Page 341 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:13 PM Page 324

324 Modern Analytical Chemistry

2+
Ca 2+ if a small amount of Mg –EDTA is added to the solution containing the ana-
2+
lyte. The Mg 2+ is displaced from the EDTA by Ca , freeing the Mg 2+ to form the
2+
red Mg –indicator complex. After all the Ca 2+ has been titrated, Mg 2+ is displaced
2+
from the Mg –indicator complex by EDTA, signaling the end point by the pres-
ence of the uncomplexed indicator’s blue form.
Finding the End Point by Monitoring Absorbance. An important limitation
when using a visual indicator is the need to observe the change in color signal-
ing the end point. This may be difficult when the solution is already colored.
For example, ammonia is used to adjust the pH of solutions containing Cu 2+
before its titration with EDTA. The presence of the intensely colored
2+
Cu(NH 3 ) 4 complex obscures the indicator’s color, making an accurate deter-
mination of the end point difficult. Other absorbing species present within the
sample matrix may also interfere in a similar fashion. This is often a problem
when analyzing clinical samples such as blood or environmental samples such
as natural waters.
As long as at least one species in a complexation titration absorbs electro-
magnetic radiation, the equivalence point can be located by monitoring the ab-
sorbance of the analytical solution at a carefully selected wavelength.* For ex-
ample, the equivalence point for the titration of Cu 2+ with EDTA, in the
presence of NH 3 , can be located by monitoring the absorbance at a wavelength
2+
of 745 nm, where the Cu(NH 3 ) 4 complex absorbs strongly. At the beginning
of the titration the absorbance is at a maximum. As EDTA is added, however,
the reaction
2+
2–
4–
Cu(NH 3 ) 4 (aq)+Y (aq) ® CuY (aq) + 4NH 3 (aq)
2+
occurs, decreasing both the concentration of Cu(NH 3 ) 4 and the absorbance.
The absorbance reaches a minimum at the equivalence point and remains essen-
tially unchanged as EDTA is added in excess. The resulting spectrophotometric
titration curve is shown in Figure 9.30a. In order to keep the individual segments
of the titration curve linear, the measured absorbance, A meas , is corrected for
dilution

V EDTA + V Cu
A corr = A meas ´
V Cu
where A corr is the corrected absorbance, and V EDTA and V Cu are, respectively, the
volumes of EDTA and Cu. The equivalence point is given by the intersection of the
linear segments, which are extrapolated if necessary to correct for any curvature in
the titration curve. Other common spectrophotometric titration curves are shown
in Figures 9.30b–f.

9 4 Representative Method
C.
Although every complexation titrimetric method has its own unique considera-
tions, the following description for determining the hardness of water provides an
instructive example of a typical procedure.

*See Chapter 10 for a further discussion of absorbance and spectroscopy.