Page 355 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:13 PM Page 338

338 Modern Analytical Chemistry

Substituting these equalities into the equation for E eq and rearranging gives
°
°
E Fe / Fe 2+ +5 E MnO 4 / Mn 2+ . 0 05916 [ 5 MnO 4 ][ Mn ]
2+
-
3+
-
E eq = - log
6 6 [ 5 Mn ][ MnO 4 ][ H 3 O ]
+ 8
2+
-
°
°
E Fe / Fe 2+ +5 E MnO 4 / Mn 2+ . 0 05916 1
3+
-
= - log
+ 8
6 6 [ HO ]
3
°
°
E Fe / Fe 2+ +5 E MnO 4 / Mn 2+ ( ( .05916 )( )
0
8
3+
-
= + log[HO + ]
3
6 6
°
E Fe 3+ /Fe 2+ + 5E ° - 2+
= MnO 4 /Mn - . 0 07888pH
6
For this titration the electrochemical potential at the equivalence point consists
of two terms. The first term is a weighted average of the standard state or
formal potentials for the analyte and titrant, in which the weighting factors are
the number of electrons in their respective redox half-reactions. The second
term shows that E eq is pH-dependent. Figure 9.36 shows a typical titration
–
curve for the analysis of Fe 2+ by titration with MnO 4 , showing the
asymmetrical equivalence point. Note that the change in potential near the
equivalence point is sharp enough that selecting an end point near the middle
of the titration curve’s sharply rising portion does not introduce a significant
titration error.
1.6
1.4
E 1.2
1
0.8
Figure 9.36
–
Titration curve for Fe 2+ with MnO 4 in 1 M 0.6
H 2 SO 4 ; equivalence point is shown by the 0 20 40 60 80 100
symbol . Volume of MnO 4 –
Finding the End Point with a Visual Indicator Three types of visual indicators
are used to signal the end point in a redox titration. A few titrants, such as
–
MnO 4 , have oxidized and reduced forms whose colors in solution are signifi-
–
cantly different. Solutions of MnO 4 are intensely purple. In acidic solutions,
2+ –
however, permanganate’s reduced form, Mn , is nearly colorless. When MnO 4
is used as an oxidizing titrant, the solution remains colorless until the first drop
–
of excess MnO 4 is added. The first permanent tinge of purple signals the end
point.
A few substances indicate the presence of a specific oxidized or reduced species.
–
Starch, for example, forms a dark blue complex with I 3 and can be used to signal
–
the presence of excess I 3 (color change: colorless to blue), or the completion of a
–
reaction in which I 3 is consumed (color change: blue to colorless). Another exam-
ple of a specific indicator is thiocyanate, which forms a soluble red-colored com-
2+
3+
plex, Fe(SCN) , with Fe .