266 HYDROGEN ION CONCENTRATION 



charge on a solid wall. And yet in higher concentrations of univalent 

 ions this effect is markedly present. This is shown in the following 

 experiment by Perrin. The figures give the electroendosmosis 

 velocity together with the sign of the charge. 



Carborundum 1/500 A^ KOH -105 



Carborundum 1/500 A^ KOH + 0.1 A^ NaBr -24 



CrCU 1/150 N HCl +100 



CrCla 1/150 A^ HCl + 0.1 A^ KBr +35 



In describing adsorption it was stated (page 237) that the adsorp- 

 tion of HCl or of NaOH is augmented by the addition of a neutral 

 salt. This was explained in the sense that the forces opposing the 

 isolated adsorption of the H- (or 0H-) ions were thereby diminished. 

 Another explanation of the same phenomenon is suggested, as can 

 be seen from these experiments, in the diminution by the presence of 

 the neutral salt of the potential which develops in the adsorption of 

 HCl or KOH. An analytic formuhzation of the regularity of this 

 phenomenon has not yet been attempted. 



According to the above the decrease of potential by univalent ions 

 could be explained without regard for their adsorbability. But they 

 doubtlessly do exhibit an adsorbability, be it ever so slight, and they 

 do enter into competition to a small extent with the H- and OH-ions 

 for adsorption on boundary surfaces. 



This competing effect is more striking in the case of the much more 

 strongly adsorbable polyvalent ions, as shown again by the following 

 of Perrin's figures : 



(a) AI2O3 1/1000 A^ HNO3 +100 



AI2O3 1/1000 A^ HCl +100 



AI2O3 1/1000 A^ H2SO4 +15 



(b) CrCl3 1/1000 N HNO3 +88 



CrCU 1/1000 A^ HNO3 + 1/1000 A^ MgS04 +23 



CrCU 1/1000 A^ HNO3 + 1/100 AT CdS04 +4 



(c) CrCU Very dilute HCl +75 



CrCls KH2PO4 of the same pH +7 



(d) CrCU in faintly alkaline water —46 



CrCla Same + 1/1000 M K3Fe(CN)6 -46 



