COLOR REACTIONS. 99 



a few drops of acetic acid are added. 2. Xanthoproteic Reaction. With 

 strong nitric acid the albuminous bodies give, on heating to boiling, 

 yellow flakes or a yellow solution. After making alkaline with ammonia 

 or alkalies the color becomes orange-yellow. 3. Adamkiewicz' 's Reaction. 

 If a little proteid is added to a mixture of 1 vol. concentrated sulphuric 

 acid and 2 vols. glacial acetic acid a reddish-violet color is obtained slowly 

 at ordinary temperatures, but more quickly on heating. According 

 to HOPKINS and COLE 1 this reaction takes place only on using glacial 

 acetic acid containing glyoxylic acid. According to them it is better 

 to use a solution of glyoxylic acid, which can be readily prepared by adding 

 sodium amalgam to a concentrated solution of oxalic acid and filtering 

 after the discharge of the gas. A dilute aqueous solution of the acid 

 or some of the solid acid is added to the proteid solution and sulphuric 

 acid allowed to flow down the side of the test-tube, when the reddish- 

 violet color will appear at the point of contact of the two liquids. Gela- 

 tin does not give this reaction. 



A similar color is obtained, according to AGREE, with formaldehyde. About 

 0.1 gm. of the substance is treated with 0.1 cc. formaldehyde solution (1:5000) 

 and then after 2-3 minutes carefully treated with 0.5 cc. sulphuric acid. At 

 the boundary of the two layers a violet coloration occurs. According to ROSEN- 

 HEIM 2 this reaction occurs in the presence of oxidizing substance only, and further 

 investigations into this reaction are necessary. 



As further color reactions we will mention: 4. Biuret Test. If a 

 proteid solution be first treated with caustic potash or soda and if then 

 a dilute copper-sulphate solution be added drop by drop, first a reddish 

 then a reddish-violet, and lastly a violet-blue, color is obtained. 5. Pro- 

 teids are soluble on heating with, concentrated hydrochloric acid, producing 

 a violet color, and when they are previously boiled with alcohol and 

 then washed with ether (LIEBERMANN 3 ) they give a beautiful blue 

 solution. This blue color is due, according to CoLE, 4 to a contamination 

 of the ether, with glyoxylic acid, which reacts with the tryptophane 

 groups split off by the hydrochloric acid. The violet color obtained 

 with proteins not purified with ether is also considered as a tryptophane 

 reaction, produced from the furfurpl formed by the action of the 

 concentrated hydrochloric acid upon the proteid. Reaction 6 with 

 concentrated sulphuric acid and sugar (in small quantities) is explained 

 in the same way. The beautiful red coloration is connected with the 

 formation of furfurol from the sugar. 7. With p-dimethylaminobenzal- 



1 Proceed. Roy. Soc., 68. 



2 Acree, Amer. chem. Journ., 37; Rosenheim, Chem. Centralbl., 1907, p. 1809. 

 See also Dakin, Journ. of Biol. Chem., 2. 



3 Centralbl. f. d. med. Wissensch., 1887. 



4 Journ. of Physiol., 30. 



