Download Potentiometry Common types of electrochemical measurements: 1

Potentiometry
Common types of electrochemical measurements:
1. Potentiometry: Measurement of a potential
(voltage) at an electrode (relative to some reference)
in the absence of current flow.
H ig h im p e d a n ce v o ltm e te r
R e fe re n c e
In d ic a to r
G a lv a n ic (o r v o ltaic ) c ell
2. Amperometry: Measurement of a limiting
current at a constant potential.
Fixed potential
+
Low impedance current meter
-
Reference
Indicator
Electrolytic cell
3. Coulometry: Measurement of the quantity of
electrical charge needed to convert an analyte from
one oxidation state to another.
Fixed potential
Coulometer
?Idt=Charge (Q)
+
-
Reference
Indicator
Electrolytic cell
4. Voltammetry: Measurement of current as a
function of applied potential.
Variable potential
+
Reference
Low impedance current meter
-
Indicator
1. Potentiometry: Measurement of a potential
(voltage) at an electrode (relative to some reference)
in the absence of current flow.
H ig h im p e d a n ce v o ltm e te r
R e fe re n c e
In d ic a to r
G a lv a n ic (o r v o ltaic ) c ell
reference electrode|salt bridge|analyte solution|indicator electrode
Direct potentiometry involves the direct measurement
of a voltage generated at an electrode in solution
relative to a potential of a reference electrode.
Potentiometry is used to measure concentrations of
specific anions and cations, sometimes in conjunction
with a titration…
Ecell = EInd – Eref + EJ
E Ind = L −
0.0592
0.0592
pX = L +
log[X ]
n
n
for metal indicator electrodes, L is usually E°, while
for membrane electrodes, L is a collection of
constants
(
E
pX = − log[ X ] = −
pX = − log[ X ] = −
pA = − log[A] =
cell
− (E J − Eref + L ))
0.0592 / n
(Ecell − K ) = − n(Ecell − K )
0.0592 / n
0.0592
(Ecell − K ) = n(Ecell − K )
0.0592 / n
0.0592
for cations
for anions
Potentiometric titrations: e.g. acid/base with a pH
electrode
or a redox titration
Ecell = Eind - Eref + Ej
Reference electrodes:
All Eº are reported relative to the Standard Hydrogen
Electrode (SHE). However, this electrode is often
inconvenient to use. Common alternatives:
The saturated calomel electrode (SCE):
Hg|Hg2Cl2(sat’d), KCl (x M)|| E°=0.2444 V @ 25 °C
Silver/silver chloride reference electrode:
Ag|AgCl(sat’d), KCl (sat’d)|| E°=0.199 V @ 25 °C
The liquid junction potential, Ej:
A potential is developed across a boundary between
electrolyte solutions of different composition:
KCl is usually used in salt bridges because the
mobilities of K+ and Cl- are similar. Therefore, only
very modest liquid junction potentials are developed
at the interfaces between the salt bridge and the other
solutions.
Indicator electrodes:
Metal indicators electrodes:
1. Pure metal electrode that is in equilibrium
with its cation in solution
Xn+(aq) + ne-
X(s)
such that,
EInd = E X° n+ / X −
[ ]
0.0592
1
0.0592
log n + = E Xo n+ / X +
log X n +
n
X
n
EInd = E Xo n+ / X +
[ ]
[ ]
0.0592
0.0592
pX
log X n + = E Xo n+ / X −
n
n
2. Pure metal electrode that responds to anions
that form sparingly soluble precipitates or stable
complexes with the electrode cations.
e.g., AgCl(s) + e°
E Ind = E AgCl
/ Ag −
Ag(s) + Cl-(aq) E°=0.222 V
[ ]
0.0592
o
log Cl − = E AgCl
/ Ag + 0.0592 pCl
n
3. Inert metallic electrodes: Inert conductors that
themselves do not engage in electrochemical
reactions under the conditions in which a redox
reaction of interest occurs.
e.g. platinum, gold, palladium, carbon
Membrane electrodes:
Example: the glass pH electrode
The glass membrane permits H+(aq) to exchange with
Na+ in the silicate structure:
H+(aq) + Na+Gl-
Na+(aq) + H+Gl-
The boundary potential arises from the equilibria
established at the interior and exterior surfaces of the
glass electrode:
H+Gl1(s)
H+(aq) + Gl1-(s)
where surface 1 is between the exterior glass and the
analyte solution
H+Gl2(s)
H+(aq) + Gl2-(s)
where surface 2 is between the interior glass and the
internal solution
Both surfaces develop a negative charge but the net
charge at each surface is dependent upon the pH of
the analyte solution at the exterior of the glass and
the pH of the interior solution at the interior of the
glass.
Even when the pH values of the solutions on either
side of the glass are equal, a small potential, referred
to as the Asymmetry Potential, is observed. (Arises
due to the fact that the two surfaces are not identical.)
Due to the existence of the asymmetry potential, pH
electrodes must be calibrated frequently.
SCE||[H+]analyte|Glass membrane|[H+]reference, [1 M Cl-], AgCl|Ag
ESCE, EJ
E1
E2
EAg,AgCl
SCE = reference electrode 1
Ag,AgCl = reference electrode 2
glass membrane + reference electrode 2 = indicator electrode
Changes between EInd and ESCE arise from changes in [H+].
[H ]
= 0.0592 log
[H ]
+
Eb = E1 − E2
analyte
+
reference
[
H ]
= 0.0592 log
[H ]
+
Eb = E1 − E2
analyte
+
reference
[H+]reference is constant, so
[ ]
Eb = L′ + 0.0592 log H +
analyte
= L′ − 0.0592 pH
where L’ = -0.0592log[H+]reference
Potential of the glass electrode includes the small
asymmetry potential, Easy,
EInd = Eb + EAg/AgCl + Easy
EInd = L – 0.0592pH
where L = L’ + EAg/AgCl + Easy
L cannot be determined theoretically because Easy is
unknown…
Therefore, use of calibrations are essential for pH
measurements with a pH electrode.
NIST as well as international scientific organizations
have established an operation definition of pH. One
or more NIST buffers are used to calibrate the pH
electrode at various specified pH values.
The meter reading Es is adjusted in accord with the
pHs value (pH of standard). The glass electrode in
the unknown solution gives a potential, Eu, and the
pH of the unknown solution (pHu) is:
pHu = pHs – (Eu-Es)/0.0592
Note that the pH electrode is inaccurate at low and high pH
A variety of other ion-selective electrodes are in use:
Liquid-membrane electrodes:
polyvalent cations, some anions
Crystalline-based membranes: mostly for anions
Ion-sensitive field effect transistors:
solid state semiconductor electrodes for various ions
Gas permeable membranes for determination of
dissolved gases (e.g., O2 and CO2 in blood):