Triprotic acids titration

TABLE 11.4 Summary of Triprotic Acid Titration Equilibria... 

Suppose we are titrating the triprotic acid H P04 with a solution of NaOH. The experimentally determined pH curve is shown in Fig. 11.13. Notice that there are three stoichiometric points (B, D, and F) and three buffer regions (A, C, and E). In pH calculations for these systems, we assume that, as we add the hydroxide solution, initially NaOH reacts completely with the acid to form the diprotic conjugate base... 

FIGURE 11.13 The variation of the pH of the analyte solution during the titration of a triprotic acid (phosphoric acid). The major species present in solution at the first two stoichiometric points (B and D) and at points when half the titrant required to reach a stoichiometric point has been added (A, C, and Ej are shown. Compare this diagram with Fig. 10.21. The labels SP1, SP2, and SP3 denote the volumes of base required to reach the three stoichiometric points. Points A through F are explained in the text. 

ES Difference plot. A solution containing 0.139 mmol of the triprotic acid tris(2-aminoethyl)amine-3HCl plus 0.115 mmol HC1 in 40 mL of 0.10 M KC1 was titrated with 0.490 5 M NaOH to measure acid dissociation constants. 

EXAMPLE 11.9 Estimating the pH during the titration of a triprotic acid... 

As noted earlier, the weaker /the acid, the shorter is the steeply rising portion of the titration curve. If the acid is very weak, the equivalence point is difficult to detect. Although H3PO4 is a triprotic acid, HPO42- is such a weak acid that the third equivalence point is not observed in the PbPC /NaOH titration curve. 

Figure 1.10. Potentiometrie titration plot of a weak triprotic acid (after Segel, 1976, with permission).

TABLE 8.4 A Summary of Various Points in the Titration of a Triprotic Acid... 

At this point it is useful to summarize the pH calculations associated with the titration of a triprotic acid H3A. Figure 8.11 shows which expression should be used for the major species in the solution at a given point in the titration. 

Titration and buffer calculations for weak diprotic and triprotic acids are done exactly as shown earlier for weak monoprotic acids. The only new consideration is which Kc or pKa value to use Very simply, we use the appropriate constant that describes the equilibrium between the species we are dealing with. For example, Figure 1-4 shows the titration of a weak diprotic acid with OH (p/ a, = 4, pKo, = 7). The pH at any point along the titration curve is given by ... 

A 14.5-mL sample of an unknown triprotic acid is titrated to the endpoint with 35.2 mL of 0.0800M Sr(OH)2. What is the molarity of the acid solution ... 

Make or copy the spreadsheet for the titration of a triprotic acid, with storage spaces for Ca, Cb, Kw, and three Ka-values, and with columns for pH, [H+], denom, and Vb/Va. (This spreadsheet can also be used for a monoprotic or diprotic acid, by setting the unused Ka s to values smaller than 10 24, i.e., the pKa s to values larger than 24.)... 

Solution, 118 Solvent, 118 Spectator ion, 126 Standard solution, 155 Strong electrolyte, 118 Titration, 155 Transmittance, 150 Triprotic acid, 130 Visible spectrophotometry, 150 Weak electrolyte, 118... 

Earlier we made an important distinction between an end point and an equivalence point. The difference between these two terms is important and deserves repeating. The equivalence point occurs when stoichiometrically equal amounts of analyte and titrant react. For example, if the analyte is a triprotic weak acid, a titration with NaOH will have three equivalence points corresponding to the addition of one, two, and three moles of OH for each mole of the weak acid. An equivalence point, therefore, is a theoretical not an experimental value. 

Since citric acid is a triprotic weak acid, we must first decide to which equivalence point the titration has been carried. The three acid dissociation constants are... 

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. 

The titration curve for H3P04 using NaOH is shown in Figure 5.8. Citric acid is also triprotic while carbonic acid (H2CQ3) is diprotic. 

Curve A in Figure 15-4 is the theoretical titration curve for triprotic phosphoric acid. Here, the ratio approximately 10, as is This ratio results in... 

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