44 THE PROTEINS. [CH. II. 



For the following five exercises use serum that has been diluted with 10 

 volumes of distilled water. 



27. Boil 5 cc. in a test-tube that has been previously rinsed 

 with distilled water. The solution becomes opalescent, but usually 

 no definite coagulum is formed. Cool the tube and add i per cent, 

 acetic acid drop by drop. A precipitate of metaprotein is formed 

 soluble in excess of acid. 



NOTE. The reaction of the mixture after boiling is distinctly alkaline to 

 the iso-electric point of the denaturised proteins formed. These are precipi- 

 tated by bringing the reaction of the solution to the iso-electric point by the 

 addition of acetic acid, but are redissolved by an excess. 



28. To 5 cc. add two drops of i per cent, acetic acid and boil. 

 A white flocculent coagulum is formed. Cool the tube and add two 

 or three drops of strong nitric or acetic acid. The coagulum does 

 not dissolve. 



NOTES. i. The amount of acid added is such that, after boiling, the 

 reaction is near to the iso-electric point of the denaturised proteins. These 

 are therefore precipitated and then coagulated. 



2. The addition of the strong acid is to ensure that the precipitate that 

 appears on boiling does not consist of calcium or magnesium phosphate, which 

 is soluble in dilute nitric acid. That such a phosphatic precipitate can be 

 formed on boiling certain solutions is shown by the following experiment. 

 Treat a solution of calcium chloride with sodium phosphate and then with 

 excess of sodium carbonate. A precipitate of Ca 3 (PO 4 ).j appears. Add acetic 

 acid drop by drop till the precipitate just dissolves owing to the formation of 

 the acid phosphate. Boil the solution for half a minute. A white precipitate 

 appears. Add a drop or two of nitric acid. The precipitate dissolves. The 

 appearance of the precipitate of Ca 3 (PO 4 ) 2 on boiling is due to the alteration of 

 reaction as the CO a is evolved. 



29. Treat 5 cc. of the solution with 0-4 per cent, hydrochloric 

 acid, drop by drop, until the precipitate obtained by the first drop 

 or two has redissolved (about five drops are usually necessary). 

 Boil. The solution remains clear. Cool the tube and add 2 per cent, 

 sodium carbonate, drop by drop. A precipitate of metaprotein is 

 formed which redissolves in excess. 



NOTE. The precipitate that first forms consists of globulin (see Ex. 32). 

 On adding enough HCl to redissolve this the reaction is such that it is acid to the 

 iso-electric point of the denaturised proteins. These are precipitated by an 

 alkali and redissolve in an excess. 



30. To 5 cc. add two drops of 2 per cent, sodium carbonate 

 and boil. The solution remains quite clear. Cool the tube and add 

 i per cent, acetic acid, drop by drop. A precipitate of metaprotein 

 is formed, soluble in excess. 



