Amphiprotic anions, salts

The reactions of salts in water become more complicated if the salt in question is amphiprotic, i.e. can function both as an acid and a base. Examples of amphiprotic anions are bicarbonate (sometimes called hydrogen-carbonate), HCOy, and bisulfite (or hydrogensulfite), HSOj. These species can donate or accept H+ ions in solution. 

Salts Hiat Yield Neutral Solutions 604 Salts Hiat Yield Acidic Solutions 604 Salts Tbat Yield Basic Solutions 604 Salts of Weakly Addic Cations and Weakly Basic Anions 605 Salts of Amphiprotic Anions 605... 

The further question we need to tackle is the pH of a solution of a salt with an amphiprotic anion, such as a solution of NaHC03. Is the solution acidic on account of the acid character of HCO3 or is it basic on account of the anions basic character As we show in Further information 4.2, under the circumstances specified there (iC 2 K i, S KJK 2> and S K i), where S is the numerical... 

The aim is to calculate the pH of a solution of a salt MHA, where HA" is an amphiprotic anion (HCOj is an example). We consider the foUowing two equilibria ... 

Amino acid zwitterions are internal salts and therefore have many of the physical properties associated with salts. They have large dipole moments, are soluble in water but insoluble in hydrocarbons, and are crystalline substances with relatively high melting points. In addition, amino acids are amphiprotic they can react either as acids or as bases, depending on the circumstances. In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yield a cation in aqueous base solution, the zwitterion is an add that loses a proton to form an anion. Note that it is the carboxylate, -C02-, that acts as the basic site and accepts a proton in acid solution, and it is the ammonium cation, -NH3+, that acts as the acidic site and donates a proton in base solution. 

The pH of the aqueous solution of an amphiprotic salt is equal to the average of the pKlts of the salt and its conjugate acid. The pH of a solution of a salt of the final conjugate base of a polyprotic acid is found from the reaction of the anion with water. 

Strategy We use Eq. 23 to determine the pH of an amphiprotic salt. This equation is used for salts of the diprotic conjugate base (H2A-) of a triprotic acid (H3A) and the monoprotic conjugate base (HA-) of a diprotic acid (H2A). However, when the solute 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 ... 

The situation is more complicated for salts containing anions that have ionizable protons, such as HS03. These salts are amphiprotic (Section 16.2), and how they behave in water is determined by the relative magnitudes of K and fCj, for the ion, as shown in Sample Exercise 16.19. If > Ky, the ion causes the solution to be acidic. If Ky > K , the Solution is made basic by the ion. 

The only salts left to consider are those in which the the cation comes from a strong base and the anion comes from a polyprotic acid with one or more ionizable protons still attached. These anions are amphiprotic— they can act as an acid and release a proton to water or as a base and abstract a proton from water. As in the previous case, to determine the overall acidity of their solutions, we compare the magnitudes of and but here we compare both the and of the same species, the anion. 

If the anion is amphiprotic (from a polyprotic acid), the strength of the anion as an acid KJ or as a base (K ) determines the acidity of the salt. 

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