Long-term stability of ion-selective valinomycin membranes.
The response characteristics of freshly prepared membranes containing valinomycin are compared to those of membranes of identical composition prepared 2 and 10 years earlier. Independent melting point and infrared spectroscopy data on valinomycin of different ages are presented to substantiate its long-term stability. The separate-solution method was employed to establish selectivity coefficients for Rb(+), Cs(+), Li(+), H(+), Na(+) and NH(4)(+) relative to K(+) for freshly prepared and aged membranes. The maximum values for Na(+) ranged from 10(-4.69) for freshly prepared membranes to 10(-4.27) for 10-year old membranes. The slope of the emf vs. potassium ion activity over the range 10(-4)-10(-1)m was 58.5 mV/decade for freshly prepared membranes and 57.7 mV/decade for aged membranes. The time needed to reach 99% of the final emf was less than 1 min for freshly prepared and 2-year old membranes but 3-6 min for 10-year old membranes. Possible measures to increase the shelf life of these and similar membranes are suggested.
Griffin
J J
JJ
D.M.T., Inc., 34 Tower St., Hudson, MA 01749, U.S.A.
Christian
G D
GD