REFRACTION OF LIGHT, REFRACTOMETRY AND INTERFEROMETRY 



are useful in calculating the "specific disper- place, but what is distilled water? and how 



sion":(nf — nc)M but they too, break down to keep it so? Specially purified toluene, 



with substances of high refractive index, high 2,2,4-trimethylpentane and methyloyclo- 



absorbance, high dispersion, or when applied hexane are also recommended. Their indices 



over too large a spectral range. Yet, the spe- are known to the 5th decimal place, but this 



cific dispersion thus defined is practically is often insufficient. We have had quite a 



useful for the rapid determination of aro- satisfactory experience with purified sodium 



matic content of gasolines and naphthas. chloride, zinc sulfate and potassium alum, 



The more general formulas of Maxwell within moderate concentration ranges. In 



and Sellmeyer, or that of Helmholtz and general, the range of indices available, the 



Ketteler, are more involved and are based temperature ranges and the wavelength spec- 



upon a number of simplifying assumptions, tra covered by the standards at present 



The fact that contemporary quantum and available are not quite sufficient to meet the 



wave theories generally substantiate these needs of this rapidly expanding branch of 



assumptions does not remove their indeter- technology. 



minate character. This indeterminacy mostly The analysis of gases and the study of 



stems from the lack of accuracy of refraction gaseous equilibria (by interferometry) were 



measurements outside the visible spectrum, pioneered by Lord Rayleigh, Le Chatelier, 



In the vicinity of absorption bands the cal- and more recently by Chaudron (126). Pres- 



culations usually become complicated. Their sure variations may be used as a means of 



intricacy places them beyond the attain- compensation. Multicomponent mixtures re- 



ment of the average chemical laboratory, and quire a more elaborate set-up. Nevertheless, 



they are then casually referred to as prob- a method devised by Thompson (127) has 



lems of "anomalous dispersion". It must be been applied to the study of respiration. An- 



emphasized, however, that anomalous dis- esthetic mixtures have also been monitored 



persion is the rule, rather than the excep- by refractometry (Bell and Krantz (128). 



tion, for all substances, in the vicinity of ab- The Bruin method (101) for the determina- 



sorption bands, any^vhere in the spectrum, tion of diffusion constants of large molecules 



and that its determination as a measure of has been mentioned. A refractometric 



the "chemical exaltation of refractivity" method for the measurement of colloidal 



acquires a meaning complementary to that particles size has been published, 



of spectrophotometric absorption. Refractometry is still a basic method in 



High-precision refractometry has been im- the sugar industry, using Schonrock (129) 

 peded by the scarcity of suitable reference calibration tables. Applications in the pe- 

 standards. Measurements referred to dry air troleimi industry stem from the observation 

 are ambiguous, for the relative index of air that the specific refractivity as well as the 

 is not exactly proportional to its density. The specific dispersion are often constant for 

 correction factor in the Sellmeyer formula many types of distillates. More recently, de- 

 used to improve the comparison itself tailed studies of the refractive dispersion in 

 varies with the wavelength. Air is also sub- the near ultraviolet range served to charac- 

 ject to a very large temperature coefficient, terize many hydrocarbons (130). 

 Solid standards for use with prismatic re- Refractometry is an empirical but con- 

 fractometers are now available from the Na- venient means of checking the qualitj^ of 

 tional Bureau of Standards, as w-ell as from soaps, fats and oils. In the case of soaps, 

 several firms. above 70°C the value obtained in solution is 



Agreement on liquid standards is not so identical with that obtained on the solid, 



easily reached. Distilled water occupies first Refractive index of oils and fats is closely 



518 



