Propionic titration curve

As our last exercise of this chapter we will fit data to a small segment of a titration curve. It is, of course, poor analytical practice to use only part of a curve, but we will use it here merely to demonstrate the general power of computer fitting of experimental data. We will use data, given by Papanastasiou etal. in Anal. Chim.Acta 277 (1993) 119, for the titration with 0.1 M NaOH of a 50 mL sample containing both formic and propionic acid. The authors show the entire titration curve in their Fig. 6, but in their Table 6 only list numerical values for the buffer region of the curve. Nonetheless, 19 low-noise data pairs are given, while the solution involves only four... 

The dissociation constants of amino acids can be determined, for example, by titration of the acid. Figure 1.2 shows titration curves for glycine, histidine and aspartic acid. Table 1.2 lists the dissociation constants for some amino acids. In amino acids the acidity of the carboxyl group is higher and the basicity of the amino group lower than in the corresponding carboxylic acids and amines (cf. pK values for propionic acid, 2-propylamine and alanine). As illustrated by the comparison of pK values of 2-aminopropionic acid (alanine) and 3-aminopropionic acid ( 3-alanine), the pK is influenced by the distance between the two functional groups. 

Fig. 3.1. Fatty acid titration curves (according to Bild et al., 1977). Aqueous solutions (0.1 mol/1) of Na-salts of propionic (1), caprylic (2) and linoleic acids (3) were titrated with 0.1 mol/1 HCl...

The pKs values for the C2-C9 short-chain acid homologues range from 04.75-4.95. The pKg of 7.9 for linoleic acid deviates considerably from this range. This unexpected and anomalous behavior, which has not yet been clarified, is clearly illustrated in the titration curves for propionic, caprylic and linoleic acids recorded under identical conditions (Fig. 3.1). 

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