Acidity polyprotic

Monoprotic weak acids, such as acetic acid, have only a single acidic proton and a single acid dissociation constant. Some acids, such as phosphoric acid, can donate more than one proton and are called polyprotic weak acids. Polyprotic acids are described by a series of acid dissociation steps, each characterized by it own acid dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions and acid dissociation constants. 

Define monoprotic acid, polyprotic acid, monobasic base, polybasic base, titration curve, and inflection point. 

Alcohols Carboxylic acids Phenols Primary and secondary amines Oximes Nitro compounds (with a-H atoms) Nitriles (with a-H atoms) Polyhydroxyalcohols Amino alcohols Hydroxy acids Polyprotic acids Polyphenols... 

Acids that contain more than one dissociable proton are called polyprotic acids. Polyprotic acids dissociate in a stepwise manner, and each dissociation step is characterized by its own acid-dissociation constant, Kal, Ka2, and so forth. For example, carbonic acid (H2C03), the diprotic acid that forms when gaseous carbon dioxide dissolves in water, is important in maintaining a constant pH in human blood. It undergoes the following dissociation reactions ... 

Several important classes of acids fall outside the classic categories of Drganic oxyacids and nitrogen acids or have special features that warrant separate discussion. This chapter covers those acids, which include carbon acids, inorganic acids, polyprotic acids, super acids, and excited-state acids. 

Citrus fruit derives its acidic qualities from citric acid, H3C6H5O7, a type of acid (polyprotic) discussed in Section 16-6. Another important constituent of citrus fruit is ascorbic acid, or vitamin C (page 679), a dietary requirement to prevent scurvy. 

Polyprotic Acids—Polyprotic acids are acids with more than one ionizable H atom that undergo a stepwise ionization and have a different ionization constant, ai/ -Kaj/ / for each ionization step. 

Polyprotic bases, like polyprotic acids, also have more than one base dissociation reaction and base dissociation constant. 

A more challenging problem is to find the pH of a solution prepared from a polyprotic acid or one of its conjugate species. As an example, we will use the amino acid alanine whose structure and acid dissociation constants are shown in Figure 6.11. 

Weltin, E. Galculating Equilibrium Goncentrations for Stepwise Binding of Ligands and Polyprotic Acid-Base Systems, ... 

This approach can be used to sketch titration curves for other acid-base titrations including those involving polyprotic weak acids and bases or mixtures of weak acids and bases (Figure 9.8). Figure 9.8a, for example, shows the titration curve when titrating a diprotic weak acid, H2A, with a strong base. Since the analyte is... 

From a chemical point of view, the common amino acids are all weak polyprotic acids. The ionizable groups are not strongly dissociating ones, and the degree of dissociation thus depends on the pH of the medium. All the amino acids contain at least two dissociable hydrogens. 

Certain weak acids are polyprotic they contain more than one ionizable hydrogen atom. Such acids ionize in steps, with a separate equilibrium constant for each step. Oxalic acid, a weak organic acid sometimes used to remove bloodstains, is diprotic ... 

The behavior of these acids is typical of all polyprotic acids in that—... 

Equilibrium Constants for Some Weak Polyprotic Acids at 25°C... 

Ordinarily, successive values of for polyprotic acids decrease by a factor of at least 100 (Table 13.3). In that case, essentially all the H+ ions in the solution come from the first step. This makes it relatively easy to calculate the pH of a solution of a polyprotic acid. 

Polyprotic acids ionise in stages. In sulphuric acid, one hydrogen atom is almost completely ionised ... 

When a polyprotic acid is dissolved in water, the various hydrogen atoms undergo ionisation to different extents. For a diprotic acid H2A, the primary and secondary dissociations can be represented by the equations ... 

Buffer mixtures are not confined to mixtures of monoprotic acids or monoacid bases and their salts. We may employ a mixture of salts of a polyprotic acid, e.g. NaH2P04 and Na2HP04. The salt NaH2P04 is completely dissociated ... 

NEUTRALISATION OF A POLYPROTIC ACID WITH A STRONG RASE... 

Polyprotic acids (or mixtures of acids, with dissociation constants AT, K2, and AT3) and strong bases. The first stoichiometric end point is given approximately... 

The theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

Acids Bronsted-Lowry theory of, 21 common, concentration of, (T) 829 dissociation constants of, (T) 832 hard. 54 ionisation of, 20 Lewis, 22 polyprotic, 20... 

Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

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