This change in the indicator’s color signals the end point. Dichlorofluoroscein now adsorbs to the precipitate’s surface where its color is pink. After the end point, the surface of the precipitate carries a positive surface charge due to the adsorption of excess Ag +. Because dichlorofluoroscein also carries a negative charge, it is repelled by the precipitate and remains in solution where it has a greenish-yellow color. Before the end point, the precipitate of AgCl has a negative surface charge due to the adsorption of excess Cl –. In the Fajans method for Cl – using Ag + as a titrant, for example, the anionic dye dichlorofluoroscein is added to the titrand’s solution. The third type of end point uses a species that changes color when it adsorbs to the precipitate. The Volhard method was first published in 1874 by Jacob Volhard. The titration must be carried out in an acidic solution to prevent the precipitation of Fe 3+ as Fe(OH) 3. The titration’s end point is the formation of the reddish-colored Fe(SCN) 2+ complex. In the Volhard method for Ag + using KSCN as the titrant, for example, a small amount of Fe 3+ is added to the titrand’s solution. The pH also must be less than 10 to avoid the precipitation of silver hydroxide.Ī second type of indicator uses a species that forms a colored complex with the titrant or the titrand. If the pH is too acidic, chromate is present as HCrO 4 – instead of CrO 4 2–, and the Ag 2CrO 4 end point is delayed. Because CrO 4 2– is a weak base, the titrand’s solution is made slightly alkaline. Subtracting the end point for the reagent blank from the titrand’s end point gives the titration’s end point. To compensate for this positive determinate error, an analyte-free reagent blank is analyzed to determine the volume of titrant needed to affect a change in the indicator’s color. As a result, the end point is always later than the equivalence point. The Mohr method was first published in 1855 by Karl Friedrich Mohr.īecause CrO 4 2– imparts a yellow color to the solution, which might obscure the end point, only a small amount of K 2CrO 4 is added. A better fit is possible if the two points before the equivalence point are further apart-for example, 0 mL and 20 mL- and the two points after the equivalence point are further apart. A comparison of our sketch to the exact titration curve (Figure 9.44f) shows that they are in close agreement.įigure 9.44 Illustrations showing the steps in sketching an approximate titration curve for the titration of 50.0 mL of 0.0500 M NaCl with 0.100 M AgNO 3: (a) locating the equivalence point volume (b) plotting two points before the equivalence point (c) plotting two points after the equivalence point (d) preliminary approximation of titration curve using straight-lines (e) final approximation of titration curve using a smooth curve (f) comparison of approximate titration curve (solid black line) and exact titration curve (dashed red line). Finally, we complete our sketch by drawing a smooth curve that connects the three straight-line segments (Figure 9.44e). Next, we draw a straight line through each pair of points, extending them through the vertical line representing the equivalence point’s volume (Figure 9.44d). The blue line shows the complete titration curve. The red points corresponds to the data in Table 9.18. Table 9.18: Titration of 50.0 mL of 0.0500 M NaCl with 0.100 M AgNO 3 Volume of AgNO 3 (mL)įigure 9.43 Titration curve for the titration of 50.0 mL of 0.0500 M NaCl with 0.100 M AgNO 3. Additional results for the titration curve are shown in Table 9.18 and Figure 9.43.
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