Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods461Table 9.10 Values of a Y 4– for Selected pH LevelspH a Y 4– pH a Y 4–1 1.9 10 –18 8 5.6 10 –32 3.4 10 –14 9 5.4 10 –23 2.6 10 –11 10 0.374 3.8 10 –9 11 0.855 3.7 10 –7 12 0.986 2.4 10 –5 13 1.007 5.0 10 –4 14 1.00Table 9.10 provides values of a Y 4– for selected pH levels. Solving equation9.11 for [Y 4– ] and substituting into equation 9.10 for the CdY 2– formationconstantKf2−[ CdY ]=2+[ Cd ] α C4−YEDTAProblem 9.42 from the end of chapterproblems asks you to verify the values inTable 9.10 by deriving an equation fora Y 4-.and rearranging givesK2−′[ CdY ]= K × α 4−=2+[ Cd ] Cf f YEDTA9.12where K f´ is a pH-dependent conditional formation constant. Asshown in Table 9.11, the conditional formation constant for CdY 2– becomessmaller and the complex becomes less stable at more acidic pHs.EDTA Co m p e t e s Wi t h Ot h e r Li g a n d sTo maintain a constant pH during a complexation titration we usually adda buffering agent. If one of the buffer’s components is a ligand that bindsCd 2+ , then EDTA must compete with the ligand for Cd 2+ . For example, anTable 9.11 Conditional Formation Constants for CdY 2–pH K f´ pH K f´1 5.5 10 –2 8 1.6 10 142 1.0 10 3 9 1.6 10 153 7.7 10 5 10 1.1 10 164 1.1 10 8 11 2.5 10 165 1.1 10 10 12 2.9 10 166 6.8 10 11 13 2.9 10 167 1.5 10 13 14 2.9 10 16