Metals potentiometric techniques

The formation of 1 1 complexes between ethylenediamine-N, N1 -diacetic acid (edda) and Zn11 or Cd11 have been studied by calorimetric (AH) and potentiometric techniques.389 Earlier studies had omitted to allow for protonation reactions of the ethanoato groups in the ligand. Thermodynamic parameters for edda lie between those of nitrilotriacetate and triethylenetetra-mine. Edda appears to undergo a slow metal-ion-catalyzed hydrolysis in aqueous acid solution. 

It is seen that the amperometric technique is more used for heavy-metal determination in comparison to potentiometric techniques. The research published during the last 5 years concerning heavy-metal determination has shown the applications especially for water samples (Table 14.1). 

A considerable amount of data on the protonation, and complex formation with metal ions of polymeric amines have been reported. A critical insight leads to the conclusion that much has to be done in order to reach a clear vision of the chemical properties of many polymers of this kind. Most protonation studies deal with the determination of basicity constants with potentiometric techniques, which alone give little information on the protonation mechanism only few studies have been substantiated by spectroscopic (nmr) and calorimetric measurements. 

A comparison of the behavior of a series of ligands in which one of the bonding atoms is replaced by another in the same group of the periodic table, helps in the understanding of the properties of these ligands and their metal chelates. For these reasons we have made a comparative study of the acid dissociation and certain metal chelate formation equilibria of 8-quino-linol, quinoline-8-thiol and quinoline-8-selenol by spectrophotometric and potentiometric techniques (Tables I and II),... 

Transition metal cations are usualty octahedrally solvent coordinated according to spectral evidence. When competitive ligands, such as bromide, chloride or azide ions are added to the solutions of transition metal perchlorates or tetra-fluoroborates, replacement of the solvent molecules together with the coordination changes involved has been followed by spectrophotometric, conductometric and potentiometric techniques (Table 67). 

Unexpected observations in this work were (a) the differences in the combining ratios of metal to ligand for the different ligand structures, and (b) the changes in the calcium complex model with relatively small variations in experimental conditions. These findings warrant further investigation with independent methods such as conductometric, polarographic, or potentiometric techniques that are sensitive to low concentrations of metal ion. 

The ion-selective electrodes (ISEs) provide rapid and selective potentiometric techniques for the determination of metals in water samples. Three categories of ISEs can be mentioned (a) solid state, (b) liquid ion exchangers, and (c) neutral carriers. 

The stoichiometry and stability constants of the complexes were determined most frequently by means of potentiometric techniques, but these data were very often completed with results of polarography. The structures of complexes were proposed on the basis of the stoichiometiy and stability of various species and via the analogy to other metal ion complexes. In some cases, the suggested structures of complexes were supported by the use of IR-spectroscopy, Cd NMR, and X-ray studies. 

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

The pH-metric technique used to determine partition coefficients was first used in the 1950s in solvent extraction of metal complexes [280-282], but it is in pharmaceutical research that it is most widely used thanks to the recent development of a fully automated and computer-controlled apparatus [125,283]. The potentiometric approach has been validated in various solvent systems [284-287], and it has become a relevant and expanding experimental technique to obtain lipophilicity descriptors [257,287-289]. 

Less complex techniques have been reported to be useful to study the acidic and alkaline treatment processes of biosorbents and the role of carboxyl and carboxylate groups in metal adsorption. Rakhshaee and coworkers101 used potentiometric titration curves to assess the content of such groups in L. minor biomass treated with NaOH and HC1. The results showed an increase (up to 25%) in the adsorption of Hg(II), Cr(III), Cr(VI), and Cu(II) with NaOH-treated biomass as a consequence of an increase of -COO- groups (0.92-2.42 mmol/g). On the contrary, the -COOH groups increase observed (1.50-2.41 mmol/g) due to the acidic treatment led to a decrease in the metal ions uptake (up to 33%) despite activation by the chloride salts. 

A wide variety of methods has been used in studies of oligomerization reactions. The most important quantitative method is potentiometric measurement of pH as a function of the total metal concentration and of the concentration of the analytical excess of acid or base. Other quantitative methods which are often used are potentiometric determination of metal ion concentration, calorimetry, spectrophotometry, and ion exchange. These, together with a number of other techniques, have recently been discussed thoroughly by Baes (22). 

The overall stability constants of several metals in solution have been determined by many workers34 employing Bjerrum s technique, pH titration and distribution and potentiometric methods. It has been shown by Irving and Williams,35 as well as by Mellor and Maley,36 that the stability of chelates of bivalent 3d metals increases regularly from Mn2+ to Cu2+ and decreases from... 

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). 

There are numerous ways to determine experimentally pK values of chemical compounds (205). Classical methods are potentiometric titration and ultraviolet (UV) spectroscopy, among others. These techniques have been widely applied for nucleobases and also for metal-nucleobase complexes. For the extremes such as negative pK values (pK < —2) of singly or multiply protonated nucleobases, or very high pK values (pK >15) for deprotonation of exocyclic amino groups of nucleobases (C, G, A), modifications have to be employed. These include the consideration of the Hammett acidity function in superacidic solvents or solvent mixtures (206), as well as extrapolative techniques according to Bunnett-Olsen and Marziano-Cimino-Passerini to be applied in polar, aprotic solvents (45, 207). 

According to previous applications to solid state potentiometric sensors, PEVD seems to be a possible technique to deposit in-situ a layer of an oxygen ion conducting phase on a metallic anode of an SOFC. Thus, a composite anode with both a PEVD product and a metallic anode could be realized to overcome the aforementioned anodic limitations. 

The interactions between the anions and the Ti02 surface have also been investigated by other, more direct techniques than the potentiometric titrations. Thus, Sanchez and Augustynski have employed X-ray photoelectron spectroscopy to examine the adsorption of a series of anions onto titanium dioxide films of two kinds the relatively thick (10-15 nm) films, obtained by thermal decomposition of an alcoholic TiCl4 solution, and the natural, thin (15-20 A) films covering the surface of titanium metal. The Ti02 films of the first kind, supported onto Ti metal, have been extensively used as electrode materials in electrochemical and photoelectrochemical studies ... 

The determination of ascorbic acid in foods is based, in part, on its ability to be oxidized or to act as a reducing agent. The most common method for determination of vitamin C in foods is the visual titration of the reduced form with 2,6-dichloroindophenol (DCIP) (4-7). Variations in this procedure include the use of a potentiometric titration (6), or a photometric adaptation (S) to reduce the diflSculty of visually determining the endpoint in a colored extract. The major criticisms of this technique are that only the reduced vitamin, and not the total vitamin C content of the food, is measured, and that there can be interference from other reducing agents, such as sulfhydryl compounds, reductones, and reduced metals (Fe, Sn, Cu), often present in foods. The DCIP assay can be modified to minimize the effects of the interfering basic substances, but the measurement is still only of the reduced form. Egberg et al. (9) adapted the photometric DCIP assay to an automated procedure for continuous analysis of vitamin C in food extracts. 

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