Titration diprotic acid

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. 

For the primary stage (phosphoric) V) acid as a monoprotic acid), methyl orange, bromocresol green, or Congo red may be used as indicators. The secondary stage of phosphoric) V) acid is very weak (see acid Ka = 1 x 10 7 in Fig. 10.4) and the only suitable simple indicator is thymolphthalein (see Section 10.14) with phenolphthalein the error may be several per cent. A mixed indicator composed of phenolphthalein (3 parts) and 1-naphtholphthalein (1 part) is very satisfactory for the determination of the end point of phosphoric(V) acid as a diprotic acid (see Section 10.9). The experimental neutralisation curve of 50 mL of 0.1M phosphoric(V) acid with 0.1M potassium hydroxide, determined by potentiometric titration, is shown in Fig. 10.6. 

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. 

FIGURE 11.14 The variation of the pH of the analyte solution during the titration ot a diprotic acid (oxalic acidl and the major species present in solution at the two stoichiometric points (B and D) and at points when half the titrant required to reach a stoichiometric point has been added (A and C). Compare this diagram with Fig. 10.20. The labels SP1 and SP2 denote the volumes of base required to reach the two stoichiometric points. 

The titration of a diprotic acid, H2 A, can be divided into six regions that differ in the major species present in solution. Shown here is the titration curve for maleic acid (50.0 mL, 0.10 M) titrated with 1.0 M NaOH. 

At the beginning of the titration, the diprotic acid (represented by H2 A) and H2 O are the only major species in the solution. As we describe in Chapter 17, the hydronium ion concentration can be calculated from the... 

At the first stoichiometric point of the titration, aii the diprotic acid has been converted to its conjugate base, H A. This amphiprotic anion can react with itseif, analogous to the self-ionization of water ... 

By recognizing species in solution and their dominant equilibrium, we can construct titration curves for other diprotic acids. Example shows how this is done for sulfurous acid. 

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 =... 

C18-0142. The amine group of an amino acid readily accepts a proton, and the protonated form of an amino acid can be viewed as a diprotic acid. The p Zg values for serine (H2 NCHRCO2 H, i = CH2 OH) are p ra(H3 N"") =9.1 and p (002 H) - 2.2. (a) What is the chemical formula of the species that forms when serine dissolves in pure water (b) If this species is titrated with strong acid, what reaction occurs (c) 10.00 mL of 1.00 M HCl is added to 200. mL of 0.0500 M serine solution. This mixture is then titrated with 0.500 M NaOH. Draw the titration curve, indicating the pH at various stages of this titration. 

If one were to titrate a weak diprotic acid such as maleic acid with the known NaOH solution, the graph would show two separate inflections — representing the neutralization of each hydrogen — assuming the pKa values differ by 4 or more pK units (see Figure 2). The dissociation of a diprotic acid occurs in two separate steps ... 

The function Data EqAH2, m simulates the pH-titration of a weak diprotic acid, AH2, in acid excess, with a strong base. The computation of the equilibria is similar to the examples Eql. m and Eq2. m given in the Chapters Example General 3-Component Titration (p.56) and Example pH Titration of Acetic Acid (p.58). From the present point of view, the important aspect is that all variables are collected in one structure s. The model is now stored in s.Model, the logP values in s. log beta, etc. Importantly, all the information contained in s is returned to the invoking programs. 

Figure 5-17. Molar absorption spectra and concentration profiles for the titration of a diprotic acid with logK values of 8 and 6.

Amino acids vary in their acid-base properties and have characteristic titration curves. Monoamino monocarboxylic amino acids (with nonionizable R groups) are diprotic acids (+H3NCH(R)COOH) at low pH and exist in several different ionic forms as the pH is increased. Amino acids with ionizable R groups have additional ionic species, depending on the pH of the medium and the pIQ of the R group. 

Point C in Figure 11-4 shows where the intermediate form of a diprotic acid lies on a titration curve. This is the least-buffered point on the whole curve, because the pH changes most rapidly if small amounts of acid or base are added. There is a misconception that the intermediate form of a diprotic acid behaves as a buffer when, in fact, it is the worst choice for a buffer. 

Titrations of many diprotic acids or bases show two clear end points, as in curve a in Figure 11 -4. Some titrations do not show both end points, as illustrated by curve b, which is calculated for the titration of 10.0 mL of 0.100 M nicotine (p/fbl = 6.15, pKbl = 10.85) with 0.100 M HC1. The two reactions are... 

Table 11-6 gives useful equations derived by writing a charge balance and substituting fractional compositions for various concentrations. For titration of the diprotic acid, H2A, ct> is the fraction of the way to the first equivalence point. When = 2, we are at the second equivalence point. It should not surprise you that, when cj> = 0.5, pH = pAj and, when = 1.5, pH pAT2. When = 1, we have the intermediate HA " and pH j(pAj -I- pA j). 

Sketch the general appearance of the curve for the titration of a weak diprotic acid with NaOH. Explain (in words) what chemistry governs the pH in each distinct region of the curve. 

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. 

Figure 11.16 shows the titration curve of a diprotic acid. Notice that there are two stoichiometric points (B and D) and that there are two buffer regions (A and C). The major species present in solution at each point are indicated. We can see that it takes twice as much base to reach the second stoichiometric point as it does to reach the first. 

TABLE 1.1.3 Summary of Diprotic Acid Titration Equilibria... 

A compound with the formula XOCI2, reacts violently with water, yielding HC1 and the diprotic acid H2XO3. When 0.350 g of XOCI2 was added to 50.0 mL of water and the resultant solution was titrated, 96.1 mL of 0.1225 M NaOH was required to react with all the acid. 

As a final example of an acid-base titration, let s consider the gradual addition of NaOH to the protonated form of the amino acid alanine (H2A+), a substance that acts as a diprotic acid. Amino acids (which are discussed in more detail in Chapter 24) are both acidic and basic and can be protonated by strong acids such as HC1, yielding salts such as H2A+C1 . The protonated form of the amino acid has two... 

Before Addition of Any NaOH The equilibrium problem at the start of the titration is the familiar one of calculating the pH of a diprotic acid (Section 15.11). The principal reaction is dissociation of H2A+, and [H30+] can be calculated from the equilibrium equation... 

The following pictures represent solutions at various stages in the titration of a weak diprotic acid H2A with aqueous NaOH. (Na+ ions and solvent water molecules have been omitted for clarity.)... 

A 25.0 mL sample of a diprotic acid is titrated with 0.240 M KOH. If 60.0 mL of base is required to reach the second equivalence point, what is the concentration of the acid ... 

When a typical diprotic acid H2A (Kal = 10-4 Ka2 = 10-10) is titrated with NaOH, the principal A-containing species at the first equivalence point is HA-. 

It should be noted that the method depends on accurate determination of the limiting chemical shifts 6a, 6au i, diprotic acids, unless the two pKas are very widely separated.29... 

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 ... 

The binding capacity for the unoccupied site, which is calculated using equation 7.6-11, is plotted versus pH in Fig. 7.8. The number of hydrogen ions bound by the catalytic site in the fumarase-L -tartrate complex is plotted in Fig. 7.9. This is steeper than the titration curve of a diprotic acid with identical and independent groups. The binding capacity for the site occupied by meso-tartrate is shown in Fig. 7.10. The slope of the binding curve is steeper than for the unoccupied site shown in Fig. 7.6, as expected since the binding is cooperative. 

Figure 1A. Potentiometric titration plot of a weak diprotic acid with pKa values of 4.0 and 7 (after Segel, 1976, with permission).

A student was given a 0.10 M solution of an unknown diprotic acid, H2A, and asked to determine the K3> and Ka, values for the diprotic acid. The student titrated... 

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