Weak acids definition

Different side chains have been found to have weak but definite preferences either for or against being in a helices. Thus Ala (A), Glu (E), Leu (L), and Met (M) are good a-helix formers, while Pro (P), Gly (G), Tyr (Y), and Ser (S) are very poor. Such preferences were central to all early attempts to predict secondary structure from amino acid sequence, but they are not strong enough to give accurate predictions. 

Buffers are solutions that tend to resist changes in their pH as acid or base is added. Typically, a buffer system is composed of a weak acid and its conjugate base. A solution of a weak acid that has a pH nearly equal to its by definition contains an amount of the conjugate base nearly equivalent to the weak acid. Note that in this region, the titration curve is relatively flat (Figure 2.15). Addition of H then has little effect because it is absorbed by the following reaction ... 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

Having considered all the possible cases, we are now in a position to give a more general definition of hydrolysis. Hydrolysis is the interaction between an ion (or ions) of a salt and water with the production of (a) a weak acid or a weak base, or (b) of both a weak acid and a weak base. 

Discussion. The hydroxides of sodium, potassium, and barium are generally employed for the preparation of solutions of standard alkalis they are water-soluble strong bases. Solutions made from aqueous ammonia are undesirable, because they tend to lose ammonia, especially if the concentration exceeds 0.5M moreover, it is a weak base, and difficulties arise in titrations with weak acids (compare Section 10.15). Sodium hydroxide is most commonly used because of its cheapness. None of these solid hydroxides can be obtained pure, so that a standard solution cannot be prepared by dissolving a known weight in a definite volume of water. Both sodium hydroxide and potassium hydroxide are extremely hygroscopic a certain amount of alkali carbonate and water are always present. Exact results cannot be obtained in the presence of carbonate with some indicators, and it is therefore necessary to discuss methods for the preparation of carbonate-free alkali solutions. For many purposes sodium hydroxide (which contains 1-2 per cent of sodium carbonate) is sufficiently pure. 

The Br0nsted theory expands the definition of acids and bases to allow us to explain much more of solution chemistry. For example, the Brpnsted theory allows us to explain why a solution of ammonium chloride tests acidic and a solution of sodium acetate tests basic. Most of the substances that we consider acids in the Arrhenius theory are also acids in the Bronsted theory, and the same is true of bases. In both theories, strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can now explain this partial ionization as an equilibrium reaction of the ions, the weak acid, and the water. A similar statement can be made about weak bases ... 

We might rephrase this statement, and say an acid is strong if its extent of ionization is high, and weak if the extent of ionization is small. Within this latter definition, both I and VI are weak acids. 

The problem the analyst has is to choose indicators that change color close enough to an equivalence point so that the accuracy of the experiment is not diminished, which really means at any point during the inflection point. (Refer to Section 4.2 for the definitions of equivalence point and end point.) It almost seems like an impossible task, since there must be an indicator for each possible acid or base to be titrated. Fortunately, there are a large number of indicators available, and there is at least one available for all acids and bases, with the exception of only the extremely weak acids and bases. Figure 5.5 lists some of these indicators and shows the pH ranges over which they change color. 

The same definitions apply to strong and weak bases as to strong and weak acids. 

Drugs cross biological membranes most readily in the unionised state. The unionised drug is 1000-10000 times more lipid-soluble than the ionised form and thus is able to penetrate the cell membrane more easily. Chemical compounds in solution are acids, bases or neutral. The Bronsted-Lowry definition of an acid is a species that donates protons (H+ ions) while bases are proton acceptors. Strong acids and bases in solution dissociate almost completely into their conjugate base and H+. Weak acids and weak bases do not completely dissociate in solution, and exist in both ionised and unionised states. Most drugs are either weak acids or weak bases. For an acid, dissociation in solution is represented by ... 

When pKa=pH, there will be equal fractions of ionised and unionised drug. This is sometimes used as an alternative definition of pKa, i.e. the pH of a solution in which a substance is 50% ionised and 50% unionised. When pH> pKa the ionised form of weak acids predominates. 

Being the opposite of an acid, a base will be defined as a compound that has a tendency to combine with protons. In this definition the base9 in an alkaline solution is the OH ion. This ion is one of the strongest bases known to exist. When combining with a proton it forms water, that itself is a weak base, since it is able to add one more proton to form an OHJ, hydronium ion, and a weak acid at the same time, since water can dissociate into OH and H+ ions. Water being a base, too, the actual dissociation reaction will be... 

More definite evidence for the transient existence of the un-cyclized l-(jS-aminoethyl)-3,4-benzoquinones has been obtained recently by Kodja and Bouchilloux,77 78 who noted that a transient yellow color (Amax ca. 385 mp) was occasionally observed during the enzymic oxidations of catecholamines (particularly in unbuffered systems at low temperatures). This phenomenon was probably due to the formation of the transient o-quinones. (The absorption maximum of o-benzoquinone, the effective chromophore of the open-chain quinones, is known to occur at ca. 390 mp.79) An absorption maximum at 390 mp is characteristic of the formation of the dopa-quinone chromophore during oxidation of small C -terminal tyrosine peptides in the presence of tyrosinase.37 48 Similar spectroscopic features were observed when the oxidations were carried out with lead dioxide in sulfuric acid solutions (pH> 1). If the initial oxidation was carried out for a short period of time, it was possible to regenerate the original catecholamines by reduction (e.g. with sodium bisulfite, potassium iodide, and zinc powder) and to show that the 385 mp peak disappeared.77,78 Kodja and Bouchilloux were also able to identify 2,4-dinitrophenylhydrazones of several of the intermediate non-cyclized quinones by paper chromatography and spectroscopy (Amax n weakly acid solution ca. 350 mp with a shoulder at ca. 410 mp).77,78... 

The "salt" or conjugate base, A, is the ionized form of a weak acid. By definition, the dissociation constant of the acid, Ka, is... 

The equilibrium constant is called Ka, the acid dissociation constant. By definition, a weak acid is one that is only partially dissociated in water. This definition means that Ka is small for a weak acid. 

The definition of pH is pH = —log[H+] (which will be modified to include activity later). Ka is the equilibrium constant for the dissociation of an acid HA + H20 H30+ + A-. Kb is the base hydrolysis constant for the reaction B + H20 BH+ + OH. When either Ka or Kb is large, the acid or base is said to be strong otherwise, the acid or base is weak. Common strong acids and bases are listed in Table 6-2, which you should memorize. The most common weak acids are carboxylic acids (RC02H), and the most common weak bases are amines (R3N ). Carboxylate anions (RC02) are weak bases, and ammonium ions (R3NH+) are weak acids. Metal cations also are weak acids. For a conjugate acid-base pair in water, Ka- Kb = Kw. For polyprotic acids, we denote the successive acid dissociation constants as Kal, K, K, , or just Aj, K2, A"3, . For polybasic species, we denote successive hydrolysis constants Kbi, Kb2, A"h3, . For a diprotic system, the relations between successive acid and base equilibrium constants are Afa Kb2 — Kw and K.a Kbl = A w. For a triprotic system the relations are A al KM = ATW, K.d2 Kb2 = ATW, and Ka2 Kb, = Kw. 

Metavanadates of the alkalis are white or colourless, and give colourless aqueous solutions which rapidly become yellow, and, on addition of acids, red or orange. These coloured solutions contain polyvanadates, the formation of which is comparable to that of the polychromates and other salts formed by condensation of weakly acid oxides of metals, e.g. molybdates and borates. Thus, under definite conditions of temperature and concentration, potassium metavanadate is converted into the acid salt 2K20.3Va05, in accordance with the equation ... 

Proton transfers in solution reach equilibrium very rapidly and for all weak acids and bases, we have to consider the reverse proton transfer reaction as well as the forward reaction. For example, the CN ion produced when HCN loses a proton to water can accept a proton from a water molecule and form HCN again. Therefore, according to the Bronsted definition, CN- is a base it is called the conjugate base of HCN. In general, a conjugate base is the species left when an acid donates a proton ... 

A weak acid is one that is only partially dissociated in water. Only a small fraction of the weak acid molecules transfer a proton to water, and the solution therefore contains mainly undissociated HA molecules along with small amounts of H30+ and the conjugate base A-. Typical weak acids are nitrous acid (HNO2), hydrofluoric acid (HF), and acetic acid (CH3C02H). In the case of very weak acids such as NH3, OH-, and H2, the acid has practically no tendency to transfer a proton to water, and the acid-dissociation equilibrium lies essentially 100% to the left. It follows from this definition that very weak acids have strong conjugate... 

Typical values of pK[nt and pfor a humic acid are 2.67 and 4.46. The introduction of the electrostatic factor into the equilibrium constant is analogous to the coulombic term used in the definition of the intrinsic surface complexation constants. In addition another binding site (WAH) is recognised which is thought to behave as a weak acidic phenolic functional group. Although this site does not contribute to the titratable acidity and, therefore, no pK is needed for proton dissociation, it is involved in metal complexation reactions. The total number of the three monoprotic sites is estimated from titratable acidity and then paired to represent the humic substance as a discrete non-interacting mixture of three dipro-tic acids, which act as the metal complexation sites. The three sites are... 

Acids and bases react to give salts and (usually) water. That is one definition of a salt. So, when we say salt solution, we are really talking about solutions that contain the conjugate acid or base of some other acid or base. The pH of a salt solution depends on the acid/base strength of the acid or base from which it was derived. There are three permutations on the problem salts of strong ac-ids/bases, salts of weak acids, and salts of weak bases. Let s consider them each in turn. 

Definition of Acids and Bases 222 Conjugate Acid-Base Pairs 222 Amphoteric Species 224 Strong Acids 225 Strong Bases 225 Weak Acids 226 Weak Bases 226 Polyprotic Acids 227 Acid and Base Strength Ka and Kb 228 Acid/Base Strength of Conjugate Acid-Base Pairs 230 Acid-Base Reactions 231... 

It is obvious from the mechanism (9.1)—(9.7) that oxidation condition variation may direct the process towards selective formation of a definite product. For example, oxygen-containing compounds can be obtained in one highly selective stage (monooxygenation) by the reaction channel (9.7). The comparative simplicity of these reactions is also associated with the fact that metal complexes, representing something like weak acids , coordinate weak bases, such as olefins, N2, CO, etc. with formation of unstable complexes, which provide for catalytic transformation of the substrate [5],... 

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