No sacrifice in response or efficiency. Choose a Sargent combination pH electrode by size alone.
Both of these Sargent combination pH electrodes cover the entire pH range from 0 to 14 pH. Both offer fast, linear responseeven in highly acid solutions. Response to alkali ions is insignificant below pH 12.5 (at higher pH, response is low and stable). Both electrodes achieve high efficiency, with a millivcit/pH response close to the Nernst coefficient. And you get this kind of performance over a range of 0 to 80°C. These electrodes also feature all-glass, shock-resistant construction. No rubber or plastic to contaminate samples. No pockets to trap solutions or hamper cleaning. Stainless-steel caps provide sure, strong clamping. What's the difference, then, between these two combination pH electrodes? Size alone. For samples as small as Vz ml or those in extremely confined volumes, choose the miniature electrode (priced at $42.00). Otherwise, your best choice is the standardsize ($40.00). Both come with connecting cable and plugs to fit all popular pH meters. Call your Sargent man or write to us for a closer look at these combination pH electrodes or for data on our complete line of glass and reference electrodes.
INSTRUMENTATION
sitive glass electrode. The enzyme urease is fixed in a layer of aerylamide gel held in place around the glass electrode bulb by porous nylon netting or thin cellophane film. The urease acts specifically upon urea in the sample solution to yield ammonium ions which diffuse through the gel to give rise to a potential at the glass electrode proportional to the original urea concentration in the sample. The electrode could be used continuously for three weeks without loss of activity and, since the catalytic efficiency of enzymes is very high, the overall system is essentially a nondestructive, urea-specific sensor. The exciting possibilities of this approach must be obvious. There are literally thousands of enzymes with high activity and selectivity. Many of these yield products measurable with existing ion-selective electrodes; thus, it should be feasible to prepare electrodes of the immobilized enzyme type with selectivity for a variety of organic and biological substances such as glutamine, asparagine, and amino acids (15). Antibiotic Electrodes
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112 A • ANALYTICAL CHEMISTRY
Some recent studies on biological membranes are directly applicable to the development of ion-selective electrodes as analytical sensors. Indeed, there are formal similarities between ion electrodes and biological membrane systems which virtually ensure that progress in one of these areas will also advance the other. Recently, it has been shown (16) that certain antibiotics display notable selectivity in their interaction with alkali metal cations. Nonactin and valinomycin, for example, preferentially associate with K + rather than Na + . Since the construction of a successful potassium-selective electrode by conventional means has so far eluded workers in the field, this observation has important practical implications and led Simon (17) to the construction of workable antibiotic-based membrane electrodes. The antibiotics nonactin and valinomycin were suspended in solvents consisting of Nujol/2-octanol and diphenylether, respectively, and incorporated into liquid membrane electrodes similar to the conventional versions described above. The potentiometric selectivity constants for the resulting electrodes are given in Tables I and II and show most remarkable electrode characteristics. The valinomycin electrode, for example, displays a selectivity for K+ over Na+ of 3800:1. This
