Redox Flashcards
Loss of electrons, oxidation number going up.
Oxidation
Gain of electrons, oxidation number going down.
Reduction
is based on an oxidation-reduction reaction between the analyte and titrant. These most commonly use a potentiometer 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
The vertical axis in oxidation/reduction titration curves is generally an electrode potential instead of logarithmic functions.
Redox Titration Curves
At the equivalence point, the conc. cannot be obtained from the stoichiometric reaction. Fortunately, equivalence-point potentials are readily obtained by taking advantage of the fact that the two reactant species have known conc. ratios at chemical equivalence.
Equivalence Point Potentials
Derivation of Redox Titration Curves
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
Types of General Redox Indicator
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
the best known specific indicator which forms a dark blue complex with triiodide ion.
Starch Indicator
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
= 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 Oxidizing Reagents
- 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
S2O82- + 2e- 2SO42- 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 4SO42- + 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
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
Standard Reductants:
The two most common indirect methods are based upon:
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
2S2O32- ⇔ S4O62- + 2e- - the end point is the disappearance of the iodine color (for
colorless analyte)
Sodium Thiosulfate
Thiosulfate tend to decompose to give sulfur and
hydrogen sulfite ion:
S2O32- + H+ ⇔ HSO3- + S(s)
provided the analyte solution is colorless, the disappearance of the
iodine color can serve as the indicator in titrations with sodium
thiosulfate.
Stability of Sodium Thiosulfate Solutions
