Diprotic acids and bases

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 treatment of diprotic acids and bases can be extended to polyprotic systems. By way of review, let s write the pertinent equilibria for a triprotic system. 

Diprotic acids and bases fall into three categories ... 

So far we have only considered monoprotic acids and bases. Fortunately, the corresponding relations for diprotic and polyprotic acids and bases are quite similar. The relations for diprotic acids and bases will be given here, while those for their polyprotic counterparts (with three or more dissociable protons) can be found in section 4.9. 

Diprotic acids and bases. Many drug substances bear more than one ionizable function acidic or basic functional groups or, in case of amphoteric substances, even both types in the same molecule. In order to evaluate their influence on solubility, the key parameter is again the pjSTa- In Table 35.2, the equations describing the pH profiles for mono- and diprotic acids and bases are given in their exponential form. As an example, the graph in Fig. 35.3 shows the profile of a diprotic base with values relatively close to each other. As upon addition of an acid to a suspension of the base the pH decreases, the solubility starts to increase by one order of... 

Table 35.2 Solubility equations for mono- and diprotic acids and bases...

TABLE 15 5 Ionization Constants of Some Diprotic Acids and Bases at 25°C a Polyprotic Acid and Their Conjugate ... 

Triprotic Acids and Bases, and Beyond The treatment of a diprotic acid or base is easily extended to acids and bases having three or more acid-base sites. For a triprotic weak acid such as H3PO4, for example, we can treat H3PO4 as if it was a mono-protic weak acid, H2P04 and HP04 as if they were intermediate forms of diprotic weak acids, and P04 as if it was a monoprotic weak base. 

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... 

Equations have been published [16] which relate pKa and p0Ka values to partition coefficient (P) values for monoprotic acids and bases, and diprotic acids, bases and ampholytes. For example, P1 for a monoprotic acid is calculated from... 

Polyprotic acids and bases are compounds that can donate or accept more than one proton. For example, oxalic acid is diprotic and phosphate is tribasic ... 

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. 

Related properties—of interest to geology, environmental science, and ceramics—are the surface acidity of a solid3 and the pH of zero charge. Mineral, clay, and even organic surfaces behave as acids and bases. The silica (Si02) surface of sand or glass can be simplistically thought of as a diprotic acid ... 

At high buffer concentrations, positive curvature may be observed in buffer dilution plots, indicating that the general acid and base are simultaneously participating in the rate-determining step.27 In such a case, the rate law must be expanded by third-order terms. Furthermore, plots of buffer slopes versus xHb may be nonlinear, when the unstable tautomer is a diprotic acid as, for example, the aci-nitro tautomer of nitrobenzene.28... 

Limited to diprotic and perfectly symmetric systems only ( apparent exact constants in ACD terminology) Limited to 5 ionization centers per molecule only Estimates only principal acid and base pK ... 

A monohydroxic base and a diprotic acid react completely to give a salt containing 44.9%, K, 18.4% S, and 36.7% O. What is the formula of the salt Name the acid and base, and write the equation for their reaction to produce this salt. 

Acids. Acidic oxides, acid anhydrides. Bases, basic oxides. Neutralization—hydrogen ion plus hydroxide ion. Hydronium ion. Equivalent weights of acids and bases. Normality. Monoprotic, diprotic, and triprotic acids. Monohy-droxic, dihydroxic, and trihydroxic bases. 

Please note that equations (4.8-5) through (4.8-7), (4.8-12) through (4.8-14), (4.8-19) through (4.8-21), and (4.8-26) through (4.8-28) are identical when the concentration fractions are written merely as a2i a1) and a0j where a2 is the concentration fraction of the fully protonated form (H2A for a dipro -tic acid, HA+for a diprotic amino acid, H2B2+for a diprotic base), a0 that of the fully deprotonated form, while is the concentration fraction of the intermediate form. For the buffer strength of the solution of a diprotic acid and/ or its conjugate bases we then have... 

We will hot constract a diprotic titration curve here, but if you want a good mental exercise, try it You just can t make the simplifying assumptions that we can usually use with monoprotic acids that are sufficiently weak or not too dilute. See your CD, Chapter 8, for auxiliary data for the spreadsheet calculation of the titration curve for 50.00 mL 0.1000 M H2C1O4 versus 0.1000 M NaOH. You can download that and enter the Kai and Kai values for other diprotic acids and see what their titration curves look like. Try, for example, maleic acid. For the calculations, we used the more exact equations mentioned above for the initial pH, the first buffet zone, and the first equivalence point. We did not use the quadratic equation for the second equivalence point since Cr04 is a quite weak base (Kbi = 3.12 X 10 ). See Ref. 8 for other examples of calculated titration curves. 

A Write stepwise equations for protonation or deprotonation of each of these polyprotic acids and bases in water, (a) C03 (b) H3ASO4 (c) NH2CH2C00 (glycinate ion, a diprotic base). 

Acid-base indicators are dyes whose acid form has one color and whose base form has another color.Thymol blue is a diprotic acid and has two different color changes (listed here in the acidic range and the basic range). 

Bronsted-Lowry acids and bases donate or accept protons. A Lewis acid or base accepts or donates a pair of electrons. Key Terms Bronsted-Lowry acid Bronsted-Lowry base Bronsted-Lowry acid-base reaction 1 lie polyprotic acid diprotic acid triprotic acid Ul IvO Lewis base Lewis acid-base reaction... 

In any acid-base titration, regardless of what acid and base are reacting, the total number of moles of H ions that have reacted at the equivalence point must be equal to the total number of moles of OH ions that have reacted. Sample Problem 4.14 explores the titration of an NaOH solution with a diprotic acid. 

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