Electrochemistry studies the processes which involve charge The charge is a source of electric field Element of charge: 1.602 10"19C The energy change is ±1.602-10~19 J if we move the charge across the potential drop of IV If we do the same with 1 mol of charges, we obtain... A 1V B 01---IO , dQ The current is the change of charge per time 1 = FARADAY'S LAWs (1834) "The chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes" "Electrochemical Equivalents coincide, and are the same, with ordinary chemical equivalents" Area under current-time curve (and freqently the current-potential curve also) is the charge!! This way can be deduced how much material was transformed. CONDUCTIVITY Electrolytes: same principle, but conductivity is preferred R * — (Q) A i E , R = —(Ohms law) A The unit for conductivity is G = X — Siemens, what is the unit ' of specific conductivity? + For species KXAY: Migration velocities can be different Stokes force is balanced by force induced by electric field Only the cations distant from the plane by vAor nearer will cross the plane. In unit field (IV) and unit area will cross the boundary xCOcFz M f cations X = xcaFzcatucat +ycaFz u an an CONDUCTIVITY I-1 fl- 0 o V Solutions can have different concentrations: A = X S m = Sm mol m mol A oo a - A A oo This is used for measurements of dissociation constants: AB c(1-oc) A+ + B" COC COC ^ A ^2 D c2a2 (l - a) 1- A A oo HOW AND WHERE THE POTENTIAL DIFFERENCE DEVELOPS J solution 1 solution 2 lembrane Junction potential (Henderson) Donnan potential Potential difference develops where a charge separation in space occurs THE NERNST EQUATION The combination of two basic physical chemistry equations: AG = zFE and t -AG = RT In K \ All processes in which charge separation occurs go to equilibrium ...but what is K? r DANIEL CELL Zn I ZnSO, I I CuSO, I Cu Zn+CuS04^ Cu+ZnS04 zFEMN = RT In [Cu\ZnS04] Zn\CuSOA BACK TO NERNST EQUATION E = E H--log zF a ox a red 303RT = 0.059 V ELECTRODES OF THE FIRST KIND The term electrode is here used in a sense of a half-cell. Metal immersed into the solution of its own soluble salt. The potential is controlled by the concentration of the salt. Zn in ZnCl2, Ag in AgN03, Cu in CuS04 etc. Non-metallic electrodes - gas electrodes (hydrogen and chlorine electrode) THE HYDROGEN ELECTRODE Kifj. 1 Fnim: introductory quantitative anafysh, L.E. Wilson, Merrill, Columbus, 1974, p. 218 (Fig. 9-1) Hi gas-- 1.0 atmosphere Pt electrode coated with ■ Pt black - L.O Fiji. 5 From Undergraduate instrumental analysis, 6th edn., J.W. Robinson, E.M. Skelly Frame, and Ci.M. Frame 11, Dekker, New York, 2005, p. 925 (Fig. 15.4) 1 -A HCl ■ pre-sarturola H, gas, 1 Sim to salt b^jdga- 1 M HCl Fiy. 2 From: Quantitative analysis, 6th edn., R.A. Day Jr. and A.L. Underwood Prentice-Hall, Engkrwood (litis. 1991 p. 262 (Fig. 10.4) H2. 1.0 atm__ • Platinum H *, 1.0 m AniiL RLu4ui4iL Cherti {200 R) 392:9-10 [X)l L0. L007/h002 Lti-00R-2227-l ANALYJ JCAL CJI ALLliNGE THE HYDROGEN ELECTRODE ELECTRODES OF THE SECOND KIND -Contact wire Argentochloride Ag | AgCl | KC11 | Calomel Hg | Hg2Cl2 | KC11 | Mercurysulphate Hg | Hg2S041 K2S04 Hg Hg2Cl2/Hg paste V Saturated KCI 197 mV 244 mV 640 mV SHE sat. Hg/Hg2S04 REDOX ELECTRODES Redox Electrode Redox wnsitive? ft band The electrode serves as an electron sink Redox combo Pt electrode ELECTRODES OF THE THIRD KIND ..just a curiosity Zn I Zn2C204 | CaC2041 CaCl2 We can measure the concentration of Ca2+, but there's a better device to do this... THE ABSOLUTE SCALE OF POTENTIALS ag RED (vacuum) A ION -agsolv(RED) OX (vacuum) A zFAE ABS agsolv(ox) Reference to vacuum (instead of hydrogen electrode) RED (solution) OX (solution) H+ (vacuum) Thermodynamic cycle for hydrogen -ag solv H+ (solution) -ag ION H (vacuum) ■FAE( ABS 1/2 H -agdiss/2 i2 The most commonly accepted value is 4.42V, but values around 4.8V are also reported -5- Vacuum 4420 mV 0 197mV 244mV —I-1—I- 640 mV H SHE % * sat. Hg/Hg2S04 X ION SELECTIVE ELECTRODES Reference electrode Reference bridge solution -B- / volts Ag/AgCI wires ^/(sample) cmem .Working elecfrode Internal solution ISE membrane Membrane potential reflects the gradient of activity of the analyte ion in the inner and outer (sample) solutions. •The trick is to find a membrane material, to which an analyte is selectively bound. The membrane must be conductive (a little bit, at least), but it should not leak PH Electrode pH sensitive glass bulb Liquid junction for reference electrode (sometimes is high) H ■Si-0 \ Si \ Nikolski eq. Li + Li + Li E = E. hydrated Haugaard layers Li ions partially free 400 MO assym RT, +-ma F H30" + X--lnß _i P Na MEMBRANES FOR ISEs •Glass membranes (H+, for other cations change in the composition of glass membrane (A1203 or B203 in glass to enhance binding for ions other than H+ (Na+, Li+, NH4+, K+, Rb+, Cs+ and Ag+) •Crystalline Membranes (single crystal of or homogeneous mixture of ionic compounds cast under high P, d~10 mm, thickness: 1-2 mm. Conductivity: doping or nonstechiometry, Ag+ in AgCl or Ag2S, Cu+ in Cu2S. Fluoride electrode: determines F, LaF3 crystal doped with EuF2). •Liquid membranes (organic, immiscible liquid held by porous (PVC) membrane with ion exchange properties or neutral macrocyclic compouds selectively binding POTENTIOMETRY E(cell) v. Ri Cell and voltmeter behaves as voltage divider circuit E(measured) POTENTIOMETRY AND PHYSICAL CHEMISTRY 1. Activity coefficients determination 2. Solubility products determination 3. Ion product of water determination Pt | H2 | HC11 AgCl | Ag Ag | AgN031 |KN0311KX, AgX | Ag Pt | H21 KOH | |KC11 AgCl |Ag Ionic product of water: 1.008-1014 (25°C) - good agreement with conductivity measurement 3-ELECTRODE CELLS AND POTENTIOSTATS Function Generator Cyclic yoltammograc: Polarizable and nonpolarizable 50 ELECTRODE MATERIALS Inert metals (Hg, Pt, Au) •Polycrystalline •Monocrystals Carbon electrodes •Glassy carbon •reticulated •Pyrrolytic graphite •Highly oriented (edge plane,) •Wax impregnated •Carbon paste •Carbon fiber •Diamond (boron doped) Semiconductor electrodes (ITO) Modified electrodes -2 -i i-r E [V vs. SCE] 0 1 Pt Hg T —r pH=0 pH = 0 pH=7 pH = 14 0.1MEt.NOH 1MHCIO. 0.1MKCI Potential window available for experiments is determined by destruction of electrode material or by decomposition of solvent (or dissolved electrolyte) ELECTRON TRANSFER PHENOMENON Electrode surface 1-10 nm —#- Solvated ions IHL OHL The double-layer region is: Where the truncation of the metal's Electronic structure is compensated for in the electrolyte. 1-10 nm in thickness ~1 volt is dropped across this region... Which means fields of order 1078 V/m ''The effect of this enormous field at the electrode-electrolyte interface is, in a sense, the essence of electrochemistry." [1] [1] Bockris, Fundamentals of Electrodics, 2000 BUTLER-VOLMER AND TAFEL EQUATIONS BUTLER-VOLMER AND TAFEL EQUATIONS ,o=0.7, =0.51 L. a=0.3 0 i0 < i0, a=0.5 X[ ( l - l 0 f(\- exp v V (l-a)nF (E-E ) ■exp ( anF J V RT (E-E ) J J Exchange current density Depends on the species undergoing redox transformation and on the electrode material In fact, large overpotential for hydrogen evolution on Hg surfaces enables us to observe reductions in aqueous solutions Also, the development of modern modified electrodes is based on finding the modifying layer which increase the exchange current density on the electrode surface BUTLER-VOLMER AND TAFEL EQUATIONS MASS TRANSFER . . We try to avoid migration by the addition excess Migration supporting electrolyte Convection Diffusion TRANSPORT BY DIFFUSION THE COTTRELL EQUATION This is how Nernst layer thickness changes over time Rotating disk electrode (RDE) Rotating ring disk electrode (RRDE) current take-off rotating cylinder solution 'rotating disk" Active area of the electrode Current (uA) versus Potential (rnV) 0.000- -100.000' -200.000 -400.000 -500.000 forward & reverse traces are on top □f each other IL = (0,620>z-F-A-D2/3-co1/2-v-1/6-c Levich Equation co speed of rotation (rad-s1), v kinematic viscosity of the solution (cm2,s4), kinematic viscosity is the ratio between solution viscosity and its specific weight. For pure water: v = 0,0100 cm2s For 1.0 moldm 3 KN03 is v = 0,00916 cmV1 (at 20°C). c concentration of electroactive species (in mol.cnr3, note unusual unit) D diffusion coefficient (crn^s1), A electrode area in cm2 -100.000 0.000 100.000 200.000 300.000 400.000 500.000 POLAROGRAPHY reference « electrode V mercury counter electrode 08 -1.0 -1.2 -1.4 -1.6 Halfwave potential Limiting diffusion current (Ilkovic equation) ld = ckFnD1/2m2/3t1/6 (Id is diffusion current (A), (c) concentration of the depolarizer (mol/cm3), k is a constant which includes % and the density of mercury, and with the Faraday constant (F) has been evaluated at 708 for max current and 607 for average current, D is the diffusion coefficient of the depolarizer in the medium (cm2/s), n is the number of electrons exchanged in the electrode reaction, m is the mass flow rate of Hg through the capillary (mg/sec), and t is the drop lifetime in seconds, and c is depolarizer concentration in mol/cm3. 14 CYCLIC VOLTAMMETRY CV - the most important electrochemical metho< Function Generator Cvclic voltammogram A • Potentio:tat .1 11 H i II -1 One or more cycles .. .CV Half cycle ... LSV I ■ verte x 1 cycle reverse scon time 0 Quiet Time NORMAL PULSE VOLTAMMETRY Pulse Widths Step E- <— Sample Period <— Pulse Period Quiet DIFFERENTIAL PULSE VOLTAMMETRY Step E-E f Pulse Amplitude Al ^Pulse Width ZU ^Sample Period ^Sample Period ^Pulse Period Quiet ^ Time y SQUARE WAVE VOLTAMMETRY S.W. Amplitude^ 1/S.W. Frequency 4\ <-> Step E ^Sample Period (if) ^Sample Period (ir) Quiet ^ Time ^ E