Amphoteric Reactions at a Charged Surface

Derivation of the mathematical relationships uses a generalized exchanger whose surface contains

S-OH groups as functional sites. In aqueous systems these surface groups can be protolysed in two different ways.

In acid media the surface may be protonated according to:

In order to maintain the condition of electroneutrality the charge on the surface has to be balanced by the negative charge of an anion, e.g.:

Therefore, in sufficiently acid media, a sorption of acids takes place similar to the uptake by weakly basic exchange resins.

In alkaline solutions the surface hydroxyl groups can dissociate and hand protons over to the liquid phase:

The negative surface charge has to be balanced by cations, e.g. by sodium ions:

Together the formal equation [3] and [4] represent a cation exchange on an arbitrary cation exchanger.

Each of the above models holds for the uptake of acids or the exchange of protons for other monovalent cations in arbitrary pure anion or cation exchange processes. If the state of equilibrium is considered for an amphoteric exchanger and a liquid phase containing different cations and anions, then the uptake of anions will decrease with increasing pH value until the sorption is completely suppressed. In the same direction the cation exchange of protons for cations increases. The pH regions of almost exclusive cation and anion sorption are separated by a maximum of noncharged surface groups. In this transition region sorption of both cations and anions occurs. At the point of zero charge, therefore, the surface is covered with equal amounts of equivalents of positively and negatively charged counterions.

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Solar Panel Basics

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