Triprotic base

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. 

STRATEGY Verify that Eq. 14 can be used by checking that S 5i> K JKal and S 5i> fC,. If so, we use Eq. 14 to determine the pH of the salts of the diprotic conjugate base (H,A ") of a triprotic acid (H SA) and the monoprotic conjugate base (HA ) of a diprotic acid (H2A). However, when the solure is a salt of an anion that has lost two protons, such as HP042-, we must adjust the expression to use the appropriate neighboring pkas. 

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. 

Each H atoms attached to an O atom is an acidic hydrogen and can be released by the acid to a base. Phosphoric acid has 3 such H atoms and thus is triprotic. Although phosphorous acid has 3 H atoms, only 2 H atoms are bonded to an O atom, the other one is attached directly to the P atom. Thus phosphorous acid is only diprotic. A way to write this is H2PH03. 

FIGURE 2-16 Conjugate acid-base pairs consist of a proton donor and a proton acceptor. Some compounds, such as acetic acid and ammonium ion, are monoprotic they can give up only one proton. Others are diprotic (H2C03 (carbonic acid) and glycine) or triprotic... 

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 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. 

Part 1 From the deprotonation equilibrium of H3A (for a triprotic acid) or H2A (for a diprotic acid), determine the concentrations of the conjugate base, H2A- or HA-, respectively, and H30+, as illustrated in Example 10.12. 

Third stoichiometric point When the number of moles of base added equals three times the number of moles of the triprotic acid originally present, the third stoichiometric point has been reached. At this point, the main species in solution is the salt consisting of the... 

Phosphoric acid, H3P04, is a triprotic acid. Show the formula of the conjugate base after two protons have been donated. 

The dependence of the equilibrium constants of polyprotic weak acids and weak bases on ionic strength allows them to be treated in exactly the same manner as monoprotic weak acids and weak bases. The first, second, and third ionization constants for triprotic weak acids in terms of activity coefficient are given by ... 

Normality (N) is the number of equivalents of the solute per liter of solution. A1.0 N solution of acid (or base) contains one equivalent of an acid (or base) per liter of solution. A 1.0 M solution of HCl is 1.0 N, but a 1.0 M solution of H2SO4 is 2.0 N. Sulfriric acid has two acidic hydrogens, and the molarity is multiplied by a factor of 2. Phosphoric acid (H3PO4) is triprotic (having three protons it can donate) and a 1.0 molar solution is 3.0 normal. 

The citric-acid case is one that points out the amplification possible from a chemical reaction—in this case, proton transfer—and the ability to design a reference to cancel out much interference, thereby permitting the interferometer to zero in on the measurement of one analyte. Ammonia is a weak base the p/sTa of the ammonium ion is 9.8, and it will react reversibly with a weak acid. Citric acid is a triprotic acid, with p/sTa s = 3.1, 4.8, and 6.4. The least acidic proton on the citric acid would be a good candidate for a reversible reaction with ammonia. The other two may be too acidic and may not reverse after exposure. 

Recently, microconstant measurements have been validated for 25 drug molecules, based primarily on multiwavelength spectroscopy. The apphed methodologies, however, are difficult and limited up to triprotic molecules when no symmetry constraints are present.In more complex cases, the microspecies resolution is still possible by applying some in silico methods for predicting ionization constants. 

Monoprotic, diprotic, and triprotic acids monohydroxic, dihvdroxic, and tri-hydroxic bases... 

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. 

For a triprotic acid such as H3PO4, the concentration of the base (P04 ) resulting from the third ionization could have been calculated in a similar manner. 

A polyprotic acid has more than one equivalence point. The first equivalence point occurs when the volume of base added is sufficient to remove one hydrogen ion from each acid molecule, Ve2 is the volume sufficient to remove two hydrogen ions from each, and so forth. A diprotic acid shows two equivalence points, and a triprotic acid, three. The equivalent volumes are related to each other by... 

In order to extend the discussion to polyprotic acids and bases, we first consider a triprotic acid such as orthophosphoric acid,... 

In picturing reactions of a polyprotic acid with a strong base, we shall assume that enough base is added to react with all of the acidic hydrogen atoms. The following complete equations describe the reactions of the diprotic acid sulfuric acid and the triprotic acid phosphoric acid with sodium hydroxide. Each equation represents the sum of a series of reactions in which the acidic hydrogen atoms are removed one at a time. 

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