Concentration–conductance relations of ionic solutes employed in this study. Conductivity of solutions aspirated into 15-cm-long Tygon tubes with an inner diameter of 0.5 mm. (A) Linear plot of the KCl concentration–conductance relation over three log units. The relation is fit to a modified Kohlrausch equation that includes a constant giving the concentration at which the molar conductivity is reduced by one-half (K₅₀). The measurements give a K₅₀ of 760 mM for KCl. (B) NaCl concentration–conductance relation determined using a patch pipette as in supplemental data. The K₅₀ is 312 mM. (C) Concentration–conductance relation and concentration–osmolarity relation for Na(Mg)ATP solutions, prepared by dilution of a 120 mM MgATP solution, titrated to pH 7.0 with NaOH. While osmolarity is directly proportional to concentration, the concentration–conductance relation is distinctly non-linear and has a K₅₀ of 129 mM. Presumably, therefore, this non-linearity reflects electrostatic interactions that reduce conductivity with increasing concentration, as described by Kohlrausch (Walter and DeWane, 1970). (D) Concentration-conductance relation of K(Mg)ATP solutions, prepared similarly to Na(Mg)ATP solutions. The K₅₀ is 135 mM.