Independence of pH values

In the context of the theoretically existing problems, a need exists to investigate the antiradical efficacy of compounds independent of pH value or polarity of the solvent, respectively. These problems are particularly relevant in the design of new pharmaceutical antioxidant preparations for specialized therapeutic applications. 

More generally, acidic depolymerization products can be leached out of many films of paint, unless suitable pigments are incorporated. Zinc dust plus zinc oxide was the original acid scavenger. Zinc phosphate, zinc ferrite, and calcium borosilicate will each exert an anodic inhibitive function at pH values exceeding 7 while zinc chromate, zinc tetroxychromate, and strontium chromate act independently of pH value (Van Eijnsbergen, 1988). In general, the pH of aqueous paint film extracts should be between 7.5 and 8.5 and certainly not below 6 or above 10. 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

Under approximately neutral conditions, where the degradation rate is independent of pH for a range of pH values characteristic of the individual penicillin, hydrolysis appears to occur through a general base-catalyzed attack of a water molecule (77JPS861) on the /3-lactam carbonyl. 

More recently Hand et al. (ref. 9) have studied the decomposition reaction of N-chloro-a-amino acid anions in neutral aqueous solution, where the main reaction products are carbon dioxide, chloride ion and imines (which hydrolyze rapidly to amine and carbonyl products). They found that the reaction rate constant of decarboxylation was independent of pH, so they ruled out a proton assisted decarboxylation mechanism, and the one proposed consists of a concerted decarboxylation. For N-bromoamino acids decomposition in the pH interval 9-11 a similar concerted mechanism was proposed by Antelo et al. (ref. 10), where the formation of a nitrenium ion (ref. 11) can be ruled out because it is not consistent with the experimental results. Antelo et al. have also established that when the decomposition reaction takes place at pH < 9, the disproportionation reaction of the N-Br-amino acid becomes important, and the decomposition goes through the N,N-dibromoamino acid. This reaction is also important for N-chloroamino compounds but at more acidic pH values, because the disproportionation reaction... 

Judge et al. [78] examined other magnetic properties of CoP. The saturation moments crs were 100 and 116 emu/g for deposits from solutions A and B, respectively. Although the P content of the deposits varied with pH, the as values were essentially independent of pH. The squareness ratio of the M-H loop showed no significant dependence on any quantity other than film thickness it ranged from 0.5 for thin films to 0.8 for thick deposits. 

The slope of the left arm of curve a is thus — 1. At intermediate pH values, the rate is independent of pH, so k0 may be determined directly from kinetic measurements in this pH regime. At high pH the only significant contribution to the rate constant is that arising from specific base catalysis. In terms of equation 7.3.5,... 

Provided due attention is paid to the potential deprotonation of the substrate, and of the cyclodextrins (VanEtten et al., 1967a,b Gelb et al., 1980, 1982 Tee and Takasaki, 1985), the value of Ks should not be pH dependent. However, for many reactions, such as the widely studied ester cleavage, ku, kc, and k2 are all dependent on the pH of the medium. This makes direct comparisons between the observed constants for different CD-mediated reactions either difficult or problematical. However, in general, the ratios kc/ku and k2/ku are independent of pH and so are more useful for comparative purposes. 

Figure 6.4 shows that the initial potential of marmatite flotation is around 0.26 V and is almost independent of pH, but the upper limit potential of flotation varies with pH, which is higher at acidic pH value. The recovery of marmatite can be above 90% only at certain pulp potential ranges at given pH. The optimal flotation potential range is 0.35 - 0.6 V at pH=4.5, 0.28 - 0.5 V at pH = 6.5 and 0.25-0.3 V at pH =9.2. It shows the stronger activation of copper ion on marmatite flotation. 

Figure 6.5 demonstrates that arsenopyrite starts flotation at a potential around 0.18 V also independent of pH and ceases flotation at different potential values dependent on pH. The recovery is about 80% in the pH range 4.5-6.5 and... 

It is seen from Table 1 that the series of halogenopyridines show quantum yields of photohydrolysis independent of pH in the neutral and alkaline region and decreasing only at pH-values where the pyridine nitrogen becomes protonated to a considerable percentage. 

Very weak acids with pATa values greater than 7.5 will be essentially non-ionized at all pH values in the range 1-8, so that absorption will be largely independent of pH. 

For most substituents, in the region of neutrality and at higher pH values, the half-wave potential becomes independent of pH. In this region, electron transfer is synchronous with bond cleavage and loss of the leaving group creates a carbon... 

In acidic aqueous buffers, secondary nitramines [116] show a six-electron po-larographic wave which is converted into two two-electron waves above pH 5 (Fig. 11.4). Nitrosamines [117,118] show a four-electron wave in acid solution and this becomes a single two-electron wave above pH 5 (Fig, 11,5). Above pH 5 the halfwave potentials are independent of pH. Primary nitramines are relatively acidic. They show a six-electron wave in acidic aqueous buffers, the height of which falls to zero around the pKg value for the nitramine because the anion is not reducible [119],... 

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