﻿328 Prof. J. N. Mukherjee on 



The general case, however, is that : 



(a) At low concentrations the density of the charge on 

 the surface at first increases to a maximum and at higher 

 concentrations falls gradually towards a null value when the 

 oppositely charged ions are monovalent. 



(b) On the other hand, when the oppositely charged ions 

 are multivalent or complex organic ions the charge passes 

 through a null value, becomes reversed in sign, and ugain 

 reaches a second maximum, after which it falls slowly 

 (Ellis, Z. Phys. Chem. lxxviii. p. &21 (1911) ; lxxx. p. 597 

 (1912) ; lxxxix. p. 115 (1914) ; Powis, Z. Phys. Chem. 

 lxxxix., pp. 91, 179 (1914) ; Piety, Compt. Rend. cliv. 

 pp. 1411, 1215 (1912); clvi. p. 1368 (1913); Young and 

 Neal, J. Phys. Chem. xxi. p. 1 (1917); Krnyt, VersL Ron. 

 Akad. v. Wetensch. Amsterdam, 27th Juin, 1914, also Koll- 

 Zeitsch. xxii. p. 81 (1918)). 



The usual explanation is as follows: — 

 The adsorption-isotherms for the two ions can be written 

 as 



^? =«_. A and ^ =«..«"*, ... (3) 



m K m A 



where the subscripts A and K refer to the anion and the 

 cation respectively. To explain the increase in the charge 

 at low concentrations it has to be assumed that 



a A >a K and fi A </3 K . .... (4) 



Thus in a paper read at the Discussion on Colloids arranged 

 by the Faraday and the Physical Societies of London, 

 Svedberg remarks : " Now as a rule, it happens that for the 

 two ions of a salt both a and /3 have different values, e.g. 



a (cation) < a (anion) 



8 (cation) >/3 (anion)." 



It is clear that the equation of the adsorption-isotherm can 

 be reconciled with the first increase in the charge. But two 

 objections can be raised against this empirical point of view. 

 In the first place, no reason is given why the constants a and 

 /3 shall have generally the relative values assumed above for 

 the cation and the anion. Secondly, these assumptions can- 

 not explain the second maximum charge and the subsequent 

 decrease observed with multivalent ions of opposite charge. 

 It will now be necessary to assume that 



« A <« K and /3a>/3k, .... (5) 



in direct contradiction to the assumptions already made 



