Dissociation constants diprotic acid

Takacs-Novak, K. Tam, K. Y., Multiwavelength spectrophotometric determination of acid dissociation constants. Part V. Microconstants and tautomeric ratios of diprotic amphoteric drugs, J. Pharm. biomed. anal. 17, 1171-1182 (2000). 

Adipic acid is a diprotic acid that is used to manufacture nylon. Its formula can be abbreviated to H2Ad. The acid dissociation constants for adipic acid are = 3.71 x 10 and = 3.87 x 10 . What is the pH of a 0.085 mol/L solution of adipic acid ... 

Similar expressions have been obtained for the particular cases of mono-protic acids and bases, diprotic acids and bases, and zwitterions (207, 208), and in each case the data conformed well to Eq. (111). It has also been shown (207) that the acid dissociation constants can be determined by using reversed phase chromatography. The pIK, values of 10 aromatic acids calculated from chromatographic data by employing Eq. (91) were... 

The standard notation for successive acid dissociation constants of a polyprotic acid is Kt, K2, K2, and so on, with the subscript a usually omitted. We retain or omit the subscript as dictated by clarity. For successive base hydrolysis constants, we retain the subscript b. The preceding examples illustrate that Kal (or K ) refers to the acidic species with the most protons, and Kbl refers to the basic species with the least number of protons. Carbonic acid, a very important diprotic carboxylic acid derived from COz, is described in Box 6-4. 

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. 

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 ... 

The protonated form of a neutral amino acid such as alanine is a diprotic acid, H2A+, with two acid dissociation constants, one for the -NH3 + group and one for the -CO2H group. 

The study of Lovgren et al. (1987) provides an example of the application of a discrete functional group approach to model the complexation of aluminium with humic substances found in bog-water. The acid-base titration behaviour of the humic material found in Swedish bog-water was modelled as a diprotic acid with the following reactions and acid dissociation constants ... 

Following the approach of Tanford (1961), it is useful to examine the ionization of a hypothetical diprotic acid (HA-BH). Thermodynamically, the acid-base properties of this acid are completely described by two acid dissociation constants ( fi and K2). However, at the molecular level, it is clearly more appropriate to define four dissociation constants as depicted in Equation (2) ... 

You can assign descriptive names to cells, such as Ka 1 and Ka2 to refer to the two acid dissociation constants of a diprotic acid. It is usually much easier to write and read formulas that contain descriptive names rather than cell addresses such as B 2 and C 2. Names can be used only to refer to individual absolute addresses. Names must start with a letter, may contain only letters, numbers, periods and underscores, and cannot be R, C, or possible cell addresses. Consequently, Cl and Ca 1 are not valid names, but Ca, caa3 and (in all current versions of Excel) Ka 1 are, since the rightmost column label is IV in future versions of Excel you might have to use Kaaland Kaa2 instead. 

Arrhenius concept Bronsted-Lowry model hydronium ion conjugate base conjugate acid conjugate acid-base pair acid dissociation constant Section 14.2 strong acid weak acid diprotic acid oxyacids organic acids carboxyl group monoprotic acids amphoteric substance autoionization... 

Equation 8-28 is formally analogous to what would be obtained at the second equivalence point of a diprotic acid titration or the last equivalence point of an N-protic acid titration. In these cases, the appropriate acid dissociation constant, K2 or must of course be used. 

Plan H2CO3 is a diprotic acid the two acid-dissociation constants, Kai and Ka2 (Table 16.3), differ by more than a factor of lO . Consequently, the pH can be determined by considering only K i, thereby treating the acid as if it were a monoprotic acid. 

Succinic acid (H2C4H5O4), which we will denote H2SUC, is a biologically relevant diprotic acid with the structure shown below. It is closely related to tartaric acid and malic acid (Figure 16.1). At 25 °C, the acid-dissociation constants for succinic acid are = 6.9 X 10 and Ff 2 = 2.5 X 10 . (a) Determine the pH of a 0.32 M solution of HjSuc at 25 °C, assuming that only the first dissociation is relevant, (b) Determine the molar concentration of Suc in the solution in part (a), (c) Is the assumption you made in part (a) justified by the result from part (b) (d) Will a solution of the salt NaHSuc be acidic, neutral, or basic ... 

In general, a diprotic acid has two acid dissociation constants, designated and K 2 (where K > K,2)-. 

Another situation in which an inflection point may be missing or difficult to detect occurs when the analyte is a multiprotic weak acid or base whose successive dissociation constants are similar in magnitude. To see why this is true let s consider the titration of a diprotic weak acid, H2A, with NaOH. During the titration the following two reactions occur. 

With a knowledge of the pH at the stoichiometric point and also of the course of the neutralisation curve, it should be an easy matter to select the appropriate indicator for the titration of any diprotic acid for which K1/K2 is at least 104. For many diprotic acids, however, the two dissociation constants are too close together and it is not possible to differentiate between the two stages. If K 2 is not less than about 10 7, all the replaceable hydrogen may be titrated, e.g. sulphuric acid (primary stage — a strong acid), oxalic acid, malonic, succinic, and tartaric acids. 

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. 

Statistical Effects. In a symmetrical diprotic acid, the first dissociation constant is twice as large as expected since there are two equivalent ionizable... 

From these equations it is possible to predict the effective lipophilicity (log D) of an acidic or basic compound at any pH value. The data required in order to use the relationship in this way are the intrinsic lipophilicity (log P), the dissociation constant (pKa) and the pH of the aqueous phase. The overaU effect of these relahonships is the effechve hpophilicity of a compound, at physiological pH, is approximately the log P value minus one unit of hpophilicity, for every unit of pH the pKa value is below (for acids) and above (for bases) pH 7.4. Obviously for compounds with mul-hfunchonal ionizable groups the relahonship between log P and log D, as weU as log D as a function of pH become more complex [65, 68, 70]. For diprotic molecules there are already 12 different possible shapes of log D-pH plots. 

In the case of a diprotic acid, the dissociation occurs in two steps and a separate equilibrium constant exists for each step ... 

Statistical effects. In a symmetrical diprotic acid, the first dissociation constant is twice as large as expected since there are two equivalent ionizable hydrogens, while the second constant is only half as large as expected because the conjugate base can accept a proton at two equivalent sites. So K IKi should be 4, and approximately this value is found... 

Uses of ionization constants to compute concentrations of the ions present in solution and the pH of the solution are illustrated in the following problems. First we consider the dissociation of a monoprotic acid, using acetic acid as an example. Later we examine the dissociation of a diprotic acid, H2S, in connection with precipitation of metal sulfides. 

Phosphorous acid (H3P03) is a weak diprotic acid because only two of its three H atoms are bonded to oxygen. The H atom bonded directly to phosphorus is not acidic because phosphorus and hydrogen have the same electronegativity and the P-H bond is nonpolar. In phosphoric acid, however, all three hydrogens are attached to oxygen, and thus phosphoric acid is a weak triprotic acid. The geometry about the P atom in both molecules is tetrahedral, as expected. Note that the successive dissociation constants decrease by a factor of about 105 (Section 15.11). 

The protonation reactions of surface hydroxyls which account for the development of surface charge are often represented as the dissociation of a diprotic acid site. Thus in water three surface sites can be present, =S—OH =S—OH0 and =S—O. The proton dissociation constants are represented by... 

Substances containing more than one acidic proton are called po/yprofic adds. Diprotic acids contain two acidic protons, and triprotic acids contain three acidic protons. Acid protons dissociate one at a time and have different Ka andpJC constants. Carbonic acid (H2CO3) is a diprotic acid. 

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