Selecting and Evaluating the End Point
Initial attempts at developing precipitation titration methods were limited by a poor end point signal. Finding the end point by looking for the first addition of titrant that does not yield additional precipitate is cumbersome at best. The feasibil- ity of precipitation titrimetry improved with the development of visual indicators and potentiometric ion-selective electrodes.
A second end point is the Volhard method in which Ag+ is titrated with SCN– in the presence of Fe3+. The end point for the titration reaction
Ag+(aq) + SCN–(aq) < = = = = > AgSCN(s)
is the formation of the reddish colored Fe(SCN)2+ complex.
SCN–(aq)+ Fe3+(aq) < = = = = > Fe(SCN)2+(aq)
The titration must be carried out in a strongly acidic solution to achieve the desired end point.
A third end point is evaluated with Fajans’ method, which uses an adsorption indicator whose color when adsorbed to the precipitate is different from that when it is in solution. For example, when titrating Cl– with Ag+ the anionic dye dichloro- fluoroscein is used as the indicator. Before the end point, the precipitate of AgCl has a negative surface charge due to the adsorption of excess Cl–. The anionic indicator is repelled by the precipitate and remains in solution where it has a greenish yellow color. After the end point, the precipitate has a positive surface charge due to the adsorption of excess Ag+. The anionic indicator now adsorbs to the precipitate’s surface where its color is pink. This change in color signals the end point.
Another method for locating the end point of a precipitation titration is to monitor the change in concentration for the analyte or titrant using an ion-selective electrode. The end point can then be found from a visual inspection of the titration curve.
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