Chapter 15
Electrodes and Potentiometry
p. 375 Harris (5th)
p. 314 Harris (6th)
Experiment 2.
Determination of Fluoride in Drinking Water by
Ion Selective Electrode Potentiometry.
The use of electrodes to measure potentials (E) and
voltages (V) that provide chemical information is called
potentiometry
Final drinking water levels set for fluoride
The maximum level of fluoride (F-) to be allowed in drinking water was
set at 2 mg/L.
This level is the best for drinking water as it protects teeth from decay
but apparently causes no harmful effects.
At 4 mg/L, dental fluorosis (a pitting or discoloration of teeth) can occur
in some persons. This is not considered an adverse health effect.
Long term exposure to fluoride at levels above 4 mg/L can lead to
changes in bone density.
According to new regulations, the 2 mg/L level requires local monitoring
of water supplies. Above 2 mg/L, local water authorities are required to
notify customers of the higher level.
Potentiometry Instrumentation
15-1 Reference Electrodes
p. 615 Harris (5th)
p. 315 Harris (6th)
15-2 Indicator Electrodes
p. 617 Harris (5th)
p. 317 Harris (6th)
Ion selective electrodes respond selectively to one ionic species
in solution
15-4 How Ion-Selective Electrodes Work
p. 320 Harris (6th)
Potentiometry Instrumentation
Resistance of the ion selective membrane is typically 100 MΩ (108 Ω), so the potential
measuring device should have an internal resistance of 1011 Ω or more.
The VWR SympHony SR40C meter allows measurement of ISE

1–5 Point Calibration

Reliable and Splash-Proof Keypad

Simple, Intuitive User-Interface
These pH/ISE meters offer the latest advances in electrochemistry measurement
techniques with a pH range of –2.000 to 19.999 and an ISE range of 0.0 to 19900.0.
The setup menu allows the user to select the parameters to be measured and
displayed, select their units of measurement, and choose between printing and
datalogging functions. A large, 3-line LCD display indicates the simultaneous
measurements being made. The meters log up to 200 data points for downloading to
a printer or computer via the RS-232 connection. Simple display prompts guide the
user through each measurement and ensure error-free readings. The meters feature
a “ready” indicator, a self-test, and a 110 V line adapter.
Electrode Types
Classes of ion-selective electrodes
glass electrodes (for measuring pH)
solid-state electrodes (for measuring F-)
liquid-based electrodes (for measuring Ca2+)
gas-sensing electrodes (for measuring CO2)
enzyme electrodes (for measuring urea)
Glass - Produces an increased affinity for various cations.
Solid State - Membrane is a single crystal.
p. 331-332 Harris (6th)
Table 15-5
Liquid - Ion Exchange - Layer of water immiscible liquid ion exchanger.
p. 333-334 Harris (6th)
Ammonia (NH3)
Ammonium (NH4+)
Bromide (Br-)
Cadmium (Cd2+)
Calcium (Ca2+)
Carbon Dioxide (CO2)
Chloride (Cl-)
Chlorine (Cl2)
Cupric (Cu2+)
Fluoride (F-)
Fluoroborate (BF4-)
Table 15-6
Iodide/Cyanide (I-/CN-)
Lead (PB2+)
Nitrate (NO3-)
Nitrite (NO2-)
Nitrogen Oxide (NOX)
Oxygen/BOD (O2)
Perchlorate (ClO4-)
Potassium (K+)
Silver/Sulfide (Ag+/S2-)
Sodium (Na+)
Surfactant (X+/X-)
Thiocyanate (SCN-)
Water Hardness (X2+)
Ion Selective Electrode for Fluoride
Electro-Chemical Devices
Yorba Linda, CA
http://www.ecdi.com/smbar.html
Combination configuration
is composed of two parts: measuring electrode and reference electrode
In-line mounting (side stream)
insertion / retraction without disturbing flow
VWR SympHony electrodes
Fluoride combination electrode
$800
Chloride combination electrode
$840
Silver/sulfide combination electrode
$825
Ammonia combination electrode
$680
Nitrate electrode (requires reference)
$780
Solid-State Electrodes
employ an inorganic salt crystal as the ion-sensitive membrane.
Membrane Potential
Ion selective electrodes (ISE) have a thin membrane separating the
sample from the inside of the electrode.
The membrane contains a ligand that can bind and transport the analyte
ions (but no other ions).
Figure 15-17
Migration of F- through LaF3 doped with EuF2
p. 331 Harris (6th)
p. 394 Harris (5th)
Fluoride ion from solution is selectively adsorbed on each surface of the
LaF3 crystal.
A F- ion from an adjacent site can jump into the vacancy, thus leaving a new
vacancy behind.
Repetition of this process move F- ions through the crystal lattice.
Migration of F- ions from one side to the other establishes a potential
difference across the crystal (membrane) necessary for the electrode to
work.
Fluoride electrode
internal (filling) solution --
0.1 M NaF + 0.1 M NaCl
Ecell = constant - 0.05916 log A1(F-)
Selectivity over other ions
Interfering species
--
OH-
--
>1000
Calibration curve for fluoride ion-selective electrode
p. 332 Harris (6th)
Figure 15-18
15-6
Using Ion-Selective Electrodes
p. 398 Harris (5th)
p. 331 Harris (6th)
Respond to the activity (a) of uncomplexed analyte ion:
a = γ[molar concentration]
where γ = activity coefficient
Standard Addition with Ion-Selective Electrodes
p. 336 Harris (6th)
In using ion-selective electrodes, it is important that the composition of
the standard solutions closely approximate the composition of the
unknown.
In cases where the unknown matrix is complex, the standard addition
method can be used.
Advantages of ion-selective electrodes
.
in-situ measurement; no sampling is required
.
non-destructive (do not destroy the sample)
.
respond to cationic, anionic and molecular species in solution and in the
gas phase
.
wide dynamic range of linear response (e.g., 10-5 - 1 M)
.
unaffected by colour or turbidity.
Ion selective electrode potentiometry reduces the analysis time when
compared to ion chromatography
It does not have to wait for the last peak to be eluted off the
column before a new sample can be injected for the next analysis
Selectivity Coefficient
p. 330 Harris (6th)
Interference
The greatest drawback to the use of ISEs is their response to ions other than the
analyte.
In the presence of an interfering ion, N, the potential is given by:
Response of ion-selective electrode
charge of analyte ion
charge of interferent ion
selectivity coefficient/constant
Eq. (15-5)
The smaller the value of kMN (or kXY), the more selective is the ISE for
analyte ion M (or X) in the presence of interferent ion N (or Y).
Thermodynamics: free energy difference ΔG between the two solutions is
ΔGm = - RT ln A1/A2
The potential difference across the membrane, or membrane potential Em, is given
by
ΔGm = -nFEm where F = Faraday constant = 9.648 5309 x 104 C
Em = - ΔGm / nF = RT/nF ln A1/A2
= 0.05916/n log A1/A2 volts
ln x = (ln 10)(log x)
at 25 C
o
The membrane potential, Em, is measured as part of an electrochemical cell which is
composed of two reference electrodes -- the internal and the external.
At equilibrium, the cell potential difference is measured by a high-impedance voltmeter:
Ecell = Eint. ref. + Em - Eext. ref. + Ejunction
= constant + Em
= constant + 0.05916/n log A1/A2 volts
A1
Note:
at 25oC
If the ISE is connected to the positive terminal of the potentiometer, the sign
before the log term is positive if X is a cation and negative if X is an anion.
p. 323-324 Harris (6th)
15-5
Glass Electrode for measuring pH = -log [H+]
Fig. 15-9
A glass combination electrode, having both the glass electrode
and a Ag-AgCl external reference electrode.
Fig. 15-10
Glass-body combination electrode with a pH-sensitive glass
bulb.
The two surfaces exposed to aqueous solution absorb water to form a
hydrated gel layer (silicate lattice).
H+ from solution can diffuse into the gel layer and occupy some of the
cation binding sites.
The glass membrane is immersed in a sample solution of unknown pH.
H+ concentration is fixed on the inside of the glass membrane.
The two Ag/AgCl reference electrodes measure the Em across the thin
glass membrane.
A change in H+ activity of the external sample solution will cause a
change in voltage, by 59.16 mV for every factor-of-10 difference in H+
activity (i.e., for every pH unit change).