PRECIPITATION REACTIONS. 97 



fluids like the blood-serum it is electro-negatively charged, which fact 

 can be explained by the presence of an excess of OH ions. 



The determination of the molecular weight of the proteids has been 

 attempted by various methods which are more or less uncertain. 1 There 

 is no doubt that the molecular weight of the proteids is very high, but 

 the statements about the size vary very considerably. For the true 

 proteids thus far investigated, values ranging from 4000 6000 10,000 

 have been found. 



The general reactions for the proteids are very numerous, but only 

 the most important will be given here. To facilitate the study of these, 

 they have been divided into the two following groups. It must be 

 remarked that the precipitation reactions are not only applicable for 

 the soluble true proteids but also, more or less, for other soluble proteins 

 in general. The color reactions are applicable to all soluble or insoluble 

 proteins with few exceptions, which will be mentioned later. 



Precipitation Reactions of the Proteid Bodies. 



1. Coagulation Test. An alkaline proteid solution does not coagulate 

 on boiling, and a neutral solution only partly and incompletely; the reac- 

 tion must therefore be acid for coagulation. The neutral liquid is first 

 boiled and then the proper amount of acid added carefully. A flocculent 

 precipitate is formed, and with proper technique the filtrate should be 

 water-clear. If dilute acetic acid be used for this test, the liquid must first 

 be boiled and then 1, 2, or 3 drops of acid added to each 10-15 cc., depend- 

 ing on the amount of proteid present, and boiled before the addition of 

 each drop. If dilute nitric acid (25 per cent) be used, then to 10-15 cc. of 

 the previously boiled liquid 15-20 drops of the acid must be added. If 

 too little nitric acid be added, a soluble combination of the acid and pro- 

 teid is formed, which is precipitated by more acid. A proteid solution 

 containing a small amount of salts must first be treated with about 1 

 per cent NaCl, since the heating test may fail, especially on using acetic 

 acid, in the presence of only a slight amount of proteid. 



2. Precipitation by Alcohol. The solution must not be alkaline, 

 but must be either neutral or faintly acid. It must, at the same time, 

 contain sufficient quantity of neutral salts. 



3. Neutral Salts, such as Na2SC>4 or NaCl, when idded to saturation 

 precipitate certain proteids but not others. Ammonium sulphate when 

 dissolved to saturation in the liquid is considered as the general pre- 

 cipitant for proteids. In the presence of free acetic or hydrochloric 

 acid the above-mentioned salts, NaCl or Na2S0 4 , in sufficient concen- 

 tration, are also geneial precipitants for the proteids. 



1 See especially F. N. Schulz, Die Grosse des Eiweissmoleciile, Jena, 1903. 



