Electrochemical properties of Na+- and K+-selective glass microelectrodes

Abstract

Electrochemical properties of Na+-selective glass microelectrodes were studied and compared with those of K+-selective glass microelectrodes. The selectivity of Na+-selective glass microelectrodes depended on the ion concentration of test solutions. With aging, resistance of Na+-selective microelectrodes increased and their selectivity for Na over K decreased. Na+-selective microelectrodes potential measured in NaCl solution remained constant with aging, while the potential measured in KCl solution decreased and became more positive. The changes in resistance and potential of Na+-selective microelectrodes may be due to the effects of the less mobile cation, i.e., H+ or K+ on the Na ion exchange in the Na-sensing region. The results indicate that Na+-selective microelectrodes must be used as soon after filling as possible. The selectivity of Na+-selective microelectrodes increased with increase of the sensitive exposed-tip length, whereas their response time became slow due to a large recessed volume, indicating requirement of an optimum exposed-tip length for intracellular applications. The changes in the properties of Na+-selective glass microelectrodes with aging contrasted with those of K+-selective glass microelectrodes in which resistance decreased and K+-selectivity increased. The K+-selective microelectrodes required aging before use for a high selectivity and low resistance. The K+-selective microelectrodes with low resistance after sufficient aging can be used without insulation to measure K+ and Na+ activities in aqueous solutions. The different properties between Na+- and K+-selective microelectrodes are understandable, because hydration of N+-selective glass is much less extensive than that of K+-selective glass.

Neutral carrier Na+- and Ca2+-selective microelectrodes for intracellular application

“…Selectivity coefficients (kNaK) of neutral carrier Na+selective microelectrodes are comparable to those of recessed-tip Na+-selective glass microelectrodes (Lee, 1979) and the microelectrode potentials are quite stable. The response time of neutral carrier Na+-selective microelectrodes are faster than those of recessed-tip Na+selective glass microelectrodes.…”

“…However, it was postulated that glass hydration may cause a reduction of the resistance through the glass wall (Lewis and Wills, 1980;Armstrong and Garcia-Diaz, 1980;Baumgarten, 1981). The resistances of glass wall of the microelectrodes have not been directly measured during hydration except those of microelectrodes made from Eisenman and co-workers (Eisenman et al, 1957;Eisenman, 1962) NAS274 and NASI, ,8 glass (Lee and Fozzard, 1974;Lee, 1979). However, Eisenman (1969) has demonstrated the existence of different thicknesses of hydrated layers in NAS274 and NAS,1,8 glasses.…”

“…Although NAS274 glass is subject to hydration, it took about one month for about a tenfold decrease in the resistance of the glass microelectrodes with sealed tips. The resistance of the microelectrodes made with NAS, !18 glass increased with aging in electrolyte solutions (Lee, 1979). Therefore hydration of glass depends on its composition, and so hydration of chemically durable glasses such as Corning 7740 may be very slow.…”

“…The tips of ion-selective microelectrodes are chemically and electrically complicated because of the small tip diameter, the possibility of poor silanization, and possible shunt through the glass wall. The selectivity coefficients and resistances of some ion-selective microelectrodes change with aging (Lee, 1979;Lee, 1981). Table II shows slopes (S), selectivity coefficients (kNaK), and resistances (R) of a neutral carrier Na+-selective microelectrode for -120 h aging after construction.…”