Figures 3 & 4.  Accumulation of lipophilic ions is used to measure DY.  If DY is interior negative, accumulation of lipophilic cations [tetraphenylphosphonium (TPP⁺), tetraphenylarsonium (TPA⁺) or methyltriphenylphosphonium (TPMP⁺)] is measured.  If DY is interior positive, accumulation of lipophilic anions [tetraphenylborate (TPB^-) or thiocyanate (SCN^-)] is measured.  The basic principle is that although charged species are generally membrane impermeant, this class of compounds is able to delocalize charge and also sufficiently hydrophobic to permeate the membrane rapidly, thereby allowing equilibration with DY.  Lipophilic ions are synthesized in radioactive form and used in micromolar concentrations, since at high concentrations, DY collapses.  Electrodes specific for TPP⁺ have also been developed in order to measure the free concentration of the lipophilic cation in the medium.  In addition, fluorescent lipophilic ions are available that quench when they are concentrated internally.