Potentiometric experiments

Papisov et al. (1974) performed calorimetric and potentiometric experiments to determine the thermodynamic parameters of the complex formation of PMAA and PAA with PEG. They investigated how temperature and the nature of the solvent affected the complex stability. They found that in aqueous media the enthalpy and entropy associated with the formation of the PMAA/PEG complex are positive while in an aqueous mixture of methanol both of the thermodynamic quantities become negative. The exact values are shown in Table II. The viscosities of aqueous solutions containing complexes of PMAA and PEG increase with decreasing temperature as a result of a breakdown of the complexes. 

Electrode potential. Potentiometric experiments determine potentials. The two components of the redox couple are only together at equilibrium at a single energy specific to the system under study and at the conditions employed. This energy, when expressed as a voltage, is termed the electrode potential, E. The electrode potential is also known as the redox potential or reduction potential . Some texts for physicists call E the electrode energy . 

We should remind ourselves that in a potentiometric experiment, we cannot measure individual electrode potentials we can only measure the emf of a cell. The emf comprises two half cells (see equation (3.3)). The value of one electrode potential will be known, while the other will be unknown the value of E for the known half cell is that of the electrode potential for the reference electrode. Accordingly, the value of E for the half cell that contains analyte is only as good as the value of E for the reference electrode. 

In this present section, we will assume that the values of used in the calculations are wholly accurate, and that all the errors are incorporated within the electrode potential, o,r- The measured term in a potentiometric experiment is the cell emf. In the simplest systems, the emf is made up of electrode potentials, one per half ceil (although see below). One potential is the electrode potential of interest, while the other is the potential of a reference electrode. As a crude generalization, the emf can be measured to within a precision of about 0.1 mV, with the exact value depending on the system under study. Fluctuations, as caused, for example, by solution stirring, may decrease the precision to as poor as 2-3 mV. 

The above three sources of error all result from the fact we assume that the emf measured in a potentiometric experiment comprises only two terms, i.e. one... 

We have seen already that the root of the word potentio- means that we are looking at a potential, so the crucial piece of apparatus in a potentiometric experiment is a potentiometer (which, in practice, will almost certainly be marketed and labelled as a voltmeter ). In the previous chapter, we looked at the simplest forms of potentiometric experiment in which the two half cells were physically separated, one from the other, in order to prevent electron transfer (et) occurring. To recap, such a separation is a key requirement for ensuring that a frustrated equilibrium holds. 

What happens during such a potentiometric experiment, i.e. what are the implications of the electron-transfer reaction ... 

One of the simplest potentiometric experiments in this context is the redox titration. We shall consider a real example, i.e. the determination... 

Let us revisit the electrochemical cell shown earlier in Figure 3.1. In this figure, two redox electrodes are immersed in solutions of their respective ions, with the half cells being connected by a salt bridge. If we were to connect an infinite-resistance voltmeter between the cells, then it would be possible to perform potentiometric experiments such as those described in the previous chapter. One electrode would be positive with respect to the other, with the separation in potential between the two electrodes being the emf - but only if the measurement was performed at equilibrium. (As before, we take the word equilibrium to imply that no charge flows.)... 

Electrode A conductor employed either to determine an electrode potential (at zero current, i.e. for potentiometric experiments), or to determine current during a dynamic electroanalytical measurement. The electronic conductivity of most electrodes is metallic. 

In voltammetric experiments a normal type of calibration is the recording of voltammetric curves for a known system, constructing plots such as variation of limiting current with the transport parameter, or of current with concentration. In potentiometric experiments the equivalent would be the variation of potential with concentration. These curves are especially important in electroanalytical experiments working curves permit the immediate conversion of a measured current or potential into a concentration. 

Finally, the detection limit of a technique, which is determined by the impossibility of separating signal from noise (blank), should be considered. In potentiometric experiments the detection limit results from a diminution down to zero variation in measured potential with concentration decrease, as discussed in Chapter 13. It is clear that reproducibility has an important effect on detection limit, and detection limits are sometimes quoted on this basis, such as three times the standard deviation. Unfortunately in the electroanalytical literature, as in many other areas, there is sometimes an incorrect use of statistical techniques that favours the authors results or hides the degree of nonreproducibility ... 

In contrast to the results from electrokinetic experiments the curves of the potentiometric experiments do not show any approach to a plateau region observed in electrokinetic experiments at pH>9. The steady slope of the exponential function os=os (pH) appears from taking the inner surface of the silica particle into account while electrokinetic experiments reflect only the charge distribution in the shear plane which is situated outside of particle surface. 

In addition to reactions involving cobalt species, protonation (3) and deprotonation (4), together with surface association of nitrate (8) and sodium ions (9) are taken into account. In doing so one deals with seven standard equilibrium constants. The parameters of basic surface reactions (3,4,8,9) were obtained by separate potentiometric experiments in absence of cobalt ions. 

Electrochemical experiments in this study were performed with a BAS-KX) Electrochemical Analyzer (Bioanalytical Systems Inc., Lafayette, Indiana). For potentiometric experiments in calm solutions, platinum disk electrodes were used in the BAS-100 cell stand. For hydrodynamic experiments a Pine Instruments Inc. rotating-disk platinum electrode was interfaced with the BAS-100. 

This is a study of the complexation of zirconium by hydroxide using the potentiometric and solvent extraction techniques. The potentiometric experiments were conducted at 25°C and the solvent extraction experiments at 20°C. All experiments were conducted using 4.0 M CIO4, with the proton concentration being varied at 0.5, 1.0 and 4.0 M. 

The potentiometric experiments used the method of competing reactions with fluoride and changes in activity coefficients between the media used being determined using Th which does not hydrolyse in the proton concentration range examined. The stabil-... 

Table 18 lists values of AGp for the partitioning of the alkali metal ions, including hydrogen ion and several other univalent ions, between water and several water-immiscible organic solvents. Some of the values listed were in fact originally determined from salt-partitioning or cation-exchange data. Other values were determined from electrochemical and potentiometric experiments. 

Our recent work has partially resolved the difficulties of interpretation of potentiometric experiments and provided a quantitative method for deciphering potentiometric data. Using this theory, we have performed quantitative analysis of the data of Belevich et al, in which the experimental amplitudes and rates are related to specific residues that exchange electrons and protons, and generate the observed membrane potential. Using this theory, we have tested proposed candidates for the proton pump site of the enzyme against experimental potentiometric data [37]. 

For a two-electrode cell, the net reaction comprises two half-reactions, involving the processes at the two electrodes. As already mentioned, usually only one of these processes is of interest, and this occurs at the working electrode in dynamic electrochemistry experiments or indicator electrode in equilibrium (potentiometric) experiments. The other electrode is made up such that it maintains a constant composition throughout the measurement, thus providing a reference potential. The most common reference for aqueous solution is the saturated calomel electrode (SCE), depicted in Fig. 1, and this provides... 

Open circuit. In an open-circuited potentiometric experiment where 1=0, both and 7ci vanish, and Eqn (11.66) becomes... 

Analogous to SECM amperometric methods, SECM potentiometric methods are based on the measurement of electrode potentials (tip and substrate, AE-j- and AEg, respectively) as a function of various parameters, including tip-substrate distance (d), XY location, and time. Again, a potentiometric tip responsive to the desired solution component is necessary to perform any SECM potentiometric experiment. To use potentiometric probes in SECM, it must be possible to exactly evaluate the tip-to-substrate distance. Different electrochemical methods to evaluate the potential-distance dependence have been investigated, for example, utilization of metal/metal oxide electrodes as both amperometric and potentiometric tips for pH measurement," double barrel tips having one... 

The weighting factors are the selectivity coefficients of the membrane toward ions j relative to ion i. The smaller the /fl", the smaller are the contributions of the interfering ions to the total cell potential. The selectivity coefficients can be determined potentiometrically and are often listed in textbooks. Their value is used as a guideline when designing ISE-based potentiometric experiments. 

Neck and Fanghanel (2008). Ekberg et al. (2004) used the species Zr2(OH)g to describe the solubility behaviour of Zr(OH)4(s). This species has a OH Zr ratio of 3 1, as does Zr3(OH)9 . It is plausible that the species identified by Ekberg et al. could equally have been Zr3(OH)9, even in their potentiometric experiments. Altmaier, Neck and Fanghanel (2008) obtained stability constants for the species in 0.51, 1.02 and 3.20molkg NaCl at 25"C and found that these stability constants were consistent with that derived by Brown, Curti and Grambow (2005) for the species at zero ionic strength. Thus, the value from this latter review is retained ... 

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