Outer Helmholz plane

A simple model of the e.d.l. was first suggested by Helmholz in which the charges at the interface were regarded as the two plates constituting a parallel plate capacitor, e.g. a plate of metal with excess electrons (the inner Helmholz plane I.H.P.) and a plate of excess positively charged ions (the outer Helmholz plane O.H.P.) in the solution adjacent to the metal the... 

Fig. 5.3 Structure of interface at equilibrium. IHP is the inner Helmholz plane through the contact adsorbed, nonsolvated anions and OHP is the outer Helmholz plane...

The inner layer (closest to the electrode), known as the inner Helmholz plane (IHP), contains solvent molecules and specifically adsorbed ions (such as Br or T that are not hydrated in aqueous solutions). It is defined by the locus of points for the specifically adsorbed ions. The next layer, the outer Helmholz plane (OHP), reflects the imaginary plane passing through the... 

Fig.n (a) Effect of distance from electrode on ion concentration. Excess concentration of attracted and repelled ions with respect to bulk concentration in the vicinity of an electrode surface calculated from Gouy-Chapman model [34]. Curves correspond to 0.025, 0.050, and 0.100 V potential differences, (b) Outer Helmholz plane (OHP) and inner Helmholz plane (IHP). Anions adsorbed on the surface have lost part of their solvation layer and are contact adsorbed (IHP) cations remain fully solvated and are water-separated adsorbed (OHP). (Reprinted with permission from Ref. 34, Copyright 2003 by John Wiley Sons Ltd). 

---