Yonsei University Chapter 14: Electrodes and Potentiometry The use of electrodes to measure voltages that provide chemical information is called potentiometry (ion-selective electrode, ion-sensing field effect transistors)
Yonsei University Potentiomtery Measurement of Cell Voltages à Chemical Information (Activity) Cell Voltage = Indicator E Reference E Reference Electrode: Fixed composition à Constant potential Indicator Electrode: Response to analyte activity à Variable potential
14-1. Reference Electrodes Reference Electrode : Ag + Cl - AgCl(s) + e - (AgCl(s) + e - Ag + Cl - ) : E - = 0.222 V E - = 0.222 0.0592 log[cl - ] : 0.197 V Indicator Electrode (Pt) : Fe 3+ + e - Fe 2+ : E + = 0.771 V E + = 0.771-0.0592 log[fe 2+ ]/[Fe 3+ ] provides a constant potential if [Cl - ] is constant (saturated KCl soluitionà fixed by KCl solubility Measuremed Cell Voltage: variable fixed E = E + - E - = (0.771-0.0592 log[fe 2+ ]/[Fe 3+ ]) (0.222 0.0592 log[cl - ]) Therefore, we can measure relative amount of Fe 2+ and Fe 3+ in a solution
Ag/AgCl Reference Electrode Ag + Cl - AgCl(s) + e - (AgCl(s) + e - Ag + Cl - ) : E - = 0.222 V E(saturated KCl) = +0.197V
Saturated Calomel Electrode (S.C.E) Saturated Calomel Electrode (SCE) ½ Hg 2 Cl 2 (s) + e - Hg(l) + Cl - E = +0.2689 E(saturated KCl) = +0.241
Potentiometry with an Oscillating Reaction Appatatus to monitor the quotient [Ce 3+ ]/[Ce 2+ ] from an oscillating reaction
Voltage Conversion between Different Reference Scales A -0.461 SHE -0.417 +0.197 +0.241 Ag/AgCl SCE -0.220 V -0.2 0 0.1 Potential versus S.H.E (volts) 0.2 +0.033-0.011 B Q : 0.461 V vs SCE A V vs Ag/AgCl (A = 0.417 V) B V vs SHE (B = 0.220 V) Q : +0.033 V vs Ag/AgCl A V vs SCE (A = 0.011 V) B V vs SHE (B = +0.230 V)
14-2. Indicator Electrodes 1. Metal Electrode: An electric potential is developed in response to redox reaction at the metal surface Most common metal indicator electrode: Pt (because inert), Au, Carbon The purpose of the metal electrode: transmit electrons to or from a species in solution 2. Ion-Selective Electrode: Not based on redox reaction Selective migration of one type of ion across the electrode membrane generates an ΔE
E lj (diffusion potential) 14-3. (Liquid) Junction Potential Develops at the interface between two liquids (electrolytes) as a result of differences in the rates with which ions move from one liquid to the other. Type 1 Type 2 Type 3 0.01 M HCl + - 0.1 M HCl Cl - H + 0.1 M HCl 0.1 M KCl H + K + - + H + 0.1 M HCl Cl - - 0.05 M KNO 3 Figure. Types of liquid junction. Arrows show the direction of net transfer for each ion, and their lengths indicate + K + NO 3 - relative mobilities. The polarity of the junction potential is indicated in each case by the circled signs. Table. Liquid Junction Potentials of 0.1 M Concentrations of Electrolytes Juncton E lj observed (mv) Juncton E lj observed (mv) Juncton E lj observed (mv) Juncton E lj observed (mv) HCl : KCl HCl : NaCl KCl : LiCl 26.78 33.09 34.86 HCl : NH 4 Cl KCl : LiCl KCl : NaCl 28.40 8.79 6.42 KCl : NH 4 Cl NaCl : LiCl 2.16 2.62 NaCl : NH 4 Cl LiCl : NH 4 Cl -4.21-6.93
Junction Potential (E j ) Any time two dissimilar electrolyte solutions are in contact, a voltage difference (called the junction potential) develops at their interface This small voltage (usually a few mv) is found at each end of a salt bridge connecting two half-cells Cl - has a greater mobility than Na + Steady-state junction potential develops
Saturated KCl is used in a salt bridge because K + and Cl - have similar mobilities Junction potentials at the two interfaces of a KCl salt bridge are slight
Cell Potential (E cell ) E cell = E right E left + E j E right = E cathod E left = E anode = (liquid) junction potential E j The liquid junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements, because we usually do not know the contribution of the junction to the measured voltage. ph electrode = 59 mv per ph unit, A ph electrode dipped into 0.1 M HCl (3M KCl in reference E): ~ 3mV junction potential à error in 0.05 ph à 12% error in [H + ]
14-4. How Ion-Selective Electrodes Work - I.S.E. responds only to one intended ion and is unaffected by other species (e.g.) glass electrode: [H + ], thus ph; Ca 2+ I.S.E. : [Ca 2+ ] -Key feature of an ideal I.S.E is a thin membrane across which only the intended ion can migrate and the other ions can not cross the membrane 0.01 M Ca 2+ 0.02 M Cl - Aqueous solution Ca 2+ Membrane (a) Ca 2+ -binding ligand soluble in membrane 0.1 M Ca 2+ 0.2 M Cl - Aqueous solution + + + + + + + (0.01+d) M Ca 2+ 0.02 M Cl - Low conc. Ca 2+ - --- Membrane (b) - - (0.1-d) M Ca 2+ 0.2 M Cl - - High conc. Scheme: Mechanism of ion-selective electrode. (a) Initial conditions prior to Ca 2+ migration across the membrane. (b) After d moles of Ca 2+ per liter have crossed the membrane, giving the left side a charge of +2d mol/l and the right side a charge of -2d mol/l.
Hydrophobic organic polymer with viscous organic solution containing ion-exchanger (ionophore), L C + : analyte cation
Thermodynamics of Ion Selective Electrode (ISE) G = -RT ln A 1 (ΔG due to activity difference) A 2 G = -nfe (ΔG due to charge imbalance) -RT ln A 1 A 2 = -nfe E memb = RT ln A 1 = 0.05916 log A 1 (volts at 25 ) nf A 2 n E memb : membrane potential (electric potential difference for ISE) 1: Sample solution, 2: Internal solution 10-fold change in [H + ] 59.16 mv change (ph electrode) 59.16 mv change per one ph unit (ph 4 unit change à 4 x 59.16 mv = 237 mv change) Ca 2+ n = 2, 10-fold change in [Ca 2+ ] 59.16/2 = 29.58 mv change A 2
The two reference electrodes serve to measure the electric potential difference across the membrane Internal reference electrode E cell = E ref ext - E ref int + E memb + E j E cell = E ref ext - E ref int + RT ln 1 RT + E lj + ln (a i ) sample nf (a i )int nf Constant mv meter External reference electrode E cell = constant + E cell = constant + RT ln (a i ) sample nf 0.05916 log (a i ) sample n at 25 Internal Filling Solution (a i ) internal = constant a i sample Potential(V) log [a i ]
14-5. ph Measurement with a Glass Electrode Glass membrane selectively binds H + (membrane potential generated) Ag(s) AgCl(s) Cl - (aq) H + (aq, outside) H + (aq, inside), Cl - (aq) AgCl(s) Ag(s) External (outer) reference E H + outside glass, analyte solution H + inside glass Internal (inner) reference E The two reference electrodes serve to measure the electric potential difference across the membrane Internal reference electrode is a part of the glass electrode (role of salt bridge) (H + inside)
Glass Membrane Glasses of certain compositions respond to ph Glass : 22% Na 2 O, 6% CaO, and 72% SiO 2 (Corning 015 glass membrane) Negatively charged oxygen atoms in glass can bind to cations of suitable size Monovalent cations, particularly Na +, can move slowly through the silicate lattice
Glass Membrane The two exposed surfaces swell as they absorb water (hydration) Most of metal cations in these hydrated gel regions of the membrane diffuse out of the glass and into solution H + is the only ion that binds significantly to the hydrated gel layer The H + sensitive membrane may be thought of as two surfaces electrically connected by Na + transport
ph Electrode (Glass Electrode) The potential difference between the inner and outer silver-silver chloride reference electrodes depends on the chloride concentration in each electrode compartment and on the potential difference across the membrane. The chloride concentration is fixed in each electrode compartment The H + concentration is fixed on the inside of the glass membrane The only variable factor is the ph of the analyte outside the glass membrane The voltage of ideal ph electrode changes by 59.16 mv for every ph-unit change of analyte activity at 25 o C The response of real glass electrode is described by the Nertian-like equation A H+ (outside) E = constant + β(0.05916) log ( ) (at 25 o C) A H+ (inside) β: electromotive efficiency, close to 1.00 (typically >0.98) constant: asymmetry potential, arises because no two sides of a real object are identical and small voltage exist even if A H+ is the same on both sides of the membrane.
Calibration of glass electrode A ph electrode should be calibrated with two (or more) standard buffers selected so that the ph of the unknown lies within the range of the standards
Errors in ph Measurement Alkaline error: sodium error Measured ph < actual ph ph electrode responds to Na + at high ph Acid error: Measured ph > actual ph The glass surface is saturated with H + and can not be protonated at any more sites
14-6. Ion-Selective Electrode Most ion-selective electrode fall into one of the following classes: 1. Glass membrane for H + (ph electrode) 2. Solid-state electrodes: based on inorganic crystals 3. Liquid-based electrode: based on hydrophobic membrane saturated with hydrophobic liquid ion exchanger 4. Compound electrode: a species selective electrode + membrane (separation or biochemical reaction)
Chem 7 Test: 70% of tests in performed in the hospital à Na +, K +, Cl -, Total CO 2, glucose, urea, and creatinine
Selectivity Coefficients in ISE No electrode responds exclusively to one kind of ion Membranes respond to a certain degree to ions other than the analyte Selectivity coefficient: relative response of the electrode to different species If an electrode used to measure ion A also respond to X k A, X = response to X / response to A The smaller the selectivity coefficient, the less the interference by X. For Na +, K + = 1/2800 Response of ion-selective electrode 0.05916 E cell = constant + β log [A A + k A,X A x )] n A A = activity of primary ion (A) A X = activity of interfering ion (X) k a,x = selectivity coefficient
Selectivity Coefficients in ISE Example: Fluoride ISE k F-,OH- = 0.1 1.0 x 10-4 M F - in ph 5.5 If ph is raised to ph 10.5 What will be the change in electrode potential? At ph 5.5, E = constant -0.05916 log[1.0 x 10-4 ] = constant + 236.6 mv At ph 10.5 [OH-] = 3.2 x 10-4 M E = constant 0.05916 log[1.0 x 10-4 + (0.1 x 3.2 x 10-4 ] = constant + 229.5 mv The change is 229.5 mv 236.6 mv = - 7.1 mv
Selectivities of a Li ion selective electrode 0 Li + -1-2 Log k Li+, M n+ -3-4 -5 K + Cs + Na + Rb + NH 4 + H + Sr 2+ Ba 2+ Cs 2+ Mg 2+
Solid- state ion-selective Electrode (Fluoride Sensor) Ionization creates a change on the membrane surface. The magnitude of the charge is dependent upon the fluoride concentration in solution LaF 3 LaF 2+ + F - Solid Solid Solution E = constant β (0.0592) loga F- (outside) (0.1M NaF) (LaF 3 doped with EuF 2 )
Liquid Based Ion-Selective Electrode Ca 2+ electrode response : E = constant + β 0.05916 2 loga Ca2+ (external)
Ionophores (neutral carriers) for ISE antibiotic
Ionophores (neutral carriers) for ISE
I. S. E. for Heparin Measurement The monitoring of the concentration of heparin is very important to prevent uncontrolled breeding during surgery Heparin : anti-clotting agent
S. C. Ma, V. C. Yang, B. Fu, and M. E. Meyerhoff, Anal. Chem. 1992, 64, 694.
Gas-Sensing Electrode (compound electrode) Compound electrodes contain a conventional electrode surrounded by a membrane that isolates (or generates) the analyte to which the electrode responds Ag/AgCl external reference electrode ph-sensitive glass membrane ph electrode Gas-permeable membrane (a) ISE Thinsolution layer Thin-solution layer Gas-permeable membrane H + + CO 2 + H 2 O HCO - 3 CO 2 Sample Fig. 28.8 Schematic representation of (a) gas-sensing electrode and (b) membrane and thin layer of electrolyte region of CO 2 electrode. (b)
Gas-Sensing Electrode (compound electrode) Gas-permeable membrane Microporous materials (hydrophobic polymers: polypropylene, Teflon) Homogeneous films (silicon rubber)
Bio-catalytic Membrane Electrode (potentiometric biosensor) Ion-selective or gas-permeable membrane NH 4+ - selective ISE Biocatalyst layer Urea s Enzyme (urease) P 2NH 4+ + CO 3 2- Semi-permeable membrane s
Bio-catalytic Membrane Electrode (potentiometric biosensor)
Microfluidic and Biosensor Chip Technology (Multibiosensor) i-stat Co. (Princeton, NJ) Sodium, Potassium, Chloride, Ionized Calcium, ph and PCO 2 by ion-selective electrode potentiometry. Urea is first hydrolyzed to ammonium ions in a reaction catalyzed by the enzyme urease. The ammonium ions are measured by an ion-selective electrode. Glucose is measured amperometrically. PO 2 is measured amperometrically. Hematocrit is determined conductometrically. Chem 7 test: Na +, K +, Cl -, total CO 2, glucose, urea, creatinine Cartridge label Sample entry well gasket Fluid channel Cartridge cover Sample entry well Tape gasket Biosensor chips Calibrant pouch (contains standard solution) Puncturing barb Blood 20 to 100 µl Cartridge base Air bladder
14-8. Solid-State Chemical Sensors ion selective field effect transistor (ISFET) - Small size - Rapid response - Ruggedness - Inertness toward harsh environment - No hydration step before use
Semiconductors Electrical resistivity of semiconductors lies between those of conductors and insulators (free to move through crystal) Doping (behaves as a positive charge carrier)
Behavior of pn Junction Diodes
Operation of a field effect transistor Nearly random distribution of holes and electrons in the base in the absence of gate potential. Positive gate potential attracts electrons that form a conductive channel beneath the gate. Current can flow through this channel between the source and drain. The potential of the gate regulates the current flow between source and drain
Operation of a chemical sensing FET AgBr (sample: AgNO3) Si3N4 (sample: H+) Ag+ is adsorbed on the AgBr giving it a positive charge and increasing current between the source and drain à the voltage must be applied by an external circuit to bring the current back to its initial value to the response to Ag+ The transistor is coated with an insulating SiO 2 layer and a second layer of Si 3 N 4 (Silicon nitride), which is impervious to ions and improves electrical stability. The circuit at the lower left adjusts the potential difference between the reference electrode and the source in response to changes in the analyte solution, such that a constant drain-source current is maintained.
Response of a silver bromide-coated chemical sensing FET