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.
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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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