14-6 Cells as chemical probes. It is important to distinguish two types of equilibrium in a galvanic cell: equilibrium between the two half-cells and equilibrium within each half cell. A galvanic cell that can be used to measure the formation constant of Hg(EDTA) 2-
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A galvanic cell that can be
used to measure the formation
constant of Hg(EDTA)2-
Hg2+ + 2e-↔ Hg(l)
Eo = 0.852 V
(a) Graph of the function labeled formal potential.
(b) Experimental polarographic half-wave reduction potential of ascorbic acid in a medium of ionic strength = 0.2 M.
At high pH (>12), the half-wave potential does not level off to a slope of 0, as Equation 14-34 predicts. Instead, a hydrolysis reaction of ascorbic acid occurs and the chemistry is more complex.
Solution: Cd2+ + 2e Cd(s) 0 = -0.403
Br2(l) + 2e 2Br-E0 = 1.065V
1. When you multiply a reaction by a coefficient, the potential of
such a reaction does not change!
2. When you subtract or add reactions, the Gibbs energy of those reactions
can always be directly added or subtracted, not their potentials!
The electrode potential
E+ = 0.771 – 0.05916 log([Fe2+]/[Fe3+])
E- = 0.222 – 0.05916 log([Cl-])
E = E+ - E-
a reference electrode.
Silver-Silver Chloride Reference Electrode
Potential for the solution saturated with KCl (is not 0.222 V):
AgCl(s) + e-↔ Ag(s) + Cl- Eo = -0.197 V
Example: Use of Ag and calomel electrodes
to measure [Ag+].
Why do we need double junction
calomel reference electrode here?
Development of the junction potential
caused by unequal mobilities of Na+ and Cl−.
A 0.1 M NaCl solution was placed in contact with a 0.1 M NaNO3. Which side of the junction is positive?
A typical pH combination electrode, incorporating both glass and reference electrodes in one body.
Glass combination electrode with a silver-silver chloride reference electrode. The glass electrode is immersed in a solution of unknown pH so that the porous plug on the lower right is below the surface of the liquid. The two silver electrodes measure the voltage across the glass membrane.15-5 pH measurements with a Glass electrode
Most ion-selective electrodes fall into one of the following classes:
1. Glass membranes for H+ and certain monovalent cations
2. Solid-state electrodes based on inorganic salt crystals
3. Liquid-based electrodes using a hydrophobic polymer membrane saturated with a hydrophobic liquid ion exchanger
4. Compound electrodes with a species-selective electrode enclosed by a membrane that is able to separate that species from others or that generates the species in a chemical reaction.
Ion exchange between heparin and Cl− associated with tetraalkylammonium ions in the membrane of the ion-selective electrode.Ion-selective electrodes-example
Rotating ion-selective heparin electrode
superimposed in the same solution.
This unusual event occurred
in a reaction that had been oscillating
normally for about 30 min