REDOX REACTIONS Flashcards
(65 cards)
is based on an oxidation-reduction
reaction between the analyte and titrant. These most
commonly use a (2)____ or a redox indicator to
determine the endpoint. Frequently either the reactants
or the titrant have a color intense enough that an
additional indicator is not needed.
Redox titration
(2) potentiometer
The vertical axis in oxidation/reduction titration curves is
generally an electrode potential instead of logarithmic
functions.
Redox Titration Curves
At the_______, the conc. cannot be obtained from
the stoichiometric reaction. Fortunately, are readily obtained by taking advantage of the fact
that the two reactant species have known conc. ratios at
chemical equivalence.
Equivalence Point Potentials (p. 344 Skoog)
Derivation of Redox Titration Curves(4)
a. initial potential
b. potential after initial addition
c. equivalence-point potential
d. potential after the excess addition
substances that change color upon beig oxidized or reduced. The color
change is independent of the chemical nature of the analyte and
titrant and depend instead upon the changes in the electrode
potential of the system that occur as the titration progresses.
General Redox Indicator
General Redox Indicator (3)
a. Iron(II) Complexes of Orthophenanthrolines
b. Diphenylamine and its Derivatives
c. Iodine Solutions
known as phenanthroline that form stable complexes with iron(II) and
certain other ions.
Iron(II) Complexes of Orthophenanthrolines
widely used in oxiddddation/reduction reactions involving iodine as an
oxidant or iodide ion as the reductant.
Iodine Solutions
a. Starch indicator
Specific Indicator
the best known specific indicator which forms
a dark blue complex with triiodide ion.
Starch indicator
➢the analyte in an oxidation/reduction titration must be
in a single oxidation state at the outset, however, steps
that precede the titration frequently convert the
analyte to a mixture of oxidation state.
Auxilliary Reducing and Oxidizing Agents
a number of metals are good reducing agents and
have been used for preredction of analytes included
among these are Zn, Al, Cd, Pb, Ni, Cu, and Ag (in the
presence of chloride ion).
Auxilliary Reducing Reagents
sticks or coils of the metals can be immersed directly
in the analyte solution. After reduction is judged
complete, the solid is removed manually and rinsed
with water. It is necessary to filter the solution to
remove granular or powdered forms of the metal. An
alternative to filtration is the use of reductor.
Auxilliary Reducing Reagents
Auxilliary Reducing and Oxidizing Agents
a. Auxilliary Reducing Reagents
b. Auxilliary Oxidizing Reagents
Auxilliary Oxidizing Reagents(3)
- Sodium Bismuthate (NaBiO3
) - Ammonium Peroxydisulfate
- Sodium Peroxide and Hydrogen Peroxide
• powerful oxidizing agent capable of converting
manganese(II) quantitatively to permanganate ion.
• a sparingly soluble salt but its exact composition is uncertain.
• oxidations are performed by suspending the bismuthate in
the analyte solution and boiling for a brief period.
• the half-reaction for the reduction of sodium bismuthate can
be written as
NaBiO3(s) + 4H+ + 2e- BiO+ + Na+ + 2 H2O
Sodium Bismuthate (NaBiO3
)
• powerful oxidizing agent that converts Cr(III) to dichromate,
Ce(III) to Ce(IV), and Mn(II) to permanganate in acidic solutions.
• the half-reaction is
S2O8
2- + 2e- 2SO4
2- E°=2.01 V
• the oxidations are catalyzed by traces of silver ion.
• the excess reagent is readily decomposed by a brief period of
boiling:
2S2O8 + 2H2O 4SO4
2- + O2(g) + 4 H+
Ammonium Peroxydisulfate
• a convenient oxidizing agent either as a solid sodium salt or
as a dilute solution of the acid.
• the half-reaction for hydrogen peroxide in acidic sol’n
H2O2 + 2H+ + 2e- 2H2O E°=1.78 V
• after oxidation is complete, the solution is freed of excess
reagent by boiling:
2H2O2 2H2O + O2(g)
Sodium Peroxide and Hydrogen Peroxide
Upon addition of
the titrant, analyte
is reduced
Analyte must be in
the oxidized state;
sample solution is
added with auxilliary
oxidizing agent; all
Fe in Fe3+ state
Titrant:
Strong Reductant
Upon addition of
the titrant, analyte
is oxidized
Analyte must be in
the reduced state;
sample solution is
added with auxilliary
reducing agent; all
Fe in Fe2+ state
Titrant:
Strong Oxidant
➢Standard solutions of most reducing agents tend to react with
atmospheric oxygen. For this reason, reductants are seldom
used for the direct titrations of oxidizing analytes; indirect
methods are used.
Application of Standard Reductants
The two most common indirect methods are based upon: Application of Standard Reductants
a. Iron(II) Solutions
b. Sodium Thiosulfate
• readily prepared from iron(II) ammonium sulfate (Mohr’s salt) or
form closely related iron(II) ethylenediamine sulfate (Oesper’s
salt)
• air oxidation of iron(II) takes place rapidly in neutral solutions but
is inhibited in the presence of acid with the most stable
preparations beig 0.5 M in H2SO4
Iron(II) Solutions
• moderately strong reducing agent that has been widely used to
determine oxidizing agents by indirect procedure that involves
iodine as an intermediate.
• with iodine, thiosulfate ion is oxidize quantitatively to
tetrathionate ion, the half-reaction being
2S2O3
<->2- S4O62- + 2e-
• the end point is the disappearance of the iodine color (for
colorless analyte)
• Example: Determination of Sodium hypochlorite in bleaches
Sodium Thiosulfate
