PH, dependence

Table XI-1 (from Ref. 166) lists the potential-determining ion and its concentration giving zero charge on the mineral. There is a large family of minerals for which hydrogen (or hydroxide) ion is potential determining—oxides, silicates, phosphates, carbonates, and so on. For these, adsorption of surfactant ions is highly pH-dependent. An example is shown in Fig. XI-14. This type of behavior has important applications in flotation and is discussed further in Section XIII-4.

A major difficulty in an inorganic text is to strike a balance between a short readable book and a longer, more detailed text which can be used for reference purposes. In reaching what we hope is a reasonable compromise between these two extremes, we acknowledge that both the historical background and industrial processes have been treated very concisely. We must also say that we have not hesitated to simplify complicated reactions or other phenomena—thus, for example, the treatment of amphoterism as a pH-dependent sequence between a simple aquo-cation and a simple hydroxo-anion neglects the presence of more complicated species but enables the phenomena to be adequately understood at this level. 

This set of calculations results in an output file containing all of the energies required to solve for the pH-dependent properties of interest. The form and content of the file are as follows ... 

It is obvious that the reaction is accelerated markedly by water. However, for the first time, the Diels-Alder reaction is not fastest in water, but in 2,2,2-trifiuoroethanol (TFE). This might well be a result of the high Bronsted acidity of this solvent. Indirect evidence comes from the pH-dependence of the rate of reaction in water (Figure 2.1). Protonation of the pyridyl nitrogen obviously accelerates the reaction. 

First, the pH-dependence of the enantioselectivity of the reaction between 3.8c and 3.9 catalysed by the copper(L-tryptophan) complex has been studied. Above pH 5 the enantioselectivity reaches a plateau value (Figure 3.3). The diminished enantioselectivities observed at lower pH most likely... 

Since the position of an acid-base equilibrium depends on the pH, the distribution ratio must also be pH-dependent. To derive an equation for D showing this dependency, we begin with the acid dissociation constant for HA. 

Cyanide is frequently used as a masking agent for metal ions. The effectiveness of CN as a masking agent is generally better in more basic solutions. Explain the reason for this pH dependency. 

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. 

Two general methods are used for homogeneous precipitation. If the precipitate s solubility is pH-dependent, then the analyte and precipitant can be mixed under conditions in which precipitation does not occur. The pH is then raised or lowered as needed by chemically generating OH or H3O+. For example, the hydrolysis of urea can be used as a source of OH . 

Free Ions Versus Complexed Ions In discussing the ion-selective electrode, we noted that the membrane potential is influenced by the concentration of F , but not the concentration of HF. An analysis for fluoride, therefore, is pH-dependent. Below a pH of approximately 4, fluoride is present predominantly as HF, and a quantitative analysis for total fluoride is impossible. If the pH is increased to greater than 4, however, the equilibrium... 

Watanabe and co-workers described a new membrane electrode for the determination of cocaine, which is a weak base alkaloid with a piC of 8.64d The response of the electrode for a fixed concentration of cocaine was found to be independent of pH in the range of 1-8, but decreased sharply above a pH of 8. Offer an explanation for the source of this pH dependency. 

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. 

Sodium cyanoborohydride is remarkably chemoselective. Reduction of aldehydes and ketones are, unlike those with NaBH pH-dependent, and practical reduction rates are achieved at pH 3 to 4. At pH 5—7, imines (>C=N—) are reduced more rapidly than carbonyls. This reactivity permits reductive amination of aldehydes and ketones under very mild conditions (42). 

The equihbrium constant of this reaction is 5.4 x 10 at 25°C, ie, iodine hydrolyzes to a much smaller extent than do the other halogens (49). The species concentrations are highly pH dependent at pH = 5, about 99% is present as elemental at pH = 7, the and HIO species are present in almost equal concentrations and at pH = 8, only 12% is present as and 88% as HIO. The dissociation constant for HIO is ca 2.3 x 10 and the pH has tittle effect on the lO ion formation. At higher pH values, the HIO converts to iodate ion. This latter species has been shown to possess no disinfection activity. An aqueous solution containing iodate, iodide, and a free iodine or triodide ion has a pH of about 7. A thorough discussion of the kinetics of iodine hydrolysis is available (49). 

The lanthanides form many compounds with organic ligands. Some of these compounds ate water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating agents). The complex formation is pH-dependent. Oil-soluble compounds ate used extensively in the industrial separation of rate earths by tiquid—tiquid extraction. The preferred extractants ate catboxyhc acids, otganophosphoms acids and esters, and tetraaLkylammonium salts. 

Fig. 3. The pH dependence, where A, B, and C represent regions corresponding to the p-K s of glutamic and aspartic acids, lysine, and argenine, respectively, of (a) protein swelling, and (b) protein acid-binding capacity. Adapted from Ref. 3.

AMMONIUM compounds). Diammonium dimolybdate [27546-07-2] (NH 2 2 7 " ble commercially as the tetrahydrate and prepared from MoO and excess NH in aqueous solution at 100°C, has an infinite chain stmcture based on MoO octahedra. In aqueous solution the behavior of Mo(VI) is extremely pH-dependent (4). Above pH 7 molybdenum(VI) occurs as the tetrahedral oxyanion MoO , but below pH 7 a complex series of concentration-, temperature-, and pH-dependent equiUbria exist. The best known of these equiUbria lead to the formation of the heptamolybdate,... 

Nloha.tes, Niobic acid is amphoteric and can act as an acid radical in several series of compounds, which are referred to as niobates. Niobic acid is soluble in solutions of the hydroxides of alkaH metals to form niobates. Fusion of the anhydrous pentoxide with alkaH metal hydroxides or carbonates also yields niobates. Most niobates are insoluble in water with the exception of those alkaH metal niobates having a base-to-acid ratio greater than one. The most weU-known water-soluble niobates are the 4 3 ad the 7 6 salts (base acid), having empirical formulas MgNb O c, (aq) and M24Nb2202y (aq), respectively. The hexaniobate is hydrolyzed in aqueous solution according to the pH-dependent reversible equiHbria (130), when the pH is ca 9. 

The process of reabsorption depends on the HpophiHc—hydrophiHc balance of the molecule. Charged and ioni2ed molecules are reabsorbed slowly or not at all. Reabsorption of acidic and basic metaboHtes is pH-dependent, an important property in detoxification processes in dmg poisoning. Both passive and active carrier-mediated mechanisms contribute to tubular dmg reabsorption. The process of active tubular secretion handles a number of organic anions and cations, including uric acid, histamine, and choline. Dmg metaboHtes such as glucuronides and organic acids such as penicillin are handled by this process. 

Fig. 3. pH Dependence of total organic carbon (TOC) decay during the photooxidation of a landfill leachate using H2O2/uv. Initial TOC concentration,... 

Dissociation. In aqueous solution, amino acids undergo a pH-dependent dissociation (37) ... 

Other ingredients besides the elastomer and the cure system itself influence cure and scorch behavior. Usually the effect of a material on cure is pH-dependent. Ingredients which are basic in nature tend to accelerate the rate of both scorch and cure, whereas acidic materials exhibit the opposite effect. 

Plasticizers (qv) can range in composition from the ester types, ie, epoxies, phosphate, amides, etc. The effect of any one of these on curing is usually pH-dependent. However, it is pmdent to investigate each on a case by case basis. 

Physical Chemical Characterization. Thiamine, its derivatives, and its degradation products have been fully characterized by spectroscopic methods (9,10). The ultraviolet spectmm of thiamine shows pH-dependent maxima (11). H, and nuclear magnetic resonance spectra show protonation occurs at the 1-nitrogen, and not the 4-amino position (12—14). The H spectmm in D2O shows no resonance for the thiazole 2-hydrogen, as this is acidic and readily exchanged via formation of the thiazole yUd (13) an important intermediate in the biochemical functions of thiamine. Recent work has revised the piC values for the two ionization reactions to 4.8 and 18 respectively (9,10,15). The mass spectmm of thiamine hydrochloride shows no molecular ion under standard electron impact ionization conditions, but fast atom bombardment and chemical ionization allow observation of both an intense peak for the patent cation and its major fragmentation ion, the pyrimidinylmethyl cation (16). 

The intermediate HCIO2 is rapidly oxidized to chloric acid. Some chlorine dioxide may also be formed. Kinetic studies have shown that decomposition to O2 and chloric acid increase with concentration, temperature (88), and exposure to light (89—92), and are pH dependent (93). Decomposition to O2 is also accelerated by catalysts, and decomposition to chlorate is favored by the presence of other electrolytes, eg, sodium chloride (94—96). 

The iodometric analysis method for CIO2 and its coproducts is based on the pH-dependant oxidation of potassium iodide to selectively distinguish the various oxychlorine species from each other (42,89). The reactions of the oxidizer species with iodide at various pH buffered conditions ate... 

The most favorable conditions for equation 9 are temperature from 60—75°C and pH 5.8—7.0. The optimum pH depends on temperature. This reaction is quite slow and takes place in the bulk electrolyte rather than at or near the anode surface (44—46). Usually 2—5 g/L of sodium dichromate is added to the electrolysis solution. The dichromate forms a protective Cr202 film or diaphragm on the cathode surface, creating an adverse potential gradient that prevents the reduction of OCU to CU ion (44). Dichromate also serves as a buffering agent, which tends to stabilize the pH of the solution (45,46). Chromate also suppresses corrosion of steel cathodes and inhibits O2 evolution at the anode (47—51). 

L/(mol-s) (39,40). QDI is also attacked by hydroxide ion (eq. 4) to produce a quinone monoimine (QMI), itself an oxidized developer derived from /)-aminopheno1. Such compounds can further react with coupler, albeit at a slower rate than QDI, to form a dye and were cited in the seminal patent as color developers (32). However, the dyes derived from this deaminated developer have different hues from the QDI dyes, and these hues are pH-dependent as a consequence of the phenoHc group contributed by the developer. Although the deamination reaction to produce QMI is fast, the rate constant is 10 to 10 L/(mol-s) (40—42), its effect is somewhat offset by the redox reaction of the QMI with the reduced developer, present in large excess, to regenerate the desired QDI. The primary net effect of the deamination reaction is to enlarge the resulting dye cloud (43). 

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

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