procedure are presented in Table 11.
