Electrochemistry

Front 1

Electrochemistry

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• The study of the exchange between electrical and chemical energy
  • Types of Charge: positive and negative

Front 2

Electrochemical principle

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• Metal strip inserted into a solution of its own metallic ions
• A electrical potential is developed between the metal and solution
• A voltage is produced as the metal dissolves
  • Produced electrons act as an electrode

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The flow of electrons in a conductor is called

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Current

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There will only be current when

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there is a potential difference across the conductor

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Potential difference is measured in

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Volts by a voltmeter

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Electromotive Force emf

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• The potential difference between two electrodes of the electrochemical cell when there is no current flowing through the cell

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Voltage refers to

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• refers to the same thing as emf but with current flowing
• Used in connection with the electrodes
  • Measurement unit of potential= volt

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E=IR

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• E: electromotive force (Volts)
• I: current (amperes)-flow of charged ions
• R: resistance (Ohms)-frictional force that opposes current flow
• Potential=currentxresistance
  • Volts=amperesxohms

Front 9

Nernst equation

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Measure the current or voltage the amount of ions can be quantified

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Ion activity and concentration

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• Ion activity can be related to concentration of the analyte
  • Ion activity=activity coefficientXion molal concentration

Front 11

Two types of flow in electrochemistry

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1. Conduction of electrons tru metal electrodes and wires
   1. Conduction of a charged species, or an ion, in a solution

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Electrochemical Cell

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• Interface between chemical system and an electrical system
  • Consists of two or more electrodes

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Electrochemical Components

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• Consista of two half cells
• Salt bridge
  • Each half cell consists of an electrode and electrolyte solution

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Electrolytes

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• Dissolve in water
  • dissociate into charged particles (cations/ions)

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Oxidation

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• Occurs at one electrode
• Anode
  • Loss of electron at the anode

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Reduction

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• Cathode
  • Gain of electrons at the cathode

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Which electrode will be the anode or cathode depends on

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Relative reduction potentials of the respective electrodes

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Electrolytic

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• External Energy is needed to bring about a physical or chemical change
  • Nonspontaneous reaction

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Galvanic

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• Cell provides electrical energy derived from a physical or chemical change
• As electrodes are connected a spontaneous flow of electrons form the electrode with lower electron affinity
  • Spontaneous reactions

Front 20

Zn and Cu Electrochemical cells

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• Zn to Zn 2+ + 2e-(oxidation)
  • Cu3+ +2e- to Cu (reduction

Front 21

Galvanic cells v. Electrolytic cell

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• Galvanic: spontaneous
  • Electrolytic: nonspontaneous

Front 22

Standard Reduction Potentials

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Zn2+ +2e- to Zn -0.7628 V

Cr2+ +2e- to Cr -0.913 V

Ni2+ + 2e- to Ni 0.257 V

2H+ + 2e- to H2 0.000 V

Cu2+ + 2e- to Cu +0.3419 V

Ag+ + e to Ag +0.7996

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Reference electrode

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• Difference in the electrodes is measured; to calculate the difference one is called the reference electrode
  • Fixed reference

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Indicator Electrode

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Half-cell potential varies in response to the analyte activity in solution

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3 Main reference Electrodes

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• Hydrogen electrode
• Calomel or Mercury electrode
  • Silver/silver chloride

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Hydrogen reference electrode

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• Primary reference electrode
• Consist of platinum wire in contact with H ions
• pH: 0
• 1 atm
  • Used to determine: accuracy of references and indicator electordes, stability of standard solutions

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Secondary Reference electrodes

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• Silver/silver chloride- can be used at high temps
  • Saturated calomel electrode- unstable at high temperatures

Front 28

Electrochemical techniques

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• Potentiometry
• Voltametry
• Amperometry
• Coulometry
• Polarography
  • Conductivity

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Potentiometry

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• Measurement of electrical potential between two electrodes in an electrochemical cell
• Voltage measured by a voltmeter
• Current maintained at 0; no current flow
  • Reference and Indicator electrode, voltmeter

Front 30

Indicator Electrodes

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• Glass
• Liquid and polymer
• Solid state membrane
• Gas permeable membrane
  • enzyme

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Ion selective Electrodes (ISE)

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• Selective for an analyte
• very sensitive and selective for ion
  • magnitude of the potential across the membrane= analytes activity
    • so therefore concentration

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Types of ISE electrodes

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1. Hydrogen (glass electrode)
2. Sodium (glass electrode)
3. Potassium ( polyvinylchloride/valinomycin)
4. Ammonia (NH4+) (glass electrode)
   1. Chloride (silver silver chloride)

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Indirect Vs. Direct ISE

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• Indirect: measures the sample analyte after it has been diluted into a solution
• Direct ISE: measure the sample analyte on undiluted sample
  • Direct more accurate

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Indirect ISE

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• There is always a sample dilution procedure involved
• Potentiometry; measurements independent of volume
  • Indirect potentiometry: Original volume is taken into account but will introduce insignificant error
    • Must be calibrated

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Sodium Error

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• Sample with large amounts of lipid or protein causes dilutional error that can be significant in indirect potentiometer
• Hyperlipidemia and hyperproteinemia: pseudo-hyponatremia by indirect pot.
  • Direct potentiometer will reveal true sodium concentration

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Glass Electrode

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• Thin walled bulb
• Composed of cation responsive to glass sealed to a stem on non-responsive glass
• Composition of the glass walled bulb determines electrode sensitivity
  • Measures pH and Na+

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pH electrode

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• Establish electrical potential
• responds to changes in H ion activity in a solution
  • Potential is dependent upon the pH of the solution

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Liquid and polymer membrane

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• Use an ionophore to select the analyte
• Valinomycin: most common used to measure Na+
  • Ionophores also available for: sodium, ammonia, calcium, lithium, magnesium, and chloride

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Solid state membrane

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• Membrane contian low solubility crystals of the ion of interest
• Usually direct ISEs
  • Dry slide technology

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Metering process

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• Equal amts of sample fluid and electrolyte reference fluid are measured out onto drop holes on potentiometric slide
• Electrolyte ref. contains a known amt of ions
  • two drop flow toward ea other and form a junction the EFR will cause charged half cells this difference can be measured

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Gas sensing electrode

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• Similar to glass but contains thin gas permeable mem and electrodes designed for a specific gas
• p02 electrode: O(g) draw electrons
  • Ammonia electrode- change in pH of ammonia gas

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Oxygen Electrode

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• Clark electrode
• Reduces oxygen at platinum cathode
• Electrons drawn from Ag/AgCl ref. electrode
  • Platinum catode requires external charge: polarizing charge from a mercury battery

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CO2 electrode

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• Severinghaus electrode
• Measures changes in pH
  • gaseous CO2 passes through the membrane and changes the pH of the carbionic acid solution

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Enzyme Electrodes

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• ISE coated with immobilized enzymes that catalyze a specific reaction
• Urease used to detect urea
  • Glucose used to detect glucose, used with a pH electrode
    • ISE is determined based on reaction product that is produced

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Glucose measurement

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• Glucose oxidase is immobilized on an electrode
• Amount of glucose is directly proportional to the increased oxygen that is produced
  • Can measure the pH or the oxygen produced

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Voltammetry

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• Study solution composition based on the current potential obtained when the potential is varied under the control of a potentiostat
• measure current amps
• potential is applied to electrochem cell and the resulting current or charge is measured
• 3 electrodes used:working, ref, auxillary
  • Potential applied between working and ref

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Amperometry

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• Type of voltammetry
• Measures current flow from ox-red rxn
• voltage is held constant and current is measured
• use gas permeable mem.
  • amount of current is proportional to analyte concentration

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Uses of Amperometry

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• Oxygen electrode
• Voltage applied between indicator and platinum electrode and silver silver chloride ref electrode; promote red of O at platinum electrode
  • electrons flow from anode to cathode
    • Glucose electrode

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Uses of Amperometry: Glucose electrode

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• Glucose oxidase-immobilized between to membranes
• Reacts with glucose to produce H2O2
• H2O2 crosses the mem and oxidized to O2
  • Current of oxidation is proportional to glucose concentration

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Biosensors

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• Covernts biological responses to an electrical signal
• Potentimetric
• Amperometric
• Thick and thin film technolgy
  • Pulse oximetry: pulse rate and oxygen saturation non-invasively

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Coulometry

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• Constant current applied to generate a titrating agent
  • Time required to titrate a sample at a constant current is related to the sample concentration

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Chloridometer

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• Anode: Ag to Ag + e
• Cathode: Ag + Cl to AgCl
  • Anode electrode will decrease over time

Front 53

Sweat chloride

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• Cystic fibrosis; high conc of salt in sweat
• Measured in a sample of sweat that is collected on a sterile gauze square
  • pilocarpine lontophoresis

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Polarography

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• Dual measurement of current flowing tru an electrochem cell
• The ele. potential between two electrodes gradually increases by an outside voltage
• analytes are reduced at specific voltage, once reduced current will flow in proportion to analyte
  • measures current

Front 55

Conductivity

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• Measurement of current flow between 2 nonpolarized electrodes
• Constant voltage
• Inverse of resistance
  • used to measure the purity of water