Page 332 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:13 PM Page 315

Chapter 9 Titrimetric Methods of Analysis 315

1946, when Schwarzenbach introduced metallochromic dyes
CH COO –
as visual indicators for signaling the end point of a complexa- 2
tion titration. – OOC H C N:
2
9C.1 Chemistry and Properties of EDTA :N CH 2 COO –
Ethylenediaminetetraacetic acid, or EDTA, is an aminocar- – OOC H 2 C
boxylic acid. The structure of EDTA is shown in Figure 9.25a.
(a)
EDTA, which is a Lewis acid, has six binding sites (the four car- O
boxylate groups and the two amino groups), providing six pairs
O
of electrons. The resulting metal–ligand complex, in which EDTA forms a cage-like O
structure around the metal ion (Figure 9.25b), is very stable. The actual number of O N
coordination sites depends on the size of the metal ion; however, all metal–EDTA M
O N
complexes have a 1:1 stoichiometry.
O
O
Metal–EDTA Formation Constants To illustrate the formation of a metal–EDTA
2+
complex consider the reaction between Cd and EDTA O
(b)
2–
4–
2+
Cd (aq)+Y (aq) t CdY (aq)
Figure 9.25
4–
where Y is a shorthand notation for the chemical form of EDTA shown in Figure Structures of (a) EDTA, and (b) a six-
9.25. The formation constant for this reaction coordinate metal–EDTA complex.
2 -
[ CdY ]
K f = 2 + 4 - = . 29 ´ 10 16 9.11
[ Cd ][ Y ]
is quite large, suggesting that the reaction’s equilibrium position lies far to the right.
Formation constants for other metal–EDTA complexes are found in Appendix 3C. Y 4–
10.17
EDTA Is a Weak Acid Besides its properties as a ligand, EDTA is also a weak acid.
2+
The fully protonated form of EDTA, H 6 Y , is a hexaprotic weak acid with succes-
sive pK a values of 3–
HY
pK a1 = 0.0 pK a2 = 1.5 pK a3 = 2.0 pK a4 = 2.68 pK a5 = 6.11 pK a6 = 10.17
The first four values are for the carboxyl protons, and the remaining two values are 6.11
for the ammonium protons. A ladder diagram for EDTA is shown in Figure 9.26. pH
4–
The species Y becomes the predominate form of EDTA at pH levels greater than 2–
4–
10.17. It is only for pH levels greater than 12 that Y becomes the only significant H Y
2
form of EDTA.
2.68
H Y –
3
Conditional Metal–Ligand Formation Constants Recognizing EDTA’s acid–base H Y 2.0
properties is important. The formation constant for CdY 2– in equation 9.11 as- 4 1.5
4–
sumes that EDTA is present as Y . If we restrict the pH to levels greater than 12, H Y +
5
2–
then equation 9.11 provides an adequate description of the formation of CdY . For H Y 2+ 0.0
2–
pH levels less than 12, however, K f overestimates the stability of the CdY complex. 6
At any pH a mass balance requires that the total concentration of unbound Figure 9.26
EDTA equal the combined concentrations of each of its forms. Ladder diagram for EDTA.
2+
–
+
4–
3–
2–
C EDTA =[H 6Y ]+[H 5Y ]+[H 4Y]+[H 3Y ]+[H 2Y ] + [HY ]+[Y ]
To correct the formation constant for EDTA’s acid–base properties, we must ac-
4–
count for the fraction, a Y , of EDTA present as Y .
4–
Y [ 4- ]
a Y 4- = 9.12
C EDTA