COLOR REACTIONS. 99 



chloride and others. On this is based the use of proteids as antidotes 

 in poisoning with metallic salts. 



5. Precipitation by Mineral Adds at Ordinary Temperatures. The 

 proteids are precipitated by the three ordinary mineral acids in proper 

 amounts, but not by orthophosphoric acid. If nitric acid be placed in a 

 test-tube and the proteid solution be allowed to flow gently thereon, a 

 white opaque ring of precipitated proteid will form where the two liquids 

 meet (HELLER'S albumin test). 



6. Precipitation by the so-called Alkaloid Reagents. To these belong 

 the precipitation by metaphpsphoric acid and by hydroferrocyanic acid, 

 which is carried out by the aid of potassium ferrocyanide in a liquid 

 containing acetic acid; precipitation by phosphotungstic acid or phos- 

 phomolybdic acid in the presence of free mineral acids; precipitation 

 by potassium-mercuric iodide or potassium-bismuth iodide in solutions 

 acidified with hydrochloric acid; precipitation by tannic acid in acetic 

 acid solutions. The absence of neutral salts or the presence of free 

 mineral acids may prevent the appearance of the precipitate, but after 

 the addition of a sufficient quantity of sodium acetate the precipitate 

 will in both cases appear; precipitation by picric acid in solutions acid- 

 ified by organic acids. Proteids are also precipitated by trichloracetic 

 acid in 2-5 per cent solutions, by phenol, salicyl sulphonic acid, nucleic 

 acid, taurocholic acid and by chondroitin sulphuric add in acid solutions. 



Color Reactions for Proteid Bodies. 



1. Millon's Reaction. 1 A solution of mercury in nitric acid contain- 

 ing some nitrous acid gives a precipitate with proteid solutions which 

 at the ordinary temperature is slowly, but at the boiling-point more 

 quickly, colored red; and the solution may also be colored a feeble 

 or bright red. Solid albuminous bodies, when treated by this reagent, 

 give the same coloration. This reaction is due to the tyrosine and is 

 also given by other monohydroxyl benzene derivatives. According to 

 O. NASSE 2 it is best to use a solution of mercuric acetate which is treated 

 with a few drops of a 1 per cent solution of potassium or sodium nitrite; 

 previous to use a few drops of acetic acid are added. 



2. Xanthoprotdc Reaction. With strong nitric acid the albuminous 

 bodies give, on heating to boiling, yellow flakes or a yellow solution. 



1 The reagent is prepared in the following way: 1 pt. mercury is dissolved in 2 pts. 

 nitric acid (of sp.gr. 1.42), first cold and then warmed. After complete solution of 

 the mercury add 1 volume of the solution to 2 volumes of water. Allow this to stand 

 a few hours and decant the supernatant liquid. 



2 See O. Nasse, Sitzungsb. d. Naturforsch. Gesellsch. zu Halle, 1879, and Pfluger's 

 Arch., 83; see also Vaubel and Blum, Journ. f. prakt. Chem. (N. F.), 57. 



