The Effect of pH

TABLE 6.2 Summary of Physical Data for the Hydrated Electron at 23°  

The reaction H + OH— eh is undoubtedly responsible for the increase of G(eh) at high pH. Similarly, the reaction eh + H+—H must be responsible for the reduction of the hydrated electron yield in acid solution. The increase of total reducing yield and water decomposition yield at pH = 1.3 is not clearly understood, but it may also be due to secondary reactions. 

The acid-base reactions that form CBPCs are aU carried out in an aqueous solution. In these reactions, either synthesis or dissociation of water may occur, involving ionized species of protons and hydroxyl ions. These reactions are given by 

EarUer in this chapter, it was mentioned that many enzymes show greatest activity over a rather narrow range of pH. The principles described in previous sections provide the basis for a more detailed explanation of the effect of pH by making use of the Michaleis-Menten procedure. In general, an enzyme has one or more sites where H+ can be removed or H+ can be added depending on the acidity of the solution. Therefore, we will represent the enzyme concentration as EH, the deprotonated enzyme as E , and 

There are three equilibrium constants to be considered, those for proton removal and addition (K2 and Ki, respectively, for dissociation of the protonated species) and that for the dissociation of the EHS complex (Kj). These can be written as 

From Eq. (6.52), substitution for the concentrations of species obtained from the expressions for the equilibrium constants gives 

Dividing each side of this equation by [EHS] gives 

Substituting [EH][S]/Ks for [EHS] on the r ht-hand side of this equation yields 

---

Stannate(II) ions are powerful reducing agents. Since, for tin, the stability of oxidation state -b4 is greater than that of oxidation state -b2, tin(II) always has reducing properties, but these are greater in alkaline conditions than in acid (an example of the effect of pH on the redox potential, p. 101). 

Note that dinitrogen oxide is the other product. In alkaline solution, however, hydroxylamine oxidises iron(II) hydroxide to iron(III) hydroxide and is itself reduced to ammonia. This is an example of the effect of pH change on oxidation-reduction behaviour (p. 101). ... 

Standardization—External standards, standard additions, and internal standards are a common feature of many quantitative analyses. Suggested experiments using these standardization methods are found in later chapters. A good project experiment for introducing external standardization, standard additions, and the importance of the sample s matrix is to explore the effect of pH on the quantitative analysis of an acid-base indicator. Using bromothymol blue as an example, external standards can be prepared in a pH 9 buffer and used to analyze samples buffered to different pHs in the range of 6-10. Results can be compared with those obtained using a standard addition. 

The ladder diagram for HF/F- also can be used to evaluate the effect of pH on other equilibria that include either HF or F-. For example, the solubility of CaF2... 

Another important parameter that may affect a precipitate s solubility is the pH of the solution in which the precipitate forms. For example, hydroxide precipitates, such as Fe(OH)3, are more soluble at lower pH levels at which the concentration of OH is small. The effect of pH on solubility is not limited to hydroxide precipitates, but also affects precipitates containing basic or acidic ions. The solubility of Ca3(P04)2 is pH-dependent because phosphate is a weak base. The following four reactions, therefore, govern the solubility of Ca3(P04)2. 

We can account for the effect of an auxiliary complexing agent, such as NH3, in the same way we accounted for the effect of pH. Before adding EDTA, a mass balance on Cd + requires that the total concentration of Cd +, Ccd, be... 

Two possible explanations for the effect of pH on the sensitivity of this analysis are the acid-base chemistry of NH4+, and, the acid-base chemistry of the enzyme. Given that the pfQ for NH4+ is 9.244, explain the source of this pH-dependent sensitivity. 

In another attempt to relate degree of ionization with antibacterial activity, the effect of pH of the medium on the antibacterial activity was studied (27,28). Activity increased with increase in pH only up to the point at which the dmg was 50% ionized, and then decreased. The interpretation of this was that sulfonamides penetrate the bacterial cell in the unionized form, but once inside the cell, the equiUbrium between ionized and unionized forms is reestabhshed, and the activity is due to the ionized form. For optimum activity, a sulfonamide should have a p that provides half-dissociation at the physiologic pH in the area where it is absorbed. This observation also provided an explanation of the paraboHc relationship between piC and MIC (24). 

The inhibitory activity of sorbates is attributed to the undissociated acid molecule. The activity, therefore, depends on the pH of the substrate. The upper limit for activity is approximately pH 6.5 in moist appHcations the degree of activity increases as the pH decreases. The upper pH limit can be increased in low water activity systems. The following indicates the effect of pH on the dissociation of sorbic acid, ie, percentage of undissociated sorbic acid at various pH levels (76,77). 

The effect of pH and the piC of the thiol has been discussed. This reaction is not of great synthetic interest, primarily because it yields a mixture of products, but it is of commercial consequence. It is also appHcable ia polysulfide synthesis, where the presence of small amounts of thiols can cause significant problems for the stabiUty of the polysulfide (51). A similar reaction between thiols and sulfides has also been described (52). In this instance, the process is heterogenous and acid-cataly2ed. 

I have carried out widespread studies on the application of a sensitive and selective preconcentration method for the determination of trace a mounts of nickel by atomic absorption spectrometry. The method is based on soi ption of Cu(II) ions on natural Analcime Zeolit column modified with a new Schiff base 5-((4-hexaoxyphenylazo)-N-(n-hexyl-aminophenyl)) Salicylaldimine and then eluted with O.IM EDTA and determination by EAAS. Various parameters such as the effect of pH, flow rate, type and minimum amount of stripping and the effects of various cationic interferences on the recovery of ions were studied in the present work. 

The effect of pH is rarely of use for pK measurement it is more often of use in identifying the site of protonation/deprotonation when several basic or acidic sites are present. Knowing the incremental substitutent effects Z of amino and ammonium groups on benzene ring shifts in aniline and in the anilinium ion (40), one can decide which of the nitrogen atoms is protonated in procaine hydrochloride (problem 24). 

The importance of the nature of the catalyst on the hardening reaction must also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7. Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration. The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio 1 2.25) is shown in Figure 23.15. 

The effect of pH alone on chlorine efficiency is shown in Figure 3. Chlorine exists predominantly as HOCl at low PH levels. Between pH of 6.0 and 8.5, a dramatic change from undissociated to completely dissociated hypochlorous acid occurs. Above pH 7.5, hypochlorite ions prevail while above 9.5, chlorine exists almost entirely as OCl. Increased pH also diminishes the disinfecting efficiency of monochloramine. 

Since hypohalous acid is a much more active disinfectant than the hypohalite ion, the effect of pH on ionization becomes important. Hypobromous acid has a lower ionization value than hypochlorous acid and this contributes to the higher disinfectant activity of BrCl compared with chlorine. 

The reasons for the above phenomena are to be found in differing configurations of hydrogen bonds, the effect of pH, differences in the structures of fluorescence indicators and binders and differences in surface area. For example, silica gel 60 possesses a surface area of 500 m /g [211] while that of Si 50 000 lies below 5 m /g [212],... 

Through all these calculations of the effect of pH and metal ions on the ATP hydrolysis equilibrium, we have assumed standard conditions with respect to concentrations of all species except for protons. The levels of ATP, ADP, and other high-energy metabolites never even begin to approach the standard state of 1 M. In most cells, the concentrations of these species are more typically 1 to 5 mM or even less. Earlier, we described the effect of concentration on equilibrium constants and free energies in the form of Equation (3.12). For the present case, we can rewrite this as... 

Small quaniiiies of acid or base may have major effects on both rate and product. Many reasons have been advanced to account for these promotional effects (65), and many examples have been cited 31,32,41,79,80). The effects of pH are very worthwhile exploring in reductions that somehow are less than satisfactory. Catalysts themselves may contain sufficient acid or base to alter the course of reduction (25). 

Fig. 2-17. The effect of pH on the retention, selectivity and resolution of coumachlor enantiomers on vancomycin CSP (250 X 4.6 mm). The mobile phase was acetonitrile 1 % tri-ethylammonium acetate (10/90 v/v). The flow rate was 1.0 mL min at ambient temperature (23 °C).

Selective extraction experiments were then performed to see transference of some transition elements (Cu ", Ni ", Co ", and Fe " ) from the aqueous phase to the organic phase by the synthesized polymeric calixarenes. Phase-transfer studies in water-chloroform confirmed that polymer 2b and 3b were Fe ion-selective as was its monomer (1). Extraction of Fe " cation with 2b and 3b was observed to be maximum at pH 5.4. Only trace amounts of other metal cations such as Cu, Ni ", and Co " were transferred from the aqueous to the organic phase (Table 3). Furthermore, the extracted quantities of these cations remained unaffected with increasing pH. The effect of pH on the extraction of 3b was lower and 56% extraction was accomplished even at pH 2.2. The extraction experiments were also performed with calix[4]arene (1) the ratio was 8.4% at pH 2.2. The polymeric calix[4]arenes were selective to extract Fe " from an aqueous solution, which contained Cu +, Ni, Co ", and Fe " cations, and it was observed that the... 

The effect of pH appears to be controversial. Some workers find a slight increase in E, with increase in pH in the acid region, whilst others report that there is practically no change. In the alkaline region, however, Ey, becomes significantly more positive with increase in pH owing to the passivating ability of the OH ion. 

The effect of pH on the corrosion of zinc has already been mentioned (p. 4.170). In the range of pH values from 5 -5 to 12, zinc is quite stable, and since most natural waters come within this range little difficulty is encountered in respect of pH. The pH does, however, affect the scale-forming properties of hard water (see Section 2.3 for a discussion of the Langelier index). If the pH is below the value at which the water is in equilibrium with calcium carbonate, the calcium carbonate will tend to dissolve rather than form a scale. The same effect is produced in the presence of considerable amounts of carbon dioxide, which also favours the dissolution of calcium carbonate. In addition, it is important to note that small amounts of metallic impurities (particularly copper) in the water can cause quite severe corrosion, and as little as 0-05 p.p.m. of copper in a domestic water system can be a source of considerable trouble with galvanised tanks and pipes. 

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

Fig. 7.1.6 Influence of pH and temperature on the luminescence of Cbaetopterus photoprotein elicited by old dioxane and Fe2+ in 20 mM phosphate buffer. Left panel the effect of pH in phosphate buffer solutions of various pH values, at 22°C. Right panel the effect of temperature at pH 7.2. Luminescence was initiated by the injection of Fe2+. The time lag of the light emission after the Fe2+ injection was also shown in the right panel. From Shimomura and Johnson, 1966.

Significant differences are also apparent for the effect of pH on the fermentation rate. The narrow pH optimum characteristic of a free cell system is replaced by an extremely broad range upon immobilisation. This effect stems from the gradient pH that exists within the bead. 

Examination of the effect of pH on the rates of protodeboronation of the 2,6-dimethoxy compound at 90 °C in malonic acid-sodium malonate buffer solutions of ionic strength 0.14 gave the data in Table 199. A plot of these data revealed the curve shown in Fig. 3 (one of the points was misplotted on the original) and the linear portions of the plot were attributed to acid and base catalysis as shown on Fig. 3, and since the rates in the region of pH 4-5 are higher than would be... 

The effect of pH and complexation on the relative stabilities of the oxidation states of Pu is discussed. A set of ionic radii are presented for Pu in different oxidation states and different coordination numbers. A model for Pu hydration is presented and the relation between hydrolysis and oxidation state evaluated, including the problem of hydrous polymerization. 

The effects of pH on electrokinetic velocities in micellar electrokinetic chromatography was studied by using sodium dodecyl sulfate solutions [179]. Micellar electrokinetic capillary chromatography with a sodium dodecyl sulfate pseudostationary phase has been used to determine the partition constants for nitrophenols, thiazolylazo dyes, and metal chelate compounds [180]. A similar technique was used to separate hydroquinone and some of its ether derivatives. This analysis is suitable for the determination of hydroquinone in skin-toning creams [181]. The ingredients of antipyretic analgesic preparations have also been determined by this technique [182], The addition of sodium dodecyl sulfate improves the peak shapes and resolution in chiral separations by micellar electrokinetic chromatography [183]. 

The literature on sucrose loss in manufacture is extensive, but not all studies have been conducted on the principles of sound scientific method. An example from the literature of a study with questionable results is a report by Cecil,62 on the effect of pH on sucrose loss during boiling in the open-pan sulfitation (OPS) process. The OPS process is used in some tropical sugar cane growing areas to produce small amounts of a sugar product known as gur, panela, or piloncillo for local consumption. Juice (massecuite from Kenyan OPS mill diluted to 20... 

Godt, R.E. (1981). A simple electrostatic model can explain the effect of pH upon the force-pCa relation of skinned muscle fibers. Biophys. J. 35, 385-392. 

---