Silver chloride, solubility curve

Precipitation titrations can be extended to mixtures that form precipitates of different solubilities. The titration curve in Fig. 2 for a chloride/iodide mixture is a composite of the individual curves for the two anionic species. Because silver iodide has a much lower solubility than does silver chloride, the initial additions of the reagent result exclusively in formation of iodide. Thus, two equivalence points are evident. 

FIGURE 16.6 Some solid silver chloride is in contact with a solution containing Ag (aq) and Cl (aq) ions. If a solubility equilibrium exists, then the product Q of the concentrations of the ions [Ag ] X [Cl ] is a constant, /Cjp (curved line). When Q exceeds K p, solid silver chloride tends to precipitate until equilibrium is attained. When Q is less than K p, additional solid tends to dissolve. If no solid is present, Q remains less than K p. 

The solid curve in Figure 11 -2 illustrates the effect of chloride ion concentration on the solubility of silver chloride data for the curve were obtained by substituting various chloride concentrations into Equation 11-41. Note that at high concentrations of the common ion, the solubility becomes greater than that in pure water. The broken lines represent the equilibrium concentrations of the various silver-containing species as a function of Ckci- Note that at the solubility minimum, undissociated silver chloride, AgCl(ag), is the major silver species in the solution, representing about 80% of the total dissolved silver. Its concentration is invariant, as has been demonstrated. 

The methods developed in the previous section for deriving titration curves can be extended to mixtures that form precipitates of different solubilities. To illustrate, consider the titration of 50.00 mL of a solution that is 0.0500 M in iodide ion and 0.0800 M in chloride ion with 0.1000 M silver nitrate. The curve for the initial stages of this titration is identical to the curve shown for iodide in Figure 13-5 because silver chloride, with its much larger solubility product, does not begin to precipitate until well into the titration. 

The data in the following table givfe the solubility of silver chloride in various aqueous solutions at 25°C. Show that these data can be plotted on the same In versus VT curve as used in Illustration 13.2-3. 

Figure 11 -2 The effect of chloride ion concentration on the solubility of AgCl. The solid curve shows the total concentration of dissolved AgCl. The broken lines show the concentrations of the various silver-containing species.

The Effect of Reaction Completeness on Titration Curves Figure 13-5 illustrates the effect of solubility product on the sharpness of the end point in titrations with 0.1 M silver nitrate. Clearly, the change in pAg at the equivalence point becomes greater as the solubility products become smaller, that is, as the reaction between the analyte and silver nitrate becomes more complete. By careful choice of indicator—one that changes color in the region of pAg from 4 to 6—titration of chloride ion should be possible with a minimal titration error. Note that ions forming precipitates with solubility products much larger than about 10 do not yield satisfactory end points. 

Curve A in Figure 13-6, which is the titration curve for the chloride/iodide mixture just considered, is a composite of the individual curves for the two anionic species. Two equivalence points are evident. Curve B is the titration curve for a mixture of bromide and chloride ions. Clearly, the change associated with the first equivalence point becomes less distinct as the solubilities of the two precipitates approach one another. In the bromide/chloride titration, the initial pAg values are lower than they are in the iodide/chloride titration because the solubility of silver bromide exceeds that of silver iodide. Beyond the first equivalence point, however, where chloride ion is being titrated, the two titration curves are identical. 

Prepare a spreadsheet to plot the titration curve of 100 mL 0.1 M chloride titrated with 0.1 M silver nitrate (Figure 11.1). See your CD for a suggested setup. Use the spreadsheet to change the concentrations of chloride and silver (e.g., 0.2 M each, 0.05 M each), and notice how the titration curve changes. Note that there is a limit to how low the concentrations can go in these calculated plots because eventually the solubility of the AgCl at 99.9 and 100.1 mL titrant becomes appreciable. 

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