Electrodes membrane, response

The mechanisms of ion-selective electrode membrane response have not been as extensively studied as the glass pH electrode, and even less is known about how their potentials are detenmned. Undoubtedly, there is a similarity of mechanisms. 

Another version of the urea electrode (Figure 11.17) immobilizes the enzyme in a polymer membrane formed directly on the tip of a glass pH electrode. In this case, the electrode s response is... 

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. 

Ion-selective electrodes (ISEs) with ionophore-based membranes allow for quantification of a large number of analytes in various matrixes. Tailoring of the composition of the membranes to comply with the analytical task, requires advanced theory of membrane response. Most of theoretical descriptions include nonrealistic extra-thermodynamic assumptions, in the first place it is assumed that some kind of species strongly predominate in membranes. Ideally, a rigorous theory of ISE response should be based on strict thermodynamics. However, real ISE membranes are too complex. Therefore, known attempts aimed at rigorous thermodynamic description of ISEs proved to be fraritless. 

Glass electrodes are responsive to univalent cations. The selectivity for these cations is achieved by varying the composition of a thin ion-sensitive glass membrane. 

Other useful solid-state electrodes are based on silver compounds (particularly silver sulfide). Silver sulfide is an ionic conductor, in which silver ions are the mobile ions. Mixed pellets containing Ag2S-AgX (where X = Cl, Br, I, SCN) have been successfiilly used for the determination of one of these particular anions. The behavior of these electrodes is determined primarily by the solubility products involved. The relative solubility products of various ions with Ag+ thus dictate the selectivity (i.e., kt] = KSp(Agf)/KSP(Aw)). Consequently, the iodide electrode (membrane of Ag2S/AgI) displays high selectivity over Br- and Cl-. In contrast, die chloride electrode suffers from severe interference from Br- and I-. Similarly, mixtures of silver sulfide with CdS, CuS, or PbS provide membranes that are responsive to Cd2+, Cu2+, or Pb2+, respectively. A limitation of these mixed-salt electrodes is tiiat the solubility of die second salt must be much larger than that of silver sulfide. A silver sulfide membrane by itself responds to either S2- or Ag+ ions, down to die 10-8M level. 

A semi-empirical, second-order response lag is used. This consists of a first-order lag equation representing the diffusion of oxygen through the liquid film on the surface of the electrode membrane... 

Tor [7] developed a new method for the preparation of thin, uniform, self-mounted enzyme membrane, directly coating the surface of glass pH electrodes. The enzyme was dissolved in a solution containing synthetic prepolymers. The electrode was dipped in the solution, dried, and drained carefully. The backbone polymer was then cross-linked under controlled conditions to generate a thin enzyme membrane. The method was demonstrated and characterized by the determination of acetylcholine by an acetylcholine esterase electrode, urea by a urease electrode, and penicillin G by a penicillinase electrode. Linear response in a wide range of substrate concentrations and high storage and operational stability were recorded for all the enzymes tested. 

When the test component content in the samples varies over a wider interval, a calibration curve must be constructed. Calibration curves with ISEs are usually linear over several concentration orders (usually from about 10" m to about 10" m) and their slope is close to the theoretical Nernstian value. Both at high and at low concentrations with respect to the linear part, the caUbration dependence becomes curved and, eventually, independent of the test substance concentration (see fig. 5.1). The upper limit of the ISE response is mostly given by saturation of the active sites in the electrode membrane (for example ion-exchange sites), whereas the lower limit is determined by solubility of the... 

This theory has been successfully verified experimentally. Buck and Shepard [51] demonstrated that electrodes of the all-solid-state type have a response that is identical to that of similar electrodes of the second kind for response to halide ions and to a silver electrode for response to silver ions, depending on the degree of saturation with silver. This is achieved by soldering a silver contact to the membrane. If however the internal contact material is more noble than silver (platinum, graphite, mercury), the electrode with response to silver ions may attain a potential between the standard potential of a silver electrode g /Ag and the value... 

Although there is no direct connection between the rules discussed in Chap. 4 for the monovalent-divalent ion selectivity of ligands in liquid membranes and the electrochemical selectivity behavior of these membranes (cf. Eqs. (22) and (31)), the effect of solvent on Na+/Ca2+ selectivity (Fig. 22) is remarkably similar to the calculated effect shown in Fig. 18 (actually for K+/Ba2+ preference). The discrimination against Mg2+ and especially HaO+ is considerably better for the electrode discussed here 117) than for liquid ion-exchange membranes responsive to Ca2+ (123). 

AgjS can also be used as a matrix for other metal sulfides, notably CuS, PbS, and CdS, giving membranes responsive to the second metal. The electrode can be regarded as responding to the equilibrium activity of Ag" ", which is given by... 

For the construction of ISMEs, solid membranes and liquid membranes are proposed. The construction of solid membrane ISEs is based on the insertion of electroactive material (ion pair complex), or only of a counter ion into a polyvinyl chloride (PVC) matrix. The PVC type as well as the plasticizer used for the membrane construction affect the reliability of the electrode s response. Thomas proposed some rules for solid membranes based on a PVC matrix construction [3-5]. The procedure most often used for solid membrane construction is known as the Moody and Thomas procedure [5], as this assures the best reliability of solid membrane electrodes. 

Membrane electrode An indicator electrode whose response is due to ion-exchange processes on each side of a thin membrane. Meniscus The curved surface displayed by a liquid held in a... 

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