Proton reference level

The BNC can be defined by a net proton balance with regard to a reference level -the sum of the concentrations of all the species containing protons in excess of the reference level, less the concentrations of the species containing protons in deficiency of the proton reference level. For natural waters, a convenient reference level (corresponding to an equivalence point in alkalimetric titrations) includes H20 and H2C03 ... 

If the water under consideration contains other acid- or base-consuming species, the proton reference level must be extended to the other components. In addition to the species given, if a natural water contains organic molecules, such as a carboxylic acid (H-Org) and ammonium ions, the reference level is extended to these species. 

The PBE can be thought of as a proton reference level relative to the aqueous solution components chosen to define the equilibrium problem. A more detailed discussion of the PBE is given by Morel (1983) and Pankow (1991). 

The proton mass balance is established with reference to a "zero level" or a "reference level" for protons (the proton reference level, PRL). The species having protons in excess of the PRL are equated with the species having less protons than the PRL. The PRL is established as the species with which the solution was prepared. 

We note that equation 40b is identical to the proton condition (iva) of Example 3.3a that is, the choice of HB as a component is equivalent to our earli( r notion of HB and H2O as reference level. In examining equations 39 and 40, we see that there are effectively four unknown species activities (B, HB, OH, and H ), the activity of H2O being constant for the dilute solution. As n Example 3.3a, there are four independent relationships 39a, 39d, 40a, and 40b, the remaining expressions being irrelevant for our conditions. Although water is always a component in our tableaux, we will omit it from subsequent tableaux for simplicity, the concentration of H2O being effectively constant for dilute solutions. 

The reference level is defined by the composition of a pure solution of HA in H2O (/ = 0 [ANC] = 0), which is defined by the proton condition, [H l = [A-] -i- [OH ]. (In this and subsequent equations, the charge type of the eicid is unimportant the equation defining the net proton excess or deficiency can always be derived from a combination of the concentration condition and the condition of electroneutrality.) Thus in a solution containing a mixture of HA and NaA, [ANC] is a conservative capacity parameter. It must be expressed in concentrations (and not activities). Addition of HA (a species defining the reference level) does not change the proton deficiency and thus does not affect [ANC]. 

In the same monoprotic acid-base system, the base-neutralizing capacity with respect to the reference level (/ = 1) of a NaA solution (proton condition [HA] -I- [H ] = [OH ]) is defined by... 

One has to distinguish between the ion concentration (or activity) as an intensity factor and the availability of the ion reservoir as given by the H-acidity or the deficiency of ions, or the alkalinity. Alkalinity and acidity are very important concepts although there are different ways to define these capacity factors, all definitions essentially relate to the proton condition at a given reference level. For the carbonate system, alkalinity [Aik] refers conceptually to the proton condition with reference to H2CO, H2O ... 

The ipso protonation deserves a special scrutiny is discussed earlier. Here we show that the additivity rule is operative for the ipso protonation too, if proper reference level is found. Let us consider multiply substituted fluorobenzenes. They are schematically depicted in Fig.7. It is obvious that the proton affinity of benzene cannot serve as a gauge value for the ipso protonation. Instead, we shall employ once again homodesmic reactions and proceed as follows. Protonation at position 1 of 1,2,3-trifluorobenzene will provide an illuminating example in this respect. The corresponding homodesmic reactions read ... 

As already stated, we have j p(H30+/H20) = 0(at standard pressure) for the proton potential. It represents the reference level, so to speak. 

Perhaps surprisingly for such an abundant element, reference levels in biological fluids have not been determined. Versieck and Comelis [7] found only one recent study [8] of Ti in serum from healthy individuals, but consider the reported value of 90 p,g/liter, determined by proton nuclear activation, to be highly questionable and likely to arise from a failure to consider contamination and sampling problems. 

A Chart of Occupied and Vacant Proton Levels. With two exceptions, each of the values of J given in Tables 9, 10, and 11 refers to the process where a proton is raised to the vacant proton level of an HsO molecule from a lower occupied proton level of a species of molecule or molecular ion in each case the value of J gives the amount by which this initially occupied level lies below the vacant level of H20. Obviously, using these values, it is at once possible to map out a chart of the proton levels of these various particles in aqueous solution, as has been done in Fig. 36. The two exceptions in Table 9 are the values derived from the KB of glycine and alanine. In these cases, as shown in (125), a proton is transferred to a vacant level from the ordinary occupied proton level in a water molecule the value of J gives the amount by which the vacant level lies above this occupied proton level of H20. 

The vacant proton level lies 0.268 electron-volt below the occupied level of (H30)+. Referring to Table 12 we see that this level lies at about the same depth as the vacant level of the chloraniline molecule. 

In Sec. 128 it was found that the vacant proton level of indicator 2 lies at 0.192 electron-volt below the occupied level of (HaO)+ in dilute aqueous solution. Using the successive increments listed in the last column of Table 39, we find, counting upward, that the value for indicator 5 is —0.052, referred to the same zero of energy. Proceeding by the same stepwise method to No. 6 we find for the energy of the vacant proton level the positive value +0.038. This still refers to the occupied level of the (II30)+ ion in dilute aqueous solution. It means that work equal to 0.038 electron-volt would be required to transfer a proton from the (H30)+ ion in very dilute solution to the vacant level of No. 6 in the concentrated acid solution in which the measurements were made. A further amount of work would be required to transfer the proton from the occupied level of No. 6 to the vacant proton level of one of the H2O molecules in the same concentrated solution. This is the situation because, as mentioned above, the changing environment has raised the proton level of the (HaO)+ ion relative to that of each of the indicator molecules. 

Table I reports the observed NMR linewidths for the H/3 protons of the coordinating cysteines in a series of iron-sulfur proteins with increasing nuclearity of the cluster, and in different oxidation states. We have attempted to rationalize the linewidths on the basis of the equations describing the Solomon and Curie contributions to the nuclear transverse relaxation rate [Eqs. (1) and (2)]. When dealing with polymetallic systems, the S value of the ground state has been used in the equations. When the ground state had S = 0, reference was made to the S of the first excited state and the results were scaled for the partial population of the state. In addition, in polymetallic systems it is also important to account for the fact that the orbitals of each iron atom contribute differently to the populated levels. For each level, the enhancement of nuclear relaxation induced by each iron is proportional to the square of the contribution of its orbitals (54). In practice, one has to calculate the following coefficient for each iron atom ...

---