Microelectrodes metal-filled glass

The use of antimony microelectrodes which are metal/metal oxide electrodes, is shown in Fig. la. They consist of antimony-filled glass capillaries, which are drawn out in order to obtain a fine bevelled tip, which is pH-sensitive and can be introduced into the tubular lumen. These electrodes have also been used to measure titratable acidity and ammonia in an in vitro" system, according to Solomon et al (16) and Karlmark (6). In this paper we will restrict ourselves to the discussion of some aspects of tubular acidification which can be studied due to the rapid response of the antimony electrode system to pH changes, permitting the kinetic study of the tubular acidification mechanisms. 

One method for producing a well-insulated microelectrode is to start with a glass capillary tube and bond it to an internal metallic wire. Several techniques exist. A low-melting-point metal can be used to fill the tube, or a wire of the same diameter as the internal diameter of the tube can be passed through the glass capillary, which is then heated to produce bonding. One must be careful to select glasses and metals which have nearly the same temperature coefficients of expansion. Three basic techniques exist as described below. 

In connection with associated electronic circuitry, fluid-filled microelectrodes behave as low-pass filters, while metal microelectrodes act as high-pass filters (Gesteland et a/., 1959). The reasons for this are simply that glass electrodes exhibit high shunt capacitance and series resistance (Figure 4.10) while electrode polarization impedance associated with metal microelectrodes produces a frequency-dependent RC combination whose series impedance decreases as frequency increases. 

The electrical connection from a glass microelectrode to its accompanying electronics is made by a metal wire inserted in the stem end of the lumen and in contact with the filling electrolyte. Early literature frequently refers to tungsten wire for this purpose, but there seems to be no valid basis for using this metal. Apparently it was available and had been used in place of antimony in certain pH electrodes. Because of its metallurgical properties, tungsten is difficult to form, and it has undesirable electrical characteristics. 

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