Transcript Electrochemistry MAE-212 Dr. Marc Madou, UCI, Winter 2015 Class V Potentiometric and Amperometric Sensors (I)

Electrochemistry MAE-212
Dr. Marc Madou, UCI, Winter 2015
Class V Potentiometric and Amperometric Sensors
(I)
Table of content
 Potentiometric Sensors
 Amperometric Sensors
Potentiometric Sensors
 Potentiometric techniques are the most widely used
electroanalytical method:
 Direct potentiometry – pH and ions (pH sensors and ion
selective probes)
 Indirect potentiometry: Enzyme sensors, Gas sensors
 Miniaturization of Potentiometric Sensors
Direct Potentiometric Sensors
 Best know example is the pH
sensor.
 Combination electrodes
(indicator+reference) for
convenience (tube within a tube)
 pH sensing component of the
indicator electrode is the glass bulb,
which is a thin glass membrane ~
0.03 – 0.1 mm thick
 When immersed, H+ ions from the
solution enter the Si-O lattice
structure of the glass membrane in
exchange for Na+
Inner tube:
pH indicator electrode
(pH sensing membrane, Ag/AgCl
reference electrode and HCl
Outer tube:
reference electrode (Ag/AgCl) and salt bridge (KCl)
Direct Potentiometric Sensors
 A traditional pH measurement with a glass electrode is the best
known potentiometric ion selective electrode (ISE) (e.g. a thin
glass layer with this composition 22% Na2O, 6% CaO, 72%
SiO2)
 There is no change in the inner solution and there is no actual
contact between inner and outer solution for any potentiometric
probe or sensor
 How to construct a combination electrode?
Direct Potentiometric pH Sensors
 The glass bulb creates an electric
‘boundary’ potential across the
membrane w.r.t. the internal
Ag/AgCl reference electrode. This
is called the Donnan potential:
E cell  constant+

RT
2.303RT
ln aH  constantpHunknown
+
F
F
H+
Where a = activity of H (=
concentration in very dilute
solutions). Slope factor
(2.303RT/F) is temperature
dependent, pH meter must be
adjusted for changes in temperature
 All modern pH meters record
potential (mV) and transform the
voltage caused by H+ into pH units
 Standard buffers (4.0, 7.0, 10.0)
are used for calibration
 Automatically recognize standard
buffers and adjust for temperature
Electrochemical Methods
Applications in Environmental Analysis
Direct Potentiometric pH Sensors
Direct Potentiometric Sensors
Measurement of Ions by Ion Selective Electrodes (ISEs)

Uses direct potentiometry to measure ion concentration
 Membrane responds selectively to a given ion
 mV reading between sensing and reference electrode
Direct Potentiometric Sensors
 There are many other types of
potentiometric ion sensors or ISE’s.
 The so-called Donnan potential is
established on both sides of any ion
selective membrane-the potential on
one side is kept constant through the
internal reference solution while the
other side is determined by the analyte
solution
 For other ions than protons (cations and
anions) other membranes are available
(see e.g. LaF3 for F- and a wide variety
of polymeric membranes
Direct Potentiometric Sensors
 An ion selective polymeric membrane is often
made by mixing an ionophore (e.g. valinomycin, a
natural occuring antibiotic) with PVC and a
plasticizer (to make the rigid plastic more
flexible)
 In these types of ISE’s one sometimes does not
use an internal reference solution at all or one
incorporates a hydrogel to replace the aqueous
solution . This makes the electrode easier to
handle and store. Especially with no internal
reference electrode drift tends to be larger!
 The polymeric ISE’s lend themselves well to
miniaturization and cost reduction (it is much
more difficult to miniaturize a glass pH
electrode)
Indirect Potentiometric Sensors: Enzyme
Base Potentiometric Sensor
 A potentiometric urea sensor may consist
of two pH sensors one with the enzyme
coated on its surface and one without
(the reference electrode)
 The electrode with the urease will sense
a local pH change
 The pH difference bewteen the two
electrodes is proportional to the urea
concentration
 As an example two IrOx electrodes may
be used
Indirect Potentiometric Sensors:
Carbon Dioxide Sensor
Ecell = E ind - Eref
(1)
As the indicator is only H+ sensitive, and the potential of the reference is a
constant (because of the constant chloride concentration in the electrolyte), we have
Ecell  K1  0.059log aH 
(at 25o C)
(2)
CO2 penetrates through the gas permeable membrane and will react with the
electrolyte in the agar hydrogel:
CO2 + H2 O = H+ + HCO3 -
(3)
a H   K' PCO2 aH 2 O / aH CO3 
(4)
As the activities for H2 O and HCO3 - are constant in the electrolyte, the
voltage of the sensor cell should be:
Ecell  K2  0.059log PCO2
(5)
Indirect Potentiometric Sensors: Carbon
Dioxide Sensor (3D)
silver spring contact
Gas permeable membrane
Ir/IrOx electrode
Ag/AgCl electrode
Dual lumen PVC tube
Hydrogel
epoxy
silver epoxy
Indirect Potentiometric Sensors: Carbon
dioxide sensor (MEMS version)
 A pH, CO2 and oxygen electrochemical sensor
array for in-vivo blood measurements was made
using MEMS techniques
 The pH and CO2 sensors are potentiometric
and the oxygen sensor is amperometric (see
further in this class)
 The pH sensor is an ISE based on a pH sensitive
polymer membrane.
 The CO2 sensor is based on an IrOx pH sensor
and a Ag/AgCl reference electrode. .
Miniaturization of Potentiometric Sensors
 By making ISE’s planar (e.g. on a
polyimide sheet) many sensors can
be made in parallel (i.e. batch
fabnrication). From 3D structures
to 2D !
 Mass production can make them
very small (e.g. 2 by 3 mm), cheap
(perhaps disposable), reproducible
and even electronics might be
integrated (see below under
ISFETs)
Miniaturziation of Potentiometric
Sensors
 Potentiometric sensors have
been made the size of a transistor
in ISFETs (almost).
Amperometric Sensors

Our first example of an amperometric sensors involves a "Fuel
cell" oxygen sensors consisting of a diffusion barrier, a sensing
electrode (cathode) made of a noble metal such as gold or silver,
and a working electrode made of a metal such as lead or zinc
immersed in a basic electrolyt (such as a solution of potassium
hydroxide).

Oxygen diffusing into the sensor is reduced to hydroxyl ions at
the cathode:
O2 + 2H2O + 4e- -------- 4 OHHydroxyl ions in turn oxidize the lead (or zinc) anode:
2Pb + 4OH- ------------- 2PbO + 2H2O + 4e2Pb + O2 ----------------- 2PbO

Fuel cell oxygen sensors are current generators. The amount of
current generated is proportional to the amount of oxygen
consumed (Faraday's Law).
Amperometric Sensors
 A second example of an amperometric sensors is
a simple (first generation) glucose sensor. This
sensor is based on the enzyme Glucose Oxidase
(GO).
 Enzymes are high-molecular weight biocatalysts
(proteins) that increase the rate of numerous
reactions critical to life itself
 Enzyme electrodes are devices in which the
analyte is either a substrate (also called reactant)
or a product of the enzyme reaction, detected
potentiometrically or amperometrically
 Here we consider an amperometric glucose
sensor where the substrate (glucose) diffuses
through a membrane to the enzyme layer where
glucose is converted and H2O2 is produced and
electrochemically detected.
Amperometric Sensors
 Amperometric glucose sensor
based on peroxide oxidation,
 The plateau of the limiting
current is proportional to the
peroxide concentration which in
turn is proportional to glucose - - typical 0.6 to 0.8 V vs Ag
cathode
 Glucose oxidase is an oxidase
type enzyme, urease is a
hydrolytic type enzyme. Other
sensors can be constructed
based on those enzymes.
Anodic
l
i
-
+
+ 0.6 V
-i
Cathodic
Urease
CO (NH2 )2  CO2 + 2 NH3
H2 O
+i
Amperometric Sensors
Measurement of Dissolved Oxygen

e.g. Polarographic Clark cell
Amperometric Sensors
Measurement of Dissolved Oxygen
e.g. Polarographic Clark cell
O2 + 2H2O + 4e- ⇌ 4OH-
(O2 reduced at gold cathode)
4Ag(s) + 4Cl-(aq) ⇌ 4AgCl(s) + 4e- (oxidation of silver at anode)

Membrane is susceptible to degradation, must be replaced if it dries out

Calibrated in air (O2), air saturated water (aerated water) or by Winkler method
Amperometric Sensors
Measurement of Dissolved Oxygen

Calibrate the probe (in air)

Place the probe below the surface of the water

Set the meter to measure temperature and allow the
temperature reading to stabilize

Switch the meter to 'dissolved oxygen‘

For saline waters, measure electrical conductivity level
or use correction feature

Re-test water to obtain a field replicate result
NOTE: The probe needs to be gently stirred to aid
water movement across the membrane