PH, significance

Figure 6-14 shows FhjS. Fhs, and Fs plotted against pH for an acid with pF, = 5.0 and pA"2 = 10.0. Evidently with such widely spaced dissociation constants the solution contains, at any one pH, significant fractions of only two species. The fraction of monoanion rises essentially to unity at one point. The pH at which the monoanion fraction achieves its maximum value is calculated by differentiating Eq. (6-74) and setting the result equal to zero this gives... 

The parent acids of common polyprotic acids other than sulfuric are weak and the acidity constants of successive deprotonation steps are normally widely different. As a result, except for sulfuric acid, we can treat a polyprotic acid or the salt of any anion derived from it as the only significant species in solution. This approximation leads to a major simplification to calculate the pH of a polyprotic acid, we just use Kal and take only the first deprotonation into account that is, we treat the acid as a monoprotic weak acid (see Toolbox 10.1). Subsequent deprotonations do take place, but provided Kal is less than about fCal/1000, they do not affect the pH significantly and can be ignored. 

Complex cyanides are compounds in which the cyanide anion is incorporated into a complex or complexes. These compounds are different in chemical and toxicologic properties from simple cyanides. In solution, the stability of the cyanide complex varies with the type of cation and the complex that it forms. Some of these are dissociable in weak acids to give free cyanide and a cation, while other complexes require much stronger acidic conditions for dissociation. The least-stable complex metallocyanides include Zn(CN)42 , Cd(CN)3 , and Cd(CN)42 moderately stable complexes include Cu(CN)2, Cu(CN)32, Ni(CN)42, and Ag(CN)2 and the most stable complexes include Fe(CN)64, and Co(CN)6. The toxicity of complex cyanides is usually related to their ability to release cyanide ions in solution, which then enter into an equilibrium with HCN relatively small fluctuations in pH significantly affect their biocidal properties. 

A further example of the use of a chiral anion in conjunction with a chiral amine was recently reported by Melchiorre and co-workers who described the asymmetric alkylation of indoles with a,P-unsaturated ketones (Scheme 65) [212]. The quinine derived amine salt of phenyl glycine (159) (10-20 mol%) provided the best platform with which to perform these reactions. Addition of a series of indole derivatives to a range of a,P-unsaturated ketones provided access to the adducts with excellent efficiency (56-99% yield 70-96% ee). The substrates adopted within these reactions is particularly noteworthy. For example, use of aryl ketones (R = Ph), significantly widens the scope of substrates accessible to iminium ion activation. Expansion of the scope of nucleophiles to thiols [213] and oximes [214] with similar high levels of selectivity suggests further discoveries will be made. 

Acid rain or precipitation refers to rain, snow, fog, or gaseous particles that have a pH significantly below 5.6. There is no absolute pH that defines acid rain, but a general guideline that can be used is that precipitation below 5.0 can be considered acidic. Although the term acid rain is used... 

This causes a slight excess of base in the reaction, but it doesn t ciffect pH significantly. You can think of the undissociated acid as a reservoir of protons that are available to neutralize any strong base that may be introduced to the solution. As we explain in Chapter 14, when a product is added to a reaction, the equilibrium in the reaction changes to favor the reactants or to undo the change in conditions. Because this reaction generates A , the acid dissociation reaction happens less frequently as a result, further stabilizing the pH. 

This causes a slight excess of acid in the reaction but doesn t affect pH significantly. It also shifts the balance in the acid dissociation reaction in favor of the products, causing it to happen more frequently and recreating the base A . 

While theoretically it is possible that botanicals with diuretic effects can increase drug excretion, most botanical diuretics are not as potent as furo-semide and are unlikely to result in significant interactions. Most botanicals also do not affect urinary pH significantly, and hence are unlikely to affect renal tubular reabsorption of drugs. Nevertheless, lithium toxicity was thought to be related to the use of a botanical diuretic mixture in a patient. If the toxicity indeed is related to the use of the botanical diuretic, the mechanism of action or the responsible constituent(s) is not known (48). 

Urinary pH significantly affects the activity of nitrofurantoin, with loss of potency as the urine becomes more alkaline. For this reason, women with lower UTI who are prescribed nitrofurantoin should be advised not to take alkalinising agents such as potassium citrate (Effercitrate). 

The size of the coefficients can inform the experimenter how significant the coefficient is. For example, does pH significantly improve the yield of a reaction Or is the interaction between pH and temperature significant In other words, does the temperature at which the reaction has a maximum yield differ at pH 5 and at pH 7 ... 

The accuracy and precision of an enzyme assay usually depend on the underlying chemical basis of techniques that are used. For example, if an assay involving reaction of an enzymatically produced aldehyde with phenylhydrazine to yield a phenylhydrazone product is carried out in buffer of the wrong pH, that is, a pH significantly above 6.0, the observed rate of phenylhydrazone formation will not accurately reflect the rate of enzymatically produced aldehyde, but rather the chemical rate of phenylhydrazone formation. The accuracy of an assay is a reflection of... 

Sanchez et al (1998) investigated the effects of i.v. (2.0mg/kg) and p.o. (6.6mg/kg) ranitidine on gastric acidity in young foals. Mean intragas-tric pH significantly increased for 5h after i.v. administration compared with baseline data. After p.o. administration, gastric pH significantly increased for up to 8 h. 

These applications are based on water dissociation at the interface of a bipolar membrane and are coupled with the action of the monopolar membrane action. Deacidification and acid production, however, entail conventional ED. In the recovery of organic acids from fermentation broths the elimination of cations has often been a major problem, as fermentation typically performs better in pHs significantly above the pfC, of the acid produced. Bipolar membranes offer a solution to the elimination... 

A further complication arises when the foulant carries a fixed charge, such as whey protein in solutions with pH significantly different from the isoelectric point. Under these conditions, the fouling layer acts as a polyelectrolyte membrane in series with the reverse osmosis membrane, and changes in the salt concentration at the surface of the reverse osmosis membrane would be expected. 

H <—> H2CO3), in which the bicarbonate is in excess of the carbonic acid, helps to maintain the correct pH. Exhalation of carbon dioxide from the lungs causes some of the bicarbonate ions in blood to combine with protons, and this would raise the pH. However, because there is an excess of bicarbonate ions and protons, the loss of a small number of protons does not influence the pH significantly. 

Figure 11.3 shows the solubilities (defined as total solute dissolved) of some /3-lactam antibiotics as a function of pH. All of these compounds exhibit a minimum in their solubility, C o, at the isoelectric point of the compound. At a pH significantly removed from the isoelectric point, the total (apparent) solubility is increased. The U-shape of the apparent solubility versus pH curve can be described by taking into account the acid-base equilibria of the antibiotic solute and assuming that the solute in the crystal is in the zwitterionic (isoelectric) form. 

Buffered solution requires a greater amount of acid for the adju.stment to a certain pH compared to the non-buffered solution. The amount of acid a buffer can absorb without changing its pH significantly depends on the initial concentration of buffer and amount of acid added. 

The effective disintegration, dissolution and eventual absorption of dipyridamole in tablet form depends upon having a low pH in the stomach. Drugs that raise the gastric pH significantly are expected to reduce the bioavailability of dipyridamole. 

A consequential conclusion is that any drug that raises the stomach pH significantly would be likely to reduce the dissolution and absorption of dipyridamole. It would therefore be reasonable to expect that proton pump... 

Solution pH significantly affects the magnitudes of both the polarographi-cally measured diffusion coefficient and the overpotential for cytochrome c reduction. The overpotential dependence on pH was first described by Griggio and Pinamonti and later by others. The former authors found that... 

Nonionic surfactant vesicles have been reported successfully, as ocular vehicles for cyclopen-tolate. In the in vivo study, niosomes, independent of their pH, significantly improved the ocular bioavailability of cyclopentolate, with respect to reference buffer solution, indicating that it can be used as an efficient vehicle for ocular drug delivery. ... 

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