Basic proton level

In addition to the acidic proton level, there is the basic proton level in basic aqueous solution which is represented by the unitary vacant proton level (the... 

Fig. 3-16. Acidic and basic proton levels in aqueous solution h (Hso /H20) = unitary energy of hydration of a standard gaseous proton to occupy the xmitary vacant acidic proton level 1h (H2cvoh-) = unitary energy of hydration of a standard gaseous proton to occupy the unitary vacant basic proton level Dh o = ionic dissociation energy of HjO.

It is interesting to point out the similarity between the proton level diagram of aqueous solutions and the electron level diagram of semiconductors as shown in Fig. 3-20. The ionic dissodation energy (1.03 eV) of water molecule H2O to form an ion pair of H30 -0H is the energy gap between the imitary acidic proton level and the unitary basic proton level this may correspond to the band gap of semiconductors. The concentration product, of the addic... 

Fig. 3-20. Comparison of the proton level diagram of aqueous solutions with the electron level diagram of semiconductors (a) proton levels in pure water, (b) electron levels in intrinsic semiconductors, (c) proton levels in week add solutions, (d) electron levels in n type semiconductors. = proton level in aqueous solutions = unitary acidic proton level of HaO /HjO = unitary basic proton level of HjO/OH" nij. , = unitary...

In general, semiconductor electrodes adsorb in aqueous solutions water molecules, hydronium ions, and hydroxide ions in addition to various solute ions. As a result, the dissociation-association equilibria of the adsorbed hydronium ions and water molecules produce, in the proton dissociation-association reactions of Eqns. 9-69 and 9-70, the acidic and basic proton levels, respectively, on the electrode interface as shown in Fig. 9-21 ... 

Fig. 9-22. Unitary proton levels of hydrated and adsorbed hydronium ions (acidic proton) and of hydrated and adsorbed water molecules (basic proton) the left side is the occupied proton level (the real potential of acidic protons), and the right side is the vacant proton level. Hi/HjO) = unitary occupied proton level of adsorbed hydronium ions (acidic proton level) H20.d = unitary vacant proton level of adsorbed hydronium ions (acidic proton level) and unitary occupied proton level of adsorbed water molecules (basic proton level) OH = unitary vacant proton level of adsorbed water molecules (basic proton level) (pHi, ) = hydrated proton level at iso-electric point pR...

For compound semiconductors, the adsorbed proton level differs with different constituents in the semiconductor thus, the distinction between the acidic and basic proton levels, pKi and pKt, is greater than in the case of elemental semiconductors. For example, on metal oxide electrodes, the acidic proton dissoci-... 

For metal oxide electrodes, the iso-electric point pH p is also located midway between the unitary acidic proton level and the unitary basic proton level of adsorbed water. Table 9-1 shows the iso-electric point pHi, of several metal oxides in aqueous solutions. 

When considering CEC or AEC, the pH of the soil solution is extremely important. There will be competition for binding sites between H30+ and other cations in the soil solution. Therefore, the observed CEC will be lower at high proton concentrations, that is, at low or acid pH levels, and higher at basic pH levels. For analytical measurements, the CEC of soil at the pH being used for extraction is the important value, not a CEC determined at a higher or lower pH. 

Fig. 3-16. Hie unitary levels of acidic proton HgOVIIjO and basic proton HgO/OH in pure water, and proton transfer between two levels Copl = occupied proton level cvpl = vacant proton level fiH cHgo-. D) = unitary occupied (donor) level of acidic proton, 1h (H20.a> = unitary vacant (acceptor) level of acidic proton.

The energy level of hydrated proton depends on the proton concentration. For an acidic proton in Eqn. 3-32 and a basic proton in Eqn. 3-34, the proton levels Hh- are, respectively, given in Eqns. 3-37 and 3-38 ... 

In general, the acidic and basic proton hydration processes may occur simultaneously giving the same proton level for both the acidic and the basic protons. In pure liquid water where WHgo- = Woh- io electroneutrality, the proton level is obtained from Eqns. 3-39 and 3-40 as shown in Eqn. 3-41 ... 

It follows, then, that the proton level in pure water is located midway between the unitary level of acidic proton and the unitary level of basic proton, leading to the hydrated proton concentration at pH 7. 

Fig. 9-21, Proton levels of adsorbed hydronium ions and of adsorbed water molecules on semiconductor electrodes (a) acidic proton dissociation of adsorbed hydronium ions, (b) basic proton dissociation of adsorbed water molecules. S = semiconductor surface atom.

In aquatic chemistry, the unitary proton level of the proton dissociation reaction is expressed by the logarithm of the reciprocal of the proton dissociation constant i.e. p = - log K here, a higher level of proton dissociation corresponds with a lower pK. When the pKy of the adsorbed protons is lower than the pH of the solution, the protons in the adsorbed hydronium ions desorb, leave acidic vacant proton levels in adsorbed water molecules, and form hydrated protons in the aqueous solution. Fig. 9-22 shows the occupied and vacant proton levels for the acidic and basic dissociations of adsorbed hydronium ions and of adsorbed water molecules on the interface of semiconductor electrodes. 

Equation 9-72 indicates that the logarithm of the ratio of the concentrations of adsorbed protons (acidic occupied proton level) to adsorbed hydroxide ions (basic vacant proton level) depends linearly on the pH of the solution. 

The pH at which the concentration of acidic occupied proton levels of adsorbed h3dronium ions equals the concentration of basic vacant proton levels of adsorbed water molecules is called the iso-electric point pHi, here, the net interfacial charge of adsorbed ions at the interface is zero. The iso-electric point pH,, is expressed in Eqn. 9-73 ... 

NMR titrations (of anion into ligand at fixed pH) and pH-potentiometric titrations (of pH at fixed anion ligand ratios) provide comparable values of the stability constants for binding of mononegative oxoanions by protonated R3Bm, R3F, and R3P hosts [15,20,21] Table 2. The weak complexation at hexaprotonated levels for tetrahedral monoanionic oxoanions makes it difficult to obtain reliable data for protonation levels below 5. This has however been achieved for nitrate with the cleft binding host R3P as well as for Re O4 with the most basic cryptand R3Bm. 

A density functional theory (B3LYP/6-311 - -G ), ab initio (HF/3-21G ) and semi-empirical (PM3) study of intrinsic basicities, protonation energies or protonation enthalpies of phosphazene bases has been reported. The study shows that the organic superbases can reach the basicity level of the strongest inorganic superbases, such as alkali metal... 

Cyclic voltammetric experiments on polyaniline in aniline-free melt show very broad peaks and on continuous scanning the polymer film loses its electroactivity. However, when the polymer film is transferred in aqueous acidic conditions it gives useful peak potential of polyaniline, suggesting that the basic structure of polymer synthesized in melt and in an aqueous medium is the same with, of course, differences in protonation levels. 

When we use any substance as a solvent for a protonic acid, the acidic and basic species produced by dissociation of the solvent molecules determine the limits of acidity or basicity in that solvent. Thus, in water, we cannot have any substance or species more basic than OH or more acidic than H30 in liquid ammonia, the limiting basic entity is NHf, the acidic is NH4. Many common inorganic acids, for example HCl, HNO3, H2SO4 are all equally strong in water because their strengths are levelled to that of the solvent species Only by putting them into a more acidic... 

As with EDTA, which we encountered in Chapter 9, o-phenanthroline is a ligand possessing acid-base properties. The formation of the Fe(o-phen)3 + complex, therefore, is less favorable at lower pH levels, where o-phenanthroline is protonated. The result is a decrease in absorbance. When the pH is greater than 9, competition for Fe + between OH and o-phenanthroline also leads to a decrease in absorbance. In addition, if the pH is sufficiently basic there is a risk that the iron will precipitate as Fe(OH)2. 

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