Proton concentration effect

The self-ionisation equilibrium (Eqn. (1)) is displaced, increasing the proton concentration effectively quantitatively in the case of relatively dilute solutions of SbF5. Weaker Lewis acids such as AsF5 and BF3, which will be described in Sec. 11.2.3.1, cause partial displacement of the equilibrium. 

Scheme 10. Mechanislic possibililies for PF condensalion. Mechanism a involves an SN2-like attack of a phenolic ring on a methylol. This attack would be face-on. Such a mechanism is necessarily second-order. Mechanism b involves formation of a quinone methide intermediate and should be Hrst-order. The quinone methide should react with any nucleophile and should show ethers through both the phenolic and hydroxymethyl oxygens. Reaction c would not be likely in an alkaline solution and is probably illustrative of the mechanism for novolac condensation. The slow step should be formation of the benzyl carbocation. Therefore, this should be a first-order reaction also. Though carbocation formation responds to proton concentration, the effects of acidity will not usually be seen in the reaction kinetics in a given experiment because proton concentration will not vary.

In resting muscle the high concentration of ADP does not decrease the proton gradient effectively and the high membrane potential slows electron transport. ADP, formed when ATP is hydrolyzed by myosin ATPase during contraction, may stimulate electron transport. However, the concentration of ATP (largely as its Mg salt) is buffered by its readily reversible formation from creatine phosphate catalyzed in the intermembrane space, and in other cell compartments, by the various isoenzymes of creatine kinase (reviewed by Walliman et al., 1992). 

Extraction rates of zinc (II) and nickel (II) with ethyldithizone, butyldithizone, or hexyldithizone in an organic phase (chloroform, CCI4, w-heptane, or benzene) showed a first-order dependence on the ligand and metal ion concentration and an inverse-first-order dependence on the proton concentration. The results were explained by chelate formation in the interfacial region [59]. The effects of stirring on the distribution equili-... 

This reaction has been shown to be very rapid77. Sulphuric and acetic acids sup press the polymerisation. Evidently their anions are ineffective as initiators, and the enhanced proton concentration provided by them must reduce the chain lifetime. The slight retarding effect of oxygen could be due to electron scavenging. However, the authors suggest that there may be a small free radical component of the chain reaction, which is inhibited in the presence of oxygen. 

Therefore, if the excited-state lifetimes r0 and Tq are known, the plot of ( / 0)/( / ) versus [H30+] yields the rate constants k3 and k i. However, it should be emphasized that corrections have to be made (i) the proton concentration must be replaced by the proton activity (ii) the rate constant k 3 must be multiplied by a correction factor involving the ionic strength (if the reaction takes place between charged particles), because of the screening effect of the ionic atmosphere on the charged reactive species. 

Pore size and dielectric constant s of water in pores exhibit a strong effect on proton distributions, as studied in Eikerling. Model variants that take into account the effect of strongly reduced s near pore walls ° and the phenomenon of dielectric saturation ° 2° lead to nonmonotonous profiles in proton concentration with a maximum in the vicinity of the pore wall. 

According to Equation 6.6, the velocity of the EOF is directly proportional to the intensity of the applied electric held. However, in practice, nonlinear dependence of the EOF on the applied electric held is obtained as a result of Joule heat production, which causes the increase of the electrolyte temperature with consequent decrease of viscosity and variation of all other temperature-dependent parameters (protonic equilibrium, ion distribution in the double layer, etc.). The EOF can also be altered during a run by variations of the protonic concentration in the anodic and cathodic electrolyte solutions as a result of electrophoresis. This effect can be minimized by using electrolyte... 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

Like Complex III of mitochondria, cytochrome b6f conveys electrons from a reduced quinone—a mobile, lipid-soluble carrier of two electrons (Q in mitochondria, PQb in chloroplasts)—to a water-soluble protein that carries one electron (cytochrome c in mitochondria, plastocyanin in chloroplasts). As in mitochondria, the function of this complex involves a Q cycle (Fig. 19-12) in which electrons pass, one at a time, from PQBH2 to cytochrome bs. This cycle results in the pumping of protons across the membrane in chloroplasts, the direction of proton movement is from the stromal compartment to the thylakoid lumen, up to four protons moving for each pair of electrons. The result is production of a proton gradient across the thylakoid membrane as electrons pass from PSII to PSI. Because the volume of the flattened thylakoid lumen is small, the influx of a small number of protons has a relatively large effect on lumenal pH. The measured difference in pH between the stroma (pH 8) and the thylakoid lumen (pH 5) represents a 1,000-fold difference in proton concentration—a powerful driving force for ATP synthesis. 

Concentration effects on 13C shifts of alcohols are small [271]. Solvent-induced shifts are enhanced on going from primary to tertiary alcohols and may be as high as 2 ppm [271]. Protonation shifts are much larger. The methanol carbon, for example, is shielded by —14.6 ppm relative to the neat liquid value when dissolved in magic acid [272], Protonation shifts of 1-alkanols in trifluoroacetic acid are shieldings for C-1 and alternating deshieldings for all other carbon atoms, e.g. <5 2 > > <5C 3 for 1-butanol [272],... 

Several reports deal with the action of heterocycle-chromate agents such as quinolin-ium dichromate on five-membered heteroaldehydes340 and quinolinium bromochromate on benzaldehydes,341,342 all in acetic acid solution. The latter studies show a second-order dependence on proton concentration, acceleration by electron-withdrawing para-substituents, and a substantial kinetic isotope effect for the deuterated aldehyde. 

Having emphasized the importance of the pH parameter, it should be recognized that its measurement is in itself a difficult task [90]. When the surface is charged, the overall ion concentrations are not constant close to the oxide-fluid interface. For instance, above the IEP, i.e. for negatively charged surfaces, there is an increase in cation and proton concentrations at the surface. Therefore, the pH is lower than in the bulk of the solution. The situation is reversed at a pH lower than the IEP. This is a typical compensation effect of the pH. 

The reaction is first-order bromide ion, first-order bromate ion, and second-order hydrogen ion. What is the rate law and overall order If the proton concentration quadruples, what is the effect on the rate ... 

If the proton concentration quadruples, then the relative effect will be 4, or 16 times faster. 

Recent studies33 of solvent and concentration effects on the NMR of indoles have established that, in addition to the 2-proton, the 7-proton resonance in certain substituted indoles also undergoes downfield shifts in more polar and more dilute solutions. The chemical shifts of protons of indole and several methylindoles in two solvents, carbon tetrachloride and tetrahydrofuran, are listed in Ref. 33. Proton chemical shifts of the... 

---