Weak acid diprotic

Strong acid Weak acid Diprotic acid Oxyacids Organic acids Carboxyl group Amphoteric substance Ionization of water Ion-product constant... 

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... 

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. 

We treat HP04 as the intermediate form of a diprotic weak acid... 

Multiprotic weak acids can be used to prepare buffers at as many different pH s as there are acidic protons. For example, a diprotic weak acid can be used to prepare buffers at two pH s and a triprotic weak acid can be used to prepare three different buffers. The Henderson-Hasselbalch equation applies in each case. Thus, buffers of malonic acid (pKai = 2.85 and = 5.70) can be prepared for which... 

Construct ladder diagrams for the following diprotic weak acids (H2L), and estimate the pH of 0.10 M solutions of H2L, HL , and Using the systematic approach, calculate the pH of each of these solutions. 

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... 

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. 

Thus, if we titrate a monoprotic weak acid with a strong base, the EW and FW are identical. If the weak acid is diprotic, however, and we titrate to its second equivalence point, the FW will be twice as large as the EW. 

If the weak acid is monoprotic, then the FW must be 58.78 g/mol, eliminating ascorbic acid as a possibility. If the weak acid is diprotic, then the FW may be either 58.78 g/mol or 117.6 g/mol, depending on whether the titration was to the first or second equivalence point. Succinic acid, with a formula weight of 118.1 g/mol is a possibility, but malonic acid is not. If the analyte is a triprotic weak acid, then its FW must be 58.78 g/mol, 117.6 g/mol, or 176.3 g/mol. None of these values is close to the formula weight for citric acid, eliminating it as a possibility. Only succinic acid provides a possible match. 

Tartaric acid, H2C4H4O6, is a diprotic weak acid with a pK i of 3.0 and a pK 2 of 4.4. Suppose you have a sample of impure tartaric acid (%purity > 80) and that you plan to determine its purity by titrating with a solution of 0.1 M NaOH using a visual indicator to signal the end point. Describe how you would carry out the analysis, paying particular attention to how much sample you would use, the desired pH range over which you would like the visual indicator to operate, and how you would calculate the %w/w tartaric acid. 

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 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. 

Like sulfuric acid, a certain diprotic acid, H,A, is a strong acid in its first deprotonation and a weak acid in its second deprotonation. A solution that is 0.020 M H,A(aq) has a pH of 1.66. What is the value of fCa2 for this acid ... 

The starting pH of the solution is calculated using i and the initial molarity of the diprotic acid. We use the standard approach to a weak acid equilibrium ... 

The calculated curve shows the general features of the pH titration curve for a diprotic acid. The pH of the solution is acidic at the first stoichiometric point (major species = weak acid HA ) and basic at the second (major species =... 

In order to maintain the complexity of the model at a level consistent with the resolution of the experimental data, the reactivity of these surface groups has been described by relatively simple models (i) as diprotic weak acids, and (ii) as monoprotic... 

Diprotic Surface Groups. According to the diprotic model, the surface is represented as an ensemble of identical diprotic weak acid groups, which react according to ... 

The representation of surface groups as diprotic weak acids is appealing because it includes a modest degree of complexity (two acidity constants), allows convenient representation of the condition of zero surface excess of hydrogen ion, and is still quite manageable mathematically. However, it must be borne in mind that this model is still a grossly simplified representation of the actual surface. It remains to be shown that this simplification is significantly better than any other simplification. 

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. 

The figure compares the titration of a monoprotic weak acid with a monoprotic weak base and the titration of a diprotic acid with strong base. 

Material balance Since H2A, HA-, and A-2 coexist at equilibrium, the sum of the concentrations of H2A, HA-, and A-2 should be equal to the amount of the diprotic weak acid initially added to the solution, Ca. 

Fractional mole equilibrium composition curves of captopril ( pKal =3.8 and a2 = 9.8) are shown in Figure 2.12. If Kal Ka2, the diprotic weak acid can ... 

There are six species present in equilibrium. Six equations are needed to solve for pH or for the concentration of H+. The same approach used for mono- and diprotic weak acids is employed here ... 

Equation (2.125) can be further simplified under certain circumstances and some of its terms ignored. The value for Ka2Ka3 is negligible when compared to the other terms. For example, for H3P04, Ka2Ka3 = 10"1953, which is much smaller than the values of Ka2[H+] and [H+]2, even at pH 8. In this case, Equation (2.125) is equivalent to Equation (2.101) for diprotic weak acids. If other approximations (i.e., [H+] [OH ] and [H+] 2Ka2) are made, then the triprotic weak acid behaves as if it were a monoprotic weak acid. 

FIGURE 2.18 Titration curve of glutamic acid ( pKal = 2.2, pKa2 = 4.3, and pK = 9.4). Thus, in the titration of a diprotic weak acid H2A, Equation (2.150) reduces to ... 

The as are given by Equation (2.157) or Equation (2.126a) through liquation (2.126d) for a diprotic weak acid. 

Let us apply the above approach for a diprotic weak acid (carbonic acid). Thus,... 

Figure 2.19 shows a plot of 0 as a function of pH for carbonic acid. As shown in Figure 2.7 for a monoprotic weak acid, the localized maxima for a diprotic weak acid occur at pH = pKal and pH = pKa2. 

If the drug is a polyprotic weak acid or weak base, the inclusion complexation process can be described as (e.g., a diprotic weak base) ... 

Thevaluesof k2+ = 2.05xl0 5sec-1M-1 and k0 = 1.50 sec 1M 1 are estimated. Substituting these values into Equation (5.174) followed by nonlinear regression analysis gives other k values. Intuitively from the profile of pH 2 to 4, one can assume kj 0 = 0. As demonstrated for monoprotic and diprotic weak acids and weak bases, the pH-rate constant profile is dependent on the kinetic pathway of the hydrolysis. It can be seen from Figure 5.32 that at pH 3 to 4 and pH 4 to 9, k2>0 and k0 0 are the predominant processes of the hydrolysis of aspartame, respectively. 

EXAMPLE. The following formula (all in one cell, of course) calculates the pKa values of a diprotic weak acid from the pH and the parameter n-bar (symbolized by nin printed equations see, e.g.. Chapter 22) using one of two different formulas, one if n is between 1.2 and 1.8, the other if n is between 0.2 and 0.8 otherwise the formula returns... 

Barbiturates are cyclic imides used as hypnotics and (in the case of pheno-barbital) as anticonvulsants. They are all derivatives of barbituric acid (which is not pharmacologically active) and differ only in their substituents on the 5-position of the ring. Barbiturates contain nitrogen atoms, but the lone pair on the nitrogen is not available for reaction with protons, so barbiturates are not basic. Instead, they behave as weak acids in solution (diprotic actually, though the second ionisation is very weak) the negative... 

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