THE PROTEINS 105 



B. METALLIC SALTS 



The following metallic salts form double insoluble compounds with 

 proteins, and therefore cause a double precipitation when added to 

 solutions of these bodies : ferric chloride, copper sulphate, mercuric 

 chloride, lead acetate, zinc acetate. 



C. ALKALOIDAL REACTIONS 



Proteins, like the polypeptides and the amino-acids of which they 

 are composed, may function either as weak acids or as weak bases, 

 according as they are treated with bases or acid radicals respectively. 

 In the presence of strong acids, therefore, proteins act like organic 

 bases, and are thrown down in an insoluble form by the various alka- 

 loidal precipitants. With certain proteins, such as the protamines, 

 where there is a preponderance of basic groups, it is not necessary to 

 add mineral acid in order to ensure the precipitation. The following 

 are the principal alkaloidal precipitants which may be employed : 



(a) Phosphotungstic acid. 



(b) Phosphornolybdic acid. 



(c) Tannic acid. 



(d) Potassium mercuric iodide. 



(e) Acetic acid and potassium ferrocyanide. 



(/) Trichloracetic acid. (In order to precipitate all the coagulable 

 proteins from a solution it is treated with an equal volume 

 of 10 per cent, trichloracetic acid, well shaken and filtered.) 

 (g) Metaphosphoric acid. 

 (h) Salicyl-sulphonic acid. 



These two latter are generally employed in a 5 per cent. 



solution. 

 (/) Picric acid. 



A mixture of picric and citric acids is largely employed, 

 under the name of Esbach's reagent, as a precipitant for 

 coagulable proteins in the urine. 



D. TESTS DEPENDING ON THE COLLOIDAL CHARACTER 



OF THE PROTEIN 



(1) HEAT COAGULATION. On boiling proteins in a very 

 slightly acid solution some are coagulated and form an insoluble white 

 precipitate. This test is applicable to albumins, globulins, and 

 under certain conditions to the derived albumins. In order that 

 the separation of protein in this way may be complete it is necessary 

 to provide for the presence of neutral salts and also for the maintenance 

 of a slight acidity. The best method of carrying out this test, therefore, 



