Page 322 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:12 PM Page 305

Chapter 9 Titrimetric Methods of Analysis 305

The phenolphthalein end point is basic, occurring at a pH of approximately 8.3
and can be reached only if the titration proceeds to the third equivalence point
(Figure 9.19b); thus, we write
3 ´moles citric acid = moles NaOH
Making appropriate substitutions for the moles of citric acid and moles of
NaOH gives the following equation
3 ´ g citric acid
V
= M b ´ b
FW citric acid
which can be solved for the grams of citric acid
M b ´ V b ´ FW citric acid (. M)(0.01762 L)(192.13 g/mol)
0 04166
=
3 3
= 0 04701. g citric acid
Since this is the grams of citric acid in a 50.00-mL sample, the concentration of
citric acid in the citrus drink is 0.09402 g/100 mL.

In an indirect analysis the analyte participates in one or more preliminary reac-
tions that produce or consume acid or base. Despite the additional complexity, the
stoichiometry between the analyte and the amount of acid or base produced or con-
sumed may be established by applying the conservation principles outlined in Sec-
tion 2C. Example 9.3 illustrates the application of an indirect analysis in which an
acid is produced.

9 3
EXAMPLE .
The purity of a pharmaceutical preparation of sulfanilamide, C 6 H 4 N 2 O 2 S, can
be determined by oxidizing the sulfur to SO 2 and bubbling the SO 2 through
H 2 O 2 to produce H 2 SO 4 . The acid is then titrated with a standard solution of
NaOH to the bromothymol blue end point, where both of sulfuric acid’s acidic
protons have been neutralized. Calculate the purity of the preparation, given
that a 0.5136-g sample required 48.13 mL of 0.1251 M NaOH.

SOLUTION
Conservation of protons for the titration reaction requires that
2 ´moles H 2 SO 4 = moles NaOH
Since all the sulfur in H 2 SO 4 comes from sulfanilamide, we use a conservation
of mass on sulfur to establish the following stoichiometric relationship.
Moles C 6 H 4 N 2 O 2 S = moles H 2 SO 4
Combining the two conservation equations gives a single equation relating the
moles of analyte to the moles of titrant.

2 ´moles C 6H 4N 2O 2S = moles NaOH
Making appropriate substitutions for the moles of sulfanilamide and moles of
NaOH gives
2 ´g sulfanilamide
V
= M b ´ b
FW sulfanilamide