Page 359 - Modern Analytical Chemistry

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342 Modern Analytical Chemistry

Table 9.19 Selected Reductions Using Metal Reductor Columns

Oxidized
Species Walden Reductor Jones Reductor
Cr 3+ –– a Cr 3+ + e ® Cr 2+
–
–
Cu 2+ Cu 2+ + e ® Cu + Cu 2+ +2e ® Cu
–
–
–
Fe 3+ Fe 3+ + e ® Fe 2+ Fe 3+ + e ® Fe 2+
TiO 2+ — a TiO 2+ +2H 3 O + e ® Ti 3+ +3H 2 O
–
+
2+ 2+ – + 2+ +4H 3 O +3e ® Mo 3+ +6H 2 O
–
+
MoO 2 MoO 2 + e ® MoO 2 MoO 2
+ VO 2 +2H 3 O + e ® VO 2+ +3H 2 O VO 2 +4H 3 O +3e ® V 2+ +6H 2 O
+
+
–
+
+
–
VO 2
a No reaction.
provides another method for oxidizing an analyte. Excess H 2 O 2 also can be de-
stroyed by briefly boiling the solution.
Selecting and Standardizing a Titrant In quantitative work the titrant’s concen-
tration must remain stable during the analysis. Since titrants in a reduced state are
susceptible to air oxidation, most redox titrations are carried out using an oxidizing
agent as the titrant. The choice of which of several common oxidizing titrants is best
for a particular analysis depends on the ease with which the analyte can be oxidized.
Analytes that are strong reducing agents can be successfully titrated with a relatively
weak oxidizing titrant, whereas a strong oxidizing titrant is required for the analysis
of analytes that are weak reducing agents.
–
4+
The two strongest oxidizing titrants are MnO 4 and Ce , for which the reduc-
tion half-reactions are
–
+
–
2+
MnO 4 (aq)+8H 3 O (aq)+5e t Mn (aq) + 12H 2 O(l)
–
4+
3+
Ce (aq)+ e t Ce (aq)
Solutions of Ce 4+ are prepared from the primary standard cerium ammonium ni-
trate, Ce(NO 3 ) 4×
2NH 4 NO 3 , in 1 M H 2 SO 4 . When prepared from reagent grade
materials, such as Ce(OH) 4 , the solution must be standardized against a primary
standard reducing agent such as Na 2 C 2 O 4 or Fe 2+ (prepared using Fe wire). Ferroin
is a suitable indicator when standardizing against Fe 2+ (Table 9.20). Despite its
availability as a primary standard and its ease of preparation, Ce 4+ is not as fre-
–
quently used as MnO 4 because of its greater expense.
–
Solutions of MnO 4 are prepared from KMnO 4 , which is not available as a pri-
mary standard. Aqueous solutions of permanganate are thermodynamically unsta-
ble due to its ability to oxidize water.
–
–
4MnO 4 (aq)+2H 2 O(l) t 4MnO 2 (s)+3O 2 (g) + 4OH (aq)
2+
This reaction is catalyzed by the presence of MnO 2 , Mn , heat, light, and the pres-
ence of acids and bases. Moderately stable solutions of permanganate can be pre-
pared by boiling for an hour and filtering through a sintered glass filter to remove
any solid MnO 2 that precipitates. Solutions prepared in this fashion are stable for
1–2 weeks, although the standardization should be rechecked periodically. Stan-
dardization may be accomplished using the same primary standard reducing agents
4+
–
that are used with Ce , using the pink color of MnO 4 to signal the end point
(Table 9.20).