76 PHYSICAL CHEMISTRY 



but the order is reversed in case of positive emulsoids. The 

 order of effectiveness of cations in precipitating negative emul- 

 soids is Li <Na <K <Rb <Cs, and with positive emulsoids 

 the reverse. 



There is not much difference in the precipitating power of the 

 ions of alkali metals, but the bivalent cations (except Mg) pre- 

 cipitate emulsoids much more effectively than monovalent cat- 

 ions, and trivalent cations more effectively than bivalent. The 

 effectiveness of the heavy metals and sometimes Ca, Sr, Ba and 

 H" in precipitating emulsoids lies partly in the fact that they 

 denature the emulsoids, making them easier to precipitate. Such 

 precipitates are irreversible, whereas the precipitations with neu- 

 tral salts of the alkali series are reversible. 



Neutral salts reduce the osmotic pressure of emulsoid solu- 

 tions, presumably by causing the particles to aggregate and form 

 larger ones, thus reducing the number. The order of effective- 

 ness of the anions is the same as given above. 



Emulsoids are precipitated by some non-electrolytes, such as 

 alcohol (see non-electrolytes). 



Many of the emulsoids are proteids and are amphoteric. These 

 are made electropositive by acid and negative by alkali. There 

 is probably a chemical reaction of the nature: Alb-OH-)-HCl= 

 Alb-Cl-f-HOH, after which the acid albumin dissociates into 

 Alb' and CI'; and Alb-H+KOH=Alb-K+HOH, after which 

 the alkali albuminate dissociates into Alb' and K*. 



Small amounts of acid or alkali increase the osmotic pressure 

 and viscosity, decreasing the surface tension of albumin solu- 

 tions. This is probably due to the fact that the acid and alkali 

 albuminates are more soluble or highly hydrated in the sense 

 that the molecules are not aggregated and hence the "molecular 

 concentration" is increased. The addition of larger amounts of 

 acid increases the effect up to a certain limit, above which the 

 albumin is denatured. 



Since the proteins are amphoteric they dissociate both H and 

 OH ions, and are either positive or negative according to whether 

 they dissociate more OH or more H ions. According to the 

 law of mass action, the greater the concentration of the H ions 

 in the solvent the less H ions will be dissociated, and the greater 

 the concentration of OH ions (or the less the concentration of 



