8 PROTEIDS. 



This reaction is particularly useful, not merely because it effects 

 a very complete precipitation of the proteids which are present (except 

 peptones), but also because the reagents employed do not produce any 

 decomposition of other substances which may be present, and do not 

 interfere with certain other tests whicli it may be necessary to apply 

 after the removal of the proteids by filtration. It is of use more par- 

 ticularly in the determination of sugar in blood. 1 



The following reactions are specially used for freeing solutions 

 from all proteids by precipitation. 



6. Acidulate faintly with acetic acid and add tannic acid. 



7. Acidulate with hydrochloric acid and add the double iodide 

 of mercury and potassium. (Briicke's reagent. 2 ) 



8. Add hydrochloric acid until the reaction is strongly acid ; 

 then add phosphotungstic acid. 



The following methods are often additionally useful for freeing 

 solutions from all proteids. 



i. Precipitate by excess of absolute alcohol, having previously 

 made the solution neutral or faintly acid. 



ii. Prepare a solution of ferric acetate by saturating acetic acid 

 with freshly precipitated ferric oxide, avoiding all excess of free acid. 

 Add this to the solution and boil ; the whole of the proteids are pre- 

 cipitated together with the iron; the latter as a basic salt. 8 In some 

 cases a mixture of ferric chloride and an excess of sodium acetate is 

 employed. 4 



iii. Boil the solution for a few minutes with a little hydrated 

 oxide of lead in presence of a little lead acetate. 6 



In recent years various neutral salts, more particularly neutral 

 ammonium sulphate, 6 have been largely employed for effecting 

 the precipitation and separation of the several proteids. 



All proteids yield a characteristic violet colouration with simul- 

 taneous slight fluorescence upon treatment with glacial acetic. 

 acid and strong sulphuric acid (Adamkiewicz' reaction). The 

 reaction is best obtained by adding to the suspected solution or 

 substance a mixture of one volume of strong sulphuric acid and 

 two volumes of glacial acetic acid and boiling. 7 The violet-col- 



1 See Gamgee's Phi/siol. Chem. Vol. i. p. 195. 



2 Sitzb. d. Wien. Akad. LXIH. 2 (1871), Feb. Hft. 



3 Hoppe-Seyler, Hdbch. S. 264. 



4 Seegen, Pfliiger's Arch. Bd. xxxiv. (1884), S. 391- 



5 Hofmeister, Zt.f. physwl. Chem. Bd. 11. (1878), S. 288. 



6 Wenz, Zt.f. Biol. Bd. xxu. (1886), S. 10. Kuhne, Verhand. d. Naturhist.-Med. 

 Ver. Heidelb. N. F. Bd. in. 1885, S. 286. See also Halliburton, Jl. of Physwl. Vol. v. 

 (1883), p. 172. 



1 Hammarsten, Pfliiger's Arch. Bd. xxxvi. (1885), S. 389. 



