Conductance measurements of diffusion in gelatin. (A) Borosilicate glass pipettes with cylindrical orifices were formed on a glass flame. Tips were filled over ∼6 mm with warm 10 g% gelatin solution and backfilled with low ionic strength solution. (A) 150–250-μm-thick gelatin coat adhered to the tip. Sinusoidal voltage oscillations were applied to monitor conductance of the tip. The pipette tip was placed directly in front of square pipette tips used to change solutions flowing directly across the orifice by moving the microscope stage. (B) Typical conductance records for addition and removal of 30 mM NaCl. Diffusion coefficients were estimated via simulations (green) with approximated inner tip dimensions (white lines in A). Optimal diffusion coefficients for Na and Cl, adjusted in a ratio of 1.5:1, were 33% less than those for free water (Robinson and Stokes, 1965). Reasonable estimates of coupled diffusion coefficients can also be made from exponential fits of the decaying conductance curves, assuming a median diffusion distance of 0.35 mm (see red line in left record).