Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods445Table 9.8 Selected Acid–Base Titrimetric Procedures for Organic Functional GroupsBased on the Production or Consumption of Acid or BaseFunctionalGroup Reaction Producing Titratable Acid or Base a Titration Details−ester RCOOR′ ( aq) + OH − ( aq) → RCOO ( aq) + HOR′( aq)titrate OH – with HClcarbonylRC= O( aq) + NHOHHCl i ( aq)→2 2RC= NOH( aq) + HCl ( aq)+ HO 2() lalcohol b [] 1 (CH CO) O+ ROH → CH COOR + CH COOH3 2 33[ 2](CH CO) O+ H O3 2 2→2CH COOH3a The species that is titrated is shown in bold.2titrate HCl with NaOHtitrate CH 3 COOH withNaOH; a blank titration ofacetic anhydride, (CH 3 CO) 2 O,corrects for the contribution ofreaction [2]b The acetylation reaction [1] is carried out in pyridine to prevent the hydrolysis of acetic by water. After the acetylation reactionis complete, water is added to covert any unreacted acetic anhydride to acetic acid [2].Example 9.2A 50.00 mL sample of a citrus drink requires 17.62 mL of 0.04166 MNaOH to reach the phenolphthalein end point. Express the sample’s acidityas grams of citric acid, C 6 H 8 O 7 , per 100 mL.(a)more basicCit 3–pK a3 = 6.40So l u t i o nBecause citric acid is a triprotic weak acid, we must first determine if thephenolphthalein end point corresponds to the first, second, or third equivalencepoint. Citric acid’s ladder diagram is shown in Figure 9.20a. Basedon this ladder diagram, the first equivalence point is between a pH of 3.13and a pH of 4.76, the second equivalence point is between a pH of 4.76and a pH of 6.40, and the third equivalence point is greater than a pHof 6.40. Because phenolphthalein’s end point pH is 8.3–10.0 (see Table9.4), the titration proceeds to the third equivalence point and the titrationreaction is−3−CHO ( aq) + 3OH ( aq) → C HO ( aq) + 3HO()l6 8 7 6 5 7In reaching the equivalence point, each mole of citric acid consumes threemoles of NaOH; thus0. 04166 MNaOH× 0.01762 LNaOH= 7.3405× 10 −5moles NaOH1 molC HO57. 3405× 10−6 8 7molNaOH×3mol NaOH= 2. 4468× 10 −4mol CHO26 8 7pH14121086420(b)pHmore acidicHCit 2–pK a2 = 4.76H 2 Cit –pK a1 = 3.13H 3 Cit0 5 10 15 20 25 30Volume of NaOH (mL)Figure 9.20 (a) Ladder diagram forcitric acid; (b) Titration curve for thesample in Example 9.2 showing phenolphthalein’spH transition region.