46 PHYSICAL CHEMISTRY IN BIOLOGY. 



another time the same cation with different anions, PAULI has arranged the cations 

 and anions in the following order in increasing precipitation ability : 



CNS < K Br < NO 3 < Cl< OCO.CH 3 < HPO 4 < SO 4 

 NH 4 <K<Na<Li. 



The protein used in these experiments was white of egg. According to PAULI 

 certain ions have a precipitating action and others a solvent action. The action 

 of a salt corresponds to the algebraic sum of the action of the ions. 1 PAULI has 

 attempted to associate the precipitation ability of the salts in relation to their 

 action upon the coagulation temperature, but without any positive results. 2 



Nevertheless SPIRO 3 has shown that the kind of protein as well as its con- 

 centration are of importance for the precipitation action, and HOBER* has recently 

 shown that the series KBr<Cl<S0 4 and Li<Na<K<Rb<Cs is valid in 

 alkaline reaction, but that the series is reversed in acid reaction. In nearly 

 neutral reaction irregularities in the ion series occur which can be considered as 

 a transition series between the two just-mentioned series. That the reaction must 

 be of great importance in the precipitation of proteins seems very probable in 

 consideration of the fact that the proteins take a decided electric charge on the 

 addition of acid or alkali (see page 42) . 5 According to PAULI 6 dialyzed protein, 

 which had no electric charge, could not be precipitated by the addition of salts 

 of Zn, Cu, Hg, Fe, Pb, while the same protein in non-dialyzed form gave heavy 

 precipitates with these. PAULI believes that the native protein of the organism 

 on account of the OH ions originating from the tissue fluids has a negative charge. 

 In regard to the precipitation by salts of the heavy metals, the hydrophile col- 

 loids do not seem to differ essentially from the suspension colloids. 7 



Theories of Precipitation Phenomenon. 



At least for the suspension colloids there is no question that they 

 are flocked out by ions which carry an electric charge opposite to the 

 colloid particles, and also by other colloids having an opposite charge. 

 This fact follows from HARDY'S theory, according to which the flocking 

 out is a neutralization process in which the charge of the colloid is 

 just neutralized and the colloid therefore precipitates. 8 The mixture 

 formed on precipitation has been shown to be electrically neutral (iso- 

 electric) as the precipitated particles show no cataphoresis. In this 

 manner it is easily understood that polyvalent ions have a stronger 

 precipitating action than monovalent, as the electrical charge in, for 

 example, a trivalent ion is 3 times greater than in a monovalent ion. 

 Otherwise greater precipitation ability of polyvalent ions can also be 

 explained by a greater hydrolytic cleavage of the salts (page 44). 



The mechanism of the precipitation of the isoelectric solution accepted 

 in HARDY'S theory is explained by BREDIG 9 as follows : At the boundary 

 between suspended particles and solvent a certain surface tension exists 



1 Holfmeister's Beitrage, 3, 225, 1902. 8 Ibid., 7, 541, 1906. 



3 Pfliiger's Arch., 78, 315, 1899. 7 Ibid., 6, 233, 1905. 



3 Hofmeister's Beitrage, 4, 300, 1903. 8 Zeitschr. f. physik. Chem., 33, 385, 1900. 



4 Ibid., 11, 35, 1908. 9 Anorganische Fermente, 1901, 15. 



5 Ibid., 7, 531, 1906. 



