Page 360 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:13 PM Page 343

Chapter 9 Titrimetric Methods of Analysis 343

9
Table .20 Standardization Reactions for
Selected Redox Titrants
Titration Reaction

Ce 4+ +Fe 2+ ® Ce 3+ +Fe 3+
2Ce 4+ +H 2 C 2 O 4 +2H 2 O ® 2Ce 3+ + 2CO 2 +2H 3 O +
–
MnO 4 + 5Fe 2+ +8H 3 O ® Mn 2+ + 5Fe 3+ + 12H 2 O
+
2MnO 4 +5H 2 C 2 O 4 +6H 3 O ® 2Mn 2+ + 10CO 2 + 14H 2 O
+
–
– 2– – 2–
I 3 +2S 2 O 3 ® 3I +S 4 O 6
2– – 2–
I 2 +2S 2 O 3 ® 2I +S 4 O 6
Potassium dichromate is a relatively strong oxidizing agent whose principal ad-
vantages are its availability as a primary standard and the long-term stability of its
4+
–
solutions. It is not, however, as strong an oxidizing agent as MnO 4 or Ce , which
prevents its application to the analysis of analytes that are weak reducing agents. Its
reduction half-reaction is
2–
+
3+
–
Cr 2 O 7 (aq) + 14H 3 O (aq)+6e t 2Cr (aq) + 21H 2 O(l)
2–
Although solutions of Cr 2 O 7 are orange and those of Cr 3+ are green, neither color
is intense enough to serve as a useful indicator. Diphenylamine sulfonic acid, whose
oxidized form is purple and reduced form is colorless, gives a very distinct end
2–
point signal with Cr 2 O 7 .
Iodine is another commonly encountered oxidizing titrant. In comparison with
–
4+
2–
MnO 4 , Ce , and Cr 2 O 7 , it is a weak oxidizing agent and is useful only for the
analysis of analytes that are strong reducing agents. This apparent limitation, how-
ever, makes I 2 a more selective titrant for the analysis of a strong reducing agent in
the presence of weaker reducing agents. The reduction half-reaction for I 2 is
–
–
I 2 (aq)+2e t 2I (aq)
Because of iodine’s poor solubility, solutions are prepared by adding an excess
–
of I . The complexation reaction
–
–
I 2 (aq)+I (aq) t I 3 (aq)
–
increases the solubility of I 2 by forming the more soluble triiodide ion, I 3 . Even
–
though iodine is present as I 3 instead of I 2 , the number of electrons in the reduc-
tion half-reaction is unaffected.
–
–
–
I 3 (aq)+2e t 3I (aq)
–
Solutions of I 3 are normally standardized against Na 2 S 2 O 3 (see Table 9.20) using
–
starch as a specific indicator for I 3 .
2–
–
4+
–
Oxidizing titrants such as MnO 4 , Ce , Cr 2 O 7 and I 3 , are used to titrate ana-
lytes that are in a reduced state. When the analyte is in an oxidized state, it can be
reduced with an auxiliary reducing agent and titrated with an oxidizing titrant. Al-
ternatively, the analyte can be titrated with a suitable reducing titrant. Iodide is a
relatively strong reducing agent that potentially could be used for the analysis of an-
–
alytes in higher oxidation states. Unfortunately, solutions of I cannot be used as a
–
–
direct titrant because they are subject to the air oxidation of I to I 3 .
–
–
3I (aq) t I 3 (aq)+2e –