International Silver

For a detailed study, see [421-424], In general, all-solid-state ISEs are used with an internal silver contact. Selectrodes (see p. 60) can also be prepared from a mixture of halide and silver sulphide [328,329]. 

The reference electrode is an internal silver-silver chloride type. It was based on a design of A. K. Agrawal, et al. (H). For supercritical water studies, the electrode body was made from 902 precision machinable ceramic (Cotronics Corporation Brooklyn, NY). 

An ISE usually is constructed with an internal silver silver chloride reference electrode. This means that the internal reference solution must contain the Cl ion together with the ion for which the membrane is selective, for example, M. Thus, a typical format for the ISE is... 

Connection to internal silver - silver chloride reference electrode... 

Figure 23-4, which is a schematic representation of the cell in Figure 23-3a, shows that this cell contains two reference electrodes (I) the external silver-silver chloride electrode (ref I) and (2) the internal silver-silver chloride electrode (ref 2). Although the internal reference electrode is a part of the glass electrode, it is noi ihc pH-sensing element. Instead, ii is the thin glass membrane at the lip of the electrode that responds to pH. 

The overall potential of the glass electrode is a contribution of several potentials (i) the potential of the internal silver/silver chloride electrode (ii) the potential developed at the inner glass surface (iti) the poterrtlal developed due to imperfections in the glass surface and (iv) the potential developed at the outer glass surface. However, potentials (i), (ii), and (Hi) are all constant... 

In the wet conditions of the developer solution however the trap sites on the surface became more effective. This meant that when exposed during development, the unexposed crystals, instead of producing internal silver, produced surface silver at sites previously uncompetitive with the internal sites. On the other hand, crystals which already had internal silver produced from the image exposure, continued to trap photoelectrons internally because the accumulation of silver at the trap sites made them more effective traps than those on the surface despite the change in exposure conditions. They therefore did not develop. A direct-positive system was thus discovered [36]. 

Fig. 2.17 The ceiling luminaire with 60 W projector lamp. The clear glass neck of the lamp gets hotter than the curved section below it which is internally silvered.

Inspection of Fig. 5.2 shows that inside the micropipette is an aqueous solution of potassium chloride which bathes a silver/silver chloride reference electrode. Accordingly when the micropipette is placed in an external solution containing an unknown level of ions and a second reference electrode is located in this external solution, a membrane potential is developed across the liquid (diphenylether) membrane. The potential difference measured between the internal silver/silver chloride electrode and the external reference electrode will be,... 

The movement of process ions into the reference electrode will also plug its junction and eventually contaminate its internal electrolyte. To reduce the rate of contamination at the internal silver-silver chloride electrode element and the shift of potential E4, smaller reference junctions, second internal junction.s, gel electrolytes, porous solid internals, and flowing junctions are used. Smaller junctions increase the rate of fractional plugging of the junction and therefore reduce the time to a totally plugged junction. Double junctions, gel electrolytes, and porous solid internals increase the time required for the reference junction potential to reach equilibrium after inserted into a process stream or for a dramatic change... 

In 1976, TBI patented a solid-state reference electrode shown in Figure 4-3b that is nearly impervious to process contamination. The reference consists of four wooden dowels saturated with potassium chloride solution. Process material must work its way up into the innermost wooden dowel, where the internal silver-silver chloride element resides to create a drastic offset in the pH reading. Teflon is substituted for the... 

---