Simulated average ion concentrations and conductance generated in a restricted column, 12 µm long. The column is open at both ends. Diffusion occurs similarly within the column and within 5-µm extensions of the column that extend from the column end, but do not contribute to the monitored conductance. (A) Left: Results for an ion pair with equal anion and cation diffusion coefficients. Initially, the concentration is 2 mM everywhere. As indicated, the concentration was increased to 40 mM at the front of the column for 3 s and then decreased again to 2 mM. Right: Equivalent results for the case that the anion diffusion coefficient was 10-fold less than the cation diffusion coefficient. Results in red are a model in which diffusion of the anion and cation were simulated separately, and results in blue are for a model assuming a diffusion coefficient for the ion pair as given in B. (B) Plot of the rate constants (x axis), determined from single exponential fits of the decaying conductance, against the relative anion diffusion coefficient (y axis), which was varied over 1,000-fold. The predicted coupled diffusion coefficient accurately describes the relationship over the entire three-log unit range.