Water acid/base properties

Acid-Base Properties of Ions in Water Solution... 

In the polyazamacrocyclic phenols [64], an acidic function, the phenol group, has an intra-annular orientation while the basic units, the nitrogen atoms of the macrocycles, have no defined orientation. In water at 25°C, the pATa values of the phenols were measured and compared with those of other macrocyclic and non-macrocyclic phenols (Kimura et al., 1987a,b). Because the nitrogen atoms and the phenol function both possess acid-base properties, more than one pXg value could be measured. By the use of UV measurements, the values of the phenol group could be distinguished from those of the amines. 

An aqueous solution of a soluble salt contains cations and anions. These ions often have acid-base properties. Anions that are conjugate bases of weak acids make a solution basic. For example, sodium fluoride dissolves in water to give Na, F, and H2 O as major species. The fluoride anion is the conjugate base of the weak acid HF. This anion establishes a proton transfer equilibrium with water ... 

The water equilibrium seldom is the dominant one when a solution contains any solute with acid-base properties, so it is reasonable that the pH is determined by. ... 

Any anion of a weak acid, including the anions of polyprotic acids, is a weak base. The acid-base properties of monoanions of polyprotic acids are complicated, however, because the monoanion is simultaneously the conjugate base of the parent acid and an acid in its own right. For example, hydrogen carbonate anions undergo two proton-transfer reactions with water ... 

A buffer soiution must contain both the acid and its conjugate base, so at least two reagents must be added to water to prepare a buffer soiution. An acetate buffer can be prepared, for example, from pure water, concentrated acetic acid, and an acetate salt. The cation contained in the salt should not have acid-base properties of its own, so sodium acetate would be an appropriate choice, but ammonium acetate would not. 

Calcium carbonate is insoluble in pure water but dissolves in weakly acidic water. The role of this solubility phenomenon in the geochemistry of caverns is described in Box. We can understand this dependence on pH by examining the acid-base properties of the species involved in the solubility equilibrium. 

Some salts have acid-base properties. Ammonium chloride, NH4C1, when dissolved in water will dissociate and the ammonium ion will act as a weak acid, donating a proton. The strong bases previously listed are all salts that dissolve yielding the hydroxide ion (which is really the base, not the salt). 

Another salt-like group of compounds that have acid-base properties is the hydrides of the alkali metals and calcium, strontium, and barium. These hydrides will react with water to form the hydroxide ion and hydrogen gas ... 

Br0nsted-Lowery acids are H+ donors and bases are H+ acceptors. Strong acids dissociate completely in water. Weak acids only partially dissociate, establishing an equilibrium system. Weak acid and base dissociation constants (Ka and Kb) describe these equilibrium systems. Water is amphoteric, acting as both an acid or a base. We describe water s equilibrium by the Kw expression. A pH value is a way of representing a solution s acidity. Some salts and oxides have acid-base properties. A Lewis acid is an electron pair acceptor while a Lewis base is an electron pair donor. 

The behavior of a salt will depend upon the acid—base properties of the ions present in the salt. The ions may lead to solutions of the salt being acidic, basic, or neutral. The pH of a solution depends on hydrolysis, a generic term for a variety of reactions with water. Some ions will undergo hydrolysis and this changes the pH. 

All studied complexes in aqueous solution are present as diaqua [Mn(L)(H20)2] species, whereas two chloro ligands are replaced by water molecules in axial positions (35,36). For studying the water-exchange processes it was necessary first to determine the acid-base properties of the complexes by potentiometric titration in order to define the pH range where the diaqua form of the studied complexes is the predominant one. The potentiometric titrations of... 

The dapsox ligand, besides high complex stability, provides appropriate acid-base properties of the complex (Fig. 3), causes an increase in the pAa values of the coordinated water molecules (pAai = 5.8 and pi a2 — 9.5) 45h,46), which are very close to the pi a values of the native Fe -SOD enzyme ( 5 and 9) (3a). Thus at the physiological pH almost 100% of the complex is in the catalytically active aqua-hydroxo form. 

The main usefulness of Eh-pH diagrams consists in the immediacy of qualitative information about the effects of redox and acid-base properties of the system on elemental solubility. Concerning, for instance, cerium, figure 8.20 immediately shows that, within the stability field of water, delimited upward by oxidation boundary curve o and downward by reduction boundary curve r, the element (in the absence of other anionic ligands besides OH groups) is present in solution mainly as trivalent cerium Ce and as soluble tetravalent hydroxide Ce(OH)2. It is also evident that, with increasing pH, cerium precipitates as trivalent hydroxide Ce(OH)3. 

Water is the prototype of an amphiprotic solvent and all other solvents with similar acid-base properties are called neutral solvents. Solvents which are much stronger acids and much weaker bases than water are called protogenic solvents, while those which are much stronger bases and much weaker acids than water are designated protophilic solvents. 

With respect to the well-known acid/base properties of water, ammonia, and phenol in aqueous solution, one has to conclude that enormous solvation energies must contribute to the differences from the behavior of isolated water molecules. 

The pH scale in water is widely used as a measure of acid-base properties in aqueous solutions. It is defined by pH=-log a(H+). In Section 3.1, we dealt with the poH value, defined by poH=-log a(H+), for solutions in amphiprotic and aprotic solvents of high permittivity. Recently, however, the symbol pH has also been used for the value of -log o(H+) in such non-aqueous solutions. Therefore, hereafter, the symbol pH is used instead of paH-... 

In the above, the pH of the solution of a(H+) = lmolkg 1 was defined equal to zero in each solvent.10 However, the solvation of H+ differs from one solvent to another and, even in a solution of pH = 0, the reactivity of H+ differs drastically by solvent. In order to compare the acid-base properties in different solvents, it is convenient to define a pH scale that is common to various solvents [20]. Figure 3.6 shows the pH windows in various solvents in such a common pH scale, using the pH in water as reference. In the figure, the left margin of the pH window corresponds to the value of -log yt(H+,W->S) (see Table 2.7) and the width of the window corresponds to the value of pKSH. If the solvent is of weaker basicity than water, the pH window expands to the left (more acidic) side than water. On the other hand, if the solvent is of weaker acidity than water, the pH window expands to the right (more basic) side than water. 

The redox reactions of organic compounds are influenced by the acid-base properties of the solvents. In protic solvents like water, many organic compounds (Q °) are reduced by a one-step two-electron process, Q°+2H+ + 2e = QH2, although... 

If the solvent is a mixture of water and organic solvent ZH, several equilibria may be established, more or less simultaneously when ZH and H20 have similar acid-base properties ... 

All these electrolytes are neutral in Bronsted acid-base properties. Although rather exceptional, an acid, a base, or a pH buffer may be added to the supporting electrolyte of neutral salts. The acid-base system to be selected depends on the purpose of the measurement. We often use trifluoromethanesulfonic acid (CF3S03F1) as a strong acid acetic acid, benzoic acid, or phenol as a weak acid an amine or pyridine as a weak base and tetraalkylammonium hydroxide (ILtNOH) as a strong base. Examples of buffer systems are the mixtures of picric acid and its R4N-salt and amines and their PlCl04-salts. Here, we should note that the acid-base reactions in aprotic solvents considerably differ from those in water, as discussed in Chapter 3. 

Hydrolysis is strong acid in which all hydroxo bridges are cleaved, followed by identification of the various mononuclear species and a determination of their molar ratios, may provide extremely valuable information. A straightforward example is the cleavage of the tetra-nuclear species Cr4(NH3)12(OH)66+, which yields Cr(H20)63+ and cis-Cr(NH3)4(H20)23+ in a ratio of 1 3 (40). Since it could be demonstrated at the same time that the polynuclear cation does not exhibit acid base properties in the pH region for terminally coordinated water, it was concluded that the only possible structure was 6 in Fig. 1, as later confirmed by a crystal-structure analysis (41). 

The pXa s of substituted 2-, 4-, and 5-substituted carboxythiazoles and rates of basic hydrolysis of substituted ethyl 2-, 4-, and 5-thiazolo-carboxylates, in water and aqueous ethanol, correlate approximately with the Hammett equation,3,0,311 but the p values are much different to their benzenoid counterparts demonstrating the presence of proximity effects such as hydrogen bonding, as well as possibly tautomeric equilibria. Other correlations of acid-base properties in this area are for... 

A good example of the use of the functional-group concept is for acid-base properties. Alcohols, ROH, are structurally related to water, HOH, in that both possess a hydroxyl function. We may then expect the chemistry of alcohols to be similar to that of water. In fact, both are weak acids because the OH group has a reactive proton that it can donate to a sufficiently strongly basic substance, written as B here ... 

I once mentioned in a lecture that acid-base properties across the interface were not important in developing water resistant bonds to metal surfaces and cited the data of Table 7 [17]. Professor Fowkes was present and commented that there are numerous examples where acid-base reactions across the interface... 

For each of the compounds in the Acid-Base Properties of Salts activity (eChapter 15.14), write the reaction that takes place when the compound is placed in water. Identify the conjugate base pairs in each reaction. 

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