Page 364 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:13 PM Page 347

Chapter 9 Titrimetric Methods of Analysis 347

Organic compounds containing a hydroxyl, carbonyl, or amine functional
group adjacent to a hydoxyl or carbonyl group can be oxidized using metaperio-
–
date, IO 4 , as an oxidizing titrant.
–
–
–
–
IO 4 (aq)+H 2 O(l)+2e ® IO 3 (aq) + 2OH (aq)
A two-electron oxidation cleaves the C—C bond between the two functional
groups, with hydroxyl groups being oxidized to aldehydes or ketones, carbonyl
functional groups being oxidized to carboxylic acids, and amines being oxidized to
an aldehyde and an amine (ammonia if the original amine was primary). For exam-
–
ple, treatment of serine with IO 4 results in the following oxidation reaction
OH NH 3 + O O
+
+ 2OH – + + NH + H O + 2e –
4
2
H C CH CO 2 – CH 2 HC CO 2 –
2
The analysis is conducted by adding a known excess of IO 4 to the solution contain-
–
ing the analyte and allowing the oxidation to take place for approximately 1 h at
room temperature. When the oxidation is complete, an excess of KI is added, which
reacts with the unreacted IO 4 to form IO 3 and I 3 .
–
–
–
–
IO 4 (aq)+3I (aq)+H 2O(l) ® IO 3 (aq)+I 3 (aq) + 2OH (aq)
–
–
–
–
The I 3 is then determined by titrating with S 2 O 3 using starch as an indicator.
2–
–
Quantitative Calculations The stoichiometry of a redox reaction is given by the con-
servation of electrons between the oxidizing and reducing agents (see Section 2C); thus
Moles of electrons lost
´ moles reducing agent
Mole reducing agent
moles of electrons gained
= ´ moles oxidizing agent
mole oxidizing agent
Example 9.13 shows how this equation is applied to an analysis based on a direct
titration.
EXAMPLE .13
9
The amount of Fe in a 0.4891-g sample of an ore was determined by a redox
titration with K 2Cr 2O 7. The sample was dissolved in HCl and the iron brought
into the +2 oxidation state using a Jones reductor. Titration to the
diphenylamine sulfonic acid end point required 36.92 mL of 0.02153 M
K 2 Cr 2 O 7 . Report the iron content of the ore as %w/w Fe 2 O 3 .
SOLUTION
3+
In this titration the analyte is oxidized from Fe 2+ to Fe , and the titrant is
2+
3+
2–
reduced from Cr 2 O 7 to Cr . Oxidation of Fe requires only a single electron.
2–
Reducing Cr 2 O 7 , in which chromium is in the +6 oxidation state, requires a
total of six electrons. Conservation of electrons for the redox reaction,
therefore, requires that
Moles Fe 2+ =6 ´moles Cr 2 O 7 2–
2+
A conservation of mass relates the moles of Fe to the moles of Fe 2 O 3
Moles Fe 2+ =2 ´moles Fe 2 O 3