THE CHARGE AND CURRENT •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 ©I---IO The current is the change of charge per time 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. Electrolytes: same principle, but conductivity is preferred R l_ A (O) E I {Ohms law) 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 vA or nearer will cross the plane. In unit field (IV) and unit area will cross the boundary xCOtFz U , cations y — xcaFz u + + ycaFz u Ay cat cat ^ an an CONDUCTIVITY 4=r fl- 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-a) A+ + B" ca ca ^ A ^2 D c2a2 (1 - a) A 1- 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 -AG = RT\n K All processes in which charge separation occurs go to equilibrium ...but what is K? Zn ZnSO, CuSO, Cu Zn+CuS04^ Cu+ZnS04 zfEmn=rtJc^z"so^ Zn\CuSOA ...is it OK? BACK TO NERNST EQUATION „0 2303RT. a E = E H--log - ox zF a red 2303RT = 0.059F 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 AgNOs? Cu in CuS04 etc. Non-metallic electrodes - gas electrodes (hydrogen and chlorine electrode) THE HYDROGEN ELECTRODE Fifj. 1 From: infrnductnry quantitative anafysh, L.E. Wilson, Merrill, Columbus, 1974, p. 218 (Fig. 9-1) H;gas-- 1.0 atmosphere Pt elecinxk coated with ■ Pt black - L.O Fiji. 5 Fnim Undergraduate instrumental analysis, 6lb edo., J.W. Robins™, E.M. Skell}' Frame, and Ci.M. Frame 11, Dekker, New York, 2005, p. 925 (Fig. 15.4) pre-sarturola H, gas, 1 Sim to salt ttfjdga- 1 M HCI 1'ijj. 2 Fnim: Quantitative analysis, 6th edn., R.A. Day Jr. and A.L. Underwood, Prerj[iue-Hall, Eoglewood CI if is, 1991 p. 262 (Fig. 10.4) -- Platinum Hi, 1.0 aim__ h\ i.O. ro __J AniiL RLu4ui4iL Cherti {200 fi) 392:9-10 DO\ LO. L007/h002 Lti-00R-2227-l ANALYJ JC'AL 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 Saturated KCI 197 mV 244 mV 640 mV SHE sat. Hg/Hg2S04 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 | ZnC204 | CaC2041 CaCl211 We can measure the concentration of Ca2+, but there's a better device to do this... THE ABSOLUTE SCALE OF POTENTIALS AGION RED (vacuum) - * OX (vacuum) i l -AGsolv(RED) -AGsolv(OX) ZFAEABS RED (solution) - » OX (solution) Reference to vacuum (instead of hydrogen electrode) H+ (vacuum) Thermodynamic cycle for hydrogen -ag solv H+ (solution) -AG ION H (vacuum) ■FAE( ABS 1/2 H -AGdiss/2 The most commonly accepted value is 4.42V, but values around 4.8V are also reported -JT- Vacuum 4420 mV 0 197mV 244mV —I-1—I- 640 mV —I \ sat. Hg/Hg2S04 ION SELECTIVE ELECTRODES Reference electrode Reference bridge solution -B- / volts Ag/AgCI wires ^/(sample) J)(inl) .Working electrode 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 hydrated Haugaard layers Li ions partially free 400 MO E = E assym RT, H--ma F H30 + X--lntf + P Na MEMBRANES FOR ISEs •Glass membranes (H+, for other cations change in the composition of glass membrane (A1203 or B2Os 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 a voltage divider circuit E(measured) POTENTIOMETRY AND PHYSICAL CHEMISTRY 1. Activity coefficients determination 2. Solubility products determination 3. Ion product of water determination Ag | AgN031 |KN0311KX, AgX | Ag Ionic product of water: 1.008 TO-14 (25°C) - good agreement with conductivity measurement 3-ELECTRODE CELLS AND POTENTIOSTATS Polarizable and nonpolarizable ELECTRODE MATERIALS Inert metals (Hg, Pt, Au) •Polycrystalline •Monocrystals Carbon electrodes •Glassy carbon •reticulated •Pyrrolytic graphite •Highly oriented (edge •Wax impregnated •Carbon paste •Carbon fiber •Diamond (boron doped) plane,) Semiconductor electrodes (ITO) Modified electrodes -2 -1 Pt Hg pH=0 pH = 7 pH = 14 pH=0 pH=7 pH = 14 O.lMEt, NOH 1MHCIO, 0.1 M KG 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 107-8 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 ,,01=0.7 IL, Quiet Time NORMAL PULSE VOLTAMMETRY Pulse Widths Step E- <— Sample Period <— Pulse Period Quiet DIFFERENTIAL PULSE VOLTAMMETRY Step E-E T 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 ^ AC voltammetry E