J . S o p o u š e k E x e r c i s e
/53/
8d
1.a. Verification of Nernst`s equation for Ce+3
/Ce+4
redox system
REDOX ELECTRODE. The main part of the redox electrode is platinum platelet. The
potential of electrode occurs if immersed in the solution with reduction oxidation
system (for example mixture of Ce+3
and Ce+4
cations). The established redox potential
is controlled by the Nernst`s equation:
𝐸𝐸𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = 𝐸𝐸𝐶𝐶𝐶𝐶+3 𝐶𝐶𝐶𝐶+4⁄
0
−
𝑅𝑅𝑅𝑅
𝑛𝑛𝑛𝑛
𝑙𝑙𝑙𝑙
𝑎𝑎𝐶𝐶𝐶𝐶+3
𝑎𝑎𝐶𝐶𝐶𝐶+4
≅ 𝐸𝐸𝐶𝐶𝐶𝐶+3 𝐶𝐶𝐶𝐶+4⁄
0
− 0,059 𝑙𝑙𝑙𝑙𝑙𝑙
�𝐶𝐶𝐶𝐶+3�
[𝐶𝐶𝐶𝐶+4]
(1.1.)
where 𝐸𝐸𝐶𝐶𝐶𝐶+3 𝐶𝐶𝐶𝐶+4⁄
0
is standard redox potential of the 𝐶𝐶𝐶𝐶+3
𝐶𝐶𝐶𝐶+4⁄ system, 𝑅𝑅 is Gas
constant, 𝐹𝐹 is Faraday`s constant, 𝑛𝑛 is the number of transmitted electrons, 𝑎𝑎 𝑀𝑀+ and
[𝑀𝑀+] are activities and molarities of 𝐶𝐶𝐶𝐶+3
or 𝐶𝐶𝐶𝐶+3
cation respectively. The value 0.059 in
equation (1.1) is the Nernst's electrode response in Volts at 𝑇𝑇 = 298𝐾𝐾 (compare
response for ISE).
The redox potential is measured by a combined redox electrode that contains the redox
electrode and the reference electrode in one unit. The eqn 𝐸𝐸𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = 𝐸𝐸𝐸𝐸𝐸𝐸 + 𝐸𝐸𝑟𝑟𝑟𝑟𝑟𝑟 is valid,
where 𝐸𝐸𝑟𝑟𝑟𝑟𝑟𝑟 is constant potential of reference electrode and 𝐸𝐸𝐸𝐸𝐸𝐸 is the electro motoric
voltage (EMV) of the combined electrode.
TASK: Verify the Nernst`s equation for the 𝐶𝐶𝐶𝐶+3
𝐶𝐶𝐶𝐶+4⁄ system. Evaluate the
experimental Nernst`s response of the redox electrode and compare it with a
theoretical value of 59 𝑚𝑚𝑚𝑚. Determine the [𝐶𝐶𝐶𝐶+3] [𝐶𝐶𝐶𝐶+4]⁄ ratio in the unknown samples
(e.g., in the Belousov-Zhabotinsky oscillating system).
LABORATORY AIDS AND CHEMICALS: combined Pt-redox electrode, potentiometer,
electromagnetic stirrer, 2 beakers (100 cm3
), 3 scale glass pipettes (25, 10
a 5 cm3
), 10 volumetric flasks (50 cm3
), storage solution for redox electrode (5·10-2
M
KCl or saturated KCl. Stock solutions: 0.006M Ce(SO4)2 in 1.5 M H2SO4, 0.006M
Ce2(SO4)3 in 1.5 M H2SO4.
INSTRUCTIONS: Get acquainted with the use of the potentiometer in mode mV
reading.
MEASUREMENT OF EMV FOR STANDARD SOLUTIONS. Pipette the 50ml of the 0.006M Ce3+
stock solution into beaker. Add the 0.006M Ce4+
stock solution in volume 0.5 ml and
measure the electro motoric voltage (EMV) using combined redox electrode after
stabilisation. Into the same solution, pipette gradually the 0.006M Ce4+
stock solution in
volumes of 2.0, 2.5, 20 and 25 ml. Measure the EMV after each addition.
MEASUREMENT OF UNKNOWN SOLUTION. Measure the EMV of combined redox electrode of
the system with unknown [𝐶𝐶𝐶𝐶+3] [𝐶𝐶𝐶𝐶+4]⁄ ratio. Alternatively, record the EMV in the
Belousov-Zhabotinsky oscillating system.
REPORT: TABLE 1: for 0.006M Ce3+
stock solution and all other prepared standard
solutions: volume of 0.006M Ce4+.
adition, total volume, concentrations of 𝐶𝐶𝐶𝐶+3
and 𝐶𝐶𝐶𝐶+3
, value log([𝐶𝐶𝐶𝐶+3] [𝐶𝐶𝐶𝐶+4]⁄ ), experimental EMV. Graph 1: Dependence of
𝐸𝐸𝐸𝐸𝐸𝐸 = 𝐸𝐸𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 − 𝐸𝐸𝑟𝑟𝑟𝑟𝑟𝑟 on value log([𝐶𝐶𝐶𝐶+3] [𝐶𝐶𝐶𝐶+4]⁄ ). NEXT: experimental Nernst`s
response, [𝐶𝐶𝐶𝐶+3] [𝐶𝐶𝐶𝐶+4]⁄ ratio of unknown sample. Alternatively, the [𝐶𝐶𝐶𝐶+3] [𝐶𝐶𝐶𝐶+4]⁄
ratios at minimum and maximum of the EMV in the Belousov-Zhabotinsky oscillating
system.
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