Transcript Redox titrations

CHEMISTRY 59-320
ANALYTICAL CHEMISTRY
Fall - 2010
Chapter 16: Redox titration
16-1 The shape of a redox titration
curve
• A redox titration is based on an oxidation-reduction
reaction between analyte and titrant.
• Consider the titration of iron(II)
with standard cerium(IV),
monitored potentiometrically
with Pt and calomel electrodes.
The potentials show above is in 1 M HClO4
solution. Note that equilibria 16-2 and 16-3
are both established at the Pt electrode.
•
There are three distinct regions
in the titration of iron(II) with
standard cerium(IV), monitored
potentiometrically with Pt and
calomel electrodes.
1.
2.
3.
Before the equivalence point,
where the potential at Pt is
dominated by the analyte redox
pair.
At the equivalence point, where the
potential at the indicator electrode
is the average of their conditional
potential.
After the equivalence point, where
the potential was determined by the
titratant redox pair.
Before the equivalence point: using analyte’s concentration to calculate E+
At the equivalence point: needs both redox pairs to calculate (why?)
E of the cell
After the equivalence point:
There are two special points during the above titration process: (1)
when V = ½ Ve, [Fe3+] = [Fe2+] and E+ = E°(Fe3+ | Fe2+) ; (2) when V =
2 Ve, [Ce4+] = [Ce3+] and E+ = E°(Ce4+ | Ce3+) = 1.70 V.
•
Summary
The greater the difference in reduction potential between analyze and
titrant, the sharper will be the end point.
•
The voltage at any point in this titration depends only on the ratio of
reactants; it will be independent of dilution.
•
Prior to the equivalence point, the half-reaction involving analyze is used to
find the voltage because the concentrations of both the oxidized and the
reduced forms of analyte are known.
•
After the equivalence point, the half-reaction involving titrant is employed. At
the equivalence point, both half-reactions are used simultaneously to find
the voltage.
For the titration of Tl+ (Thallium) by IO3- in 1.00 M HCl solution
• The curve is not symmetric
about the equivalence point.
This is because the
stoichiometry of reactants is
2:1, not 1:1.
• Still, the curve is so steep near
the equivalence point that
negligible error is introduced if
the center of the steep part is
taken as the end point.
16-2 Finding the end point
• A redox indicator is a
compound that changes
color when it goes from its
oxidized to its reduced
state.
or
For ferroin, with E° = 1.147 V
we expect the color change to
occur in the approximate range
1.088 V to 1.206 V with respect SHE
A redox titration is feasible if the difference between analyte and titrant
is > 0.2 V.
If the difference in the formal potential is > 0.4 V, then a redox
indicator usually gives a satisfactory end point.
Starch-Iodine Complex
•
•
•
Starch is the indicator of choice for those
procedures involving iodine because it forms an
intense blue complex with iodine. Starch is not a
redox indicator; it responds specifically to the
presence of I2, not to a change in redox
potential.
The active fraction of starch is amylose, a
polymer of the sugar α-d-glucose.
In the presence of starch, iodine forms I6 chains
inside the amylose helix and the color turns dark
blue
16-3 Adjustment of analyte
oxidation state
• Sometimes one needs to adjust the oxidation state of analyte before
it can be titrated.
• Pre-adjustment must be quantitative and one must eliminate excess
pre-adjustment reagent so that it won’t interference the subsequent
titration.
• Pre-oxidation: powerful oxidants that can be easily removed after
preoxidation include peroxydisulfate, silver(II) oxide, sodium
bismuthate.
Definition:
•
•
•
Disproportionation: a reactant oxidizes and reduces itself, such as H2O2 in
boiling water.
Pre-reduction: Process of reducing an analyte to a lower oxidation state
prior to performing a titration with an oxidizing agent.
Amalgam: a solution of anything in mercury.
16-4 Oxidation with potassium
permanganate
• KMnO4 is a strong oxidant with an intense
violet color. In strongly acidic solutions (pH
< 1), it is reduced to Mn2+.
• In neutral or alkaline solution, it is reduced
to brown solid MnO2.
• In strongly alkaline solution ( 2 M NaOH),
green manganate ion (MnO42-) is
produced.
16-5 Oxidation with Ce4+
• Reduction of Ce4+ to Ce3+ proceeds cleanly in acidic
solutions.
• The aquo ion, Ce(H20)n4+, probably does not exist in any
of these solutions, because Ce(IV) binds anions (ClO4-,
SO42- NO3-, Cl-) very strongly.
16-7 Methods Involving Iodine
• Iodimetry: a reducing analyte is titrated directly with
iodine (to produce I−).
• iodometry, an oxidizing analyte is added to excess I− to
produce iodine, which is then titrated with standard
thiosulfate solution.
• Iodine only dissolves slightly in water. Its solubility is
enhanced by interacting with I-
• A typical 0.05 M solution of I2 for titrations is prepared by
dissolving 0.12 mol of KI plus 0.05 mol of I2 in 1 L of
water. When we speak of using iodine as a titrant, we
almost always mean that we are using a solution of I2
plus excess I−.
Preparation and Standardization of Solutions
• Acidic solutions of I3- are unstable because the excess
I− is slowly oxidized by air:
• In neutral solutions, oxidation is insignificant in the
absence of heat, light, and metal ions. At pH ≳ 11,
triiodide disproportionates to hypoiodous acid (HOI),
iodate, and iodide.
• An excellent way to prepare standard I3- is to add a
weighed quantity of potassium iodate to a small excess
of KI. Then add excess strong acid (giving pH ≈ 1) to
produce I2 by quantitative reverse disproportionation:
Problems
• Would indigo tetrasulfonate be a suitable redox indicator
for the titration of Fe(CN)64- with Tl3+ in 1 M HCl?
• Solution:
Standard potentials: indigo tetrasulfonate is 0.36 V;
Fe(CN)63-/ Fe(CN)64- is 0.356 V;
Tl3+/Tl+, 0.77 V.
The end-point potential will be between 0.356 and
0.77 V. Indigo tetrasulfonate changes color near 0.36 V.
Therefore it will not be a useful indicator for this titration.
• Given the following data select the best indicator for the titration
of iron(II) with thallium(III), using a S.C.E. reference electrode.
(a) methylene blue
(b) diphenylamine sulfonic acid
(c) diphenylamine
• Which of the following titrations would give a titration curve
symmetric around the equivalence point?
(a) Na2S2O3 titrated with I2.
(b) ascorbic acid titrated with I2.
(c)As4O6 titrated with I2.