Acids triprotic acid

Bronsted-Lowry acids and bases donate or accept protons. A Lewis acid or base accepts or donates a pair of electrons. Key Terms Bronsted-Lowry acid Bronsted-Lowry base Bronsted-Lowry acid-base reaction 1 lie polyprotic acid diprotic acid triprotic acid Ul IvO Lewis base Lewis acid-base reaction... 

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. 

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 , ... 

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). 

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 ... 

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. 

Commercial arsenic acid is usually the orthoarsenic acid [7774-M1-6] corresponding to the above hemihydrate formula. The aqueous solution of this acid behaves as a triprotic acid the dissociation constants, Ki, K2 and K3 being 5.6x10-3, 1.7x10 and 3.OxlO-i, respectively. The meta and pyro forms... 

The aqueous solution of arsenic pentoxide is arsenic acid which probably corresponds to the hemihydrate formula H3ASO4 O.5H2O. See Orthoarsenic acid. It behaves as a triprotic acid forming various arsenate derivatives of metals. 

Phosphoric acid behaves as a triprotic acid, having three ionizable hydrogen atoms. The hydrogen ions are lost sequentially. The chart below provides the ionization constants and their logs. 

Challenge Derive the following results for a triprotic acid ... 

Our strategy is to substitute expressions into the charge balance to obtain an equation in which the only variable is [H+l. For this purpose, we use the fractional composition equations for the triprotic acid, H,P04, which we abbreviate as H3P ... 

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