Page 9 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GJB Revised Pages
Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

548 Analytical Chemistry

TABLE II Common Oxidizing and Reducing Agents Em-
MY (n−4)+
K MY = , ployed as Standard Solutions in Oxidation–Reduction Titra-
[M ][Y 4− ] tions
n+
Note the analogy of this result with that outlined for Oxidizing Reduction
the effect of solubility product in precipitation titrations Reagent species Product
discussed above. A great many compounds have been
proposed as indicators for metal ions in EDTA titrations. Oxidizing
These species are generally organic compounds that form Potassium permanganate, KMnO 4 MnO − 4 Mn 2+
colored chelates with metal ions in a range of pM that Potassium bromate, KBrO 3 BrO − 3 Br −
is characteristic of the cation and dye. One example is Cerium ammonium nitrate, Ce 4+ Ce 3+
Ce(NO 3 ) 4 2NH 4 NO 3
Eriochrome black T, which is blue at pH 7 and red when 2− 3+
complexed with a variety of metal ions. Potassium dichromate, K 2 Cr 2 O 7 Cr 2 O 7 Cr −
−
EDTA titrations are still widely used because of their Potassium, iodate, KlO 3 IO 3 I
great versatility with respect to the analysis of a large num- Reducing Oxidation
ber of different metal cations. Furthermore, the technique species product
can be made more selective by adjusting the pH or by
Reducing
the use of compounds that effectively remove interfering 2− 2−
Sodium thiosulfate, Na 2 S 2 O 3 S 2 O 3 S 4 O 6
cations from the titration (masking agent). The method is
Ferrous ammonium sulfate, Fe 2+ Fe 3+
inexpensive and reasonably accurate. Fe(NH 4 ) 2 (SO 4 ) 2 ·6H 2 O
4. Oxidation–Reduction Titrations
derive
Here, we are dealing with the reaction of an oxidizing
(reducing) reagent as titrant with a reducing (oxidizing) E 0 3+ + 5E 0
Fe MnO − 4
species as unknown. The equivalent weight of a partici- E eq =
6
pant in this type of system is the weight that directly or
− 2+
indirectly produces or consumes 1 mol of electrons. Thus, 0.0591 5 MnO [Mn ]
4
− log .
−
+ 8
the equivalent weight for the permanganate ion (MnO ) 6 5[Mn ] MnO [H ]
2+
−
4 4
in its oxidation of oxalate anion in the reaction
Thus,
−
5C 2 O 2− + 2MnO + 16H + 0 0
4 4 E 3+ + 5E
Fe MnO − 4 0.0591 1
→ 10CO 2 + 2Mn 2+ + 8H 2 O E eq = 6 − 6 log [H ] .
+ 8
−
is the molecular weight of MnO divided by 5. In much the same manner as for the other types of
4
A great variety of both oxidizing and reducing agents titration described above, it is the electrode potential dur-
have been employed for this type of titration, and some ing the procedure that exhibits an abrupt change on ad-
commoncompoundsaregiveninTableII.Theequivalence dition of reagent. Accordingly, we require indicators that
point of oxidation–reduction titrations can be computed change color during this change (i.e., that show oxidation–
from a knowledge of solution concentrations and electri- reduction behavior themselves). Two examples are 1, 10-
cal potentials. For example, in the titration of ferrous ion phenanthroline–iron (II) complex, which changes from
against KMnO 4 according to the following reaction pale blue to red at an electrical potential of +1.11 V (in
1M H 2 SO 4 ), and diphenylaminesulfonic acid, which
5Fe 2+ + MnO + 8H → 5Fe 3+ + Mn 2+ + 4H 2 O,
−
+
4 changes from colorless to violet at 0.85 V.
we can write Nernst electrode potentials for each
oxidation–reduction system:
II. INSTRUMENTAL METHODS
[Fe ]
2+
0
E = E 3+ − 0.0591 log
Fe A. Absorption of Electromagnetic Radiation
[Fe ]
3+
2+
0.0591 [Mn ] Matter interacts with incident electromagnetic radiation
0
E = E − − log , by the three distinct processes of transmission, scattering,
MnO 4 5 MnO [H ]
+ 8
−
4
or absorption. The nature of any interaction is a function of
At the equivalence point we know that these two poten- the properties of the radiation, such as energy, phase, po-
tials, now called E eq , are equal; therefore, on adding we larization, and the chemical properties of the matter under