  We use space-charge (SC) theory (also called the capillary pore model) to
describe the ionic conductance, $G$, of charged carbon nanotubes (CNTs). Based
on the reversible adsorption of hydroxyl ions to CNT pore walls, we use a
Langmuir isotherm for surface ionization and make calculations as function of
pore size, salt concentration $c$, and pH. Using realistic values for surface
site density and pK, SC theory well describes published experimentally data on
the conductance of CNTs. At extremely low salt concentration, when the electric
potential becomes uniform across the pore, and surface ionization is low, we
derive the scaling $G\sim \sqrt{c}$, while for realistic salt concentrations,
SC theory does not lead to a simple power law for $G(c)$.
