Specific Electrodes

In this chapter we examine the general problem of determining the amount of a specific ion in solution. Direct potentiometric measurement of ion activity is based upon the cell 

Most ion-specific electrodes operate on a potentiometric principle rather than an amperiometric or polarographic principle that is, a change in potential, rather than a change in current, is sensed. 

The number and variety of ion-specific electrodes is rapidly increasing with no end in sight. At the present writing, it is possible to use such electrodes to determine, either by direct or indirect measurement, ionic concentrations of the following species ammonia, bromide, cadmium, calcium, chloride, cupric, cyanide, fluoride, fluoroborate, iodide, lead, nitrate, perchlorate, potassium, sulfide, sodium, sulfur dioxide, and thiocyanate, all by direct measurement, and by titration methods aluminum, boron, chromium, cobalt, magnesium, mercury, nickel, phosphate, silver, sulfate, and zinc. 

Historically the glass pH electrode was the first of the ion-specific membrane electrodes. In terms of the cell referred to above, the glass pH electrode is described by 

In biological studies, we are normally interested in cationic determinations. The membrane-electrode potentials which are observed are described by the Nernst relation 

Although pH determination is the most often used and well-established potentiometric method, electrodes capable of selectively responding to 

Material Detected Concentration Range (Af) Interfering Compounds and Ions 

Measured Species Diffusing Species Equilibria in Electrolyte Sensing Electrode 

LUCITE HOLDER EPOXY PLUG SILVER ANODE 

The circuit is completed by the following reaction occurring at the silver anode  

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M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

Ion-specific electrodes can be used for the quantitative determination of perchlorates in the parts per million (ppm) range (109) (see Electro ANALYTICAL techniques). This method is linear over small ranges of concentration, and is best appHed in analyzing solutions where interferences from other ionic species do not occur. 

There are two types of fluoride lon-selective electrodes available [27] Onon model 96-09-00, a combination fluoride electrode, and model 94-09-00, which requires a reference electrode The author prefers to use Onon model 94-09-00 because it has a longer operational life and is less expensive When an electrode fails, the reference electrode is usually less expensive to replace The Fisher Accumet pH meter, model 825 MP, automatically computes and corrects the electrode slope It gives a direct reading for pH, electrode potential, and concentra tion in parts per million The fluoride lon-specific electrode can be used for direct measurement [2S, 29] or for potenPometric titration with Th" or nitrate solutions, with the electrode as an end point indicator... 

ELECTRODEjcls Fig. 4.24 The operation of an ion-specific electrode with a slope of 59.16 mV per decade for mono-valent ions (29.58 mV/dec for di-valent ions) is simulated under the assumption that a digital volt meter with a resolution of, say, 0.1 mV is used. The sample volume and the concentration of the metered titration solution are known. Normally, one would add a few milliliters of the concentrated titration solution and do the calculation spelled out in lines 140-150 in Table 4.22 here, because the sample concentration is known, the result can be normalized to it. The operation of short-cuts (volume correction), unknowns (volume bias, deviation of true slope from theoretical), and equipment shortcomings (digitization) can be studied. 

Frant, M. S. and Ross, J. W., Jr. Potassium ion specific electrode with high selectivity for potassiim over sodiim. Science (1970), 167> 987 - 988. 

Such a result may be put in parallel with the determination of the bound cations obtained by ion-specific electrode or dual-wavelength spectrophotometric method, and analysed in terms of cooperativity [7,29,31]. 

In the Antek Fluoride Analyzer, a pyrolysis furnace is combined with an ion-specific electrode cell (ISE). Table 8.9 compares this specific analyser to a conventional combustion bomb. 

Further examples of diffusion processes characterized by boundary conditions connected with specific electrode processes will be considered in Section 5.4. 

The development of lithium-specific electrodes has assisted greatly in monitoring patient compliance. The toxicity profile of lithium carbonate is now well established and the drug is safely administered and well tolerated. It is of limited use in other psychiatric disorders such as pathological aggression, although additional benefit may also include a reduction in actual or attempted suicide. 

Figure 6. Plot of the specific electrode pore volume vs. the specific pore volume measuredfor the corresponding carbon powders.

M. Perry, E. Lobel, and R. Bloch, Mechanism of a polymeric valinomycin-based potassium specific electrode. J. Membr. Sci. 1, 223-235 (1976). 

Another approach that has been used to extract rate constants from ultrasonic relaxation data involves using independently determined equilibrium constants to determine concentrations of each species at equilibrium163 or, for special cases where molecule-specific electrodes exist, direct determination of equilibrium concentrations.167,168... 

In soil analysis, pH, specific ion, oxidation-reduction (redox), electrical conductivity (EC) cells, and oxygen electrodes are commonly used. For each of these measurements, a different specific electrode along with a separate or integral reference electrode is needed. In some cases, with extended use or long exposure to soil or soil-water suspensions, electrodes may become polarized. When this happens, erroneous results will be obtained and depolarization will need to be carried out using the electrode manufacturers directions [3],... 

ISEs were originally and sometimes still are called ion-specific electrodes, and this term may still be encountered. 

Because of the complex nature of soil and soil solutions, it is rarely possible to directly determine specific soil constituents by titrating soil or soil solutions using a pH meter, ion-specific electrode, or a platinum electrode (with appropriate reference electrode) [3],... 

An appropriate ion-specific electrode was found to provide a convenient, precise and relatively inexpensive method for potentiometry of copper(II) ion in copper-complex azo or formazan dyes. Copper(II) ion in copper phthalocyanine dyes can be quantified after anion exchange. Twelve commercial premetallised dyes evaluated using this technique contained copper(II) ion concentrations in the range 0.007 to 0.2%. Thus many copper-complex direct or reactive dyes are likely to contribute low but possibly significant amounts of ionic copper to textile dyeing effluents [52]. 

The lead content of biological samples is usually very small, rendering gravimetric methods impracticable, and methods have often relied upon the formation of coloured complexes with a variety of dyes. More recently, the development of absorption spectroscopy using vaporized samples has provided a sensitive quantitative method. Oxygen measurements using specific electrodes offer a level of sensitivity which is unobtainable using volumetric gas analysis. 

Reversible cell potentials have been the source of much thermodynamic data on aqueous electrolytes. In recent years, this technique has been extended to nonaqueous solutions and to molten salt systems. Its use for aqueous solutions, relative to other techniques, has decreased. Various ion specific electrodes have been developed in recent years. These are used primarily in analytical chemistry and have not produced much thermodynamic data. 

Amino acids may be determined by measuring the amines obtained after the action of a carboxylase with a specific electrode for amines, which is based on a poly(vinyl chloride) membrane containing sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (166) as ion exchanger and tricresyl phosphate as solvent mediator. LOD was 20 and 50 i-M for tyroxine and phenylalanine, determined as tyramine (5) and phenethylamine (33), respectively364. 

The properties of this electrode approach those of an ideal specific electrode. It has an anionic response almost exclusively for sulphide ions in a broad activity interval (up to 10 molar concentration in hydrogen sulphide solutions). Only the cyanide ion interferes here at high concentrations. This electrode has a cation response for silver ions in a wide activity range (in Ag(I) complexes up to 10 m Ag [325]), where only Hg interferes [417]. 

Development of lithium selective electrodes (LiSE) and their application in clinical chemistry have been amply reviewed Several models of lithium ion specific electrodes are commercially available. The central problems in developing such sensing devices are their dynamic range, the accuracy and precision by which the signals are correlated to the concentration of the analyte and the selectivity towards that species, especially in relation to other alkali metal cations. Additional problems of practical importance are the times of response and recovery and the durability of the electrode in the intended service. 

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