Analytical Chem istry - DePauw University

Chapter 6 Equilibrium Chemistry253pHpLE4.17F –11.69Ca 2+0.184Sn 4+pK a= 3.17logK1 = 10.69E o = 0.1542.17HF9.69Ca(EDTA) 2-0.124Sn 2+(a) (b) (c)Figure 6.14 Ladder diagrams showing buffer regions for (a) an acid–base buffer forHF and F – ; (b) a metal–ligand complexation buffer for Ca 2+ and Ca(EDTA) 2– ;and (c) an oxidation–reduction (redox) buffer for Sn 4+ and Sn 2+ .1 1pL = logK± 1 or pL = log β ±nnn npL = –log[L]where K n or b n is the relevant stepwise or overall formation constant. Foran oxidizing agent and its conjugate reducing agent, a redox buffer existswhen1 RT 0.05916E = Eo ± × = Eo ± (at25C)on FnLadder diagrams showing buffer regions for several equilibria are shown inFigure 6.14.6H.3 Preparing BuffersBuffer capacity is the ability of a buffer to resist a change in pH whenadding a strong acid or a strong base. A buffer’s capacity to resist a change inpH is a function of the concentrations of the weak acid and the weak base,as well as their relative proportions. The importance of the weak acid’s concentrationand the weak base’s concentration is obvious. The more moles ofweak acid and weak base a buffer has, the more strong base or strong acidit can neutralize without significantly changing its pH.The relative proportions of a weak acid and a weak base also affects howmuch the pH changes when adding a strong acid or a strong base. Buffersthat are equimolar in weak acid and weak base require a greater amount ofstrong acid or strong base to bring about a one unit change in pH. Consequently,a buffer is most effective against the addition of strong acids orstrong bases when its pH is near the weak acid’s pK a value.Although higher concentrations of bufferingagents provide greater buffer capacity,there are reasons for using smaller concentrations,including the formation of unwantedprecipitates and the tolerance ofcells for high concentrations of dissolvedsalts.A good “rule of thumb” when choosing abuffer is to select one whose reagents havea pK a value close to your desired pH.