Triprotic

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. 

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... 

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. 

Arsenic Acids and the Arsenates. Commercial arsenic acid, corresponds to the composition, one mole of arsenic pentoxide to four moles of water, and probably is the arsenic acid hemihydrate [7774-41-6] H AsO O.5H2O. It is obtained by treatment of arsenic trioxide with concentrated nitric acid. Solutions of this substance or of arsenic pentoxide in water behave as triprotic acids with successive dissociation constants = 5.6 x 10 , ... 

Phosphoric acid, a common ingredient of cola drinks, is triprotic ... 

Aspartic acid acts as a triprotic acid with successive dissociation constants of 8.0 x 10-3, 1.4 X 10-4, and 1.5 X 10-10. Depending upon pH, aspartic acid can exist in four different forms in water solution. Draw these forms and calculate the pH range over which each form is the principal species. 

A triprotic acid H3A (e.g. phosphoric(V) acid) will similarly yield three dissociation constants, Klt K2, and K3, which may be derived in an analogous manner ... 

Neutralisation reactions. The equivalent of an acid is that mass of it which contains 1.008 (more accurately 1.0078) g of replaceable hydrogen. The equivalent of a monoprotic acid, such as hydrochloric, hydrobromic, hydriodic, nitric, perchloric, or acetic acid, is identical with the mole. A normal solution of a monoprotic acid will therefore contain 1 mole per L of solution. The equivalent of a diprotic acid (e.g. sulphuric or oxalic acid), or of a triprotic acid (e.g. phosphoric( V) acid) is likewise one-half or one-third respectively, of the mole. 

Phosphoric acid, H P04, is a triprotic acid. Write the equation for complete reaction with KOH.)... 

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. 

Note that the fully protonated form (H2C03) is dominant when pH < pKa] and the fully deprotonated form (C032-) becomes dominant when pH > pka2. Similar calculations can be carried out for triprotic acids (Fig. 10.21). 

For each of the following polyprotic acids, state which species (H2A, HA, or A2 of the diprotic acid or H3A, H,A, HA2, or A for triprotic acids) you expect to be the form present in highest concentration in aqueous solution at pH = 5.0 (a) tartaric acid (b) hydrosulfuric acid (c) phosphoric acid. 

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. 

Although its formula suggests that it should be a triprotic acid, H3P03 is in fact diprotic because one of the H atoms is attached directly to the P atom and the P-H bond is nonpolar (Section 10.10). 

Phosphate, PO, is the fully dissociated anion of triprotic phosphoric acid, H3PO4 ... 

An acid that contains more than one acidic hydrogen atom is caiied a polyprotic acid. Figure 17-8 shows the structures of two poiyprotic oxoacids, suifuric acid (H2 SO4, diprotic) and phosphoric acid (H3 PO4, triprotic). Many carboxyiic acids are poiyprotic, too. Tabie 17-2 lists several examples of polyprotic acids. 

Tam, K. Y. Multiwavelength spectrophotometric resolution of the micro-equilibria of a triprotic amphoteric drug methacycline. Mikrochim. Acta 2001, 136, 91-97. 

Certain acids can give up two or three protons these are called diprotic (for example, H2S04) or triprotic (for example, H3P04) acids, respectively. According to the Lowry-Bronsted concept, each acid is regarded as giving up only one proton. Diprotic or triprotic acids are perceived as giving out protons in different steps. 

Consider phosphoric acid, H3P04 (a triprotic acid), and phosphorous acid, H3P03 (a diprotic acid). Two structures that show this acidic behavior are ... 

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. 

If the acid contains more than one ionizable hydrogen, the acid is known as a polyprotic acid. Specifically, diprotic acids, such as sulfuric acid, H2S04, contain two ionizable hydrogens and triprotics, such as phosphoric acid, H3P04, contain three ionizable hydrogens. 

C) H3PO4 is a triprotic acid that is, there are 3 moles of H+ ions produced for each mole of H3PO4 that completely ionizes. Normality is the number of equivalents per liter. Assuming complete or 100% ionization, a 1-molar HC1 solution is 1 normal. A 1-molar H2S04 solution is 2 normal, and a 1-molar solution of H3P04 is 3 normal. 

The triprotic corroles 64 form monoorganotin complexes of the type RSnlv(Cor). PhSn(Cor) is formed when octaethylcorrole reacts with diphenyltin oxide, or phenylmagnesium bromide reacts with ClSn(Cor). The tin is five-coordinate, and is placed 72.2(3) pm above the plane of the four nitrogen atoms, with the corrole assuming a domed structure. The compound is reversibly reduced by transfer of one electron into the 7r-system.511... 

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