Effects of pH

A reader of this book might have expected that the compilation of the PZCs would be followed by a statistical analysis similar to that in [2]. Indeed, the original plan 

It may very well be that, indeed, the most frequently studied specimens in each category of materials are representative of the best PZC, but critical, fundamental studies in this direction are rare. In view of the tendency to study only a few selected specimens, the average over all values of PZCs/IEPs found in the literature may be misleading. 

Consider the following process involving the uptake of m-protons and consumption of n-electrons  

The limiting cases correspond to those of electrochemical reversibility and irreversibility. Here we consider the electrode process being fully electrochemi-cally reversible, thus for the relevant Nemst equation we can write [10]  

Experimentally, the cyclic voltammetric response is recorded over a range of pH s with the (or more commonly peak potential ) plotted as a function of pH. 

A real example from the hterature is shown in Fig. 2.25 which utilises a cat-echin-immobihsed poly(3,4-ethylenedioxythiophene)-modified electrode towards the electrocatalysis of NADH in the presence of ascorbic acid and uric acid [11]. Interestingly, catechin has a quinone moiety in its oxidised state and the effect of pH on the redox properties of the modified electrode is shown in Fig. 2.25 over the pH range of 2-10 where the redox couple of the catechin molecules are shifted to less positive values with the increase in pH. The insert in Fig. 2.25 shows a plot of the half-wave potential of the catechin molecule as a function of pH. Note it 

The enzymes in living systems function at nearly constant pH because they are in an environment that contains buffers (Chapter 1). 

CHAPTER6 Enzymes I General Properties, Kinetics, and Inhibition 

Plot of substrate concentration versus initial velocity of an enzyme-catalyzed reaction. Segment A At low substrate concentration, the reaction follows first-order kinetics with respect to substrate concentration i.e., V — [S], where k is a reaction rate constant. Segment B At high substrate concentration, maximum velocity (Umax) is attained (saturation kinetics), and any further increase in substrate concentration does not affect the reaction rate the reaction is then zero-order with respect to substrate but first-order with respect to enzyme. is the value of [S] corresponding to a velocity of j Vmax- 

The density of observed DNA-SWCNTs as a function of pH, at a fixed incubation time and ionic strength, is shown in Fig. 16.4c. 

As expected, deposition is faster at low pH values and slows at increased pH. We now fit our kinetics models to the experimental 

For charge accumulation limited deposition (case 1), using the fact that the initial surface potential, varies approximately linearly with pH from nearly 0 mV at pH 2 to about 80 mV at pH 8 in Eq. 16.6 and applying it to Eq. 16.8, we conclude that the potential barrier will vary linearly in both the pH and surface density of deposited nanotubes, which we may write as 

We substitute Eq. 16.15 in Eq. 16.10. If we now expand Eq. 16.10 about t = 0, we expect an exponential decay in deposition density with increasing pH for small times (Jotfp 1), which is qualitatively consistent with the data in Fig. 16.4c. Shown is a fit using Eq. 16.10 with the value for p = 0.8 o.m/ a.m, obtained by fitting the kinetics experiment. It can be seen that the fit is able to capture well the observed reduction in deposited density. 

We now consider site-limited (case 11) kinetics. In order to fit the equation to the pH dependence data, we use a relation similar to Eq. 16.13 (but without the surface accumulation term), AE = Ci pH + 2- The fit is made using the value pt = 0.8 o.m/ a.m from the kinetics experiment (Fig. 16.4a). Note that just as in case 1, this model predicts exponential decay of ps with pH for small Jotfpt- Both models fit the data adequately here, although only case 11 is shown. 

In moderately alkaline solutions (pH 8) oxidation of Fe solutions proceeds via Fe(OH)2 and usually yields magnetite (David Welch, 1956 Sidhu et al., 1977). Under these conditions the solubility product of magnetite is exceeded so the mixed oxide is more stable than the pure Fe oxides (see Chap. 8). Tamaura et al. (1981) monitored the transformation of Fe(OH)2 at pH 11 and 65 °C. Initially both goethite 

1) Formation of Fe oxides most commonly involves oxidation with air or oxygen, but other oxi- 

In 1925, Welo and Baudisch found that magnetite formed upon bringing a solution with Fe /Fe - 2, i.e. the ratio of magnetite, up to pH 9-10. Misawa et al. 

They are stable only at low redox potential. They form either by direct precipitation from an Fe salt solution upon oxidation once their solubility product is exceeded (eqn. 13.13), or by interaction between 2-line ferrihydrite precipitated initially and Fe in solution (eqn. 13.14) in the presence of a sufficiently high [Fe ], the green rust is more stable than 2-line ferrihydrite 

1) In these experiments, two main approaches have been followed either a constant pH has been maintained by addition of base or, alter- 

The corrosion rate often depends more on the ion that alters the pH than on the pH itself. For example, aluminium is not rapidly attacked by concentrated nitric acid at a pH of 1, or by glacial acetic acid at a pH of 3, but is corroded rapidly by hydrochloric or phosphoric acid at a pH of 4. On the alkahne side, aluminium is resistant to ammonium hydroxide at pH13, but is rapidly corroded by sodium hydroxide at pHll. The reason for this behaviour is probably the composition of the corrosion product and its abihty to form protective films on the surface of the aluminium [2.6]. 

The reason that the rate of enzyme reaction is influenced by pH can be explained as follows  

Enzyme is a protein which consists of ammo acid residues (that is, amino acids minus water). 

9 Literally, in glass pertaining to a biological reaction taking place in an artificial apparatus. 

On the other hand, an amino acid, lysine, is basic in the range of higher pH value. As the pH is decreased, lysine is ionized as 

Similarly, the pK value of lysine is 10.0, at which half of the residues are ionized. 

Since the isoelectric point of most proteins is between a pH of 4 and 5, it would be expected that operation in this pH range would result in the lowest gel concentration. Further, since changes in pH are not expected to change the 

There is an interesting effect of pH on the solubility of acid gas in water. In a solution with a high pH (a basic solution), the solubility of the acid gas components is dramatically increased. This is due to the acid-base reactions that occur between the dissolved acid gas and the base in the original solution. 

On the other hand, the effect of a low pH (acidic) solution is less dramatic. In low pH solutions, the effect is more like the salting-out seen with neutral electrolyte solutions. In fact, at low pH the type of acid present is more significant than the pH. Kendall and Andrews (1921) showed that solubility of H2S in hydrochloric acid increases slightly as the pH decreases. However, at a pH of about 0.7 (a very acidic solution) the solubility only increased by 6% over the solubility in pure water. Doubal and Riley (1979) showed essentially no difference in the solubility of H2S in pure water versus a 5 molal solution of H.SO,. 

The formation of an aqueous phase is critical in acid gas injection scheme. The presence of the aqueous phase greatly increases the possibility of corrosion. In this chapter, methods are presented for estimating water content of acid gas, both in the gas and liquid phases. 

As always, the design engineer should verify the accuracy of the models used. This is particularly true for the water content of acid gases where simple models give erroneous results and may lead to poor design decision. The appendix to this chapter presents a review of the available experimental data for these systems. 

Monnery, and W. Svrcek. 2005. Model predicts equilibrium water content of high-pressure acid gases. Oil Gas J. (July 11). 

After the CNTs were added to the solution, as shown in the inset of Fig. 10.3(a) and (b), pH increased in all the solutions due to the adsorption of protons on the surface of the CNTs, which caused the rapid drop in E. For the solutions of initial pH 2.5 and pH 7, the solution pH instantly jumped to pH 9 and pH 8 immediately after the addition of CNT, respectively, and then gradually decreased to pH around 7 overtime. For the solutions of the initial pH 1, however, their pH increased gradually to pH 7 over time without an imminent sharp jump in pH after the addition of CNT because of the very high proton concentration in the initial solution. 

It should be pointed out that the reduction reaction time between the moment of CNT addition and the time for the negative shift of E in the third stage, that is, the time required for the complete removal of MnOJ ions from the solutions, decreased with decreasing pH. This indicates that the reduction reaction rate increases with decreasing solution pH. As more protons in the solution become available for 

It can be expected that the Helmholtz potential of a silicon electrode in an aqueous electrolyte is a strong function of pH since hydrogen adsorption is a dominant process on the surface of silicon. Table 2.15 shows the dependence of flatband potential on pH 

TABLE 2.14. Platband Potentials of Silicon Materials Determined in the Dark in Various Solutions 

Nucleosides play a vital role in many biological systems, not only as precursors to DNA and RNA but also in their own right, functioning as metabolic regulators. Consequently, the quantification and HPLC analysis of nucleosides in biological fluids can provide important information regarding their function. The analysis of the major and minor components of DNA and RNA is most conveniently carried out at the nucleoside level and will be considered as a separate section (Davis et al., 1979). 

In the past HPLC separations of nucleosides have been carried out in a variety of modes, including anion-exchange (Floridi et al., 1977) and cation-exchange (Breter et al., 1977) however, since the introduction of stable packings reversed phase has become the preferred method. Typical chromatographic conditions for the separation of nucleosides include the use of dilute phosphate buffers with organic modifiers such as methanol or acetonitrile on ODS stationary phases. The effects of variations in these parameters is described below. 

It should be noted that the zeta potential is an average value for the surface potential, and does not account for individual surface functional groups. TTierefore it is possible that there are remaining positive groups even in the presence of organics, which do interact with the membrane surface. 

the operation of MF at a very high flux may bring the colloids in very close contact with the membrane surface. Then the repulsive forces might be overcome, inducing the colloids to adhere to the membrane due to short range forces (Van der Waals interactions). 

Alternatively, the continued adsorption, even in the presence of electrostatic repulsive effects, may indicate strong specific binding between colloid-metal centres and membrane functional groups. Once the flux has declined to 2 - 20 /o of its initial value, electrokinetic interactions could increase rejection. 

1 Prepare 0.25 M pH buffer solutions ranging from pH 0.5 to 9. (Note that phosphate buffer is only good for pH = 4.5-9 due to the dissociation constant.) Before coming to the lab, review how to make a pH buffer solution in a freshman chemistry textbook and calculate the relative amounts of KH2PO4 (monobasic phosphate) and K HPO O (dibasic phosphate) needed to make these phosphate buffer solutions. 

2 Add an equal volume of one of the above buffer solutions to 2.0 ml of the 20 g 1 1 starch solution prepared in step 1. The resulting solution should contain 10 g 1—1 of starch in a buffered environment. 

4 After 3 min, stop the enzymatic reaction by adding 0.1 ml of the reacted starch solution to 1 ml of the HC1 stopping solution (0.1 N). 

5 Then, add 0.2 ml of the above mixture to 2 ml of iodine solution to develop the color. Shake and mix. The solution should turn deep blue if there is any residual, unconverted starch present in the solution. The solution is brown-red for partially degraded starch but clear for totally degraded starch. 

7 Carry out the same procedure for the other starch solutions buffered at different pHs. Use your time wisely all the solutions can be handled simultaneously if you are familiar with the procedure. Slightly stagger the sequential sample withdrawal so that there is enough time for sample preparation and handling. 

It has been observed that the enzymes are active over a limited range of pH and most of the enzymes have a definite optimum pH. The optimum pH is obtained due to the effect on the affinity of the enzyme with the substrate stability may be irreversibly destroyed 

Most colloidal silicas are stabilized in pH range of 9-10, but usually more alkali is added to the polishing slurry to maximize the alkali content However, the ability to add alkali is very limited. As discussed earlier, the effect of excess alkali is to tend to dissolve the colloidal sUica particles and convert them back to sodium silicate. If enough sodium silicate is present it acts like any other salt and causes the colloidal silica particles to agglomerate and/ or gel. In any case, the increase in pH is short lived as the pH falls as the particles begin to dissolve and form sodium silicate. One might consider the particles as acting as a slow-release silicic acid. 

Alumino-silicates are known to be less soluble in alkaU than pure silica and Sears [9] used this fact to produce and patent a product having a high degree of alumina surface modification which resisted alkaline attack and therefore would retain a high pH for a longer period. This allowed it to be a more effective polishing agent. 

Pressure, temperature, and flow rate are considered together here as they are all interconnected in their effect on most polishing machines. When pressure is increased. 

Using lots of slurry, particularly if the slurry is not recycled, but discarded, is not only expensive, but the lower temperature reduces the rate at which silicon is removed from the wafer. However, using a very low slurry flow to increase the temperature and therefore the chemical attack rate runs the risk of developing so much heat that the slurry dries, and the silica particles aggregate into scratch-producing sediments. Therefore, as with Goldilocks and the Three Bears one flow is too hot, one is too cold and one is just right . 

In fact, the rate of chemical attack and the rate of abrasive polishing of the colloidal silica need to be well matched, if the chemical attack is so fast compared to the abrasive character that the abrasive action cannot level the high spots left on the etched surface then pits will be left in the polished surface. If these cannot be smoothed out in the final polish, then they will represent defects in the finished wafer surface. 

Elevated arsenic concentrations in oxic aquifers in Arizona (US) were linked to pH-dependent desorption (Robertson, 1989). Similar results exist for metamorphic aquifers in New England (US), where moderately alkaline waters (pH 7.5-9.3) were found to have elevated concentrations of arsenic (Robinson and Ayotte, 2006). Conversely, (BGS (British Geological Survey), 1989) suggested that arsenic concentrations of 4pgF-1 in water of the Lincolnshire Limestone (UK) cannot be explained by pH values of 7.0-9.5. McArthur et al. (2004) commented that the observations of pH increases with arsenic mobilization by Welch, Lico and Hughes (1988) and Robertson (1989) are not by themselves sufficient to prove that arsenic is mobilized by increasing pH. Arsenic may be mobilized by extended residence times, evaporation, and/or weathering, any of which could lead to both increases in pH and dissolved arsenic concentrations. 

Knowledge of the Maillard reaction is being extended very actively in many different ways. The participation of free radicals has already been dealt with in Chapter 2 and work on colour and flavour aspects is being deferred to Chapters 4 and 5, respectively. This chapter deals with a number of relatively disparate topics, namely, the effects of pH, high pressure, 7g, and the use as reactants of amines other than amino acids, of lipids, and of oligo- and polysaccharides, as well as the determination of a-dicarbonyl intermediates, control of aldol/retroaldol reactions, fluorescence, kinetic aspects, and sites of protein glycation. 

A good example of the effect of pH98 is that observed on a xylose-lysine model system (1 M each, refluxed 1 h with diethyl ether in a Likens and Nickerson apparatus, initial pH 4.9, either kept at pH 5 with NaOH additions or left, when the final pH is 2.6) 54 and 28 volatiles were identified, respectively, 2-furaldehyde dominating with 52.2 and 99.9% (w/w). Total yield and number of nitrogen-containing compounds were greater at higher pH values of the former system, and monocyclic pyrroles, pyridines, and 2,3-dihydro-lH-pyrrolizines were identified only in that system. 

Therefore, furanone can be taken as symptomatic of 2,3-enolisation similar to furfural and HMF being symptomatic of 1,2-enolisation (see Table 3.1).54 

The work reported by Amoldi and Boschin with equimolar aqueous solutions of xylose-glycine, heated at 100 °C for 2 h at different pH values, maintained by additions 

Amadori from 2-Furaldehyde Furanone 2-Furaldehyde Furanone 

This reaction proceeds rapidly at room temperature, whereas for iron a similar reaction forming NaFeO and Na2pe02 requires concentrated alkali and high temperatures. 

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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 Cosmetics. Amino acids and their derivatives occur in skin protein, and they exhibit a controlling or buffering effect of pH variation in skin and a bactericidal effect (216). Serine is one component of skin care cream or lotion. Ai-Acylglutamic acid triethanolamine monosalt is used for shampoo. Glucose glutamate is a moisturizing compound for hair and skin (234). 

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). 

Fig. 11. Effects of pH in the colloidal siUca-water system (1), where A represents the point of zero charge regions B, C, and D correspond to metastable gels, rapid aggregation, and particle growth, respectively. Positive and negative correspond to the charges on the surface of the siUca particle.

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. 

Fig. 6. Effect of pH on corrosion of iron in aerated water at room temperature (22). Point A is where evolution begins. To convert mm/yr to mils per...

FIG. 28-2 Effect of pH on the corrosion rate, a) Iron, (h) Amphoteric metals (aluminum, zinc), (c) Noble metals. 

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. 

Effect of pH, dye concentration, size of pores of paper filters and their hydrophobic characteristics, filtration rate, nature and hydrocarbon radical length cSurf on sensitivity of their determination was studied. 

The effect of concentration of cationic (cetylpyridinium chloride, CPC), anionic (sodium dodecylsulfate, SDS) and nonionic (Twin-80) surfactants as well as effect of pH value on the characteristics of TLC separ ation has been investigated. The best separ ation of three components has been achieved with 210 M CPC and LIO M Twin-80 solutions, at pH 7 (phosphate buffer). Individual solution of SDS didn t provide effective separation of caffeine, theophylline, theobromine, the rate of separ ation was low. The separ ation factor and rate of separ ation was increase by adding of modifiers - alcohol 1- propanol (6 % vol.) or 1-butanol (0.1 % vol.) in SDS solution. The optimal concentration of SDS is 210 M. 

Figure 8.1 Effect of pH on corrosion of 1100-H14 alloy (aluminum) by various chemical solutions. Observe the minimal corrosion in the pH range of 4-9. The low corrosion rates in acetic acid, nitric acid, and ammonium hydroxide demonstrate that the nature of the individual ions in solution is more important than the degree of acidity or alkalinity. (Courtesy of Alcoa Laboratories from Aluminum Properties and Physical Metallurgy, ed. John E. Hatch, American Society for Metals, Metals Park, Ohio, 1984, Figure 19, page 295.)...

Figure 11.10 Effect of pH of distilled water on erosion-corrosion of carbon steel at 122°F (50°C) (velocity, 39 ft/s, 12 m/s). (SOURCE M. G. Fontana and N. D. Greene, Corrosion Engineering, 2d ed., 1978, p. 75. Reprinted with permission from McGraw-Hill, Inc.)...

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). 

Figure 23.15. Effect of pH on the gel time of a P F cast resin. (After Apley )...

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. 

Luk, C. K., and Dulfano, M. J. (1983). Effect of pH, viscosity and ionic-.srrengtb changes on ciliary beating frequency of human bronchial explants. Clin. Sc. 64, 449-451. 

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],... 

FIGURE 8.6 Comparison of hexafluoro-2-propanol (HFIP) with formic acid as a denaturing agent in SEC. Eiution positions of neutral amino acids were similar with both agents. The elution positions of Lys and Asp shifted dramatically in C, as shown by the tie lines, but this was an effect of pH (see Fig. 8.7). The elution positions of a-MSH and formic acid are shown to demonstrate that the amino acids eluted within Vo and V,. Column Same as Fig. 8.1. Flow rate 1.0 ml/min. Mobile phase As noted. Detection Aiij = 0.1 AUFS. 

FIGURE 8.7 Effect of pH on retention of amino acids. Column and flow rate Same as Fig. 8.1. Mobile phase 10 mA1 potassium phosphate with SO mM HFIP pH as indicated (adjusted prior to the addition of HFIP). 

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