The Precipitation Titration Curve

We now turn our attention to details of precipitation titrations as an illustration of principles that underlie all titrations. We first study how concentrations of analyte and titrant vary during a titration and then derive equations that can be used to predict titration curves. One reason to calculate titration curves is to understand the chemistry that occurs during titrations. A second reason is to learn how experimental control can be exerted to influence the quality of an analytical titration. For example, certain titrations conducted at the wrong pH could give no discernible end point. In precipitation titrations, the concentrations of analyte and titrant and the size of Ksp influence the sharpness of the end point. For acid-base titrations (Chapter 11) and oxidation-reduction titrations (Chapter 16). the theoretical titration curve enables us to choose an appropriate indicator. 

The titration curve is a graph showing how the concentration of one of the reactants varies as titrant is added. Because concentration varies over many orders of magnitude, it is most useful to plot the p function  

Because the equilibrium constant for the titration reaction 7-9 is large (K = 1 /KSJ = 1.2 X 1016), the equilibrium lies far to the right. It is reasonable to say that each aliquot of Ag reacts completely with I-, leaving only a tiny amount of Ag+ in solution. At the equivalence point, there will be a sudden increase in the Ag+ concentration because all the I has been consumed and we are now adding Ag+ directly to the solution. 

What volume of Agr titrant is needed to reach the equivalence point We calculate this volume, designated Fe, with the fact that 1 mol of Ag+ reacts with 1 mol of I-. 

The titration curve has three distinct regions, depending on whether we are before, at, or after the equivalence point. Let s consider each region separately. 

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As with the other reaction types, the precipitation titration curve can be described by a single equation. If of a chloride solution is titrated with C gM AgN03, then at any point after the addition of mL, balance equations can be written for [Cl ] and [Ag ]. The total Ag content is distributed between dissolved Ag and precipitated AgCl. Similarly, total Cl is the sum of dissolved Cl" and precipitate. 

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