dr. augustine ofori agyeman assistant professor of chemistry department of natural sciences clayton...
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DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences
Clayton state university
INSTRUMENTAL ANALYSIS CHEM 4811
CHAPTER 15
- The study of the relations between chemical reactions and electricity
- The study of the interconversion of chemical energy and electrical energy
- The study of redox reactions
- Electrochemical processes involve the transfer of electrons from one substance to another
ELECTROCHEMISTRY
Electroactive Species
- Species that undergoes an oxidation or a reduction during reaction
- Species may be complexed, solvated, molecule, or ion
- Species may be in aqueous or nonaqueous solution
ELECTROCHEMISTRY
- The use of electrochemical techniques to characterize a sample
- Deals with the relationship between electricity and chemistry
- Analytical calculations are based on the measurement of electrical quantities (current, potential, charge, or resistance)
and their relationship to chemical parameters
ELECTROANALYTICAL CHEMISTRY
Advantages
- Measurements are easy to automate as they are electrical signals
- Low concentrations of analytes are determined without difficulty
- Far less expensive equipment than spectroscopy instruments
ELECTROANALYTICAL CHEMISTRY
FUNDAMENTAL CONCENPTS
Redox Reaction
- Oxidation-reduction reaction
- Reactions in which electrons are transferred from one substance to another
Oxidation- Loss of electrons
Reduction- Gain of electrons
Oxidized Species
- The species that loses electrons
- The reducing agent (reductant)
- Causes reduction
Fe(s) ↔ Fe2+(aq) + 2e-
FUNDAMENTAL CONCENPTS
Reduced Species
- The species that gains electrons
- Oxidizing agent (oxidant)
- Causes oxidation
Cu2+(aq) + 2e- ↔ Cu(s)
FUNDAMENTAL CONCENPTS
Half Reactions
- Just the oxidation or the reduction is given
- The transferred electrons are shown
Oxidation Half-Reaction
- Electrons are on the product side of the equation
Reduction Half-Reaction
- Electrons are on the reactant side of the equation
FUNDAMENTAL CONCENPTS
Half Reactions
Oxidation half reactionFe(s) ↔ Fe2+(aq) + 2e-
Fe2+ ↔ Fe3+ + e-
Reduction half reaction Cu2+(aq) + 2e- ↔ Cu(s)
Cl2(g) + 2e- ↔ 2Cl-
FUNDAMENTAL CONCENPTS
- Many redox reactions are reversible
- Reduction reaction becomes oxidation reaction when it is reversed and vice versa
- Sum of oxidation and reduction half-reactions gives the net redox reaction or the overall reaction
- No electrons appear in the overall reaction
FUNDAMENTAL CONCENPTS
The Overall Reaction- Both an oxidation and a reduction must occur in a redox reaction
- The oxidizing agent accepts electrons from the reducing agent
Cu2+(aq) + Fe(s) ↔ Cu(s) + Fe2+(aq)
- Oxidizing agent- Reduced species
- Electron gain
- Reducing agent- Oxidized species
- Electron loss
FUNDAMENTAL CONCENPTS
Charge (q)
Charge (q) of an electron = - 1.602 x 10-19 CCharge (q) of a proton = + 1.602 x 10-19 C
C = coulombs
Charge of one mole of electrons = (1.602 x 10-19 C)(6.022 x 1023/mol) = 96,485 C/mol
= Faraday constant (F)
- The charge (q) transferred in a redox reaction is given by q = n x F
FUNDAMENTAL CONCENPTS
Current (i)
- The quantity of charge flowing past a point in an electric circuit per second
i = q/time
Units Ampere (A) = coulomb per second (C/s)
1A = 1C/s
FUNDAMENTAL CONCENPTS
Voltage or Potential Difference (E)
- The amount of energy required to move charged electrons between two points
- Work done by or on electrons when they move from one point to another
w = E x q or E = w/q
Units: volts (V or J/C)
1V = 1J/C
FUNDAMENTAL CONCENPTS
Electrode
- Conducts electrons into or out of a redox reaction system
- The electrode surface serves as a junction between an ionic conductor and an electronic conductor
Examplesplatinum wire
carbon (glassy or graphite) GoldSilver
FUNDAMENTAL CONCENPTS
Electroactive Species- Donate or accept electrons at an electrode
- Can be made to oxidize or reduce
Electrochemical Measurements- Occur at the electrode – solution interface
Chemical Measurements- Involve homogeneous bulk solutions
FUNDAMENTAL CONCENPTS
- Made up of the electrodes and the contacting sample solution
- Electrical conductor is immersed in a solution of its own ions
- A potential difference (voltage) is created between the conductor and the solution
- The system is a half-cell
- The metal conductor is an electrode and the solution is an electrolyte
ELECTROCHEMICAL CELL
Electrode Potential
- A measure of the ability of the half-cell to do work(the driving cell for the half-cell reaction)
Anode- Electrode where oxidation occurs
Mo → Mn+ + ne-
- Metal loses electrons and dissolves (enters solution)Cd(s) → Cd2+ + 2e-
Ag(s) → Ag+ + e-
ELECTROCHEMICAL CELL
Cathode
- Electrode where reduction occursMn+ + ne- → Mo
- Positively charged metal ion gains electrons
- Neutral atoms are deposited on the electrode
- The process is called electrodeposition
Cd2+ + 2e- → Cd(s)Ag+ + e- → Ag(s)
ELECTROCHEMICAL CELL
ELECTROLYSIS
- Voltage is applied to drive a redox reaction thatwould not otherwise occur
Examples- Production of aluminum metal from Al3+
- Production of Cl2 from Cl-
ELECTROLYTIC CELL
- Nonspontaneous reaction
- Requires electrical energy to occur
- Consumes electricity from an external source
- Spontaneous reaction
- Produces electrical energy
- Can be reversed electrolytically for reversible cells
ExampleRechargeable batteries
Conditions for Non-reversibility- If one or more of the species decomposes
- If a gas is produced and escapes
GALVANIC CELL
- Also known as voltaic cell
- A spontaneous redox reaction generates electricity
- One reagent is oxidized and the other is reduced
- The two reagents must be separated (cannot be in contact)
- Electrons flow through a wire (external circuit)
GALVANIC CELL
Oxidation Half-Reaction- Loss of electrons
- Occurs at anode (negative electrode)- The left half-cell by convention
Reduction Half-Reaction- Gain of electrons
- Occurs at cathode (positive electrode)- The right half-cell by convention
GALVANIC CELL
GALVANIC CELL
Salt Bridge
- Connects the two half-cells (anode and cathode)
- Filled with gel containing saturated aqueous salt solution (KCl)
- Ions migrate through to maintain electroneutrality (charge balance)
- Prevents charge buildup that may cease the reaction process
For the overall reactionCu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)
Anode Oxidation
Zn(s) → Zn2+(aq) + 2e-
Cathode Reduction
Cu2+(aq) + 2e- → Cu(s)
Salt bridge (KCl)
Cl-
K+
Voltmeter
- +
e- e-
Cu electrodeZn electrode
Cu2+Zn2+
GALVANIC CELL
Line Notation
Phase boundary: represented by one vertical line
Salt bridge: represented by two vertical lines
Fe(s) FeCl2(aq) CuSO4(aq) Cu(s)
GALVANIC CELL
STANDARD POTENTIALS
Standard Reduction Potential (Eo)
- Used to predict the voltage when different cells are connected- Potential of a cell as cathode compared to
standard hydrogen electrode- Species are solids or liquids
- Activities = 1
- We will use concentrations for simplicityConcentrations = 1 M
Pressures = 1 bar
STANDARD POTENTIALS
Standard Hydrogen Electrode (SHE)
- Reference electrode half-cell
- Used to measure Eo for half-reactions (half-cells)
- Connected to negative terminal (anode)
Assigned Eo = 0.000 under standard state conditions(T = 25 oC, concentration = 1M, pressure = 1 bar,
pure solid or liquid)
STANDARD POTENTIALS
Standard Hydrogen Electrode (SHE)
Consists of - Platinized Pt electrode immersed in a solution of 1M HCl
- H2 gas (1 bar) is bubbled over the Pt electrode
2H+(aq, 1 M) + 2e- ↔ 2H2 (g, 1 bar)
CELL POTENTIALS
- The potential for a cell containing a specified concentrationof reagent other than 1 M
Standard Cell PotentialEo
cell = Eocathode – Eo
anode
Cell PotentialEcell = Ecathode – Eanode
- Ecell is positive for spontaneous reactions
- Half-reaction is more favorable for more positive Eo
CELL POTENTIALS
Junction Potential
- Is produced when there is a difference in concentrationor types of ions of the two half-cells
- Is created at the junction of the salt bridge and the solution
- Is a source of error
- Minimized in KCl salt bridge due to similar mobilities of K+ and Cl-
Half ReactionF2 + 2e- ↔ 2F-
MnO4- + 5e- ↔ Mn2+
Ce4+ + e- ↔ Ce3+ (in HCl)O2 + 4H+ + 4e- ↔ 2H2O
Ag+ + e- ↔ Ag(s)Cu2+ + 2e- ↔ Cu(s)2H+ + 2e- ↔ H2(g)Cd2+ + 2e- ↔ Cd(s)Fe2+ + 2e- ↔ Fe(s)Zn2+ + 2e- ↔ Zn(s)Al3+ + 3e- ↔ Al(s)
K+ + e- ↔ K(s)Li+ + e- ↔ Li(s)
Eo (V)2.8901.5071.2801.2290.7990.3390.000-0.402-0.440-0.763-1.659-2.936-3.040In
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ower
Incr
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Oxidizingagents
Reducingagents
CELL POTENTIALS
- Elements that are more powerful reducing agents than hydrogen show negative potentials
- Elements that are less powerful reducing agents than hydrogen show positive potentials
- Metals with more negative Eo are more active
- More active metals displace less active metals from solution
Fe will displace Cu2+ out of solutionZn dissolves in HCl but Cu does not
CELL POTENTIALS
NERNST EQUATION
Gives relationship between the potential of an electrochemical cell and the concentration of reactants and products
O + ne- ↔ R
R
Olog
nF
2.3RTEE O
E = electrode potentialEo = standard potential for the redox reaction
R = gas constant = 8.314 J/K-molT = absolute temperature in Kelvin
F = Faraday’s constant = 96,485 C/moln = number of electrons transferred
NERNST EQUATION
a
bO
A
Blog
n
0.05916EE
For the half reaction
aA + ne- ↔ bB
The half-cell potential (at 25 oC), E, is given by
a
bO
A
Bln
nF
RTEE
a
bO
A
Blog
nF
2.3RTEE
NERNST EQUATION
For the overall reaction
aA + bB ↔ cC + dD
The potential at 25 oC is given by
ba
dcO
BA
DClog
nF
2.3RTEE
ba
dcO
BA
DCln
nF
RTEE
ba
dcO
BA
DClog
n
0.05916EE
NERNST EQUATION
- E = Eo when [O] = [R] = 1M
- Concentration for gases are expressed as pressures in bars or atm
- Concentrations for pure solids, liquids, and solvents are omitted (activity = 1)
- Reduction is more favorable on the negative side of Eo
- When a half reaction is multiplied by a factorEo remains the same
REFERENCE ELECTRODES
- An ideal reference electrode
- Has a fixed potential over time and temperature
- Long term stability
- Ability to return to the initial potential after exposure to small currents (reversible)
- Obey the Nernst equation
REFERENCE ELECTRODES
Standard Hydrogen Electrode (SHE)E = 0.000 V
Saturated Calomel Electrode (SCE)- Composed of metallic mercury in contact with saturated
solution of mercurous chloride (calomel, Hg2Cl2)
- Pt wire is in contact with the metallic mercury
- Calomel is in contact with saturated KCl solution
E = +0.244 V at 25 oC
REFERENCE ELECTRODES
Silver/Silver Chloride Reference Electrode (Ag/AgCl)
- Consists of silver metal coated with silver chloride paste
- Immersed in saturated KCl and AgCl solution
E = +0.199 V at 25 oC
ELECTROANALYTICAL METHODS
Two main types - Potentiometric and Potentiostatic
- The type of technique reflects the type of electrical signal used for quantitation
- Techniques require at least two electrodes and an electrolyte (containing solution)
ElectrodesWorking (indicator) electrode, reference electrode, counter electrode
Potentiometric Technique
- Based on a static (zero-current) situations
- Based on measurement of the potential established across a membrane
- Used for direct monitoring of ionic species (Ca2+, Cl-, K+, H+)
ELECTROANALYTICAL METHODS
Potentiostatic Technique
- Controlled-potential technique
- Based on dynamic (non-zero-current) situations
- Deals with the study of charge transfer processes at the electrode-solution interface
- Chemical species are forced to gain or lose electrons
ELECTROANALYTICAL METHODS
- Potentiometry
- Coulometry
- Voltammetry
- Polarography
- Methods are classified according to the variable being measured
- One variable (current, voltage, charge) is measured and the others are controlled
ELECTROANALYTICAL METHODS
- Based on static (zero-current) measurements
- Involves measurement of potential (voltage) of an electrochemical cell
- Used to obtain information on the composition of an analyte
- Potential between two electrodes is measured (indicator electrode and reference electrode)
- Indicator (sensing) electrode responds to the concentration of the analyte species
POTENTIOMETRY
- The analyte concentration is related to the potential difference between the indicator electrode and the reference electrode
(by applying the Nernst equation)
- Indicator electrode is connected to a reference electrode (SCE, Ag/AgCl) to form a complete cell
- Implies Etotal = Eindicator – Ereference
- Reference electrode is connected to the negative terminal of the readout device (potentiometer)
POTENTIOMETRY
Applications
- Environmental monitoring
- Clinical diagnostics (blood testing, electrolytes in blood)
- Control of reaction processes
POTENTIOMETRY
- Electrode that responds to change in analyte activity
- Generally show high degree of selectivity
Types of indicator electrodes- Metallic electrodes (metal wire, mesh, or strip)
- Metal coated with its sparingly soluble salt (Ag/AgCl)- Electrode whose equilibrium reaction responds to nalyte cation
- Redox indicator electrode (measures redox reactions)
INDICATOR ELECTRODE
- Are 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)
- If C+ is the preferential ion
- [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)
ai = γici
where γi = activity coefficient (between 0 and 1)
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)
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 saturated in AgCl
- 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
)log(a 0.05916 -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
SOLID-STATE ELECTRODES
- Solid membranes that are selective primarily to anions
Solid-state membrane may be - single crystals (most common)
- polycrystalline pellets or
- mixed crystals
SOLID-STATE ELECTRODES
- Ionic solid contains the target ion
- Solid is sealed to the end of a polymer tube
- Contains internal reference electrode and filling solution
- Concentration difference across the membrane causes migration of charged species across the membrane
- Can measure concentrations as low as 10-6 M
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
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
)log(a2
0.05916KE Ca
LIQUID MEMBRANE ELECTRODES
- Cell potential is given by
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
Anion-Selective Electrodes
- For sensing organic and inorganic anions
Examples of Anions- Phosphate- Salicylate
- Thiocyanate- Carbonate
LIQUID MEMBRANE 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
- For monitoring gases such as CO2, O2, NH3, H2S
- Device is known as compound electrode(probe is usually used in place of electrode)
- Highly sensitive and selective for measuring dissolved gases
- For environmental monitoring for clinical and industrial applications
GAS SENSING PROBES
- Gas permeable membrane (teflon, polyethylene) is immobilized on a pH electrode or ion-selective electrode
- Thin film of electrolyte solution is placed between electrode and membrane (fixed amount, ~0.1 M)
- Inbuilt reference electrode
- The target analyte diffuses through the membrane and comes to equilibrium with the internal electrolyte solution
GAS SENSING PROBES
- The target gas then undergoes chemical reaction and the resulting ion is detected by the ion-selective electrode
- Electrode response is directly related to the concentration of gas in the sample
- Two types of polymeric materials are usedMicroporous and Homogeneous
- Membrane thickness is ~ 0.01 – 0.10 mm
- Membrane is impermeable to water and ions
GAS SENSING PROBES
CO2 Sensors
- Consists of pH electrode covered by a CO2 selective membrane (silicone)
- Electrolyte between electrode and membrane is NaHCO3-NaCl solution
- pH of inner solution lowers when CO2 diffuses through membrane
- Inner glass electrode senses changes in pH
- Overall potential is determined by CO2 concentration in sample
GAS SENSING PROBES
CO2 Sensors
]ln[COF
RTKE 2
pH glass electrodeHCO3- solution
CO2 + H2O ↔ H+ + HCO3-
H+ lowers pH
Membrane (silicone)
GAS SENSING PROBES
NH3 Sensors
- Consists of pH electrode covered by NH3 selective membrane (teflon or polyethylene)
- Electrolyte between electrode and membrane is NH4
+-KCl solution
- NH3 goes through membrane and raises pH
- Inner glass electrode senses changes in pH
- Increase in pH is proportional to amount of NH3 in sample
GAS SENSING PROBES
Other Gas Sensing Devices
NO2 and SO2
- Makes use of modified pH electrode
H2S- Makes use of S2- ISE or modified pH electrode
HF- Makes use of F- ISE or modified pH electrode
GAS SENSING PROBES
- Enzymes are proteins that catalyze chemical reactions in living things
- Based on coupling a layer of an enzyme with an electrode(enzyme is immobilized on an electrode)
- Electrode serves as a transducer
- Very efficient and extremely selective
IMMOBILIZED ENZYME MEMBRANE ELECTRODES
- Enzyme (biocatalytic) layer immobilized on an electrode
Electrode
Biocatalytic Layer
IMMOBILIZED ENZYME MEMBRANE ELECTRODES
Applications
- Useful for monitoring clinical, environmental, food samples
- For determination of glucose in blood (glucose sensors)
For amperometric sensing of ethanol (ethanol electrodes)
For sensing urea in the presence of urease enzyme (urea electrodes)
IMMOBILIZED ENZYME MEMBRANE ELECTRODES
- Known as ion-selective field effect transistors (ISFET)
- Are semiconductor devices
- Surface of transistor is covered with silicon nitride
- Absorbs H+ from solution (results in change of conductivity)
- Provides the ability to sense several ions (Na+, Ca2+, K+, pH in blood samples, etc)
- For detection of hydrocarbons and NOx in exhaust
SOLID-STATE DEVICES
- External reference electrode is required
- Does not require hydrating
- Has rapid response time
ExamplesNa+ ISFETNH3 ISFETCl- ISFET
SOLID-STATE DEVICES
- Employs a potential measuring device (handheld device)(high-impedance circuit)
- Example is the pH meter (or pIon meter)
- Designed to work with various electrodes
- Have built-in temperature measurement and compensation
- Three-point or more auto calibration
- Two-electrode system (auxiliary reference electrode and working electrode)
POTENTIOMETRY INSTRUMENTATION
APPLICATIONS OF POTENTIOMETRY
- 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
- For studying chemical reactions (kinetics, equilibria, mechanism, solubility product constant, stability constant of complexes)
- For characterization of materials
- Quality control of raw materials and finished products
- Pharmaceutical and biological studies
- Elemental and molecular analysis
- Environmental monitoring
APPLICATIONS OF POTENTIOMETRY
- Electronics
- Electrochemical sensors
Advantages of controlled potential processes- High sensitivity and selectivity
- Very low detection limits- Wide range of electrode types
- Wide range of linearity- Portable and low cost instrumentation
APPLICATIONS OF POTENTIOMETRY
- Electrostatic technique
- Measurement of the current response to an applied potential
- Various combinations of potential excitations exist(step, ramp, sine wave, pulse strain, etc)
CONTROLLED POTENTIAL TECHNIQUES
Instrumentation
- Potentiostat (Voltammetric Analyzer)
- Electrochemical cell with a three-electrode systemWorking Electrode (WE)Reference Electrode (RE)
Counter/Auxiliary Electrode (CE/AE)
- Plotter
- Other components may be required depending on the type of experiment
CONTROLLED POTENTIAL TECHNIQUES
Potentiostat- Instrument that controls the potential at a working electrode
- Connects the three electrodes
Electrochemical Cell- Covered glass container of 5 – 50 mL volume
- Contains the three electrodes immersed in the sample solution
- Electrodes are inserted through holes in the cell cover
- N2 gas used as deoxygenated gas
CONTROLLED POTENTIAL TECHNIQUES
Working Electrode (WE)- Electrode at which the reaction of interest occurs
(Pt, Au, Ag, C)
Reference Electrode (RE)- Provides a stable and reproducible potential
- Independent of the sample composition(Ag/AgCl, SCE)
Counter/Auxiliary Electrode (CE/AE)- Current-carrying electrode made of inert conducting metal
(Pt wire, Graphite rod)
CONTROLLED POTENTIAL TECHNIQUES
- RE is placed as close as possible to WE to minimize potential drop caused by the cell resistance (iR)
- Flow cannot occur through RE hence the need for CE to complete the current path
- Current flows through solution between WE and CE
- Voltage is measured between WE and RE
CONTROLLED POTENTIAL TECHNIQUES
MASS TRANSPORT
- Three modes of mass transport
Diffusion- Spontaneous movement as a result of concentration gradient
- Movement from regions of high concentration to regions of low concentration
MASS TRANSPORT
- Three modes of mass transport
ConvectionTransport to the electrode by gross physical movement
Forced Convection- Driving force is an external mechanical energy
- Solution stirring or flowing- Electrode rotation or vibration
Natural Convection- Physical movement as a result of density gradient
MASS TRANSPORT
- Three modes of mass transport
Migration- Movement of charged particles along an electric field
- Charge is carried through the solution as a result of movement of ions
- Inert
- Decreases the resistance of the solution
- Eliminates electromigration effects
- Maintains a constant ionic strength
- Concentration range in usually 0.1 M – 1.0 M
- Should be in large excess of analyte concentration
SUPPORTING ELECTROLYTE
- Medium for electrochemical measurements
- Contains a supporting electrolyte
- Choice of solvent depends on the solubility and the redox activity of the analyte
Solvent Properties- Electrical conductivity
- Electrochemical activity - Chemical reactivity
SOLVENTS
- Purging with an inert gas for about 10 minutes
- Nitrogen gas is usually used
- Purging is done just before voltammetric measurements
- Necessary as oxygen complicates interpretation
Other Methods- Formation of peroxides followed by reduction of peroxides
-Reduction by addition of sodium sulfite or ascorbic acid
OXYGEN REMOVAL
- Method in which charge is measured
- Species being measured is converted quantitatively to a new species
The Methods Based on Electrolysis- Electrogravimetry
- Constant-potential coulometry- Constant-current coulometry (coulometric titrimetry)
Electrolysis- A process causing a thermodynamically nonspontaneous
oxidation or reduction reaction to occur by application of potential or current
COULOMETRY
Electrogravimetry
- Product of electrolysis is plated on a pre-weighed electrode
- Electrode is weighed again after process and the amountplated is determined by difference
- Metal dissolves from the anode and deposits on the cathode(electroplating, electrowinning, or electrorefining)
Examples of metals commonly determinedCd, Bi, Co, Cu, Sb, Zn, Ni, In, Ag
COULOMETRY
Controlled Potential Coulometry
- Three electrode system
- Permits applied potential pulse or ramp at the working electrode
- Metal elements are deposited as potential is increased which increases charge passing through cell
- The instrument is the coulometer which measures q
COULOMETRY
Controlled Potential Coulometry
Applications
- Used to eliminate interferences from other reactions that take place at different potentials
- Used to determine the number of electrons involved in a reaction
- Used for coulometric titrations
COULOMETRY
Conductometric Analysis
- Measures electrical conductivity between two electrodes by ions in solution
Applications- To determine the ionic content of drinking water,
deionized water, solvents, beverages
- Used as a detector for ion chromatography, HPLC
- Used for conductometric titrations (end point determination)
COULOMETRY
Instrumentation
Apparatus comprises of- Potentiostat with DC output voltage
- Inert cathode and anode- Stirring rod set-up
- Solution may be heated
- Working electrode can be either anode or cathode
- Controlled potential conditions
COULOMETRY
- Voltage between two electrodes is varied as current is measured
- Solid working electrodes are used
- Oxidation-reduction takes place at or near the surface of the working electrode
- Graph of current versus potential is obtained
- Peak current is proportinal to concentration of analyte
VOLTAMMETRY
VOLTAMMOGRAM
- Current versus potential plot
- Current on vertical axis and excitation potential on horizontal axis
- Electrode reactions involve several steps and can be complicated
- The rate is determined by the slowest step and depends on the potential range
- Involves linear scanning of potential of a stationary electrode using a triangular waveform
- Solution is unstirred
- The most widely used technique for quantitative analysis of redox reactions
Provides information on- the thermodynamics of redox processes
- the kinetics of heterogeneous electron transfer reactions- the kinetics of coupled reactions
CYCLIC VOLTAMMETRY
- Is a three electrode system
- Pretreatment (polishing) of working electrode is necessary
- The current resulting from an applied potential is measured during a potential sweep
- Current-potential plot results and is known as cyclic voltammogram (CV)
CYCLIC VOLTAMMETRY
O + ne- ↔ R
- Assume only O is present initially
- A negative potential sweep results in the reduction of O to R(starting from a value where no reduction of O initially occurs)
- As potential approaches Eo for the redox process, a cathodic current is observed until a peak is reached
- The direction of potential sweep is reversed after going beyond the region where reduction is observed
CYCLIC VOLTAMMETRY
- This region is at least 90/n mV beyond the peak
- R molecules generated and near the electrode surface are reoxidized to O during the reverse (positive) scan
- Results in an anodic peak current
- The characteristic peak is a result of the formation of a diffusion layer near the electrode surface
- The forward and reverse currents have the same shape
CYCLIC VOLTAMMETRY
- Increase in peak current corresponds to achievement of diffusion control
Characteristic Parameters- Anodic peak current (ipa)
- Cathodic peak current (ipc)- Anodic peak potential (Epa)
- Cathodic peak potential (Epc)
CYCLIC VOLTAMMETRY
Reversible Systems
- Peak current for a reversible couple is given by the Randles-Sevcik equation (at 25 oC)
n = number of electronsA = electrode area (cm2)
C = concentration (mol/cm3)D = diffusion coefficient (cm2/s)
ν = potential scan rate (V/s)
1/21/23/25p νACDn10x2.69i
CYCLIC VOLTAMMETRY
Reversible Systems
ip is proportional to C
ip is proportional to ν1/2
- Implies electrode reaction is controlled by mass transport
ip/ic ≈ 1 for simple reversible couple
- For a redox couple
2
EEE pcpao
CYCLIC VOLTAMMETRY
Reversible Systems
- The separation between peak potentials
Vn
0.059EEΔE pcpap
- Used to determine the number of electrons transferred
- For a fast one electron transfer ∆Ep = 59 mV
- Epa and Epc are independent of the scan rate
CYCLIC VOLTAMMETRY
Irreversible Systems- Systems with sluggish electron transfer
- Individual peaks are reduced in size and are widely separated
- Characterized by shift of the peak potential with scan rate
Quasi-reversible Systems- Current is controlled by both charge transfer and mass transport
- Voltammograms are more drawn out
- Exhibit larger separation in peak potentials compared to reversible systems
CYCLIC VOLTAMMETRY
Applications
For analyzing - drugs
- herbicides- insecticieds
- foodstuff additives- pollutants
CYCLIC VOLTAMMETRY
- Voltammetry in which the working electrode is dropping mercury
- Makes use of potential ramp
- Conventional DC
- Wide cathodic potential range and a renewable surface
- Hence widely used for the determination of many reducible species
POLAROGRAPHY
- Initial potential is selected such that the reaction of interest does not take place
- Cathodic potential scan is applied and current is measured
- Current is directly proportional to the concentration-distance profile
- Reduction begins at sufficiently negative potential[concentration gradient increases and current rises
rapidly to its limiting value (iL)]
POLAROGRAPHY
- Diffusion current is obtained by subtracting response due to supporting electrolyte (background current)
- Analyte species entering region close to the electrode surface undergo instantaneous electron transfer reaction
- Maximum rate of diffusion is achieved
- Current-potential plot provides polarographic wave (polarogram)
POLAROGRAPHY
DC POLAROGRAPHY
- Three electrode system
WE = dropping mercury electrode (DME)
CE = Pt wire or foil
RE = SCE
The Ilkovic Equation
DC POLAROGRAPHY
D = cm2/s C = mol/cm3 m = g/s t = s
iL is current at the end of drop life (the limiting current)
iL is a measure of the species concentration
Ctm708nDi 1/62/31/2
L
Half Wave Potential (E1/2)
- Potential at which the current is one-half its limiting value
- E1/2 is independent of concentration of species
DC POLAROGRAPHY
DR = diffusion coefficient of reduced speciesDO = diffusion coefficient of oxidized species
- Experimental E1/2 is compared to literature values to identify unknown analyte
1/2
O
Ro1/2 D
Dlog
nF
RTEE
Half Wave Potential (E1/2)
At 25 oC
DC POLAROGRAPHY
- A graph of E versus log[(iL-i)/i] is linear if reaction is reversible(Nernstian behavior)
- Slope = 0.05916/n and intercept = E1/2
E = E1/2 when [Ox] = [Red]
i
ilog
n
0.05916EE L
1/2i
STRIPPING ANALYSIS
Two step technique
1. Deposition Step (Preconcentration step)- Involves preconcentration of analyte species by reduction
(anodic stripping) or oxidation (cathodic stripping) into a mercury electrode
2. Stripping Step- Measurement step
- Rapid oxidation or reduction to strip the products back into the electrolyte
STRIPPING ANALYSIS
- Very sensitive for trace analysis of heavy metals
- Favorable signal to background ratio
- About four to six metals can be measured simultaneously at levels as low as 10-10 M
- Low cost instrumentation
- There are different versions of stripping analysis depending on the nature of the deposition and stripping steps
STRIPPING ANALYSIS
Anodic Stripping Voltammetry (ASV)
- The most widely used stripping analysis
- Preconcentration is done by cathodic deposition at controlled potential and time
- Metals are preconcentrated by electrodeposition into a small-volume Hg electrode
- Deposition potential is usually 0.3 – 0.5 V more negative than Eo for the analyte metal ion
STRIPPING ANALYSIS
Anodic Stripping Voltammetry (ASV)
- Metal ions reach the Hg electrode surface by diffusion and convection
- Electrode rotation or solution stirring is employed to achieve convection
- Metal ions are reduced and concentrated as amalgams
Mn+ + ne- + Hg → M(Hg)
- Hg film electrodes or Hg drop electrodes may be used
STRIPPING ANALYSIS
Anodic Stripping Voltammetry (ASV)
Following preselected deposition period:
- Forced convection is stopped
- Anodic potential scan is employed (may be linear or pulse)
- Amalgamated metals are reoxidized (stripped off electrode)
- An oxidation (stripping) current then flows
M(Hg) → Mn+ + ne- + Hg
STRIPPING ANALYSIS
Cathodic Stripping Voltammetry (CSV)
- Mirror image of ASV
- Involves anodic deposition of analyte and subsequent stripping by a potential scan in the negative direction
An- + Hg ↔ HgA + ne-
(Deposition to the right and stripping to the left)
- Useful for measuring organic and inorganic compounds that form insoluble salts with Hg (thiols, penicillin, halides, cyanides)
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