Acidity constants potential-determining ions

The addition of acid (Ca) or base (Cb) to a CaCO system (while pco2 = constant) will change the alkalinity in solution and produce (i) a shift in the HCO3, CO3", Ca2+ equilibrium (and in pH), (ii) an adsorption of potential determining ions on the CaC03 surface, and (iii) a dissolution or precipitation of CaC03. 

Schindler and Stnmm (1987) snmmarized the intrinsic acidity constants of various metal oxides/ minerals under solution chemistry. In this system, H+ and OH" are potential determining ions and the surface charge and surface potential are defined by the pH of the solution. There is a particular pH at which the surface charge becomes zero, that is, the point of zero charge (pzc) or the isoelectric point (iep). is correlated to the pzc, which is as follows (Bin et al., 2011) ... 

Although a single value is given in Table 16 for Ee (Ce3+(aq)/Ce4+(aq)), the oxidation potential of Ce3+ ion in aqueous media varies considerably with the nature of the counterion, doubtless due to stabilization of Ce4+ to different extents by complexation. Thus in 1M H2S04, Ee= 1.4435 V,680 but in perchloric acid solutions, = 1.6400-1.7310 V depending on concentation,681 while a range of 1.6085-1.6104 V was observed in nitric acid solutions.682 There is some evidence for the formation of polymeric species in aqueous nitric acid solutions, and this may explain some anomalies in electrode potential determinations. Values for association constants of 17 and 2 respectively for Ce4+-Ce4+ and Ce4+-Ce3+ interactions were obtained.683... 

A large volume of work64 has been published on the determination of stability constants for complexes of hydroxamic acids, e.g. acetohydroxamic acid.65 The stability of 3d transition metal ions (Mn2+ to Zn2+) with salicylhydroxamic and 5-methyl-, 5-chloro-, 5-bromo-, 5-nitro-, 4-chloro-, 4-bromo- and 3-chloro-salicylhydroxamic acids,66 as well as with methyltolylbenzohydroxamic acid,67 has been studied potentiometrically. Stability constants of iron(III) with a number of hydroxamic acids have been determined by redox potential studies.68... 

Jacobsen and Langmuir (1974) determined a value for pKSp (25°C) for calcite of 8.42 0.01, whereas Berner s (1976) value was 8.45 0.01. Berner also determined the pKSp for aragonite at 25°C to be 8.28 0.03. An aspect of particular interest of both studies was that to obtain internal consistency for the carbonic acid system or constant values for the solubility products over the range of conditions studied, it was necessary to neglect ion pair formation. The potentially important ion pairs that could have formed in the experimental solutions are CaHCC>3+ and CaC03°. The former is by far the most important species, and a vast body of previous literature supported its existence (see Plummer and Busenberg, 1982, for summary). 

The results obtained in acid solutions indicate that there are two distinct mechanisms. At low overpotentials, the atom-atom recombination step (59F) is believed to be rate determining, This should yield a Tafel slope of b = - 23RT/2F = - 30 mV and a reaction order (at constant potential) of = 2 in agreement with experiment. As the overpotential is increased, the fractional coverage 0 must also increase (cf. Eq. 41F). This increases the rate of the atom-alom recombination step, but also that of the ion-atom recombination, which occurs in parallel. As 0 approaches unity, the rale of step 40F can no longer increase but the rale of the ion—atom recombination step can grow, since it depends on potential (cf. Eq. 20F). This step then becomes rate... 

Considering the reaction between Ce(IV) and chloride ion, it appears that the observed formal potential of 1.28 V in 1 M hydrochloric acid is actually a mixed potential determined partly by the chlorine-chloride couple. Consequently, measured values of the potential cannot be used to calculate the formation constants of Ce(rV)-chloride complexes. From a practical analytical viewpoint, however, it is important that Ce(IV) can be used as a titrant for solutions containing up to 3 Af hydrochloric acid without loss of chlorine. 

Cu, have shown toxic effects on a diverse assortment of aquatic biota (6 8). The very same metal ions, on the other hand, portray a reduction or complete eradication of toxic effects when complexed with natural organic matter. Fate and transport of metal ions in the environment are also governed by associations with fulvic acid material. Therefore, determination of stability constants between FA ligand sites and potentially hazardous metal ions should be considered fundamentally important. 

Voltammetric methods also provide a convenient approach to establish the thermodynamic reversibility of an electrode reaction and for the evaluation of the electron stoichiometry for the electrode reaction. As outlined in earlier sections, the standard electrode potential, the dissociation constants of weak acids and bases, solubility products, and the formation constants of complex ions can be evaluated from polarographic half-wave potentials, if the electrode process is reversible. Furthermore, studies of half-wave potentials as a function of ligand concentration provide the means to determine the formula of a metal complex. 

A mercury cathode finds widespread application for separations by constant current electrolysis. The most important use is the separation of the alkali and alkaline-earth metals, Al, Be, Mg, Ta, V, Zr, W, U, and the lanthanides from such elements as Fe, Cr, Ni, Co, Zn, Mo, Cd, Cu, Sn, Bi, Ag, Ge, Pd, Pt, Au, Rh, Ir, and Tl, which can, under suitable conditions, be deposited on a mercury cathode. The method is therefore of particular value for the determination of Al, etc., in steels and alloys it is also applied in the separation of iron from such elements as titanium, vanadium, and uranium. In an uncontrolled constant-current electrolysis in an acid medium the cathode potential is limited by the potential at which hydrogen ion is reduced the overpotential of hydrogen on mercury is high (about 0.8 volt), and consequently more metals are deposited from an acid solution at a mercury cathode than with a platinum cathode.10... 

The presence of residual unbound transition-metal ions on a dyed substrate is a potential health hazard. Various eco standards quote maximum permissible residual metal levels. These values are a measure of the amount of free metal ions extracted by a perspiration solution [53]. Histidine (5.67) is an essential amino acid that is naturally present as a component of perspiration. It is recognised to play a part in the desorption of metal-complex dyes in perspiration fastness problems and in the fading of such chromogens by the combined effects of perspiration and sunlight. The absorption of histidine by cellophane film from aqueous solution was measured as a function of time of immersion at various pH values. On addition of histidine to an aqueous solution of a copper-complex azo reactive dye, copper-histidine coordination bonds were formed and the stability constants of the species present were determined [54]. Variations of absorption spectra with pH that accompanied coordination of histidine with copper-complex azo dyes in solution were attributable to replacement of the dihydroxyazo dye molecule by the histidine ligand [55]. 

The use of ISEs in non-aqueous media(for a survey see [125,128]) is limited to electrodes with solid or glassy membranes. Even here there are further limitations connected with membrane material dissolution as a result of complexation by the solvent and damage to the membrane matrix or to the cement between the membrane and the electrode body. Silver halide electrodes have been used in methanol, ethanol, n-propanol, /so-propanol and other aliphatic alcohols, dimethylformamide, acetic acid and mixtures with water [40, 81, 121, 128]. The slope of the ISE potential dependence on the logarithm of the activity decreases with decreasing dielectric constant of the medium. With the fluoride ISE, the theoretical slope was found in ethanol-water mixtures [95] and in dimethylsulphoxide [23], and with PbS ISE in alcohols, their mixtures with water, dioxan and dimethylsulphoxide [134]. The standard Gibbs energies for the transfer of ions from water into these media were also determined [27, 30] using ISEs in non-aqueous media. 

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