Iodide ISE

These types of electrodes for chloride and bromide ions have resistances of less than 30Mf2, while that of the iodide ISE lies between 1 and 5 [263]. 

Model 97-70 electrode from Orion Research, an electrode for residual chlorine [321 ], is based on a cell consisting of a platinum electrode which reacts to the I2 /I" system, and an iodide ISE. The EMF of this cell is given by the relationship... 

Cyanide ion-selective electrode As demonstrated in chapter 3.4, the dissolution of a halide ISE in the presence of some complexing agents, especially cyanide, can be used for determination of these agents [312, 392, 434]. An iodide ISE can be used as a cyanide ISE. The principal application of this electrode is in the determination of cyanides in waters [60, 126, 281, 336] and in galvanic baths [222, 225]. 

Enzyme electrodes with amperometric indication have certain advantages over potentiometric sensors, chiefly because the product of the enzymic reaction is consumed at the electrode and thus the response time is decreased. For this reason, the potentiometric glucose enzyme electrode, based on reaction (8.1) followed by the reaction of HjO, with iodide ions sensed by an iodide ISE [39], has not found practical use. 

The application of potentiometric detection in ion-chromatography is favoured by the progress in the field of membrane ion-selective electrodes (ISE). The electrodes with solid-state membranes were mostly employed for determination of halides, pseudohalides and some other anions binding silver ions. The use of fluoride electrode in multidetector, chromatographic system offers very low detection limit of 1.2 ng fluoride in injected samples. Application of bromide electrode in the same system provides even five-fold better detectability. The same level of detectability was reported by Butler and Gershey for iodide with iodide ISE. In the system with preconcentration step the detectability can be lowered by an order... 

Other possibilities include absorption of SO2 in a 3% solution of hydrogen peroxide followed by titration of the solution with a lead(II) standard solution (indication - lead(II) ISE), or the reduction of iodine by SO2 with determination of the iodide formed with an iodide ISE. 

Chlorine can be determined using a gas-sensitive probe (Table 2) or by absorption in an Ag(CN)2" solution and measurement with an iodide ISE or, better, a sulfide ISE. 

Other types of ISE with silver halides are based on homogeneous membranes [6, 383]. With silver chloride or bromide, a single crystal or membrane from a salt melt can be prepared, while silver iodide membranes are prepared from... 

NaH (1.55 g, 0.032 mol, 50% oil dispersion) was placed in a three-necked flask under argon. The NaH was washed several times with pentane by decantation. The flask was then fitted with a condenser, a drying tube and a dropping funnel equipped with an argon inlet tube. Anhyd DMSO (17 mL) was added dropwise, and then the mixture was heated at a bath temperature of 75 °C for 1 h. The mixture was cooled in ice, and a solution of methyltriphenylphosphonium iodide (13.3 g, 0.033 mol) in warm DMSO (30 mL) was added dropwise. The solution was then stirred at 25 C for 15 min and cyclobutanonc (2.25 g, 0 032 mol) in DMSO (5 mL) was added dropw ise. After stirring the mixture for 1 h. bulb-to-bulb distillation at 20 Torr was carried out while the bath temperature was maintained at < 75 C the distillate was collected in a dry ice cooled receiver yield 1.94g (90%). 

A nitrate-selective potentiometric MIP chemosensor has been devised [197, 198]. For preparation of this chemosensor, a polypyrrole film was deposited by pyrrole electropolymerization on a glassy carbon electrode (GCE) in aqueous solution of the nitrate template. Potentiostatic conditions of electropolymerization used were optimized for enhanced affinity of the resulting MIP film towards this template. In effect, selectivity of the chemosensor towards nitrate was much higher than that to the interfering perchlorate ( o3 cio4 = 5.7 x 10-2) or iodide ( N03, r = x 10 2) anion. Moreover, with the use of this MIP chemosensor the selectivity of the nitrate detection has been improved, as compared to those of commercial ISEs, by four orders of magnitude at the linear concentration range of 50 pM to 0.5 M and LOD for nitrate of (20 10) pM [197]. 

Table 12.6 sets out other standard methods for the determination of nonmetallic substances chloride is determined by titration,50 51 73 VIS spectrophotometry,50 74 and ISE 51 chlorine by titration and VIS spectrophotometry 75-77 fluoride by VIS spectrophotometry50 and ISE 50 78 79 iodide by VIS spec-trophotomtery 50 cyanide by titration, VIS spectrophotometry, and ISE 50 80 81 sulfate by gravimetry50 51 and turbidimetry 50 sulfite by titration and VIS spectrophotometry 51 and sulfur by titration and VIS spectrophotometry.50 51... 

Figure 3. Selectivity pattern of an ISE based on ionophore 2. The electrode was exposed to the following anions salicylate (1), thiocyanate (2), nitrite (3), perchlorate (4), iodide (5), benzoate (6), bromide (7), bicarbonate (8), hydrogen phosphate (9), nitrate (10), chloride (11), sulfate (12). (Reproduced with permission from ref. 10. Copyright 1989 Alan R. Liss.)...

The potentiometric behavior of electrodes based on these films was studied (Figure 8). These ISEs presented sub-Nemstian slopes for thiocyanate (from -40 to -53 mV/decade, depending on the buffer used), and had detection limits of 5xl0 7 M. The response time of the electrodes was typically less than 25 s. The selectivity pattern observed was thiocyanate > perchlorate > iodide > nitrite - salicylate bromide > chloride > bicarbonate > phosphate. This anion-selectivity behavior does not follow the Hofmeister series, with thiocyanate and nitrite being the ions that deviate the most from it. This indicates that there is a selective interaction of the immobilized porphyrin with the two anions. 

Conventional ISEs have been used as detectors for immunoassays. Antibody-binding measurements can be made with hapten-selective electrodes such as the trimethylphenylammonium ion electrode. Enzyme immunoassays, in which the enzyme label catalyzes the production of a product that is detected by an ISE, take advantage of the amplification effect of enzyme catalysis in order to reach lower detection limits. Systems for hepatitis B surface antigen and estradiol use horseradish peroxidase as the enzyme label and an iodide electrode as the detector. The horseradish peroxidase catalyzes the oxidation of p-fluoroanadine with the fluoride detected by the ISE. Biotin and cyclic have been determined... 

Ion-selective electrodes are used for many applications but most commonly for the measurement of pH and metal ions such as tin , lead silver and nickeT as well as various other analytes such as surfactants". The samples investigated range from surface waters, e.g. lakes and streams, ground waters, rain, to soil and food. ISEs for anion detection are not as prevalent due to solubility issues but more are becoming available, such as one for iodide with a detection limit of 5.3 X 10 and with the help of ionic liquids another has been developed for sulfate detection". 

The transducer can be an ammonium ISE ionophore (nonactin-based), a gas sensor for ammonia or carbon dioxide, and an iodide-selective sensor. The latter can be coupled with the enzymes L-aminooxidase (l-AOx) and peroxidase (e.g., coimmobilized in a polyacrylamide gel), which catalyses reactions [V] and [VI] ... 

The potentiometric detection of the endpoint of precipitation titrations is very often used because not many visual indicators are available, in particular when mixtures of analytes are titrated. Halides, cyanide, sulfide, chromate, mercaptans, and thiols can be titrated with silver nitrate, using the silver sulfide-based ISE. Also complex mixtures, such as sulfide, thiocyanide, and chloride ions, or chloride, bromide, and iodide ions, can be titrated potentio-metrically with silver(I) ions. When the solubility of a compound formed during titration is too high, nonaqueous or mixed solvents are used, for example,... 

The real move towards ISEs based on non glass membranes came in 1961 when Pungor and Hollos-Rokosinyi (1 6) produced a membrane by incorporating silver iodide into paraffin. This led on to the development of heterogeneous membrane electrodes, especially those based on silicone rubber matrices (17 -20). 

Measurements of nitrate in soils and waste waters using the TOAN-DBP electrode and the brucine method agreed closely. All these nitrate ISEs (66,67) are subject to serious interference from iodide, chlorate and perchlorate. This feature can in turn be exploited, e.g., the Corning nitrate exchanger may be readily converted to a viable chlorate-sensing cocktail using an extraction technique as described for the uranyl phosphate sensor (section 3.2.10). 

An ISE prepared from [9]mercuracarborand-3 (MC3) yielded an electrode selective for chloride (33). Indeed, the MC3-based electr e demonstrated improvements in selectivity over all anions in the Hofrneister series, with only iodide and bromide being interferences (log > 0). Iodide and bromide are less interfering, however, for this electrode than common Ag/AgCl-based ISEs for chloride. Preliminary results from electrodes constructed from the hexamethyl form of MC3 (-R = -CH3) show that carborane substitution has a profound effect on the acidl)ase properties of the mercuiy centers of the ionophore, yielding different selectivity patterns than for MC3-based ISEs. Aside from providing a marked improvement in selectivity, the MC3-based ISEs demonstrate retention of response characteristics for approximately two months (Figure 9). 

Detailed analysis suggested that the strong tendency of 5 to form a 2 1 sandwich complex with iodide in solution [19] was also the source of selectivity in the ISE. 

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