The role of pH

Hydrogen ions are involved in the reduction of most organic species. Since the reduction often involves adding electrons to a neutral molecule to form a reduced species, which itself is a neutral molecule, then an equal number of hydrogen ions are required to ensure neutrality. 

Since the hydrogen ions are involved, the hydrogen ion concentration must be included in the Nernst equation for the reaction. 

It can therefore be seen that the potential is dependent on the hydrogen ion concentration, that is on the pH of the solution. The Nernst equation applies only to a thermodynamically reversible system but an equivalent expression involving the hydrogen ion concentration applies to all reductions or oxidations. 

The half wave potential of the polarographic wave for such a reaction will also be dependent on pH. The reaction above shows an equal number of hydrogen ions consumed as electrons. For the more general case involving n electrons and p hydrogen ions the half wave potential will be  

For a thermodynamicaiy irreversible process the equation must be modified by the addition of a constant a with a value between 0 and 1. 

Organic acids are most active at an environmental pH equal to or lower than their pKa value (ionization constant) (Brul et al., 2002). Bacteriostatic effects of acids vary, and the type of acid, as well as the pH, are of utmost importance in controlling pathogens in foods. Antibacterial activity of organic acids is related to the reduction in pH, as well as its ability to dissociate, which is determined by the pKa value and pH of the surrounding area, and as a result activity increases with a decrease in pH (Jung and Beuchat, 2000). 

To demonstrate the organic acid activities at various pH values, a study done on the salts calcium propionate, sodium benzoate, and potassium sorbate is herewith discussed. These salts were found to be effective and to inhibit some isolates from bakery products at pH 4.5 when applied at concentration of 0.3%. In a sponge cake (an analogue of pH 6) addition of these same weak organic acids salts appeared to be effective only at low aw levels (Guynot et al., 2005). 

An important factor in food safety is that bacteria can only survive in acidic environments by regulating their cytoplasmic pH (pH ). This is primarily driven by controlled movement of cations across the cell membrane. However, this ability to maintain the pH close to neutrality can be 

Source International Commission of Microbiological Specifications for Foods (ICMSF) Microorganisms in Food, Vol. 5. New York Blackie Academic and Professional, 1996. 

In vitro assays are often conducted at low pH to show the antibacterial effectiveness of organic acids, ensuring that the acids are not dissociated, as it is known that at pH below 3-3.5, almost all organic acids are very efficacious in controlling bacterial growth. However, it is often necessary to simulate the real environment, such as the GIT of poultry or pigs (Gauthier, 2005). 

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Magnesium is a relatively reactive metal, and can be chromated in nearly neutral solutions as well as in acid solutions. The range of treatments possible illustrates well the role of pH, activating anion, temperature and duration of treatment in promoting the breakdown of passivity in the chromate solution and the consequent formation of a chromate him. 

Further, for studying the role of pH and salt concentrations on bulk-electrostatic and non-bulk electrostatic contributions the same approach was made to experiments on the influence of the alcohols mentioned above on the oxygen affinity at various KC1 concentrations and pH-values 144,146). The results obtained indicate that at a low alcohol concentration the bulk-electrostatic contributions are dominant and that with increasing size of the alkyl group, alcohol and KC1 concentration, the nonbulk electrostatic, hydrophobic contributions increase. Recent results of kinetic measurements of 02 release show that cosolvents such as alcohols and formamide influence mainly the allosteric parameter L, i.e. -the equilibrium between T and R conformation and that the separation of the alcohol effects into bulk-electrostatic and hydrophobic (non-bulk electrostatic) contributions is justified. 

Clayton, T. D., Byrne, R. H., Breland, J. A. et al. (1995). The role of pH measurements in modern oceanic C02-system characteristics precision and thermodynamic consistency. Deep-Sea Res. II42,411 30. 

The BCS scheme can be made more useful by incorporating a further improved basis of physicochemical profiling. For example, the role of pH in permeability measurements could be better defined. The use of simulated intestinal fluids for solubility measurements could be better promoted. The effects of fed/fasted states on absorption could be better address, in methods that have optimum clinical relevance. 

Torres, P., Guzman-Ortiz, M. and Ramirez-Wong, B., Revising the role of pH and thermal treatments in aflatoxin content reduction during the tortilla and deep frying processes, J. Agric. Food Chem., 49, 2825, 2001. 

The above-discussed equilibria are of fundamental importance for an understanding of alteration processes, with special emphasis on the role of pH in water-rock interaction events. 

Sassman S.A. and L.S. Lee (2005). Sorption of three tetracyclines by several soils Assessing the role of pH and cation exchange. Environmental Science and Technology 39 7452-7459. 

The second novelty of our work is the elucidation of the role of pH-buffers in swelling equilibria and kinetics. It is interesting to note that most physical chemists choose not to use buffers because they complicate the system under study. On the other hand, any ultimate biological application of gels is likely to involve some sort of buffered medium. As we have shown, buffer properties such... 

That there may be metal ion uptake in the interlayer region is consistent with the above crystal structure experimental confirmation of this process is provided by the work of Wadsley (12) and Buser et al. (2). However, these and other workers did not isolate the role of pH in such... 

In 1945, Marcel Pourbaix submitted a Ph.D. dissertation entitled Thermodynamics in dilute solutions graphical representation of the role of pH and potential. It was initially rejected, or so the legend goes. Fortunately for corrosion scientists... 

Another important parameter that has to be deeply considered is the pH. It is well known the role of pH on silica chemistry it affects dissolution and polymerization rate, gel or precipitate formation and the textural properties of the final silica (11). Also for surfactant micelle or cluster templated syntheses of silica-aluminas, the effect of pH on porosity remains relevant and it is strongly influenced by the kind of material and by the synthesis route selected for its preparation. 

CaC03(s) -t UsO iaq) —> Ca (aq) + UCOJ aq) + HiOff) causes this increase. This section examines the role of pH in solubility. 

Stabenau IR, Warren KS, Rail DP. 1958. The role of pH gradient in the distribution of ammonia between blood and cerebro-spinal fluid, brain and muscle. J Clin Invest 38 373-383. 

The role of pH in polarography can be very varied. An entire textbook would be needed to cover all the many possibilities. The best that we can do in our present study is to look at a few examples to illustrate the range of common behaviour. For detailed information a textbook such as Zuman (1967) should be consulted. 

In order to highlight the effect of the cathodic sulfate reduction reaction, the model does not include the role of pH. 

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