Titrations and Buffers

Beyond simple pH and pOH lie titrations and buffers. Titrations allow you to determine the concentration of acids and bases. Buffers maintain the pH of a solution by reacting to changes and neutralizing them. 

Chapter 17 Achieving Neutrality with Titrations and Buffers 239... 

Titration and buffer calculations for weak diprotic and triprotic acids are done exactly as shown earlier for weak monoprotic acids. The only new consideration is which Kc or pKa value to use Very simply, we use the appropriate constant that describes the equilibrium between the species we are dealing with. For example, Figure 1-4 shows the titration of a weak diprotic acid with OH (p/ a, = 4, pKo, = 7). The pH at any point along the titration curve is given by ... 

Titration and buffer calculations involving peptides are done exactly as shown earlier for polyprotic acids. We must remember that the amino acid carboxyl groups used in forming the peptide bond are no longer available for titration. The structure of the fully protonated glutamylserylglutamylvaline is shown below. It is assumed that the ct-COOH group retains a pKa of — 2.5... 

Iron (and nickel, if present) can be determined by adding an excess of standard EDTA to the cold solution, and then back-titrating the solution with lead nitrate solution using xylenol orange as indicator provided the solution is kept cold, chromium does not react. The solution from the back-titration is then acidified, excess of standard EDTA solution added and the solution boiled for 15 minutes when the red-violet Cr(III)-EDTA complex is produced. After cooling and buffering to pH 6, the excess EDTA is then titrated with the lead nitrate solution. 

The pH is governed by the major solute species present in solution. As strong base is added to a solution of a weak acid, a salt of the conjugate base of the weak acid is formed. This salt affects the pH and needs to be taken into account, as in a buffer solution. Table 11.2 outlines the regions encountered during a titration and the primary equilibrium to consider in each region. 

Acid-base buffers comprise both a weak acid or base and its respective salt. Calculations with buffers employing the Henderson-Hasselbach equation are introduced and evaluated, thereby allowing the calculation of the pH of a buffer. Next, titrations and pH indicators are discussed, and their modes of action placed into context. 

Equations 16-9 and 16-10 are analogous to the Henderson-Hasselbalch equation of acid-base buffers. Prior to the equivalence point, the redox titration is buffered to a potential near E+ = formal potential for Fc 1 Fe2+ by the presence of Fe 1 and Fe2+. After the equivalence point, the reaction is buffered to a potential near E+ = formal potential for Ce4+ Ce3+. [R. de Levie Redox Buffer Strength, J. Chem. Ed. 1999, 76, 574.]... 

Differences in pH and buffering capacity among individual lots of fresh milk reflect compositional variations arising from the functions of the mammary gland. In general the pH is lower in colostrum (down to pH 6.0 McIntyre et al. 1952) and higher in cases of mastitis (up to pH 7.5 Prouty 1940) than in normal milk of mid-lactation. As discussed in Chapter 1, colostrum and mastitis milks are known to differ radically in their proportions of the proteins and certain salts. Milks of lower phosphorus, casein, and Ca2+ tend to be low in titratable acidity, while excessive acidity is related to hyperketonemia, inadequate calcium and excessive concentrates in the ration (Bonomi 1978). 

Now consider the overall shape of the pH curve. The slow change in pH about halfway to the stoichiometric point indicates that the solution acts as a buffer in that region (see Section 11.3). At the halfway point of the titration, [HA] = [A ] and pH = pKa. The flatness of the curve near pH = pKa illustrates very clearly the ability of a buffer solution to stabilize the pH of the solution. Moreover, we can now see how to determine pKa all we need to do is plot the pH curve during a titration and then identify the pH halfway to the stoichiometric point (Fig. 11.8). To obtain the pFCb of a weak base, we find pKa the same way but go on to use pfCa + pKb = pfCw. The values recorded in Tables 10.1 and 10.2 were obtained by this method. 

Acid-Base. The pH of natural waters is determined primarily by the carbonate equilibria. However, organisms may produce amounts of organic matter or ammonia sufficient to influence the pH and buffer capacity of the waters. It would be of interest to determine titration curves of high organic, high color, low alkalinity waters leached from some marshes. It is possible that these waters contain sufficient amounts of organic acids to be significant. 

Although it is generally recognized that sour taste is a property of the hydrogen ion, there is no simple relationship between sourness and acid concentration. Acids have different tastes the sourness as experienced in the mouth may depend on the nature of the acid group, pH, titratable acidity, buffering... 

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