Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods459and the analyte’s and interferent’s contributions to the equivalence pointvolume can not be separated.Tim e , Co s t, a n d Eq u i pm e n tAcid–base titrations require less time than most gravimetric procedures, butmore time than many instrumental methods of analysis, particularly whenanalyzing many samples. With an automatic titrator, however, concernsabout analysis time are less significant. When performing a titration manuallyour equipment needs—a buret and, perhaps, a pH meter—are few innumber, inexpensive, routinely available, and easy to maintain. Automatictitrators are available for between $3000 and $10 000.9CComplexation TitrationsThe earliest examples of metal–ligand complexation titrations are Liebig’sdeterminations, in the 1850s, of cyanide and chloride using, respectively,Ag + and Hg 2+ as the titrant. Practical analytical applications of complexationtitrimetry were slow to develop because many metals and ligandsform a series of metal–ligand complexes. Liebig’s titration of CN – withAg + was successful because they form a single, stable complex of Ag(CN) 2 – ,giving a single, easily identified end point. Other metal–ligand complexes,such as CdI 4 2– , are not analytically useful because they form a series ofmetal–ligand complexes (CdI + , CdI 2 (aq), CdI 3 – and CdI 4 2– ) that producea sequence of poorly defined end points.In 1945, Schwarzenbach introduced aminocarboxylic acids as multidentateligands. The most widely used of these new ligands—ethylenediaminetetraaceticacid, or EDTA—forms strong 1:1 complexes with manymetal ions. The availability of a ligand that gives a single, easily identifiedend point made complexation titrimetry a practical analytical method.9C.1 Chemistry and Properties of EDTAEthylenediaminetetraacetic acid, or EDTA, is an aminocarboxylic acid.EDTA, which is shown in Figure 9.26a in its fully deprotonated form, isa Lewis acid with six binding sites—four negatively charged carboxylategroups and two tertiary amino groups—that can donate six pairs of electronsto a metal ion. The resulting metal–ligand complex, in which EDTAforms a cage-like structure around the metal ion (Figure 9.26b), is verystable. The actual number of coordination sites depends on the size of themetal ion, however, all metal–EDTA complexes have a 1:1 stoichiometry.Recall that an acid–base titration curve fora diprotic weak acid has a single end pointif its two K a values are not sufficiently different.See Figure 9.11 for an example.(a)− OONONO O − O(b)− OO – OO –OO −NNM 2+O –Figure 9.26 Structures of (a) EDTA, in its fully deprotonatedform, and (b) in a six-coordinate metal–EDTAcomplex with a divalent metal ion.OO –O