Single-site titration model

Figure 10-4. The double- and single-site titration models for His and Asp groups [42]. (A) In the double site model, only one X is used for describing the equilibrium between the protonated and deprotonated forms, while the tautomer interversion process is represented by the variable x. (B) In the single-site model, protonation at different sites is represented by different X variables. HSP refers to the doubly protonated form of histidine. HSD and HSE refer to the singly protonated histidine with a proton on the h and e nitrogens, respectively. ASP1 and ASP2 refer to the protonated carboxylic acid with a proton on either of the carboxlate oxygens...

Fig. 12.12 Top Interaction of 4e with human serum albumin as probed by isothermal titration calorimetry, a single-site model yielded a stoichiometry of 5 1 of 4e HSA with a kn of 2 pM. Middle Inhibition of hemolysis of 4e by HSA. Bottom Identical potency of NO inhibition of 4e solubilized in DMSO or in HSA...

Figure 7 shows pH dependent eleetrokinetic profiles in 7.5 mM NaCI for APS-coated quartz. The APS surface is best described by a two acid-single base model of the surface. The acid sites appear to be silanols of the same pK l as for clean glass, and the basic site an APS amine of pKb I0, which is consistent with a value obtained from thermometric enthalpy titration 24 ... 

A detailed discussion of the FITEQL optimization of TLM surface parameters for a-Al203 has been discussed previously (Hayes et al., 1991). The analysis demonstrated that the TLM version of the surface complexation model can fit titration data reasonably well using a range of TLM surface parameter sets (see Fig. 7-5 as an example). While it was not possible to identify a single optimum set of parameters, it was possible to narrow the range of parameters. Valid sets of surface parameters for the TLM were found for ApXa between 0 and 6, C, between 0.8 and 2.0 F m-2, and Ns between 1 and 10 sites nm 2. In all cases the value of C2 was set equal to 0.2 F m-2. 

The quantitative pH shift model [23] combined (1) a proton balance between the surface and bulk liquid with (2) the protonation-deprotonation chemistry of the oxide surface (single amphoteric site), and (3) a surface charge-surface potential relationship assumed for an electric double layer. Given the mass and surface area of oxide, the oxide s PZC, its protonation-deprotonation constants Kj and Kj (Figure 13.2), and the hydroxyl density, these three equations are solved simultaneously and give the surface charge, surface potential, and final solution pH. The mass titration experiment of Figure 13.4 can be quantitatively simulated, but perhaps the most powerful simulation is a comprehensive prediction of final pH versus initial pH, as a function of... 

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