Ion electrodes

The necessity to calculate the electrostatic contribution to both the ion-electrode attraction and the ion-ion repulsion energies, bearing in mind that there are at least two dielectric ftmction discontinuities hr the simple double-layer model above. 

That short-range contributions to both the ion-ion and ion-electrode interactions must be included. 

Specific breakage rate Specific conductivity Specific ion electrode Specific properties... 

The JnitedSfates Pharmacopeia (76) specifications for sodium monofluorophosphate require a minimum of 12.1% fluoride as PO F (theoretical 13.2%) and a maximum of 1.2% fluoride ion reflecting unreacted sodium fluoride. Analysis for PO F is by difference between total fluoride ia the product less fluoride ion as determined by a specific ion electrode. The oral LD q of sodium monofluorophosphate ia rats is 888 mg/kg. 

Oxygen and nitrogen also are deterrnined by conductivity or chromatographic techniques following a hot vacuum extraction or inert-gas fusion of hafnium with a noble metal (25,26). Nitrogen also may be deterrnined by the Kjeldahl technique (19). Phosphoms is determined by phosphine evolution and flame-emission detection. Chloride is determined indirecdy by atomic absorption or x-ray spectroscopy, or at higher levels by a selective-ion electrode. Fluoride can be determined similarly (27,28). Uranium and U-235 have been determined by inductively coupled plasma mass spectroscopy (29). 

A number of simple, standard methods have been developed for the analysis of ammonium compounds, several of which have been adapted to automated or instmmental methods. Ammonium content is most easily deterrnined by adding excess sodium hydroxide to a solution of the salt. Liberated ammonia is then distilled into standard sulfuric acid and the excess acid titrated. Other methods include colorimetry (2) and the use of a specific ion electrode (3). 

Instmmental methods are useful for the determination of the total silver ia a sample, but such methods do not differentiate the various species of silver that may be present. A silver ion-selective electrode measures the activity of the silver ions present ia a solution. These activity values can be related to the concentration of the free silver ion ia the solution. Commercially available silver ion-selective electrodes measure Ag+ down to 10 flg/L, and special silver ion electrodes can measure free silver ion at 1 ng/L (27) (see Electro analytical techniques). 

Specific-Ion Electrodes In addition to the pH glass electrode specific for hydrogen ions, a number of electrodes that are selective for the measurement of other ions have been developed. This selectivity is obtained through the composition of the electrode membrane (glass, polymer, or liquid-liquid) and the composition of the elec trode. Tbese electrodes are subject to interference from other ions, and the response is a function of the total ionic strength of the solution. However, electrodes have been designed to be highly selective for specific ions, and when properly used, these provide valuable process measurements. 

Reference electrodes are used in the measurement of potential [see the explanation to Eq. (2-1)]. A reference electrode is usually a metal/metal ion electrode. The electrolyte surrounding it is in electrolytically conducting contact via a diaphragm with the medium in which the object to be measured is situated. In most cases concentrated or saturated salt solutions are present in reference electrodes so that ions diffuse through the diaphragm into the medium. As a consequence, a diffusion potential arises at the diaphragm that is not taken into account in Eq. (2-1) and represents an error in the potential measurement. It is important that diffusion potentials be as small as possible or the same in the comparison of potential values. Table 3-1 provides information on reference electrodes. 

There are five major methods for the determination of fluonde ions m aqueous solution (1) volumetric, (2) gravimetnc, (3) null pointpotentiometry, (4) fluonde lon-selecti ve electrode (also known as specific ion electrode), and (5) ion chromatography... 

Similar to the pH meter, gas meters employ specific ion electrodes. The electrodes generate a potential proportional to the activity of a specific ion in solution. The calibration is achieved in standard solution and results read in mV or concentration in mg/L or ppm on the meter. The water can be adapted to monitor the concentration of carbon dioxide, hydrogen sulfide, ammonia, chloride, calcium, potassium and sodium to name a few. 

By contrast, it appears from Figs. 10.4 and 10.6 that a potential measurement would be more reliable. Specifically that (the equilibrium potential for the iron/ferrous-ion electrode given a suitably low ferrous ion concentration) would always represent the achievement of full protection for iron. 

It must not be assumed that the protection potential is numerically equal to the equilibrium potential for the iron/ferrous-ion electrode (E ). The standard equilibrium potential (E ) for iron/ferrous-ion is -0-440V (vs. the standard hydrogen electrode). If the interfacial ferrous ion concentration when corrosion ceases is approximately 10 g ions/1 then, according to the Nernst equation, the equilibrium potential (E ) is given by ... 

Specific ion electrodes, similar in design to the glass electrode, have been developed to analyze for a variety of cations and anions. One of the first to be used extensively was a fluoride ion electrode that is sensitive to F- at concentrations as low as 0.1 part per million and hence is ideal for monitoring fluoridated water supplies. An electrode that is specific for Cl- ions is used to diagnose cystic fibrosis. Attached directly to the skin, it detects the abnormally high concentrations of sodium chloride in sweat that are a characteristic symptom of this disorder. Diagnoses that used to require an hour or more can now be carried out in a few minutes as a result, large numbers of children can be screened rapidly and routinely. 

The general approach illustrated by Example 18.7 is widely used to determine equilibrium constants for solution reactions. The pH meter in particular can be used to determine acid or base equilibrium constants by measuring the pH of solutions containing known concentrations of weak acids or bases. Specific ion electrodes are readily adapted to the determination of solubility product constants. For example, a chloride ion electrode can be used to find [Cl-] in equilibrium with AgCl(s) and a known [Ag+]. From that information, Ksp of AgCl can be calculated. 

Potentiometric titration using a selective ion electrode (Sections 15.7, 15.8) responsive to the ion being titrated. 

Indicator electrodes for anions may take the form of a gas electrode (e.g. oxygen electrode for OH- chlorine electrode for Cl-), but in many instances consist of an appropriate electrode of the second kind thus as shown in Section 15.1, the potential of a silver-silver chloride electrode is governed by the chloride-ion activity of the solution. Selective-ion electrodes are also available for many anions. 

A single crystal electrode is exemplified by the lanthanum fluoride electrode in which a crystal of lanthanum fluoride is sealed into the bottom of a plastic container to produce a fluoride ion electrode. The container is charged with a... 

The chlorine content can be determined by either chlorine elemental analysis or a potentiometric titration using a chloride-ion electrode. For titration, about 0.2 g. of polymer is heated in 3 ml. of pyridine at 100° for 2 hours. This suspension is then transferred to a 50-mi. beaker containing 30 ml. of aqueous 50% acetic acid and 5 ini. of concentrated nitric acid, and the resulting mixture is titrated against aqueous 0.1 N silver nitrate. 

It was found that chlorides, chlorates, and nitrates interfere by forming in sol ppts (Ref 6). Later it was found that w sol perchlorates could be analyzed gravimetrically by pptn of the perchlorate ion as tetraphenylarsonium perchlorate (Ksp at 20° = 2.60 0.14 x 10"9). The ppt can be (Tried at 105° and weighed directly (Ref 16). Still more recently a specific perchlorate ion electrode has been developed (Ref 17) which can be used for the potentiometric titration of the perchlorate ion using a soln of tetraphenylarsonium chloride as the titrant (Ref 15). Tetra-phenylphosphonium chloride has been similarly used, but the corresponding Sb compd is too insol in w for practical use (Ref 8), For routine assay of perchlorates for use by the armed forces, a Na carbonate fusion procedure is described in Mil Specs MlL-A-23946 (19 Aug 1964) MIL-A-23948 (19 Aug 1964) (see also under Ammonium Perchlorate). The tetraphenylarsonium chloride procedure has also been proposed for use in Mil Specs (Ref 11)... 

NOTE The moisture content of steam can be measured by means of a throttling calorimeter or by analysis of the sodium content in a sample of condensed steam, using perhaps a specific ion electrode or flame photometer. 

As the DI plant becomes exhausted, silica leakage occurs in the treated MU water and the pH falls abruptly. Furthermore, where sodium ion leakage occurs from the cation bed, it produces a regenerative effect on the anion bed, which may also lead to silica leakage. To differentiate between the two phenomena, sodium ion leakage from the cation bed should be assessed directly using selective sodium-ion electrodes, flame photometric analysis, or other appropriate test methods. 

J. Koryta, Ions, Electrodes and Membranes, Wiley, New York, 1982. 

These forms are useful if [L] is known, either from an in situ analysis (e.g., a chloride ion electrode to measure [Cl-]) or because the experiment was designed with [L] [A]. If, on the other hand, the concentration of free A is known or more eas-... 

Studies of ligands which might provide specificity in binding to various oxidation states of plutonium seems a particularly promising area for futher research. If specific ion electrodes could be developed for the other oxidation states, study of redox reactions would be much facilitated. Fast separation schemes which do not change the redox equilibria and function at neutral pH values would be helpful in studies of behavior of tracer levels of plutonium in environmental conditions. A particularly important question in this area is the role of PuOj which has been reported to be the dominant soluble form of plutonium in some studies of natural waters (3,14). 

Long range, many determinations which are presently being performed by chemical procedures, will be performed in the future with specific ion electrodes. Using the iodide electrode the high sensitivity of iodimetry is augmented manyfold. 

Assay of calcium and magnesium by specific ion electrodes (37) is not practicable on micro quantities today, although it is quite possible that these may become available in the near future, especially with the developments that are taking place in foreign countries in this area. 

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