Selecting and Evaluating the End Point - Precipitation Titrations

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.

Finding the End Point with a Visual Indicator 

The first important visual indicator to be developed was the Mohr method for Cl– using Ag+ as a titrant. By adding a small amount of K₂CrO₄ to the solution containing the analyte, the formation of a precipitate of reddish-brown Ag₂CrO₄ signals the end point. Because K₂CrO₄ im- parts a yellow color to the solution, obscuring the end point, the amount of CrO 2– added is small enough that the end point is always later than the equivalence point. To compensate for this positive determinate error an analyte-free reagent blank is analyzed to determine the volume of titrant needed to effect a change in the indica- tor’s color. The volume for the reagent blank is subsequently subtracted from the experimental end point to give the true end point. Because CrO 2– is a weak base, the solution usually is maintained at a slightly alkaline pH. If the pH is too acidic, chromate is present as HCrO₄–, and the Ag₂CrO₄ end point will be in significant error. The pH also must be kept below a level of 10 to avoid precipitating silver hydroxide.

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.

Finding the End Point Potentiometrically 

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.