Sulphide ISE

The solution pH affects the function of all ISEs, either through interference of hydroxonium or hydroxide ions in the membrane reaction (for example the interference of OH" with the function of fluoride ISEs at pH values greater than about 5.5), or through chemical interference in solution (for example, formation of poorly dissociated HF and HF in acid solutions, which are not sensed by a fluoride ISE or formation of HS and H2S with decreasing pH, which are not sensed by a sulphide ISE), or both. Moreover, the pH value can affect the equiUbria of interferents in the solution. The pH must thus be adjusted with all these effects in mind. Fortunately, it is usually sufficient to maintain the pH within a certain region rather than at a single precise value. 

Linear relation between peak height and concentration was also found in chromatographic detection with Ag/AgCl electrode, with sulphide ISE used for cyanide... 

Homogeneous polycrystalline membrane electrodes [see Fig. 2.10 (3)J. The relatively high electrical conductance of monoclinic / -Ag2S and its extremely low solubility product led to the development of halide and other metal ISEs with addition of silver sulphide. 

For the halide ISEs on the basis of silver sulphide we may assume, e.g., in the case of chloride, the following exchange reaction ... 

For metal ISEs based on silver sulphide we can assume for other divalent metals analogously the exchange reaction... 

Good results are obtained for electrodes with sulphides of Pb, Cd and Cu(II), but with certain other sulphides the response time is unsatisfactory. Interference occurs in highly acidic solutions (H2S formation) and in alkaline solutions (at pH > 11) other metal ions sometimes disturb determinations with the metal ISE also, anions may cause difficulties, e.g., in a Cu(H) determination at a Cu(H) ISE if Cu2+ and Cl" are simultaneously present in the... 

An ideal ISE would exhibit a specific response to a certain ion J and the effect of interferents would be excluded. Except for the silver sulphide electrode, which is specific for sulphide or silver ions, no ion-selective electrode has this property. The others exhibit selectivity only for a particular ion with respect to the others. The selective behaviour of an ISE follows from (3.1.7). If the activity of the interferent is sufficiently low, i.e. if... 

Fig, 5.12. Continuous concentration gradient titration of sulphide with mercuric nitrate, using an S ISE [40]. 

For a detailed study, see [421-424], In general, all-solid-state ISEs are used with an internal silver contact. Selectrodes (see p. 60) can also be prepared from a mixture of halide and silver sulphide [328,329]. 

Silver sulphide exists in two modifications, a-Ag2 S, the cubic form, which is an electronic conductor and is stable above 176°C, and monoclinic/l-AgjS, an ionic conductor, which is stable at lower temperatures [316]. In this latter modification, Ag is almost the only charge carrier [141, 325,428], The good conductivity and negligible solubility of the compact membrane make the Ag2 S ISE one of the most reliable sensors. 

The standard potential of these ISEs, analogous to halide ISEs, depends on the activities of silver and sulphur in the membrane. When metallic silver is used as a contact soldered directly onto the membrane, the silver activity in the membrane equals one and the Ag2 S ISE has properties identical with a silver electrode of the first kind or with a silver sulphide electrode of the second kind, depending on the solution with which it is in contact [203], The membrane that is in contact with electrolyte on both sides behaves similarly [106],... 

If the graphite contact in cell (6.2.1) is replaced by a silver contact, quantity 6 corresponds to saturation of silver sulphide with silver and the silver activity in Ag2 S equals unity. Then juQ = ju and E=0. The standard potential of the Ag2 S ISE is identical with the standard potential of the silver electrode, O.ISE AgVAg = + 0.799 V versus SCE. On the other hand, when Ag2 S is in equilibrium with elementary sulphur (for example, when Ag2 S is prepared in an oxidizing medium [417]) then... 

The Ag2 S ISEs used in practice are of three kinds with a heterogeneous membrane containing Ag2 S precipitate (preferably precipitated from excess sulphide or by the action of H2 S [228]) in a matrix of silicon rubber [346] or a thermoplastic material [248], with a pressed pellet as a membrane [235,... 

The Ag2 S ISE has Nemstian response dE/d log a( = 0.0296 V in the sulphide concentration range 10" to 10" M and silver ions from 10 to 10 M if the solutions are prepared from pure salts, as a further concentration decrease is prevented by adsorption on the glass (see p. 76 and [87, 163]). After prolonged use, the limit of the Nemstian behaviour shifts to about 10" m [130] as a result of formation of mixed potentials on accumulation of metallic silver in the membrane surface. An analogous deterioration in the membrane function in the presence of iodine results from surface oxidation [23]. Cyanide interferes only at large concentrations the equilibrium constant of the reaction... 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

A detailed study of I iSe and 3lp nuclear spin relaxation in tri(tertbutyI)phosphine selenide has been reported 6 and the kinetics and mechanism of formation of tetracoordinate P(V) sulphides from the reaction of tricoordinate phosphorus compounds with diaryl trisulphides have been investigated.1 ... 

An ion-selective electrode (ISE) determination of sulphide from sulphate-reducing bacteria was compared with a gravimetric determination. The results obtained were expressed in milligrams of sulphide. 

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