Barium Ion-Selective Electrode Based on a Neutral Carrier Complex SIR: Correspondence with W. Simon ( I ) indicates that while our experimental data are in excellent agreement, the selectivity ratios for monovalent cations in Table I1 of my recent paper (2) are in error. The error stemmed from an improper reduction of the Eisenman equation for determining the selectivity ratios of ions of mixed valence. The general Eisenman equation and the reduced forms as derived by Simon are :
Table I. Approximate Selectivity Ratios for Various Cations Ion
KB&X
Ba2+ Ca2+ MgZ+
Ni2+ co2+ Zn2+
Fez+ K+ "4'
= =
Na+
activity of ion to be determined of charge n activity of interfering ion of charge z
EMF of cell assembly with activity a of ion to be determined (mV) EMF of cell assembly with activity a of interfering ion (mV> e.g., at a = 0.1 and Ba*+/Na+:
e.g., at a =
1.0 10-3 10-4 10-4 10-4 lo-' 10-4 10-4 10-3 10-4 10-4 10-4 10-4
x x
