THE PROTEINS 369 



(2) Alkaloidal Reagents in Acid Solution. 



(a) Hydroferrocyanic acid. A few drops of glacial acetic acid are 

 added to a little of the protein solution and then, drop by drop, potas- 

 sium ferrocyanide solution. A voluminous precipitate is formed. 

 This precipitation is less complete in the presence of neutral salts and 

 'does not occur in neutral solutions. 



(&) Picric acid. A yellowish precipitate is formed on adding picric, 

 acid to egg-white solution. 



(c) Potassio-mer curie iodide (Bruckes reagenf]. A whitish pre- 

 cipitate is formed when the protein solution is acidified with dilute 

 hydrochloric acid and a few drops of potassio-mercuric iodide are added. 



(d) Trichlor acetic acid. A white precipitate is formed on adding 

 an equal volume of 10 per cent, trichloracetic acid. 



(e) Tannic acid. A brownish precipitate is formed. 



(f) Bromine water gives a white precipitate. 



(#) Phosphotungstic acid. A white precipitate is produced when 

 phosphotungstic acid is added to the protein solution pre- 

 viously acidified with hydrochloric or sulphuric acid. 



These reagents are the most commonly employed for 

 removing proteins from solution, e.g. in the analysis of blood- 



The tannic acid compound is of commercial importance. 

 Leather is made by tanning skins. 



The hydroferrocyanic acid reaction is often used clinically 

 for detecting protein ("albumin") in the urine. 



ESTIMATION OF PROTEIN. 



Picric acid is used in the estimation of protein by Es- 

 bach's method. 



Some of the solution is poured into the Esbach tube (Fig. 

 54) up to the mark U and then Esbach's reagent up to the 

 mark R. The tube is corked, the contents are mixed by FIG. 54. 

 inverting 2 or 3 times without shaking and allowed to stand for 24 

 hours. The tube is graduated in amounts of protein in grams per 

 litre ; the height of the deposit gives the amount. This method is 

 often employed in estimating " albumin " in urine. 



