Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods417pumptitranttitrandFigure 9.5 Typical instrumentation for an automated acid–base titration showing the titrant, the pump, andthe titrand. The pH electrode in the titrand’s solution is used to monitor the titration’s progress. You can seethe titration curve in the lower-left quadrant of the computer’s display. Modified from: Datamax (commons.wikipedia.org).9BAcid–Base TitrationsBefore 1800, most acid–base titrations used H 2 SO 4 , HCl, or HNO 3 asacidic titrants, and K 2 CO 3 or Na 2 CO 3 as basic titrants. A titration’s endpoint was determined using litmus as an indicator, which is red in acidicsolutions and blue in basic solutions, or by the cessation of CO 2 effervescencewhen neutralizing CO 3 2– . Early examples of acid–base titrimetryinclude determining the acidity or alkalinity of solutions, and determiningthe purity of carbonates and alkaline earth oxides.Three limitations slowed the development of acid–base titrimetry: thelack of a strong base titrant for the analysis of weak acids, the lack of suitableindicators, and the absence of a theory of acid–base reactivity. Theintroduction, in 1846, of NaOH as a strong base titrant extended acid–base titrimetry to the determination of weak acids. The synthesis of organicdyes provided many new indicators. Phenolphthalein, for example, wasfirst synthesized by Bayer in 1871 and used as an indicator for acid–basetitrations in 1877.Despite the increasing availability of indicators, the absence of a theoryof acid–base reactivity made it difficult to select an indicator. The developmentof equilibrium theory in the late 19th century led to significantimprovements in the theoretical understanding of acid–base chemistry, and,in turn, of acid–base titrimetry. Sørenson’s establishment of the pH scalein 1909 provided a rigorous means for comparing indicators. The determinationof acid–base dissociation constants made it possible to calculatea theoretical titration curve, as outlined by Bjerrum in 1914. For the firstThe determination of acidity and alkalinitycontinue to be important applicationsof acid–base titrimetry. We will take acloser look at these applications later inthis section.