CALCULATIONS OF BIOELECTRIC POTENTIALS : I. EFFECTS OF KCl AND NaCl ON NITELLA

The P.D. of 0.01 M vs. 0.001 M KCl (concentration effect) in Nitella is 54.7 mv. and that of NaCl is 20.9 mv. The P.D. of 0.01 M NaCl vs. 0.01 M KCl is 82.9 mv. If we assume that diffusion potentials predominate we may calculate the relative mobilities of the ions in the outer layer of the protoplasm. If we put v (Cl) = 1 we obtain u (K) = 85.45 and u (Na) = 2.18. (These values depend upon the fact that the mean ionic activities in this layer must be equal at equilibrium to the mean ionic activities in the external solution.) Using these values the calculated P.D. of 0.01 M NaCl vs. 0.005 M NaCl + 0.005 M KCl is 66 mv. (the observed value is 68.8 mv.) and the conductance of KCl in the outer layer of the protoplasm is 27 times as great as that of NaCl (the observed conductance for the whole protoplasm is 25 to 50 times greater for KCl than for NaCl). Calculations on the basis of phase boundary potentials are less satisfactory.