Potentiometric behaviour

E. Desimoni, L. Sabbatini and P.G. Zambonin, Oxygen Electrodes in Fused Salts. Potentiometric Behaviour of the System C02,02/C0f in Molten Alkali Nitrates, J. Electroanal. Chem. 71 (1976) 73-79. 

In the light of response time experiments the basic potential determining process in both KCl and KSCN solutions seems to be the ion transfer rates at the membrane-solution interface. These transfer rates are characterized by the exchange current densities and were found to be high for ionophore based membranes. The slow inner membrane processes, however, do not influence the short-term potentiometric behaviour of the solvent polymeric membranes. 

Anomalous titration curves for PLL in pure water are observed as illustrated by Figure 1 which shows the effect of various counterions on the potentiometric behaviour of PLL [12, 14]. A conformational change between a very compact form (stable at low a values) and an extended coil-like form clearly appears. Stabilization of the compact conformation is found to be more pronounced as the length of apolar chains of tetraalkylammonium counterion chains increases. The transition between the two forms, which is initiated at a critical oc of 0.15-0.20 (in the presence of Na" or K ), is shifted to higher values of a in the presence of tetraalkylammonium counterions attaining an of about 0.7 with bulky (and hydrophobic) N-tetrabutylammonium ions (Bu4N ). 

For PLL, one may ask whether hydrogen bonds may be not held responsible for the compact form. The comparison between the potentiometric behaviour of PLL and PLDAB indicates that the latter behaves as a stronger acid than the former. 

In attempts to establish a relationship between cavity sizes of crown-4 derivatives and lithium selectivities, Kitazawa et al (58) examined the potentiometric behaviour of a series of fourteen highly lipophilic crown-4 compounds with 12-, 13-, 14-, 15- and 16-member rings respectively and the fixed compositions of each of the otherwise identical PVC sensor membranes based on (24-27) make for a facile comparison of their respective potentiometric behaviour (Table 3.15). In general, the 13-membered ring compounds are much less selective than 14- and 15-membered ring compounds. Benzo-13-crown-4 derivatives also showed poor lithium selectivities, and... 

There are now some indications that the ordered form of the polymer might be a j3 structure. These indications arise for instance from the potentiometric behaviour of [L-His] [50] which is very similar to that of other polypeptides in the P structure, such as [Lys] and (Tyr] . 

In the case of [L-His] and [L-Tyr] in aqueous solution, CD studies, potentiometric behaviour and infrared results indicate that these polymers in the charge free state assume the /3-structure. In no case theoretical calculations are available predicting the CD properties of a /3-structure in the presence of side chain aromatic contributions. 

Extensive data are given in the Uterature for the potentiometric titration of polymer acids which may be used to study the behaviour of polyelectrolyte systems under different conditions. For poly(a-D) galacturonic acid there are few data of this kind, especially in connection with the occurrence of a conformational transition induced by pH variations, or with the effect brought about by the addition or the exchange of counterions. Since for a polyacid not exhibiting a conformational transition in the course of titration, pK K denoting the apparent dissociation constant) increases monotonously with degree... 

Thermodynamics describes the behaviour of systems in terms of quantities and functions of state, but cannot express these quantities in terms of model concepts and assumptions on the structure of the system, inter-molecular forces, etc. This is also true of the activity coefficients thermodynamics defines these quantities and gives their dependence on the temperature, pressure and composition, but cannot interpret them from the point of view of intermolecular interactions. Every theoretical expression of the activity coefficients as a function of the composition of the solution is necessarily based on extrathermodynamic, mainly statistical concepts. This approach makes it possible to elaborate quantitatively the theory of individual activity coefficients. Their values are of paramount importance, for example, for operational definition of the pH and its potentiometric determination (Section 3.3.2), for potentiometric measurement with ion-selective electrodes (Section 6.3), in general for all the systems where liquid junctions appear (Section 2.5.3), etc. 

The now familiar alternatives of visual and potentiometric detection are available. A number of organic dyes form coloured chelates with many metal ions. These coloured chelates are often discernible to the eye at concentrations of 10 6-10 7 mol dm 3 and can function as visual indicators. Most metal ion indicators will also undergo parallel reactions with protons bringing about similar colour changes. Hence, a careful consideration of pH is prudent when selecting an indicator. Some typical indicators appear in Table 5.9. Of these, eriochrome black T, which forms red complexes with over twenty metal ions, is amongst the most widely used. Its behaviour will serve as a general example of indicator function. 

Their electrochemical properties serve to regulate the coagulation rates, catalysis behaviour and electron transfer reactions of iron oxides (Mulvaney et ah, 1991). Two major methods of characterizing electrochemical behaviour are potentiometric titration and electrophoresis. 

Some ISEs containing no inner reference solution, as well as tubular potentiometric sensors, has been used in conjunction with FI systems for the determination of vitamins B, and Bg in pharmaceutical preparations. The membranes used for this purpose were prepared from the vitamin tetra(2-chlorophenyl)borate dissolved in o-nitrophenyloctyl ether and immobilized in PVC. The intrinsic behaviour of the tubular electrodes was assessed by using a low-dispersion single-channel FI manifold and compared with those of conventionally-shaped electrodes using the same membrane the results provided by both were very similar [119]. 

Sirieix-Plenet, ]., Gaillon, L., and Letellier, R, Behaviour of a binary solvent mixture constituted by an amphiphilic ionic liquid, l-decyl-3-methylimidazolium bromide and water. Potentiometric and conductimetric studies, Talanta, 62, 979,... 

Solid electrolyte electrochemical cells can be operated in a variety of ways (the three modes of operation are illustrated schematically in Figure 2). Such a cell may be operated potentiometrically in order to investigate the behaviour of a catalyst of interest. This technique has become known as solid electrolyte potentiometry (SEP). The catalyst itself is deposited in the form of an electrode... 

The cross-redox reaction discussed above and formation of the Vm dimers has also been reported in the case of the edta complexes. Scheme 12 summarizes part of the equilibria proposed to explain potentiometric and spectroscopic data at t = 25 °C and I = 0.20 M in NaC104.336-338 The polarographic behaviour of V111 complexes of edta and analogous ligands has also been studied.339... 

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. 

Because of its biological and therapeutic importance L-Dopa and its coordination properties have been extensively studied. The weight of potentiometric and spectroscopic evidence points to the ligand chelating either as a normal bidentate amino acid (N,0) or through the two phenolate oxygens (0,0) the ligand is therefore classed as ambidentate (a similar situation occurs for tyrosine in which it is sterically impossible to achieve tridentate behaviour). 

Solvent effects on the dissociation of 11 2,6-disubstituted benzoic acids have been analysed by chemometric analysis.66 The acid-base behaviour of the three zwitterionic pyridinecarboxylic acids (picolinic, nicotinic, and isonicotinic acid) has been studied. The cationic form of picolinic acid converts partially into the corresponding zwitterion within a borderline acidity range (pH/acidity function). The various pXa values were determined for the three isomers by spectrophotometric and potentiometric methods and reasonable agreement was found.67... 

Potentiometric sensors (ion-selective electrodes) These sensors are able to directly determine the activity of the ion of interest in the sample. In the following section, their behaviour is described in more detail. 

V Surface Characterisation, A quantitative analysis of the amounts of PAA associated with the latices was obtained by conductometric titration and a qualitative analysis of its behaviour by potentiometric titration. Titrations were performed under a nitrogen atmosphere using a Radiometer Autoburette system and a M64 pH meter together with a Wayne Kerr B642 Bridge. 

Most polymers belonging to this class are water-soluble only in their protonated form. As a consequence, most potentiometric titrations of these polymers have been performed with OH starting from their hydrochlorides, either in water or in mixed solvents, such as water/dioxane, water/sulfolane, water/acetonitrile 64). Many poly[thio-l-(N,N-dialkyl-aminomethyl)ethylene]s are optically active, and in these cases ORD and CD techniques may be used to study their protonation behaviour, in addition to potentiometric techniques. 

The partitioning behaviour of an ionisable compound can also be followed directly using a two-phase potentiometric titration using commercial instrumentation (Sirius analytical Instruments) as shown by Avdeef et al. (1998). 

These results will be selected although it is not clear if a correction for the formation of hydrogen selenate was applied to the spectrophotometric data and the variation of the medium is considerable. The behaviour of the ion-selective electrode also poses questions on the reliability of the potentiometric data and hence the value of logio/ r at/=0. 

We can expect that the eombination of the kinetic and potentiometric measurements will give a strong hint concerning the role of the different phases within the reaction network and may lead to practical correlations between the oxidation state of a catalyst and its kinetic behaviour (selectivity and activity) during oxidation. 

Zeolite films were also explored for their use in polenliomctric reactions.[93] The authors determined the potentiometric response of zeolite-modified electrodes towards cations in aqueous phase. Three different preparation routes were used for the formation of the zeolite films zeolites Y (FAU), A (LTA) and mordenite (MOR) were pressed into discs, sodalite (SOD) was grown in a free-standing membrane, and for the pressed discs of LTA a secondary growth phase was added in order to heal defects in the pressed discs. The authors could demonstrate size-selective behaviour in these systems, i.e., cations with diameters exceeding those of the zeolite window openings resulted in no detectable potential response. 

Thus, the highest oxides or chromium(VI) and molybdenum(VI) were established to destroy molten KN03 with subsequent evolution of N02 termination of this destruction resulted in stabilization of the pO values of the melt [67], This value determined by the method of potentiometric titration was found to reduce as the temperature of the melt was elevated considerably. Such a behaviour of the acidic solution in the nitrate melt was explained by the formation of nitrogen pentoxide, N2Os, or nitronium cation, NOj, in the acidic solution, together with the corresponding oxoanions of Cr(VI) and Mo(VI). Since the added acids are very strong, their reaction with the melt... 

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