Measuring Electrodes Used in Potentiometric Analysis

Electrodes made of different metals have been used for long times for the location of end point in potentiometric titrations. In redox titrations often platinum electrodes, in argen-tometric titration silver electrodes are used. The measuring surface of these electrodes is usually made of the needle, disk, plate, wire, and so on, shaped platinum or silver metal. They are attached into the nonconductive electrode body. 

For pH measurements, different metal-metal oxide electrodes such as oxide film cowered antimony, stainless steel, iridium [11] have also been used. 

Frant and Ross [15] invented an ion-elective fluoride-type electrode. Its measuring membrane was made of europium-doped homogeneous lanthanum fluoride crystal membrane. Measuring fluoride ion concentration is not an easy task on the other hand, the contradiction concerning the beneficial and the toxic physiological effects of fluorine ion intake created intensive debates. Therefore, the electrode that made the simple potential measuring technique applicable for fluoride measurements became very popular in practice. 

the liquid state measuring membrane was changed by an elastic plastic disk. This made the electrode form mechanically more stable. In the early times, well working potassium [16] and ammonium [17] ISEs were made with a silicon rubber matrix. Later, however, plasticized polyvinyl chloride (PVC) matrices became the most popular in ISE fabrication. Their preparation procedure was well worked out and clearly described by Shatkay [18] and Thomas [19], The high molecular weight PVC used in membrane preparation can be dissolved in tetrahydrofurane. In this solution, the plasticizer, the selective ionophore, and other ingredients like special lipophilic salt are dissolved. The solution is transferred into a ring-shaped flat bottomed mold. Care is taken to let the tetrahydro furane (THE) slowly evaporate. In a few days, a circular, flexible membrane is obtained. A disk with appropriate diameter is cut from the membrane and pasted on the tubular electrode body. 

The PVC matrix provides the mechanical stability. It is chemically inert and does not dissolve in water. The ionophores are lipophilic organic molecules that selectively interact with the analyte ions. The plasticizer and the lipophilic salt also play an important role in the ion-selective function. They decrease the electric resistance, increase selectivity, and influence the detection limit. As a plasticizer, bis-(2-ethylhexyl)-sebacate (DOS) or 2-nitrophenyl-octyl-ether (oNPOE) is used, while for lipophilic salt potassium-[tetrakis-4-chlorophenyl]-borate is employed most often. 

---