Hydrogen ion dependencies

Step (2.3), and AS, are 1.52 kcal.mole and —37.3 cal.deg . mole , respectively. It is noteworthy that the following reduction reactions of the analogous M02 actinide ions show a similar first-order hydrogen-ion dependence (NpOi- +Fe + ) , (Np02-"- -Np + ) , (Pu02++Pu02 ") and (U02 "-l-In Table 1 the activation parameters of the V(V)-t-Fe(II) reaction are... 

Adin and Sykes S have re-examined the hydrogen-ion dependence of the V(III)- -Cr(lI) system over a broad range of H concentrations from 0.45 M down to 0.016 M. They confirm the type of acid dependence quoted by Espenson and support the interpretation given by Haim, equations (2.16) to (2.18). At 25 °C and n = 0.5 M, the experimental parameters q and r are 0.50 and 0.10, respectively, so that k = 3.57x 10 l.mole". sec"S and k Jk = 0.1 mole.P. Adin and Sykes S find no evidence for the existence of the more complex hydrogen-ion dependence originally suggested by Sykes in a re-analysis of Espenson s data °. 

On this basis Cr(V), not Cr(IV), is the kinetically important intermediate such that k = 3 k4 and k = k Jk. The hydrogen-ion dependence of the reaction rate has been discussed. Furthermore, comparisons are drawn with the rate of the Cr(VI)+Fe(phen)3 reaction, and Sullivan has speculated on the intimate nature of both mechanisms in the light of Marcus theory... 

Scheme B) is considered unlikely on the grounds that, at the low concentrations of U(V) involved, the latter would disappear by oxidation with Fe(ni) rather by dismutation. The hydrogen-ion dependence suggests that the rate-controlling step between Fe(irf) and U([V) can be visualised in terms of a series of competitive reactions of hydrolysed species of both reactants viz.

The inverse hydrogen-ion dependence of the rate of reaction (13.22) is exceptionally complex and is summarised by... 

The process is first order in each of the reactants and the rate is unaffected by the presence of Fe(lII). However, there is some uncertainty about the inclusion of a term in Pu(lII) in the rate law. From the observed hydrogen-ion dependence, Newton and Cowan conclude that the principal reaction path has an activated complex formed from Pu, Fe and water with the prior loss of one hydrogen ion. The probable form of rate law is... 

The concentration of hydrogen ions depends on the ionic strength through the ratio of activity coefficients appearing in the dissociation equilibrium expression. Any change in ionic... 

The aqueous chemistry of plutonium may well be unique in that four oxidation states co-exist in appreciable quantities. As with the trend set at NpOj, PuOJ is quite stable and its stability increases with decreasing acidity since the couples are strongly hydrogen ion dependent. 

The experimental observations on the actinide oxidation-reduction reactions are described, and the empirical results are tabulated. The rate laws have been interpreted in terms of net activation processes, and these have been tabulated togther with the associated activation parameters— aF, AH, and AS. An electrical analog is described which has been useful in interpreting complicated rate laws. Empirical correlations have been found between the formal entropies of the activated complexes and their charges, and for sets of similar reactions, between the hydrogen ion dependence and AF°, between AF and aF°, and between AH and AH°. The kinetic and physical evidence for binuclear species is discussed. 

Ti(III) (71). These analogous reactions show diflFerent hydrogen ion dependences. The Ti(III) reaction is almost exclusively inverse first power while for V(III), terms inverse first power and inverse second power in acid concentration are important in the rate law. 

Fe(II) reacts with Pu(IV) at rates which show inverse first power hydrogen ion dependence (54). In chloride solutions a term first power in chloride but zero power in hydrogen ion becomes important. Thus, for this reaction, Cl can replace OH in the activated complex. 

The net reaction for the reduction of Pu(VI) to Pu(V) by Fe(II) is quite simple in spite of this a complicated three-term hydrogen ion dependence was found (56). A mechanism which involves both outer-sphere and inner-sphere activated complexes is favored. The inner-sphere complexes are supported by evidence for consecutive reactions and a binuclear intermediate. 

Hydrogen Ion Dependences. The hydrogen ion dependences given in Table I and indicated by the net activation processes in Table II are... 

The Fe(II)-Pu( VI) reaction is interesting in that although the overall reaction appears to be a simple electron transfer, an accurate representation of the data requires a three-term rate law —d[Pu(VI)]/df = [Pu( VI)] [Fe(II)] [A -f (B -f C[ff]) ]. This rate law can be rearranged to the form for either pattern 3-1-a or 3-1-b, in which the hydrogen ion dependence of both Bi and R2 or B3 is zero and that of the remaining B-term is -1. The intermediate in this reaction appears to be analogous to the Fe(III) Np(V) species mentioned above. 

The reverse of some of the above reactions may be seen in the decomposition by heating, perhaps in oxygen, of polyatomic non-metallic cations which have been ion exchanged into the zeolite, or which were present as templates from the synthesis. These include the ammonium, hydronium, and alkylammonium ions. In all cases, the final products are hydrogen ions. Depending upon the temperature and the zeolite, these may leave with oxide ions of the zeolite fi-amework as water vapor, so that the fiamework which remains is deficient in oxygens. 

The PEPTs are hydrogen ion-dependent transporters that transport small peptides and proteinlike compounds such as cephalosporins, penicillin, enalapril, and cap-topril. Humans express two distinct PEPTs designated PEPT1 and PEPT2. PEPT1... 

The primary emphasis in this paper is on the three "slow" dissociation paths. In Table II, the variation of these terms with pH is given. These values correspond to hydrogen ion dependencies of 0.102, 0.172, and 0.173, respectively, for IC3, IC4, and IC5. The percent dissociation for each path varied when the mixing time prior to dissociation was less than 2 days for longer times, they were invariant. 

If there is a hydrogen ion dependence in the indicator reaction. Equation 14.3, then this will appear in the corresponding Nemst equation. Equation 14.4, and the potential at which the indicator changes color wiU be displaced from by the hydrogen ion term. 

Provided the equilibrium (12) is set up rapidly both these schemes will account for the earlier results on the chlorine and bromine reactions expressed by Eq. (b) above. For iodine an additional hydrogen ion dependent term is required (Eq. c) and Abel suggests that this arises through the reaction of peroxide as Ch- in addition to H02 in reaction (13)-... 

The accelerating effect of cupric ions on the ferric ion catalysis which was observed by Bohnson and Robertson is considered by Barb el al. to be due to reaction (5 ), as was the analogous effect of cupric salts on the ferrous ion catalysis. For conditions in which Scheme A applies reaction (4 ) is in effect catalyzed by (5 ). At high cupric ion concentrations (5 ) will eliminate (3), since effectively all the radical HO2 will react in (5 ). In these conditions the enhancement reaches a limit as was observed by Bohnson and Robertson. However (1) now becomes the operative chain-terminating step, and hence the kinetics of Scheme B should apply (Eq. g). Unfortunately no data on the peroxide dependence is available, but analysis of the data (43,66) shows the rate to be proportional to [Fe+++]w as required by (g). There is the same discrepancy in hydrogen ion dependence as was found in the simple ferric reaction. 

This ability of water to donate or accept hydrogen ions, depending on whether it reacts with a base or an acid, is referred to as amphiprotic. The conjugate acid-base pairs in this reaction are NH3/NH4 and H20/0H. ... 

Couples involving oxygen-transfer, for example, U02++4H++2e = U4++2H20 are irreversible and are of course hydrogen ion dependent. Couples such as Pu02 +/Pu02 are reversible. 

These values are to be considered tentative since there is some drift in the values with concentration, and it is likely that the above equilibria do not completely define the system. Self-diffusion coefficients of the zirconium-EDTA complex in slightly acidic solution also indicate a considerable degree of polymerization [417) as do the hydrogen ion dependence data of Ermakov [172). In the direct analytical titration of zirconium with EDTA, polymeric species must be depolymerized. This may be accomplished by boiling the solution with 5 N sulfuric acid [430). 

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