Stoichiometry, proton

Honeyman, B. D., and J. O. Leckie (1986), "Macroscopic Partitioning Coefficient for Metal Ion Adsorption Proton Stoichiometry at Variable pH and Adsorption Density", in J. A. Davis and K. F. Hayes, Eds., Geochemical Processes at Mineral Surfaces, ACS Symposium, Washington, DC. 

Proton stoichiometries change as pH is altered and are influenced by oxo group... 

Current estimates are that three protons move into the matrix through the ATP-synthase for each ATP that is synthesized. We see below that one additional proton enters the mitochondrion in connection with the uptake of ADP and Pi and export of ATP, giving a total of four protons per ATP. How does this stoichiometry relate to the P-to-O ratio When mitochondria respire and form ATP at a constant rate, protons must return to the matrix at a rate that just balances the proton efflux driven by the electron-transport reactions. Suppose that 10 protons are pumped out for each pair of electrons that traverse the respiratory chain from NADH to 02, and 4 protons move back in for each ATP molecule that is synthesized. Because the rates of proton efflux and influx must balance, 2.5 molecules of ATP (10/4) should be formed for each pair of electrons that go to 02. The P-to-O ratio thus is given by the ratio of the proton stoichiometries. If oxidation of succinate extrudes six protons per pair of electrons, the P-to-O ratio for this substrate is 6/4, or 1.5. These ratios agree with the measured P-to-O ratios for the two substrates. 

Fig. 10. A plot of the absorbance at 450 nm of ferric enterobactin solutions in 50 % aqueous methanol as the pH is changed. The data are from Figure 10 of Ref. 148. The vertical axis is absorbance (AobJ. The horizontal axis is the function (Ao — A0J/[H+] and has been multiplied by 104. A linear relationship implies a single-proton stoichiometry in the reaction. A least-squares fit (the line shown) gives a corelation coefficient of 0.978, a slope of 1.18 x 10-5 (the inverse of the protonation constant) and an intercept of. 425 (Aw)...

In view of the pH effects induced by hydrolysis in solution, the blank curve method (titration of the initial solution without the absorbent, Fig. 3.3) is only applicable, when the titration curves of the initial solution without the adsorbent and of the supernatant are identical, i.e. when the hydrolysis is negligible over the pH range of interest. Otherwise the proton adsorption can be obtained by back titration of the supernatant (Section 3.I.B. 2). In both methods (blank curve, back titration) the results need a correction for the acid or base associated with the original adsorbent, which is obtained from titrations at different ionic strengths under pristine conditions (Fig. 3.3). The description of the experimental procedure in the papers on the proton stoichiometry of specific adsorption is often not complete enough to assess if all necessary precautions have been taken into account, and the discrepancies in the results reported by different authors for similar systems are probably due in part to differences in the experimental procedure and interpretation of results. 

Fokkink et al. [35] argue that for divalent metal cations the proton stoichiometry coefficient as a function of (PZC-pH) should give one bell shaped master curve for all oxides and all metal cations, with a maximum r value slightly below 2 for PZC-pH = 0 and r = 1 at PZC-pH = 4. They support their calculations by experimental data from six sources covering seven adsorbents and six metal cations. 

FIG. 4.13 (A) The difference in the proton charge of alumina induced by the presence of 10 mol dm" of phosphate (total concentration), calculated from the charging curves in Fig. 4.11 (A). (B) The proton stoichiometry coefficient calculated from the curves shown in Fig. 4.13 (A). 

The examples shown is Section D indicate that the shape of calculated uptake curves (slope, ionic strength effect) can be to some degree adjusted by the choice of the model of specific adsorption (electrostatic position of the specifically adsorbed species and the number of protons released per one adsorbed cation or coadsorbed with one adsorbed anion) on the one hand, and by the choice of the model of primary surface charging on the other. Indeed, in some systems, models with one surface species involving only the surface site(s) and the specifically adsorbed ion successfully explain the experimental results. For example, Rietra et al. [103] interpreted uptake, proton stoichiometry and electrokinetic data for sulfate sorption on goethite in terms of one surface species, Monodentate character of this species is supported by the spectroscopic data and by the best-fit charge distribution (/si0,18, vide infra). 

The proton stoichiometry coefficients taken from literature are compared with reciprocal slope of log-log adsorption isotherms at constant pH The agreement wa.s good with Cd and Cu (r = 1 8 and I 9. respectively), but rather poor wtth Zn... 

S per Cu adsorbed, almost pH independent In the presence of chlorides tlic proton stoichiometry factor is lower and increases when pH increases... 

Modeling divalent metal ion sorption requires estimation of the proton stoichiometry (the number of protons released per metal ion sorbed), the type of surface complex (inner or outer sphere) formed, and the formation constants for each reaction selected. Table 7-2 presents a list of various reactions that may be incorporated into the TLM. Because a variety of combinations of different sorption reactions and constants may fit various aspects of the sorption data equally well (see, e.g., Westall Hohl, 1980 Hayes et al., 1991 Katz Hayes, 1995a), protocols are needed to insure the best choice of reactions and a more universally accepted set of guidelines to allow reproducibility from one laboratory to another. The strategy used in modeling Co(II) sorption to a-Al203 involved ... 

The non-integer stoichiometry we propose may be put together with the results of Marbti and Wraight [8], and Me Pherson et al [9],. who reported non-integer proton stoichiometries with isolated bacterial reaction centers. This was interpreted in terms of a number of residues with distributed pK values and electrostatic interactions with the redox centers. A similar situation seems to be involved in the OES. 

The above-cited example on Cd/hematite indicates that some groups perform titrations in the presence of solutes different from innocent electrolytes. Such titrations may yield important macroscopic information on the proton balance of the suspension in the presence of such a solute (Table 2). However, the exact proton stoichiometry of some surface complex can rarely be inferred, because this would require that only one complex exists and that the protonation states of the surface groups, which are not contributing to that particular surface complex, are not affected by the adsorption process. This can, at best, be assumed in a quaUtative interpretation but can be quantitatively handled with the mean field approximation and the corresponding assumptions inherent to the respective computer programs. In fitting some models to adsorption data, proton data will constitute an independent and very valuable dataset representative of the system however, they may be restricted to sufficiently high solute to sorbent ratios. 

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