﻿336 Prof. J. N. Mukherjee on 



hydroxyl ions the surface can adsorb per unit area when it is 

 saturated. 



Since the chemical adsorption of hydrogen ions is assumed to- 

 be absent, on the addition of an acid the negative charge will 

 decrease owing to electrical adsorption till the surface becomes 

 neutral. At this concentration of the acid, the surface has 

 an adsorbed layer of water, and an equal number of hydrogen 

 and hydroxyl ions, An increase in the positive charge 

 cannot be due to electrical adsorption of the univalent 

 hydrogen ions (cp. previous paper). The increase in the 

 charge is due to the neutralization of the hydroxyl ions 

 being formed in the surface by impinging hydrogen ions, as 

 represented in scheme 2 (/>) above. 



The maximum charge E 9rt for an acid will, therefore, be 

 equal to " x, 9i 2 he maximum charge due to acids thus gives 

 a measure of the hydration of the surface. The difference 

 between the maximum charge observed with acid and with 

 alkali gives a measure of the amount of hydroxyl ions that 

 is required to saturate the surface. 



In trie preceding discussion, the chemical and electrical 

 adsorption of the anion of the acid lias been left out of 

 account for the sake of simplicity. If the initial negative 

 charge of the surface in contact with pure water is consider- 

 able the electrical adsorption can be complete only at high 

 concentrations of the acid, i. <?., the surface will be neutral at 

 a high concentration of the acid. The electrical adsorption 

 of the anion, is no longer negligible. A reversal of the charge, 

 though theoretically possible, may not be actually observed 

 owing to the great concentration of the anion. 



The reversal is thus dependent on : — 



(1) a. large value of x, and 



(2) a small value of y. 



A non-reversal is to be expected when the opposite is the 

 case, ?'. <?., 



(1) a small value of " x" and 



(2) a large value of "?/•" 



A regular transition from marked reversal to non-reversal 

 can be observed in Ferrin's work. With cellulose he also 

 does not record a reversal of the charge. It is to be expected 

 from the preceding considerations that non-reversal will not 

 be observed when the concentration of the acid required to 

 render the surface neutral is comparatively high, i. <?., the 

 anion concentration is high. The concentration of the acid 

 in the case of cellulose is the greatest recorded by Perrin. 



