100 



METHODS OF ANALYSIS. 



(c) LEACH AND LYTHGOE METHOD WITH IMMERSION ^EFEACTOMETEB. 



PROVISIONAL. 



Determine at 20 C. the refraction of the distillate obtained in the determina- 

 tion of alcohol by the immersion refractometer. If on reference to the table 

 the refraction shows the percentage of alcohol agreeing with that obtained from 

 the specific gravity, it may safely be assumed that no methyl alcohol is present. 

 If, however, there is an appreciable amount of methyl alcohol the low refrac- 

 tometer reading will at once indicate the fact. If the absence from the solu- 

 tion of other refractive substances than water and the alcohols is assured, this 

 qualitative test by difference in refraction is conclusive. 



The addition of methyl to ethyl alcohol decreases the refraction in direct pro- 

 portion to the amount present; hence the quantitative calculation is readily 

 made by interpolation in the table, using the figures for pure ethyl and methyl 

 alcohol of the same alcoholic strength as the sample. 



Example. Suppose the distillate made up to the original volume of the 

 measured portion taken for the alcohol determination has a specific gravity of 

 0.97350, corresponding to 18.38 per cent alcohol by weight, and has a refraction 

 of 35.8 at 20 C. by the immersion refractometer; by interpolation in the 

 refractometer table the readings of ethyl and methyl alcohol corresponding to 

 18.38 per cent alcohol are 47.2 and 25.4, respectively, the difference being 21.8; 

 47.235.8=11.4; (11.4-^-21.8) 100=52.3, showing that 52.3 of the alcohol present 

 is methyl alcohol. 



Scale readings on Zeiss immersion refractometer at 20 C., corresponding to each 

 per cent by weight of methyl and ethyl alcohols. 



