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ELECTROCHEMISTRY
CHEM 4700
CHAPTER 5
DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences
Clayton state university
CHAPTER 5
POTENTIOMETRY
- Based on static (zero-current) measurements
- Used to obtain information on the composition of an analyte
- Potential between two electrodes is measured
Applications- Environmental monitoring
- Clinical diagnostics (blood testing, electrolytes in blood)- Control of reaction processes
POTENTIOMETRY
- Also known as indicator electrodes
- Respond directly to the analyte
- Used for direct potentiometric measurements
- Selectively binds and measures the activity of one ion (no redox chemistry)
ExamplespH electrode
Calcium (Ca2+) electrodeChloride (Cl-) electrode
ION-SELECTIVE ELECTRODES (ISE)
Advanteages
- Exhibit wide response
- Exhibit wide linear range
- Low cost
- Color or turbidity of analyte does not affect results
- Come in different shapes and sizes
ION-SELECTIVE ELECTRODES (ISE)
- Made from a permselective ion-conducting membrane(ion-exchange material that allows ions of one electrical
sign to pass through)
- Reference electrode is inbuilt
- Internal solution (solution inside electrode) contains ion of interest with constant activity
- Ion of interest is also mixed with membrane
- Membrane is nonporous and water insoluble
ION-SELECTIVE ELECTRODES (ISE)
- Responds preferentially to one species in solution
Internal reference electrode
Ion-selective membrane
Internal (filling) solution
ION-SELECTIVE ELECTRODES (ISE)
- Selective (preferential) ion is C+
- Membrane is made of poly(vinyl chloride) (PVC)
- Membrane is impregnated with nonpolar liquid
- Membrane contains ligand L (ion-selective ionophore)
- Membrane contains the complex LC+
- Membrane contains hydrophobic anion R- (ion exchanger)
ION-SELECTIVE ELECTRODES (ISE)
- [C+] inside the electrode ≠ [C+] outside the electrode
- Results in a potential difference across the membrane
Generally (at 25 oC)- 10-fold change in activity implies 59/zi mV change in E- zi is the charge on the selective ion (negative for anions)
- zi = +1 for K+, zi = +2 for Ca2+, zi = -2 for CO32-
ION-SELECTIVE ELECTRODES (ISE)
inner
outer
i ][C
][Cln
Fz
RTE
inner
outer
i
o
][C
][Clog
z
0.05916EC, 25At
- Let ci = molarity of C+
- Activity (ai) rather than molarity is measured by ISEs
- Activity is the effective (active) concentration of analyte(effective concentration decreases due to ionic interactions)
- zi = ionic charge (±)
ai = γici
where γi = activity coefficient (between 0 and 1)
ION-SELECTIVE ELECTRODES (ISE)
Debye-Hückel Equation
- Relates activity coefficients to ionic strength (at 25 oC)
α = size of ion in picometers (1 pm = 10-12 m)
µ = ionic strength
/305)μ(α1
μ0.51zγlog
2i
ION-SELECTIVE ELECTRODES (ISE)
Ionic strength- A measure of the concentration of all ions in solution
with their charges taken into account
.........zczczc2
1zc
2
1μ 2
ii2ii
2ii
i
2ii
ci = the concentration of the ith species
Ionic strength of electrolytes1:1 electrolytes (NaCl) µ = molarity
2:1 electrolytes (CaCl2) µ = 3 x molarity3:1 electrolytes (AlCl3) µ = 6 x molarity2:2 electrolytes (MgSO4) µ = 4 x molarity
ION-SELECTIVE ELECTRODES (ISE)
- For very dilute solutions ai ≈ ci
- Activity coefficient decreases as ionic strength increases
For zi = 1- 1 mV change in potential implies 4% change in activity
For zi = 2- 1 mV change in potential implies 8% change in activity
- This is known as Nernstian behavior
ION-SELECTIVE ELECTRODES (ISE)
Selectivity Coefficient (k)
- A measure of the ability of ISE to discriminate against an interfering ion
- It is assumed that ISEs respond only to ion of interest
- In practice, no electrode responds to only one specific ion
- The lower the value of k the more selective is the electrode
- k = 0 for an ideal electrode (implies no interference)
ION-SELECTIVE ELECTRODES (ISE)
Selectivity Coefficient (k)
For k > 1- ISE responds better to the interfering ion than to the target ion
For k = 1- ISE responds similarly to both ions
For k < 1- ISE responds more selectively to ion of interest
ION-SELECTIVE ELECTRODES (ISE)
Empirical Calibration PlotP
oten
tial
(m
V)
p[C+]
Slope = 59/zi mV
zi = charge of ion
Called Nernstian slope
- Used to determine the unknown concentration of analytes
- Departure from linearity is observed at low concentrations
ION-SELECTIVE ELECTRODES (ISE)
Three groups of ISEs
- Glass electrodes
- Liquid electrodes
- Solid electrodes
ION-SELECTIVE ELECTRODES (ISE)
GLASS ELECTRODES
- Responsive to univalent cations
- Employs thin ion-selective glass membrane
pH GLASS ELECTRODE
- The most widely used
- For pH measurements (selective ion is H+)
- Response is fast, stable, and has broad range
- pH changes by 1 when [H+] changes by a factor of 10
- Potential difference is 0.05196 V when [H+] changes by a factor of 10
For a change in pH from 3.00 to 6.00 (3.00 units)Potential difference = 3.00 x 0.05196 V = 0.177
pH GLASS ELECTRODE
- Thin glass membrane (bulb) consists of SiO4
- Most common composition is SiO2, Na2O, and CaO
Glass membrane contains - dilute HCl solution
- inbuilt reference electrode (Ag wire coated with AgCl)
pH GLASS ELECTRODE
Glass Electrode Response at 25 oC (potential across membrane with respect to H+)
ΔpH = pH difference between inside and outside of glass bulb
β ≈ 1 (typically ~ 0.98)(measured by calibrating electrode in solutions of known pH)
K = assymetry potential (system constant, varies with electrodes)
ΔpHβ(0.05916)KE
)(a0.05916log -KEH
pH GLASS ELECTRODE
- Equilibrium establishes across the glass membrane with respect to H+ in inner and outer solutions
- This produces the potential, E
- Linearity between pH and potential
- Calibration plot yields slope = 59 mV/pH units
- Electrode is prevented from drying out by storing in aqueous solution when not in use
pH GLASS ELECTRODE
Sources of Error
- Standards used for calibration- Junction potential- Equilibration time
- Alkaline (sodium error)- Temperature- Strong acids
- Response to H+ (hydration effect)
OTHEER GLASS ELECTRODES
Glass Electrodes For Other CationsK+ -, NH4
+-, Na+-selective electrodes- Mechanism is complex
- Employs aluminosilicate glasses (Na2O, Al2O3, SiO2)- Minimizes interference from H+ when solution pH > 5
pH Nonglass Electrodes- Quinhydrone electrode (quinone – hydroquinone couple)
- Antimony electrode
LIQUID MEMBRANE ELECTRODES
- Employs water-immiscible substances impregnated in a polymeric membrane (PVC)
- For direct measurement of polyvalent cations and some anions
- The inner solution is a saturated solution of the target ion
- Hydrophilic complexing agents (e.g. EDTA) are added to inner solutions to improve detection limits
- Inner wire is Ag/AgCl
Ion-Exchange Electrodes
- The basis is the ability of phosphate ions to form stable complexes with calcium ions
- Selective towards calcium
- Employs cation-exchanger that has high affinity for calcium ions(diester of phosphoric acid)
- Inner solution is a saturated solution of calcium chloride
- Cell potential is given by )log(a2
0.05916KE
aC
LIQUID MEMBRANE ELECTRODES
Other Ion-Exchange Electrodes
- Have poor selectivity and are limited to pharmaceutical formulations
Examples- IEE for polycationic species (polyarginine, protamine)
- IEE for polyanionic species (DNA)- IEE for detection of commonly abused drugs
(large organic species)
LIQUID MEMBRANE ELECTRODES
Neutral Carrier Electrodes
- Employs neutral carriers such as crown ethers and cyclic polyesters
- Carriers envelope target ions in their pockets
Used for clinical analysis- detection of blood electrolytes
- detection alkali and alkaline earth metal cations
LIQUID MEMBRANE ELECTRODES
Neutral Carrier Electrodes
Examples of Carriers- Monensin for sodium
- Macrocyclic thioethers for Hg and Ag- Valinomycin for potassium ions- Calixarene derivatives for lead- 14-crown-4-ether for lithium
LIQUID MEMBRANE ELECTRODES
Anion-Selective Electrodes
- For sensing organic and inorganic anions
Examples of Anions- Phosphate- Salicylate
- Thiocyanate- Carbonate
LIQUID MEMBRANE ELECTRODES
SOLID-STATE ELECTRODES
- Solid membranes that are selective primarily to anions
Solid-state membrane may be - single crystals
- polycrystalline pellets or
- mixed crystals
SOLID-STATE ELECTRODES
Examples - Most common is fluoride-ion-selective electrode
(limited pH range of 0-8.5)(OH- is the only interfering ion due to similar size and charge)
- Iodide electrode (high selectivity over Br- and Cl-)
Chloride electrode (suffers interference from Br- and I-)
Thiocynate (SCN-) and cyanide (CN-) electrodes
OTHER ELECTRODES
- Coated-wire electrodes (CWE)
- Solid-state electrodes without inner solutions
- Made up of metallic wire or disk conductor (Cu, Ag, Pt)
- Mechanism is not well understood due to lack of internal reference
- Usually not reproducible
For detection ofamino acids, cocaine, methadone, sodium
APPLICATIONS OF ISEs
- Used as detectors for automated flow analyzers(flow injection systems)
- High-speed determination of blood electrolytes in hospitals(H+, K+, Cl-, Ca2+, Na+)
- For measuring soil samples (NO3-, Cl-, Li+, Ca2+, Mg2+)
- Coupling ion chromatography with potentiometric detection
- Micro ISEs as probe tips for SECM
- Column detectors for capillary-zone electrophoresis