Anion amphiprotic anions

For the acid-base properties of amphiprotic anions such as HCO3 otHJPOt. see the discussion at the end of this section. 

A similar argument can be used to decide whether an amphiprotic anion such as HC03 or H2PO4- is acidic or basic. In principle, these ions can act as either weak acids or weak bases ... 

At the first stoichiometric point of the titration, aii the diprotic acid has been converted to its conjugate base, H A. This amphiprotic anion can react with itseif, analogous to the self-ionization of water ... 

At intennediate pH values, the solution contains a relatively high concentration of the amphiprotic anion HA". 

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. 

Predicting Whether the Solution of an Amphiprotic Anion Is Acidic or Basic... 

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. 

These examples of acid-base reactions show that water can act as either an acid or a base Water accepts a proton fixtm an HCl molecule, but it donates a proton to a PO4 anion. As an acid, water donates a proton to a base and becomes a hydroxide anion. As a base, water accepts a proton from an acid and becomes a hydronium cation. A chemical species that can both donate and accept protons is said to be amphiprotic. Water is an amphiprotic molecule. 

For all the amphiprotic solvents in the pure state the cation (or acid) concentration is by definition equal to the anion (or base) concentration in other words, the pure solvent is neutral therefore, adding a solute such as HC1 makes the solution acidic, whereas a solute such as NaOH makes it basic. 

Comparison of reactions 4.9, 4.10, 4.12, 4.13 and 4.15 leads to another important conclusion, viz., in an amphiprotic solvent its own solvonium cation represents the strongest acid possible, and its own anion the strongest base. Even when a very strong foreign acid or base is dissolved, excessive proton donation to and proton abstraction from the solvent molecule yield the respective acid or base this phenomenon is generally known as the levelling effect, which in an amphiprotic solvent takes place on both the acid and the basic... 

If the dielectric constant of an amphiprotic solvent is small, protolytic reactions are complicated by the formation of ion pairs. Acetic acid is often given as an example (denoted here as AcOH, with a relative dielectric constant of 6.2). In this solvent, a dissolved strong acid, perchloric acid, is completely dissociated but the ions produced partly form ion pairs, so that the concentration of solvated protons AcOH2+ and perchlorate anions is smaller than would correspond to a strong acid (their concentrations correspond to an acid with a pK A of about 4.85). A weak acid in acetic acid medium, for example HC1, is even less dissociated than would correspond to its dissociation constant in the absence of ion-pair formation. The equilibrium... 

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 ... 

Autoprotolysis constant — The ion-product calculated from the ion activities of the conjugate acidic and basic species of an -> amphiprotic solvent (SH). The chemical equation of such self-ionization reactions can be schematized as 2HS H2S+ + S , where H2S+ is the conjugate cation, S the conjugate anion. The autoprotolysis constant can be formulated as JCauto = [H2S+] ... 

The autoprotolysis constant The extent of ionization (4-19) of a pure amphiprotic solvent is measured by the autoprotolysis constant SH> defined as the product Since the autoprotolysis reaction results in the formation of both solvent cations and solvent anions, the autoprotolysis constant is a measure of the differentiating ability of a solvent. If a solvent has a large Ash value, the existence in it of a wide range of strengths of either adds or bases is not possible. In contrast, if the autoprotolysis constant is small, adds and bases of varying strengths show titration curves distinctly different from each other. 

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