Sulphide ion selective electrode

For the determination of sulphides thin-layer chromatography [55, 56] is used, as well as spectrophotometry in the IR region [57], AAS [58], and a sulphide ion selective electrode [59-61]. 

M. G. Glaister, G. J. Moody, and J. D. R. Thomas, Studies on Flow Injection Analysis with Sulphide Ion-Selective Electrodes. Analyst, 110 (1985) 113. 

Many sulphur compounds can be assayed without prior combustion owing to certain subtle properties of organic sulphur bonds [409—415]. Thus, thiourea [410] can be assayed directly in sodium hydroxide media with a sulphide ion-selective electrode. The assay is supported by reactions (24) and (25) ... 

Lindell H, Jappinen P, Savolainen H. 1988. Determination of sulphide in blood with an ion-selective electrode by pre-concentration of trapped sulphide in sodium hydroxide solution. Analyst 113 839-840. 

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... 

L. Bailey, J. Wilson, S. Kaipel and M. Riley, Application of chloride electrodes based on mercurous chloride/mercuric sulphide, ia Ion-Selective Electrodes (ed. E. Pungor and L Buz5s), Conference 1977, Akad6miai Kiad6, Budapest (1978), p. 201. 

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

Chao and Cheng [76CHA/CHE] studied the determination of a number of anions, single or in mixtures, by a stepwise potentiometric titration with silver nitrate at pH = II. The silver ion activity was measured with a silver ion selective electrode based on silver sulphide. The data were also used to estimate the solubility products of the silver salts formed during a titration. The method is only sketched in the paper but appears to have proceeded along the following course. The potential of incipient precipitation ( prec) was estimated graphically from the shape of the titration curve. p,ec would thus be a measure of the silver ion activity at the nominal and known concentration of the anion in the presence of its silver salt. 

It can be concluded in general, that electrochemical properties of copper(II) sulphide are influenced by chloride ions and consequently that copper(II) sulphide plays an active role in build up of chloride interference whenever this substance is present in the membrane. The redox reaction responsible for the interference discussed above shows the importance of consideration and inclusion of such mechanism in the general picture of interfering pathways on solid-state ion-selective electrodes. Explanation of the mechanism has so far dominated by traditional reasoning restricted only to simple ion-exchange reactions at the membrane surface. 

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. 

Solid-state membrane electrodes include H+, F , CN , Cl , 1, Br, H2S, CN , thiourea, Pb, Cu, and Cd selective electrodes. In this category, silver halide and silver sulphide electrodes are found. The macroscopic ion-selective electrodes made with these systems have been applied in many fields, such as medicine, food industry, and environmental studies (56, pp. 93,96,103,104) but their full potential has not yet been exploited in terms of SECM studies. 

Direct binding of the ion-selective component to the electrode has also been studied. For example, graphite combined with an antimony compound has been screen printed and the resultant electrodes shown to give selective responses to sulphide ion in simulated wastewater samples (0.01-0.7 mM sulphide) with high stability to repeated testing and low interference from other compounds [31]. 

Similarly, doping Ag2S with a divalent metal sulphide enables the electrode to respond to the corresponding metal (e.g. Pb, Cu or Cd). In ail cases the electrode responds by virtue of the solubility product equilibria involved, the movement of Ag ions within the membrane being the potential-governing factor. Selectivity ratios for some of these electrodes are given in table 6.2. 

With metal sulphide/sUver sulphide crystal membranes, the silver ions are again the mobile species. The cadmium ion-selective crystal membrane electrode (CdS/Ag2S membrane) can be used as an example of the mechanism involved. CdS wUl be more soluble than Ag2S. The activity of the sulphide ion wiU be controUed at the interface by that of the cadmium ion ... 

Terminal alkynes can be determined by potentiometric titration with silver nitrate solution, in aqueous or non-aqueous media, using an ion-selective silver sulphide electrode. ... 

In a similar manner, as described for fluoride electrode above, pofyciystalline Ag S membrane gives a good sulphide ion electrode. Mixed crystals of AgX-Ag S compose the anion selective electrodes for chloride, bromide, iodide, and thiocyanate. 

The electrode is, therefore, selective for ions in solution assuming that (a) the solubility product MS is much larger than (h) the solubility of MS is much less than the concentration of M " ions, (c) the exchange reaction between MS and Ag2 S is sufficiently fast, which is fulfilled only for certain sulphides. 

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. 

The membrane is a conducting solid. Both single crystal and peUet-pressed crystalhne substance mixtures can be used in membrane construction. Table 2, accumulated from data available from several electrode manufactmers data sheets, shows information relative to crystal membrane electrodes and then-application capabilities. As can be seen, crystal membrane electrodes, with the exception of that selective to F, involve Ag2S or crystal mixtures where one component is Ag2S and the other the sulphide of the selective ion of interest. The membranes are generally produced by pressing the polycrystalline substance in a pellet press. 

Where the crystal membrane contains both Ag2S and a silver hahde, the electrode is then selective to the hahde involved. The silver hahde wiU have higher solubhity than the silver sulphide. It wdl, however, have an equihbrium solubihty such that the hahde activity in the membrane wiU be significantly less than its activity in the test solution. The extent of the difference in activities on either side of the interface generates a potential difference resulting in the movement of the conducting silver ions. The extent of the difference in potential relates through this movement to the activity (concentration) of the hahde in the test solution. 

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