Protons release

In an oversimplified way, it may be stated that acids of the volcanoes have reacted with the bases of the rocks the compositions of the ocean (which is at the fkst end pokit (pH = 8) of the titration of a strong acid with a carbonate) and the atmosphere (which with its 2 = 10 atm atm is nearly ki equdibrium with the ocean) reflect the proton balance of reaction 1. Oxidation and reduction are accompanied by proton release and proton consumption, respectively. In order to maintain charge balance, the production of electrons, e, must eventually be balanced by the production of. The redox potential of the steady-state system is given by the partial pressure of oxygen (0.2 atm). Furthermore, the dissolution of rocks and the precipitation of minerals are accompanied by consumption and release, respectively. 

Procedures to compute acidities are essentially similar to those for the basicities discussed in the previous section. The acidities in the gas phase and in solution can be calculated as the free energy changes AG and AG" upon proton release of the isolated and solvated molecules, respectively. To discuss the relative strengths of acidity in the gas and aqueous solution phases, we only need the magnitude of —AG and — AG" for haloacetic acids relative to those for acetic acids. Thus the free energy calculations for acetic acid, haloacetic acids, and each conjugate base are carried out in the gas phase and in aqueous solution. 

Equation (4) indicates that the slope n of the log D versus the pH plot corresponds to the number of protons released upon extraction. If the logarithm of the ratio between Fe content in aqueous phase and organic phase is plotted as a function of pH, a linear relation was obtained between pH 3.5-5.4, which deviated from linearity at lower pH values (2.2). The fact that the slope of the curves were very close to unity indicates that only one proton has separated from the ligand (Eq. [5]). 

The authors formulate the mechanism in two steps, first an electron transfer from phenoxide ion to diazonium ion forming a radical pair, followed by attack of the diazenyl radical at the 4-position of the phenoxy radical and a concerted proton release, i. e., without involving the o-complex. Admittedly, there is no experimental evidence against such a concerted process, but also none for it It seems that those authors wanted only to demonstrate the occurrence of radical intermediates, but did not consider the question of the mechanism of the proton release. 

Figure 6. Pathways of protons and electrons during mitochondrial oxidations. The diagrams show the pathways of electrons which enter the electron chain at the level ofcomplexi (a)orcomplex II (b). Complexes I, III, and IV usethefreeenergy of electron transport to pump protons out of the matrix. This diagram also distinguishes formally between protons released by dehydrogenation and those which are pumped out of the matrix, although they all enter or leave the same pool.

P. Imas, B. Bar-Yosef, U. Kakafi, and R. Ganmore-Neumann, Phosphate induced carboxylate and proton release by tomato roots. Plant Soil /9/ 35 (1997). 

R. Pinton, S. Cesco, S. Santi, and Z. Varanini, Soil humic substances stimulate proton release by intact oat. seedling roots. J. Plant Nutr. 20 857 (1997). 

First of all, the mesomerism of HBI is rendered complex by the presence of several protonable groups actually, HBI might exist, depending on pH, under cationic, neutral, zwitterionic, anionic, and possibly enolic forms (Fig. 3a). The experimental p/sTa s of model analogs of HBI in aqueous solutions have been studied. Titration curves follow two macroscopic transitions at pH 1.8 and pH 8.2, each corresponding to a single proton release [69]. Comparison of theoretical... 

The stoichiometries of both oxygen consumption and of proton release subsequent to Fe(III) hydrolysis have been determined by using a combined oximeter and pH stat (Yang et ah, 1998). The overall reaction at the ferroxidase centre is postulated to be ... 

The protons released in these oxidations react with the hydrides in subsequent chemical steps ... 

The Kurbatov plot is a convenient tool to display in a simple way adsorption (surface complex formation) data. But care must be exercised in the interpretation of the data, because n varies with pH and may vary with the adsorption coverage. For an exact analysis of the proton release stoichiometry, see Hohl and Stumm (1976) or Honeyman and Leckie (1986). 

As Fig. 5.17 illustrates aggrading vegetation (forests and intensive crop production) produces acidity (see Eq. (viii) of the Appendix) since more cations are taken up by the plants (trees) H+ is released through the roots. The protons released react with... 

The determination of the ligand number (27) for adsorption reactions has been discussed by Hohl and Stumm (22). The following example illustrates the relationship between the net proton release and ligand number. 

SOH represents any surface site unassociated with any species of M, SOM a metal/surface-site complex and x the apparent ratio of moles of protons released or consumed per mole of adsorbate removed from solution. 

In surface-complexation models, the relationship between the proton and metal/surface-site complexes is explicitly defined in the formulation of the proposed (but hypothetical) microscopic subreactions. In contrast, in macroscopic models, the relationship between solute adsorption and the overall proton activity is chemically less direct there is no information given about the source of the proton other than a generic relationship between adsorption and changes in proton activity. The macroscopic solute adsorption/pH relationships correspond to the net proton release or consumption from all chemical interactions involved in proton tranfer. Since it is not possible to account for all of these contributions directly for many heterogeneous systems of interest, the objective of the macroscopic models is to establish and calibrate overall partitioning coefficients with respect to observed system variables. 

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