GALACTOSE 105 



which galactose predominates. Pure galactose is, however, 

 more easily prepared from lactose ; for this purpose i kg. 

 of lactose is boiled for two hours with 2*5 litres of water 

 containing 50 grams of sulphuric acid ; the solution is 

 neutralized with barium carbonate, filtered, and concentrated ; 

 the galactose is crystallized by the addition of a mixture of 

 I part of ethyl with 2 parts of methyl alcohol.* 



The estimation of galactose depends upon its oxidation 

 by nitric acid, under specified conditions, to mucic acid and 

 weighing the latter ; the results obtained vary in the hands of 

 different workers, and considerable practice is required to 

 obtain consistent values. 



Properties. 



Galactose crystallizes in minute hexagonal crystals, which 

 melt at 164°. It is strongly dextro-rotatory, a^ = 8i'5°, and 

 exhibits muta-rotation. Galactose after association with 

 ordinary yeast for some time is fermentable, but it is not 

 acted upon by 5. Ludimgii and S. anonialus. 



Detection. 



1. The hexagonal form of the crystals is characteristic of 

 galactose. 



2. It gives a methylphenylhydrazone (m.p. 190-191°). 



3. It reduces Fehling's solution somewhat more slowly 

 than glucose ; 10 c.c. Fehling's solution =0*5 1 gram galactose. 



4. On oxidation with nitric acid it yields mucic acid. 

 Five grams of substance are heated in a beaker with 6 c.c. of 

 nitric acid (sp. gr. I'I5) until two-thirds of the liquid have been 

 evaporated off. After twelve hours the mucic acid formed 

 will have separated, and may be washed with lO c.c. of water. 

 If other insoluble substances are present, place the filter paper 

 with the solid in a dilute solution of ammonium carbonate 

 to extract the mucic acid as ammonium salt. Filter once 

 more, and evaporate the filtrate almost to dryness, and 

 acidify with nitric acid ; the precipitate is pure mucic acid. 



* Dept. of Commerce, Bur. of Standards, Washington, No. 416. 



