PHYSIOLOGY OF FUNGI 35 



when dissolved are nonconductors in the absence of moisture, we must postuUite some change 

 in the solute. Without going any further into the proofs for electrolytic dissociation, we 

 will state here very simply the long accepted fact that if a substance when dissolved in water 

 is found to be a conductor of electric current, it dissociates from the uncharged form into 

 ions capable of carrying current and yet balancing one another in electric charges. The 

 graphic representation of this fact may be made as follows: XY :^ X+ + Y". Arrows are 

 used to indicate that this is an equilibrium, not a completed reaction, that must adjust itself 

 to whatever other equilibria other solutes may necessitate. 



Let us take, now, the hypothetical acid HA. It dissolves in water and dissociates accord- 

 ing to the following representation: 



HA ^ H^ + A- 



At the instant of solution we may visualize the undissociated acid breaking up into H* and A- 

 ions and the dissociated ions recombining to form HA at such rates that the equilibrium for 

 the given acid will be established. Thereafter the rates of dissociation and reassociation must 

 be equal. If, now, we add to the solution, some salt BA, then the concentration of the 

 A" ions will be materially increased. If by any chance we have doubled the concentration 

 of A" we have doubled the possibility of collisions between H* and A" and doubled tlie velocity 

 of the reaction from right to left. If now, we add another acid so that the concentration 

 of H* will be doubled, we have, obviously, quadrupled the possibility of collision. The 

 velocity, then, in a given direction is proportional to the products of the concentrations of 

 the reacting groups. Eepresenting concentrations by square brackets, we may express this 

 as follows: 



Velocity from right to left kj [H+] [A~] where k^ is the proportionality factor. 



At the same time, however, we are having HA redissociating at a rate proportional, 

 more or less, to the concentration. 



Velocity from left to right k^ [HA]. Since, as we have already stated, equilibrium is 

 established, the velocity in one direction must be equal to the velocity in the other. Equating 

 these two expressions, we get: 



k, [H^] [A-] = k, [HA] 

 or 



[Hi [A-] ^^^;g- 



[HA] k, 



which will be a determinable quantity, characteristic of the solute for which it is determined 

 and referred to as equilibrium or ionizing or dissociation constant. 

 Similarly, a hypothetical base BOH, dissociating as follows: 



BOH ^ B* + OH- 



will have a characteristic ionization constant expressed by the equation: 



Kb = [B-] [0H-] 

 [BOH] 

 And the salt BA: 



BA ^ B^A- 



Kba = [Bi [A-] 

 [BA] 



Assume, now, that we mix equal quantities of two solutions, one of the acid and one 

 of the base, in equivalent concentrations. We will have then in solution the four ions: 

 H+, A", B*, OH". These will adjust themselves so that the constants, Ka, Kb, Kba, 'wiU be 

 satisfied and, in addition, another constant which will express the equilibrium between the 

 H+ and OH" ions as follows : 



^ _ [Hi [0H-] 



