Acids, polybasic, ionization

Dibasic acids, polybasic acids, and proteins, with two or more ionizable groups, give more complex titration curves than the sinqile monobasic adds (Schulz, 1994). Consider the titration curve of a simple amino acid which has no charge in its side chain. In aqueous solution, a monoamino, monocarboxylic add is distributed between three species, namely the fully protonated acid, the zwitterion, and the fully dissociated base ... 

Polybasic acids like HP3O4 dissociate in stages—the first atom of hydrogen ionizes easily, but HP2O4 is a weak acid and ionizes slowly, and HPO4 is a very weak acid. 

Moreover, many hydrogen compounds, when dissolved in such solvents, ionize more or less completely lo give solvated protons and anions, In the case uf polybasic acids, ionization constants are reported for each step in this dissociation. 

For block copolymers with a polyacid or polybase block, the structure and properties of micellar solutions depend on the pH. For example, Morishima et al. (1982b) found that for a poly(9-vinylphenanthrene)-poly(methacrylic acid) (PVPT-PMA) diblock in water, the rate constant for the fluorescence quenching of phenanthrene groups by oxidative non-ionic quencher is pH dependent. These authors suggested that at low pH the polyacid units are not fully ionized and may participate in the formation of hydrophobic domains, cooperatively with PVPT. An alternative explanation is that the PM A chains are less solvated when... 

It is usually true with polybasic acids that one hydrogen radical ionizes with greater facility than the remaining ones. Thus phosphoric acid ionizes primarily as a monobasic acid... 

As long as the base is in excess both stages of the ionization of carbonic acid can proceed to completion because of the removal of H+ ions (see Ionization of Polybasic Acids, Chapter III, page 116),... 

Review in Chapter III the section on the ionization of polybasic acids, page 116 in Chapter IV, Preparations 8 and 14, and Experiments 15 and 25 in Chapter VIII, Experiments 11 and 12. 

Pauling suggests that successive ionization constants of a given polybasic acid have ratios about 1 10 b 10 10, but such an estimate is obviously a very rough approximation. 

Dissociation Constants of Polybasic Acids Conductance Method.— A polybasic acid ionizes in stages, each stage having its own characteristic dissociation constant for example, the ionization of a tribasic acid HsA, such as phosphoric acid, may be represented by ... 

Dissociation Constants of Dibasic Acids by E.M.F. Measurement.— If the ratio of the dissociation constants of a dibasic acid, or of any two successive stages of ionization of a polybasic acid, is greater than about 10 or 10 , it is possible to treat each stage as a separate acid and to determine its dissociation constant by means of cells without liquid junction in the manner already described. In a mixture of the free dibasic acid H2A with its salt NallA, the essential equilibria are... 

The ratio of successive ionization constants for a polybasic acid is usually about 10 , as in this case. We see that with respect to its first hydrogen phosphoric acid is a moderately strong acid—considerably stronger than acetic acid. With respect to its second hydrogen it is weak, and to its third very weak. 

Solution of polybasic add A solution containing only a polybasic add and water can almost always be treated by considering only tKe first step of ionization. To illustrate, if the dibasic acid HjA is added to water, we have the equilibria... 

For detailed understanding of many chemical reactions, more knowledge about ionization constants is needed than has been considered so far. The macroscopic constants K yKi,--. of polybasic acids can have several components arising from the detailed ionization processes. It has been shown that, if a dibasic add has two equivalent and independent ionization processes, the minimum value of KJK2 is 4. 

In polybasic acids, the effect of chelation appears between later /C s. For example, in ethylenediaminetetra-acetic acid the first two hydrogen ions can dissociate readily, but the third and later ionizations axe greatly hindered by H bonding. 

It is important to be able to recognize the predominant species which will be present in solutions of polybasic acids at various pH values. The two ionizations for oxalic acid may be written as... 

In the case of weakly ionizable PEs, the fraction of charged monomer units, a, is controlled by an ionization equilibrium and is affected by the local proton concentration and ionic strength. Weak polyacids such as poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA), or polybases such as PVP and poly(dimethyl-aminoethyl methacrylate)(PDMAEMA) are typical examples of these pH-sensitive or annealing PEs. 

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