Potentiometric data of

D.5.5 Potentiometric data of [2001EKB/BRO], [2004EKB/KAL] in perchlorate media... 

The paper [1958LEF] does not contain new experimental data and the proposed speciation scheme and equilibrium constants are not accepted by this review. The potentiometric data of [1954H1E] and [1954KRA/HOL] have been evalnated snbse-quently by Baes et al. [1965BAE/MEY] and Hietanen and Sillen [1968H1E/S1L] who found a consistent model and equilibrium constants for the hydrolysis of thorinm in 1 M NaC104 solntion. Their model, including the complexes (n,m) = (1,1), (2,1), (2,2), (8,4) and (15,6), is selected in the present review. 

Table A-45 Equilibrium constants logm y ,(Th (OH) " ) in nitrate media calculated by Baes and Mesmer [1976BAE/MES] from the potentiometric data of Milic [1971MIL2] at25°C.

Bases on the data of curve of the potentiometric titrations of [AuBr ] ions by thiourea (Thio), it consistently replaces bromide ions in [AuBr ] ion. They are formed mixed bromide-thiourea complexes of Au(III) AuBr Thio, AuBiyr/iio AuBrThio, AuBrThioJ. ... 

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... 

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... 

However, for m-cresol purple, thymol blue and o-nitroaniline, many estimated K, values yielded straight lines with eqn. 4.77, so that with these indicators the spectrophotometric method failed. This result led Bos to check some of the above results by means of the potentiometric method of Tanaka and Nakagawa64 applied to titrations in glacial acetic acid also he obtained the following data ... 

Alcock, R. M. Hartley, F. R. Rogers, D. E., A damped non-linear least-squares computer program (dalsfek) for the evaluation of equilibrium constants from spectrophotometric and potentiometric data, J. Chem. Soc. Dalton Trans. 115-123 (1978). 

The N-terminal peptide fragment of des-angiotensinogen Val-Ile-His-Asn contains two strongly hydrophobic amino acid residues on the N-terminal site of His-3. The potentiometric data have shown that the NiH.2L complex with this albumin-like sequence is more than two orders of magnitude more stable than the respective complex with Gly-Gly-His.1744 The NMR-based molecular structure has shown that the side chains of Val-1 and lie-2 form a well-ordered hydrophobic fence (Figure 21) shielding one side of the coordination plane from the bulk of... 

The sorption data of Cd2+ and Pb2+ by B. subtilis and E. coli were well described by a one-site complexation model (r2 > 0.9) with Cd2+ showing somewhat lower sorption affinities than Pb2+ (Kulczycki et al. 2002). A two-site sorption model yielded an improved fit but only for the E. coli data. The stability constants for the high- and low-affinity sorption sites differed by several orders of magnitude. The total metal sorption capacity of E. coli increased, and moved closer to the value of B. subtilis when the presence of low-affinity sorption sites was allowed. Ngwenya et al. (2003) used potentiometric titrations to assess the different types of sites present... 

Complex stability constants are often determined by pH-potentiometric titration of the ligand in the presence and absence of the metal ion (129). This method works well when equilibrium is reached rapidly (within few minutes), which is usually the case for linear ligands. For macrocyclic compounds, such as DOTA and its derivatives, complex formation is slow, especially at pH-s where the formation is not yet complete, therefore a batch method is used instead of direct titration (130,131). A few representative examples of stability constant data mainly collected from Ref. (132), on MRI relevant Gdm complexes are presented in Table IV. 

Considering the abundant evidence for carbene protonation, some quantitative estimate for the base strength of carbenes is clearly desirable. The conventional spectrometric or potentiometric methods of determining the pKa in solution are not applicable, with the exception of some onium ions 1 and their conjugate bases 2 (Section V.B). In favorable cases, equilibria of carbenes with the conjugate carbenium ions have been studied in the gas phase. Proton affinities of various carbenes can be obtained from their enthalpies of formation, and by ab initio computation (Section V.A). Kinetic data have been evaluated to obtain the pKa of carbenes in solution (Section V.B). 

The titration of metal ions in alcohol solvents28 follows the same sort of rules as titrations of metal ions in water29 but poses additional problems due to the lower polarity that increases ion pairing and oligomerization of the metal ions. We have performed several such titrations with the analysis of the potentiometric data depending on the level of information one requires. More complete and time-consuming analyses are reserved for the most effective catalytic metals, namely La3 +, and for the transition metal ion Zn2+ and Cu2+ along with some simple complexes of the latter two which we describe a little later. For the other metal ions described in our titration papers,7,8 we only present the data in terms of the... 

For other cases, such as La3+ where more detail is required about the nature of the species present in solution, titration data can be computer fit to more complicated multi-equilibrium models containing Mx 1 v( OR)v forms whose stoichiometry is suggested by information gained from independent spectroscopic or kinetic techniques. One must be mindful of the pitfalls of simply fitting the potentiometric data to complex multi-component models for which there is no independent evidence for the various species. Without some evidence for the species put into the fit, the procedure simply becomes an uncritical mathematical exercise of adding and removing various real and proposed components until the goodness of fit is satisfactory. 

The ion [MoW5Oi9]2 has been isolated in the solid state as a tet-rabutyl ammonium salt (183). The formation of [Mo3W3Oi9]2 in an aqueous solution was proposed on the basis of potentiometric data (184). 

In this chapter, the potentiometric method of obtaining electroanalytical data has been discussed at some length. 

Trimers (except in chloride solutions) and tetramers have not been identified directly, although a number of authors in the past and again more recently (Daniele et ah, 1994) claimed to have identified a trimer on the basis of potentiometric data. Schneider has developed a model for the structural variety of trimers that could form from the hydrolysis of dimers in chloride solutions (Schneider Schwyn, 1987). 

FIGURE 2.2S Score plot on the two LDA discriminant functions, showing the discrimination among honey samples, on the basis of potentiometric data, according to the predominant pollen variety Er, Erica Ec, Echium L, Lavandula (reproduced from Dias et at., 2008, with permission). 

Dehydrohalogenation of polyvinylidene fluoride and polyvinylidene chloride was used previously for synthesis of a new form of a hnear chain carbon - carbyne [1,2], We have found that dehydrochlorination proceeded with a decreasing rate and completed in 6 h at 80°C. According to potentiometric data for chloride ions, only 31% of chlorine contained in the initial polymer systan reacted under these conditions (Fig. 4.1). 

A more accurate way to use potentiometric data is to prepare a Gran plot6-7 as we did for acid-base titrations in Section 11-5. The Gran plot uses data from well before the equivalence poinl (Ve) to locate Ve. Potentiometric data taken close to Ve are the least accurate because electrodes are slow to equilibrate with species in solution when one member of a redox couple is nearly used up. 

In aqueous solution, thiocyanato complexes of the type Agm(SCN) 2 2 have been inferred from solubility and potentiometric data. For high concentrations of thiocyanate the complex having m = 1 was predicted to be the most abundant. Thermodynamic data are collected in Table 23.140141... 

Not mentioned in Table 2 (and often not in the original papers ) is the optical form (chirality) of the amino acids used. All the amino acids, except for glycine (R = H), contain an asymmetric carbon atom (the C atom). In the majority of cases the optical form used, whether l, d or racemic dl, makes little difference to the stability constants, but there are some notable exceptions (vide infra). Examination of the data in Table 2 reveals (i) that the order of stability constants for the divalent transition metal ions follows the Irving-Williams series (ii) that for the divalent transition metal ions, with excess amino acid present at neutral pH, the predominant spedes is the neutral chelated M(aa)2 complex (iii) that the species formed reflect the stereochemical preferences of the metal ions, e.g. for Cu 1 a 2 1 complex readily forms but not a 3 1 ligand metal complex (see Volume 5, Chapter 53). Confirmation of the species proposed from analysis of potentiometric data and information on the mode of bonding in solution has involved the use of an impressive array of spectroscopic techniques, e.g. UV/visible, IR, ESR, NMR, CD and MCD (magnetic circular dichroism). 

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