Aqueous solutions polyprotic acids

Acids are described as monoprotic, diprotic, ortriprotic depending on whether they can donate one, two, or three protons per molecule, respectively, in aqueous solutions. Polyprotic acids include both diprotic and triprotic acids. 

The theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

Suppose we need to estimate the pH of an aqueous solution of a fully deproto-nated polyprotic acid molecule. An example is a solution of sodium sulfide, in which sulfide ions, S2-, are present another example is a solution of potassium phosphate, which contains P04 ions. In such a solution, the anion acts as a base it accepts protons from water. For such a solution, we can use the techniques for calculating the pH of a basic anion illustrated in Example 10.11. The K, to use in the calculation is for the deprotonation that produces the ion being studied. For S2, we would use Ki2 for H2S and, for P043-, we would use Kai for H3P04. 

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. 

For each of the following polyprotic acids, state which species (H2A, HA, or A2-) you expect to be the form present in highest concentration in aqueous solution at pH = 6.50 ... 

STRATEGY In an aqueous solution of a polyprotic acid, we assume that the acid is the solute species present in largest amount. We also assume that only the first deprotonation contributes significantly to [H30+] and that the auto- protolysis of water does not contribute significantly to [H30+] or [OH], Find the acidity constants in Table 10.9. 

We can calculate pH titration curves using the principles of aqueous solution equilibria. To understand why titration curves have certain characteristic shapes, let s calculate these curves for four important types of titration (1) strong acid-strong base, (2) weak acid-strong base, (3) weak base-strong acid, and (4) polyprotic acid-strong base. For convenience, we ll express amounts of solute in millimoles (mmol) and solution volumes in milliliters (mL). Molar concentration can thus be expressed in mmol/mL, a unit that is equivalent to mol/L ... 

To illustrate the variation in composition of an aqueous solution of a polyprotic weak acid species, it is useful to plot a species distribution curve such as the one for citric acid shown in Figure 20-1. The parameter plotted versus pH for each species is a, the fraction of the total citric acid concentration in the form of a particular species. 

FIGURE 15.17 A titration curve for the titration of a polyprotic acid (phosphoric acid) by a strong base. The curve shown is for 100.0 mL of 0.1000 M H3PO4 titrated with 0.1000 M NaOH. No clear third equivalence point is seen at 300 mL because for HP04 is not much greater than for H2O in aqueous solution. 

The first step in the ionization of H2SO4 is complete in dilute aqueous solution. The second step is nearly complete in very dilute aqueous solutions. The first step in the ionization of a polyprotic acid always occurs to a greater extent than the second step, because it is easier to remove a proton from a neutral acid molecule than from a negatively charged anion. 

Some acids have two or more protons that can be released upon dissociation. Such acids are called polyprotic acids. For example, phosphoric acid (H3PO4) can lose up to three protons in aqueous solution. 

In the preceding section, we dealt only with acids releasing one HsO ion or proton. Some acids, however, have two or more such protons these acids are called polyprotic acids. Sulfuric acid, for example, can lose two protons in aqueous solution. One proton is lost completely to form HsO (sulfuric acid is a strong acid). 

These relative concentrations are true only in an aqueous solution of phosphonc add that contains no other dissolved compounds. We will look in detail at how to determine concentrations in aquecxis solutions of polyprotic acids in Chapter 16-... 

The goal is to calculate the concentrations of all solute species of a polyprotic acid in aqueous solution. 

This is a titration of a polyprotic weak acid with a strong base, and the titration curve for this problem should look very similar to Figure 17-12. In the titration of a weak acid by a strong base we know that at the point of half-neutralization, pH = pfCa and therefore pK should be the pH at 8.12 mL. For pK, we will use expression (17.10) since at this point in the titration we will have an aqueous solution of HOC6H4COONa, which is a salt of a polyprotic acid. The pH of the first equivalence point is given and to find the pH of the second equivalence point we must perform an ICE calculation similar to the one in Example 16-14. 

At the first equivalence point, the solution is HOC6H4COONa(aq) with pH = 7.02. Recognizing this as the salt produced in neutralizing a polyprotic acid in its first ionization, we use equation (17.10) to solve for pK - aqueous solu-... 

In the same manner as described in Section 2.2.5, mathematical expressions for the buffer capacity of polyprotic weak acid/base systems can be developed. When one adds a strong base such as NaOH to an aqueous buffer solution containing a polyprotic weak acid (HnA), the electroneutrality would be ... 

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