Analytical Chem istry - DePauw University

442 Analytical Chemistry 2.01412(a) 14 (b) 14 (c)1212101010pH86pH86pH8644422200 20 40 60 80 100Volume of NaOH (mL)Figure 9.19 Titration curves for 50.0 mL of (a) 0.10 M NaOH, (b) 0.050 M Na 2 CO 3 , and (c) 0.10 M NaHCO 3 using 0.10M HCl. The dashed lines indicate the fixed pH end points of 8.3 and 4.5. The color gradients show the phenolphthalein(red colorless) and bromocresol green (blue green) endpoints. When titrating to the phenolphthalein endpoint, the titrationcontinues until the last trace of red is lost.Solutions containing OH – and HCO 3–alkalinities are unstable with respect to theformation of CO 32– . Problem 9.15 in theend of chapter problems asks you to explainwhy this is true.00 20 40 60 80 100Volume of NaOH (mL)0 20 40 60 80 100Volume of NaOH (mL)When the sources of alkalinity are limited to OH – , HCO 3 – , and CO 3 2– ,separate titrations to a pH of 4.5 (or the bromocresol green end point)and a pH of 8.3 (or the phenolphthalein end point) allow us to determinewhich species are present and their respective concentrations. Titrationcurves for OH – , HCO 3 – , and CO 3 2– are shown in Figure 9.19. For a solutioncontaining only OH – alkalinity, the volumes of strong acid needed toreach the two end points are identical (Figure 9.19a). When the only sourceof alkalinity is CO 3 2– , the volume of strong acid needed to reach the endpoint at a pH of 4.5 is exactly twice that needed to reach the end point at apH of 8.3 (Figure 9.19b). If a solution contains only HCO 3 – alkalinity, thevolume of strong acid needed to reach the end point at a pH of 8.3 is zero,but that for the pH 4.5 end point is greater than zero (Figure 9.19c).Mixtures of OH – and CO 3 2– , or of HCO 3 – and CO 3 2– also are possible.Consider, for example, a mixture of OH – and CO 3 2– . The volumeof strong acid to titrate OH – is the same whether we titrate to a pH of 8.3or a pH of 4.5. Titrating CO 3 2– to a pH of 4.5, however, requires twice asmuch strong acid as titrating to a pH of 8.3. Consequently, when titratinga mixture of these two ions, the volume of strong acid to reach a pH of 4.5is less than twice that to reach a pH of 8.3. For a mixture of HCO 3 – andCO 3 2– the volume of strong acid to reach a pH of 4.5 is more than twicethat to reach a pH of 8.3. Table 9.6 summarizes the relationship betweenthe sources of alkalinity and the volumes of titrant needed to reach the twoend points.Acidity is a measure of a water sample’s capacity for neutralizing base,and is conveniently divided into strong acid and weak acid acidity. Strongacid acidity, from inorganic acids such as HCl, HNO 3 , and H 2 SO 4 , iscommon in industrial effluents and acid mine drainage. Weak acid acidityis usually dominated by the formation of H 2 CO 3 from dissolved CO 2 , butalso includes contributions from hydrolyzable metal ions such as Fe 3+ , Al 3+ ,0