The interface features the following species nodes and are added at the Electrolyte node:

The Ampholyte, Weak Acid, Weak Base, and Carbonic Acid nodes are dissociation species and can define an arbitrary number of dissociation steps. Each dissociation step is defined by its pKa (the acid equilibrium constant) parameter. For the weak bases the pKa refers to the acid constant of the conjugate acid. Each dissociation step adds one additional subspecies concentration variable so that the concentration dependent variable represents the sum of all subspecies, and initial and boundary conditions are defined with respect to this total concentration.

All species node have a setting for the Species name, which needs to be unique. The species name is used for naming of all related variables of the species. For species nodes not defining any subspecies, the concentration variables are named as <name>.c_<species_name> where <name> is the name of the interface (default is el), set on the interface top node, and <species_name> is the Species name. For dissociation species nodes defining multiple subspecies, the concentration nodes are named as <name>.c<X>_<species_name> where <X> is the integer from 1 up to the total number of subspecies. Note that the Electrolyte node automatically defines the concentration variables for protons (<name>.cH) and hydroxide ions (<name>.cOH).

All species except the Uncharged Species carry charge and contribute to the total electrolyte current which is used in the equation for solving the electrolyte potential.

The Immobile Species checkbox can be used to lock the concentration of a species, to, for instance, define the immobile charges in an ion-selective membrane or a gel. When the checkbox is enabled the concentration of the species is not added as a dependent variable to the model; instead the concentration will be set to the value provided in the Concentration field. The contribution to the electrolyte current for immobile species is zero.