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Electrochem group 2


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Electrochem group 2

  2. 2. ELECTROANALYTICAL METHOD Study an analyte by measuring the potential (volts) and/or current (amperes) in an electrochemical cell containing the analyte. 3 main categories : - potentiometry : the difference in electrode potential is measured - AMPEROMETRY : the cell current is measured over time - voltammetry : the cell current is measured while actively altering the cells potential.
  3. 3. POTENTIOSTAT Potentiostat are amplifiers used to control a voltage between two electrode, a working electrode and a reference electrode, to a constant value ~ Reference electrode is used to maintain a constant voltage referred to the potential of the hydrogen electrode ~ Counter electrode is used to maintain a constant potential difference between the reference electrode and the working electrode
  4. 4. Function of Potentiostat To measure the potential difference between the working electrode and reference electrode without polarizing the reference electrode To compare the potential difference to a preset voltage and force a current through the counter electrode towards the working electrode in order to counteract the difference between preset voltage and existing working electrode potential
  5. 5. Characteristics of Potentiostat1) Control speed Speed of a potentiostat is measured in terms of „small - signal rise – time‟, bandwidth rate. Potentiostats have bandwidths from 100 kHz to some MHz.2) Accuracy The potentiostat counterbalances voltage difference between set control voltage and existing cell3) Current range and dynamics High currents are required from the potentiostat.
  6. 6. 4) Noise In potentiostats, the most sensitive circuit is the input stage, producing noise in the input resistor and the first amplifier stage. Good potentiostats are equipped with low-noise amplifiers.5) Stability
  7. 7. VOLTAMMETRIC ANALYZER Advantages of the voltammetric analysis: i) High absolute sensitivity ii) Low cost of singular analysis iii) Multielement determination in one sample iv) Express analysis v) Safety for the operator vi) Excluding work with the metallic mercury (determination on the solid electrodes)
  8. 8. POTENTIOSTATIC CIRCUITRY Referred also as control circuitry. Main purpose:i. To maintain a voltage between the reference electrode and the working electrodeii. To control the electro-chemical reactioniii. To deliver an output signal proportional to the WE current• -Also provides the current to the counter electrode to balance the current required by WE
  10. 10.  A circuit that generates a sweep voltage which increases linearly in value during one cycle of sweep, then returns to zero suddenly to start the next cycle. Voltage ramp generator is formed- feedback resistor of the inverting voltage amplifier is replaced by a capacitor. If the input voltage, V1 is constant and RC = 1s then the output voltage Vit after a time t is given Vo = Vo by: The output voltage rises steadily with time.
  11. 11. OPERATIONAL AMPLIFIER Is an extremely versatile electronic device. Versatility stems from the very high voltage gain together with high input resistance and low output resistance. Op-amp are direct coupled devices such that the input signal may be either AC or DC.
  12. 12. • All op-amp have two inputs connected in a differential mode, so that output voltage:V₀ =A(V₊ - V₋)V₊ = voltage at non inverting inputV₋ = voltage at inverting inputA = open loop gain of the op-amp
  13. 13. Operational Amplifier (Op-Amp) Very high differential gain +V cc In p u t 1 High input impedance + V o Low output impedance V d O u tp u t Provide voltage changes  In p u t 2 (amplitude and polarity) R ~ in f in -V R ~ 0 cc out Used in oscillator, filter and instrumentation Vo  G dVd Accumulate a very high G d : differenti al gain normally gain by multiple stages 5 ver y large, say 10 Operational Amplifier
  14. 14. Single-Ended Input + • + terminal : Source V o • – terminal : Ground~ V i • 0o phase change  + V o • + terminal : Ground • – terminal : Source  • 180o phase change ~ V i Operational Amplifier
  15. 15. Double-Ended Input • Differential input +V d V o • Vd  V  V ~ • 0o phase shift change  between Vo and Vd + V o~ V 1  ~ V 2Operational Amplifier
  16. 16. Common-Mode Operation• Same voltage source is +applied V o at both terminals • Ideally, two input are equally V i ~ amplified• Output voltage is ideallyzero due to differential voltage is zero• Practically, a small output signal can still be measured Operational Amplifier
  17. 17. Ideal Op-Amp ApplicationsAnalysis Method :Two ideal Op-Amp Properties:(1) The voltage between V+ and V is zero V+ = V(2) The current into both V+ and V termainals is zero +V in V o  Rf Ra Operational Amplifier
  18. 18. Non-inverting Amplifier(1) Kirchhoff node equation at V+ yields, V  V V in +  i V o (2) Kirchhoff node V o V   0 V   equation at V  0 Rf yields, a R Rf Ra(3) Setting V+ = V– yields Vi Vi  Vo Vo Rf  0 or  1 Ra Rf Vi Ra Operational Amplifier
  19. 19. v+ v+vi + vi + vo R1 vo R2 v- v-   Ra Rf Ra Rf Noninverting amplifier Noninverting input with voltage divider Rf Rf R2 v o  (1  )vi v o  (1  )( )vi Ra Ra R1  R 2 v+ v+ + v i + vi v vo R 1 v- o v- R 2   Rf R f Less than unity gain Voltage follower R2 vo  vi vo  vi R1  R 2 Operational Amplifier
  20. 20. Inverting Amplifier(1) Kirchhoff node equation at V+ Rf yields, V   0 Ra  V o(2) V in  V Kirchhoff _nodeequation at V V V  o 0 V ~in + yields, a R Rf(3) SettingV = fV– yields Vo + R Notice: The closed-loop gain Vo/Vin is V in Ra dependent upon the ratio of two resistors, and is independent of the open-loop gain. This is caused by the use of feedback output voltage to subtract from the input voltage. Operational Amplifier
  21. 21. COMPUTER READOUT Computer use digital signals (0 & 1) instead of analog signals (0-10 V) Interfacing a potentiostat with a computer requires translation back and forth Modern potentiostats have on-board DAC (digital to analog converters) and ADC (analog to digital converters)
  22. 22. External (strirrer, T, …)01001010… 0-10 V DAC P-stat ADC10010100… 0-10 V External ( spectro, pH, …)
  23. 23. DAC Digital to analog converter -Defines the smallest possible step - Multiple channels working as indipendent function generators
  24. 24. ADC Analog to digital converter -ADC is a digital filter - Multi-channel ADC to convert several analog signals into digital
  25. 25. ANALYTICAL SOFTWARE - Autolab potentiostat
  26. 26. Autolab potentiostat External (RDE, strirrer, T, …) 01001010… 0-10 V DAC 01001010… 0-10 V MODULE P-stat 10010100… 0-10 V ADC External (QCM, spectro, pH, …)
  27. 27. Autolab potentiostat other D/A modules Scangen module: true linear scan generator  Generates an analog scan (no staircase) with scan rates up to 250,000 V/s FRA module: frequency response analyzer  Digital to analog sine wave generator
  28. 28. FARADAY CAGE 3 conditions: - Very small currents are to be measured (current down to a few picoamperes) - The electrolyte has low conductivity - The reference electrode system has high source resistance make fast measurements of small currents. useful for eliminating electrical interference, especially line frequency noise. If the electrochemical cell is picking up electrical noise from the environment, the additional use of Faraday cage is strongly recommended
  29. 29. • Protect not just from static electrical charge but also fromelectromagnetic waves; this is known as electromagneticshielding.• This is how cars protect you from lightning: The charge isconducted along the outer layer of the metal but does not
  30. 30. OXYGEN REMOVAL Most experiments require dissolved oxygen be removed from the cell. This is because:i. Electrochemically active across the cathodic potential rangeii. Very likely to react with products formed by electron transfer To remove the dissolved oxygen:i. The solution is purged with an inert gas (N2 or Ar) for 5 – 10 mins prior to the experimentii. A “blanket” of inert gas maintained above the solution during the experiment
  31. 31. Both cells have a fritted sparge tube to allowdeoxygentation of the solution with inert gas