Potentiometric studies

An early use of oxygen-ion conducting solid electrolytes was in potentiometric devices for the measurement of oxygen partial pressure. In Section 2 it was shown that the e.m.f. of the cell relates the oxygen partial pressures on both sides through the Nemst equation, 

This equation relies on the assumption that adsorbed oxygen on the electrode surfaces is in equilibrium with gas phase oxygen. This has lead to a number of applications of this type of cell as a sensor. The principle applications are listed below 4 

In the case where one of the electrodes is exposed to a reacting gas mixture this no longer need be true and the e.m.f. of the cell becomes a reflection of the thermodynamic activity of oxygen adsorbed on the electrode in question,  

The use of equation (3.2) to study the behaviour of catalysts is known as solid electrolyte potentiometry (SEP). Wagner38 was the first to put forward the idea of using SEP to study catalysts under working conditions. Vayenas and Saltsburg were the first to apply the technique to the fundamental study of a catalytic reaction for the case of the oxidation of sulfur dioxide.39 Since then the technique has been widely used, with particular success in the study of periodic and oscillatory phenomena for such reactions as the oxidation of carbon monoxide on platinum, hydrogen on nickel, ethylene on platinum and propylene oxide on silver. 

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It is assumed that NaAlCl4 is dissociated into sodium and tetrachloroaluminate ions. Torsi and Mamantov [35] have investigated a further dissociation of the tetrachloroaluminate ion in molten sodium chloroaluminates by potentiometric studies. For the 50 50 mol% composition of NaCl-AlCl3 the principal anion is A1C14. In acidic melts it was assumed that the reaction A1C13 + A1C14 <-> A12C17 is quantitative. More important is the con-... 

Dremlyuzhenko SG, Voloshchuk AG, Zakharuk Zl, Yurijchuk IN (2008) Thermodynamic evaluation and potentiometric study of Cdi xMnxTe and Cdi xZnx Te dissolution in acid and alkaline solutions. Inorg Mater 44 21-29... 

Deiana S., Micera G., Muggiolu G., Gessa C. Pusino A. (1983) Interaction of transition-metal ions with polygalacturonic acid a potentiometric study. Colloids Surf. 6,17-25. 

The nature of mass transport in MEMED has been confirmed with both ampero-metric and potentiometric studies of bromine transfer from an aqueous phase to DCE [79]. Figure 17 shows typical amperometric data for this case, in which the DCE phase acts as a sink for Br2, and a depleted region of Br2 is measured adjacent to the droplet in the aqueous phase. Video images are also provided, which correspond to particular times during the amperometric transient at position (3) the edge of the developing concentration boundary layer, around the drop, reaches the electrode the concentration profile is then mapped out between points (3) and (4). The measured current, i, can be related to the local concentration, c, via... 

The aforementioned application of conductometry in Lewis titrations was an incentive, in addition to our potentiometric studies, to investigate also conductometric titration in non-aqueous media more thoroughly. Figs. 4.10 and 4.11 show two selected examples of the study. 

It was proposed that mono-protonation of [CuL]2+ to yield [Cu(HL)]2+ occurs initially at the axial nitrogen atom since this Cu-N bond is expected to be weaker as a result of Jahn-Teller distortion. With respect to this, it should be noted that species such as [Cu(HL)]3+ are commonly observed during potentiometric studies of the formation of Cu(n) polyamine complexes. From the proposed mechanism, the following rate law can be deduced ... 

Koryta, J., Dvordk, J. and Kavan, L., Principles of Electrochemistry, 2nd Edn, Wiley, Chichester, 1993. The release of this second edition of a now-classic work was much welcomed. The text is well structured and always worth reading. Some of the examples are particularly relevant, although the voltammetric sections are inferior to those concentrating on potentiometric studies. 

A potentiometric study of polymeric anions of molybdenum in nitrate melts has resolved apparent contradictions in earlier work and confirmed that [Mo207] ", rather than [Mo30,o] , is the polymeric anion in equilibrium with [MoOj ] The crystal structure of K2[Mo,07] has been shown to involve chains of M0O4 tetrahedra and MoO octahedra " and the dissociation of molten Na2[W207] to Na and a mixture of anions including [W04], [W207] , and has been confirmed. " The results of other... 

Br HjO [PtBrjlNHjU] [PtBr Cl ] - [PtBr lNO U - [PtBr lCNU] - potentiometric Studied replacement of one Br by HjO. Results show that thermodynamic cis effect is small. Order of increasing cis influence parallels trans influence order. h... 

Cl, Br H2O [PtXjCDMSO)]- potentiometric Studied replacement of one X by H2O. This is the most strongly held of three halide ions. d... 

A number of pendant arm ligand derivatives based on hexamine macrocyclic backbones have been reported." One such species was formed by the [2 -f 2] Schiff base condensation between 3,3-iminobis(propylamine) and diformyl-p-cresol followed by sodium borohydride reduction of the four imine functions so generated. Potentiometric studies indicate that a range of both mononuclear and dinuclear species are formed in solution with manganese(II) incorporating various both protonated and nonprotonated forms of the ligand. 

In Figure 4, we plot the variation of specific viscosity of the mixture against the concentration ratio r. The polymers, their molecular weights and concentrations are the same as those of Figure 1 concerning potentiometric study. 

M. Rizzotto, V. Moreno, S. Signorella, V. Daier, and L. F. Sala, Electron spin resonance and potentiometric studies of the interaction of Cr(VI) and Cr(V) with D-ribose-5-phosphate and nucleotides, Polyhedron, 19 (2000) 417 123. 

The selectivity factors determined in potentiometric studies (K[,ot) should therefore be identical to the ones (Kjj) determined in transport experiments. In Fig. 4 selectivities obtained potentiometrically on a membrane containing ligand 11 (3 wt.% carrier 11, 65 wt.% dibutyl sebacate 32 wt.% polyvinyl chloride, thickness =100 /xm) are compared with those obtained in electrodialytic transport experiments.55 Although widely different methods have been used to determine the ion selectivity, the agreement between the two sets of data is evident and corroborates the model presented. The deviation for CsH may possibly be due to kinetic limitations suggesting a loss in transport selectivity (see Section IV.D). 

A series of ion-selective membrane electrodes based on neutral carrier solvent polymeric membranes has been designed for the potentiometric determination of ion activities (for reviews see Refs. 52, 65). Systems with analytically relevant selectivities for Li+, Na+, K+, NHJ, Ca2+, and Ba2+, are available. In agreement with the treatment given in Sections III and IV, the ions preferred in potentiometric studies may be transported preferentially through the same membranes in electrodialytic experiments. So far, selective carrier transports have been realized for Li+, Na+, K+, and Ca2+. 

Many potentiometric studies have been carried out to obtain information about the compositions and formation constants of metal complexes in non-aqueous solutions. Ion-selective electrodes and metal (or metal amalgam) electrodes are used as indicator electrodes. 

Electrochemical methods play important roles in studying ion solvation. In this section, we deal with the potentiometric study of ion solvation and the difficulties associated with it [28]. 

Various potentiometric indicator electrodes work as sensors for ion solvation. Metal and metal amalgam electrodes, in principle, respond in a thermodynamic way to the solvation energy of the relevant metal ions. Some ion-selective electrodes can also respond almost thermodynamically to the solvation energies of the ions to which they are sensitive. Thus, the main difficulty in the potentiometric study of ion solvation arises from having to compare the potentials in different solvents, even though there is no thermodynamic way of doing it. In order to overcome this difficulty, we have to employ a method based on an extra-thermodynamic assumption. For example, we can use (1) or (2) below ... 

Aromatic polyalcohols act as strong coordinating agents and Table 17 summarizes reported formation constants. The complexes are quite stable this behaviour has been used for the qualitative and quantitative determination of vanadium (e.g. refs. 494 and 495). At pH 3-4, an initial vanadyl catechol complex slowly converts to a tris complex.496 In fact complexes with 1 3 metal-ligand stoichiometry have been isolated (see below), but since in the equilibrium (30) no protons are consumed or liberated, [VO(cat)2]2- and [V(cat)3]2 are not distinguishable by potentiometric studies. 

In a potentiometric study in propylene carbonate, using Pb11 or Tl1 as auxiliary ions, stability constants have been determined for a variety of crown ethers. Some results464 are shown in Table 8. They show that the wrap-around ligand dibenzo-30-crown-10 is relatively quite effective, while the 2 1 complexes, presumably of the sandwich type, are favoured for larger lanthanides and smaller crowns. 

A potentiometric study of the formation of cadmium-imidazole complexes has been reported and a number of species, including [CdL]2+, [Cdl ]24, [CdL3]2+, [CdL4]2+ and [Cd(OH)L]+, have been characterized.420... 

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