Page 330 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:13 PM Page 313

Chapter 9 Titrimetric Methods of Analysis 313

Substituting the molarity and volume of titrant for moles, and rearranging gives
1
V b = ´ moles H 2 SO 3 9.10
M b
where k is equivalent to
1
k =
M b
There are two ways in which the sensitivity can be increased. The first, and most ob-
vious, is to decrease the concentration of the titrant, since it is inversely propor-
tional to the sensitivity, k. The second method, which only applies if the analyte is
multiprotic, is to titrate to a later equivalence point. When H 2 SO 3 is titrated to the
second equivalence point, for instance, equation 9.10 becomes

1
V b = 2 ´ ´ moles H 2 SO 3
M b
where k is now equal to
2
k =
M b
In practice, however, any improvement in the sensitivity of an acid–base titration
due to an increase in k is offset by a decrease in the precision of the equivalence
point volume when the buret needs to be refilled. Consequently, standard analytical
procedures for acid–base titrimetry are usually written to ensure that titrations re-
quire 60–100% of the buret’s volume.

Selectivity Acid–base titrants are not selective. A strong base titrant, for example,
will neutralize any acid, regardless of strength. Selectivity, therefore, is determined
by the relative acid or base strengths of the analyte and the interferent. Two limiting
situations must be considered. First, if the analyte is the stronger acid or base, then
the titrant will begin reacting with the analyte before reacting with the interferent.
The feasibility of the analysis depends on whether the titrant’s reaction with the in-
terferent affects the accurate location of the analyte’s equivalence point. If the acid
dissociation constants are substantially different, the end point for the analyte can
be accurately determined (Figure 9.24a). Conversely, if the acid dissociation con-
stants for the analyte and interferent are similar, then an accurate end point for the
analyte may not be found (Figure 9.24b). In the latter case, a quantitative analysis
for the analyte is not possible.
In the second limiting situation the analyte is a weaker acid or base than the in-
terferent. In this case the volume of titrant needed to reach the analyte’s equivalence
point is determined by the concentration of both the analyte and the interferent. To
account for the contribution from the interferent, an equivalence point for the in-
terferent must be present. Again, if the acid dissociation constants for the analyte
and interferent are significantly different, the analyte’s determination is possible. If,
however, the acid dissociation constants are similar, only a single equivalence point
is found, and the analyte’s and interferent’s contributions to the equivalence point
volume cannot be separated.

Time, Cost, and Equipment Acid–base titrations require less time than most gravi-
metric procedures, but more time than many instrumental methods of analysis,
particularly when analyzing many samples. With the availability of instruments for