MUTUAL PRECIPITATION 05 



power. In some cases the globulin is not increased, but carries 

 an increased positive electric charge. This increases its precipi- 

 tating action, (Method given in Part II.) 



Precipitation by Colloids of Opposite Charge. 



Colloids that are present together in the same medium may 

 mutually precipitate one another, either because being of opposite 

 sign they neutralise their charges, or because they sensitise each 

 other to electrolytes. As an example of the former action we may 

 consider the usual method of getting a protein-free filtrate from 

 serum by the addition of a calculated quantity of colloidal iron or 

 of tannic acid. The serum proteins are, as found, slightly alkaline, 

 and carry a small negative charge. This is neutralised by the 

 positive charge on the iron or on the tannin (Part II.). 



A similar method could not be used for whole blood because the 

 haemoglobin at pH 7-4 carries only a feeble negative charge and 

 would very readily take on a + charge. A small quantity of 

 colloidal iron is added which distributes itself over the blood 

 colloids. If one now precipitates the liq. ferri oxidati dialysati by 

 an electrolyte to which it is sensitive, e.g. KgSO^, it will rapidly 

 separate out, carrying with it the blood proteins. This is an 

 example of the second type of mutual precipitation. 



A special instance of this kind is found in the precipitation of 

 electro-positive dyes on filter paper, — colloidal cellulose with a 

 negative charge in water, intensified by the presence in it of 

 electrolytes, especially calcium silicate (Part II.). 



Of great interest in this connection is the reaction of proteins to 

 dyes. In histological technique, various " basic " and " acidic " 

 dyes (see p. 53) are used to obtain a differential staining of various 

 tissues or to indicate cell structure. Gortner has found that at 

 physiological concentrations of hydrogen ions the dye combines 

 chemically with the protein, the dye anions forming a salt with the 

 protein cations, and vice versa. For example, the nuclear material 

 contains a predominant amount of acidic protein and so attracts 

 dye cations. If the cation of the dye is coloured, i.e. if the dye is 

 basic, it will stain the nucleus. On the other hand, at greater 

 acidities {pH 2-5-1) true adsorption takes place with a neutralisa- 

 tion of the electro-kinetic potential on the surfaces of the colloids. 

 That is, at hydrogen ion concentrations not far removed from the 

 isoelectric point of the proteins the amount of dye fixed is deter- 

 mined by the chemical composition of the proteins, and differs, of 

 course, for the various proteins concerned. When the deviation 

 from the neutral point is greater, more dye is taken up by adsorp- 

 tion. This extra amount is independent of the ehcmieal composi- 



