REDOX REACTIONS Flashcards
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
_______ tend to decompose to give sulfur and
hydrogen sulfite ion: S2O3
2- + 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.
Thoisulfate
Stability of Sodium Thiosulfate Solutions
Brown-yellow
Negative test (starch absent)
Blue/purple
Positive test (starch present)
Most iodine titration are performed with a suspension of
Starch as an indicator
Variables that influence the rate of thiosulfate reaction include
- pH
- presence of microorganisms
- concentration of the solution
- presence of copper (II)
- exposure to sunlight
• _____ is an excellent primary standard for thiosulfate
solutions. The reaction for the standard mixture in a strong acid is
IO3
- + 5I- + 6H+ 3I2 + 2H2O
• The liberated iodine is then titrated with the thiosulfate solution.
The stoichiometry of the reaction is
1 mol IO3- = 3 mol I2 = 6 mol S2O3²–
Potassium iodate
Standardization of Thiosulfate solutions
A solution of sodium thiosulfate was standardized by dissolving
0.1210 g KIO3
(214.00 g/mol) in water, adding a large excess of KI,
and acidifying with HCl. The liberated iodine required 41.64 mL of
the thiosulfate solution to decolorize the blue starch/iodine
complex. Calculate the molarity of Na2S2O3.
IO3
- + 5I
- + 6H+ 3I2 + 2H2O
1 mol IO3
- = 3 mol I2 = 6 mol S2O3
Ans: 0.08147 M
COMMON OXIDANTS USED AS
STANDARD SOLUTIONS
a. The strong Oxidants – Potassium Permanganate and Cerium(IV)
The choice among the two depends upon
• the strength of the analyte as a reducing agent,
• the rate of reaction between oxidant and analye,
• the stability of the standard oxidant solutions,
• the cost
• the availability of indicator.
The intense purple color of potassium permanganate solution is
sufficient to serve as an indicator for most titrations. The
permanganate ions react slowly with the large conc. of
manganese(II) ions present at the ____
End points
Solutions of Cerium(IV) are yellow-orange but the color is not
intense enough to act as an indicator in titrations. The most
widely used indicator is the ______(2) complex of 1,10-
phenanthroline or one of its derivatives.0
End points
(2) iron(II)
__________(1) when prepared properly, are stable
because decomposition reaction is slow. It is catalyzed by light,
heat, acids, bases, manganese(II) and manganese dioxide.
Standardized permanganate solutions should be stored in the dark.
Preparation and Stability of Standard Solution
(1)Permanganate solutions
Describe how would you prepare 2.0 L of an approximately 0.010 M
solution of KMnO4
(158.03 g/mol).
PRACTICE EXERCISE 1 Determine mass of
titrant/solution
Ans: 3.16 g KMnO4
PRIMARY STANDARDS
Name Formula
Cerium(IV) ammonium nitrate-Ce(NO3)4•2NH4NO3
Cerium(IV) ammonium sulfate- Ce(SO4)2•2(NH4 )2SO4•2H2O
Cerium(IV) hydroxide- Ce(OH)4
Cerium(IV) hydrogen sulfate
- Ce(HSO4)4
Primary Standards for KMnO4 and Ce4+
a. Sodium Oxalate
used to standardize permanganate and
cerium(IV) solutions. In acidic solution, the oxalate is converted to
the undissociated acid.
2MnO4
- + 5H2C2O4 + 6H+ 2Mn2+ + 10CO2(g) + 8H2O
The same oxidation products are formed with Cerium(IV).
Sodium Oxalate
You wish to standardize the 0.010 M KMnO4
solution against primary
standard sodium oxalate (134.00 g/mol). If you want to use between
30 and 45 mL of the reagent for the standardization, what range of
masses of primary standard should you weigh?
Ans: 0.10 - 0.15 g primary standard
Standardization:
calculate the concentration
Exactly 33.31 mL of the KMnO4
solution were required to titrate a
0.1278 g sample of primary standard Na2C2O4
. (134.00 g/mol) What
was the molarity and normality of KMnO4
reagent.
Ans: 0.01145 M
2MnO4
- + 5C2O4
2- + 16H+ 2Mn2+ + 10CO2(g) + 8H2O
• dichromate ion is reduced to green chromium(III) ion.
Cr2O7
2- + 14H+ + 6e- 2Cr3+ + 7H2O E° = 1.33V
• carried out in solutions that are 1 M in HCl or H2SO4
where E° is 1.0 to 1.1 V.
• indefinitely stable , can be boiled w/o decomposition and do
not react with HCl.
• primary standard reagent is available commercially and at
modest cost.
• the disadvantage of ________over cerium(IV)
and ________(2) is its lower electrode potential and the
slowness of its reaction with certain reducing agents.
Potassium Dichromate
(2) permanganate ion
• reagent of ________(1) is pure
• it has an orange color and not intense for use as an end point
detection
• ________(2) is an excellent indicator for this titration. The
oxidized form of the indicator is ____(3), and the reduced form is
essentially _____(4); thus the color change observed at the end point in
a direct titration is from _____(5) of chromium(III) to violet.
The Preparation, Properties and Uses of Dichromate Solutions
(1) potassium dichromate
(2)diphenylamine sulfunic
(3)violet
(4) colorless
(5) green
• The principal use of dichromate is for the volumetric titration of iron(II)
based upon the reaction
Cr2O7
2- + 6Fe2+ + 4H+ 2Cr3+ + 6FE3+ + 7H2O
• titration in the presence of moderate conc. of HCl.
• the reaction of dichromate has been widely used for the indirect
determination of a variety of oxidizing agents. This method has been
applied to the determination of nitrate, chlorate, permanganate, and
dichromate ions as well as organic peroxides and several oxidizing
agents.
Applications of Potassium Dichromate Solutions
% Purity with back-titration
A 5.00 mL sample of brandy was diluted to 1.00 L in a volumetric
flask. The ethanol (C2H5OH) in 25.00 mL aliquot of the diluted solution
was distilled into 50.00 mL of 0.0200 M K2Cr2O7 and oxidized to
acetic acid with heating. The reaction is
3C2H5OH +2Cr2O7
2- +6H+ 4Cr3+ +3CH3COOH +11H2O
After cooling, 20.00 mL of 0.1253 M Fe2+ were pipetted into the flask.
The excess Fe2+ was then tritated with 7.46 mL of the standard
K2Cr2O7
to a diphenylamine sulfonic end point.
Cr2O7
2- + 6Fe2+ + 4H+ 2Cr3+ + 6FE3+ + 7H2O
Calculate the percent (w/v) C2H5OH (46.07 g/mol) in the brandy.
Ans: 40.4 % (w/v) C2H5OH
• solutions are weak oxidizing agents that are used for
the determination of strong reductants.
I3
- + 2e- 3I- E° = 0.536 V
• advantageous of the use of solutions although it has low
potential is that it imparts a degree of selectivity that makes
possible the determination of strong reducing agents in the
presence of weak ones.
• an important advantage of this solution is the availability of sensitive
or reversible indicator for the titrations.
• solutions lack stability and must be restandardized
regularly.
Iodine
• not very soluble in water; it is ordinarily dissolved in moderately
concentrated solutions of KI and the reaction is
I2(s) + I
-
I3
- K=7.1 x 102
• the volatility of ______ makes the ______ lack
stability; iodine slowly attacks most organic materials; cork and
rubber stoppers are never used to close containers of reagent.
• air oxidationof iodide ions can cause changes in the molarity of
an _____:
4I
- + O2(g) + 4H+ 2I
- + 2H2O
• this reaction causes molarity of iodine solution to increase. Air
oxidation is promoted by acids, heat and light.
The Preparation and Properties of Iodine Solutions.
• can be standardized against anhydrous sodium thiosulfate or barium
thiosulfate monohydrate, both which are available commercially.
The Standardization and Application of Iodine Solutions
SOME SPECIALIZED OXIDANTS
- Potassium bromate
- Periodic acid
- Karl Fischer reagent
• used for the determination of organic compounds that contain
oleffinic and certain type of aromatic functional groups
Potassium bromate
• reacts selectively with organic compounds having hydroxyl,
carbonyl, or amine groups on adjacent carbon atoms.
Periodic acid
• widely employed for the determination of water in a variety of
organic or inorganic samples.
Karl Fischer reagent
Karl Fischer reagents
a. Potassium Bromate as a Source of Bromine
b. Peiodic Acid
• available for commercial uses and can be used to prepare
standard solutions that are stable indefinitely.
• widely used as a stable source of bromine
• standard bromine solutions lack stability
BrO3
- + 5Br + 6H+ 3Br2 + 3 H2O
• an excess potassium iodide is introduced to determine the
excess bromine, the reaction occurring is
2I- + Br2 2Br- + I2
• the liberated iodine is then titrated with standard sodium
thiosulfate.
• bromine is incorporated into an organic molecule either by
substitution or by addition.
Potassium Bromate as a Source of Bromine
Indirect method
% Purity with back-titration
A 0.2981 g of an antibiotic powder containing sulfanilide was
dissolved in HCl and the solution diluted to 100.0 mL. A 20.00 mL
aliquot was transferred to the flask, and followed by 25.00 mL of
0.01767 M KBrO3
. An excess of KBr was added to for m Br2
, and the
flask was stoppered. After 10 minutes, during which tome the Br2
brominated the sulfanilamide, an excess of KI was added. The
liberated iodine was titrated with 12.92 mL of 0.1215 M sodium
thiosulfate. The reactions are
BrO3
- + 5Br- + 6H+ → 3Br2 + 3H2O
sulfanilamaide + 2Br2 → sulfanilamide derivative + 2H+ + 2Br-
Br2 + 2I
- → 2Br- + I2
(excess KI)
I2 + 2S2O3
2- → S4O6
2- + 2I
-
Calculated the percent sulfanilamide (172.21 g/mol) in the powder.
Ans: 78.04 % sulfanilamide
the reaction of ______to iodate ion is best described by
the half-reaction
H5
IO6 + H+ + 2e- IO3
- + 3H2O E˚ = 1.6 V
Periodic Acid
•_______ is available for the preparation of standard solutions;
it is a crustalline, readily soluble, hygroscopic solid.
• another is _____
, which is soluble in water,
_________
, is not siufficiently soluble but converted
to soluble metaperiodate by recrystallization from concentrated nitric
acid.
• ______ vary in stability, depending on their mode of
preparation.
In what preparation is this?
Paraperiodic acid
sodium metaperiodate, NaIO4
sodium paraperiodate, Na5
IO6
Periodate solutions
The Preparation and Properties of Periodic Acid Solutions
• can be standardized by buffering aliquots of the reagent with
standard borax or sodium hydrogen carbonate to ensure they
remain slightly alkaline. An excess iodide is introduced in the
formation of a mol of iodine for each mole of periodate
H4
IO6
- + 2I
-
IO3 + I2 + OH- + H2O
The Standardization of Periodate Solutions
• it is widely used because it reacts remarkably selectively with
organic compounds containing certain combinations of
functional groups.
• ________ are usually carried out in aqueos solutions,
although solvents such as ____, _____or _____ may be
added to enhance the solubility of the sample.
Applications of Periodic Acid
periodate oxidations
Methanol, ethanol or dioxane
• relatively specific for water
Karl Fischer Reagent for Water Determination
• composed iodine, sulfur dioxide, pyridine and methanol.
• stoichiometry involves the consumption of 1 mol of iodine, a mol
of sulfur dioxide, and 3 mol of pyridine for each mole of water.
The Reaction and Stoichiometry
• ______ decomposes on standing that’s why it is
common practice to prepare the reagent a day or two before it is
to be used.
• its strength must be established at least daily against a standard
solution of water in methanol.
• keep atmospheric pressure from containing the reagent
and the sample.
• all glassware must be carefully dried before use and the standard
solution must be stored out of contact with air.
• to minimize the contact between the atmosphere and the solution
during titration.
Properties of Fischer Reagent
• signaled be the appearance of the first excess of pyridine/iodine
complex when all water has been consumed.
• the color of the reagent is intense enough for the visual end-point;
the change is from the yellow of the reaction products to the brown
of the excess reagent.
End-Point Detection
• applied to the determination of water in numerous samples.
• several variations of the basic technique are applied depending
upon the solubility of the material, the state in which the water is
retained, and the physical state of the sample.
• if the sample can be completely dissolved in methanol, a direct
and rapid titration is usually feasible. This method has been applied
to the determination of water in many organic acids, alcohols,
esters, ethers, anhydrides and halides. The hydrated salt of most
organic acids as well as the hydrates of a number of inorganic salts
that are soluble in methanol can be also determined by direct
titration.
Applications of KARL FISCHER REAGENT
