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Analytical Chemistry > Electroanalytical Methods > Flashcards

Electroanalytical Methods Flashcards

Potentiometry, Coulometry, Electrogravimetry, and Voltammetry

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1
Q

Define potentiometric method of analysis

A

measuring galvanic cell potentials (electrochemical cells in the absence of appreciable current) to obtain chemical information (concentration)

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2
Q

Basic components of potentiometric analysis and usual arrangement

A
  1. Reference electrodes
  2. Indicator electrode
  3. Salt Bridge
  4. Potential Measuring Device

Ref Electrode | Salt Bridge | Analyte Solution | Indicator Electrode

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3
Q

Define (liquid) junction potential

A

Voltage difference (small voltage; mV; cation rich and anion rich) that develops at the interface when 2 dissimilar electrolyte solutions are in contact (found at each end of a salt bridge connecting 2 half-cells)

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4
Q

Define reference electrode and its desired characteristics (5)

A
  1. Known and stable potential
  2. Constant response
  3. Independent of the type of solution in which it is immersed
  4. Rugged and easy to assemble
  5. reversible and obey Nernst equation
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5
Q

Explain SHE equation and its component

A

Standard Hydrogen Electrode
2H+ (aq) + 2 e- <===> H2 (g)

  1. 1M HCl solution with Pt foil coated with black Pt
  2. H2 gas is passed through a tube and into the solution (bubbled continuously at 1 atm)
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6
Q

Why platinum is used in SHE?

A

Pt is inert and will not react much with hydrogen

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7
Q

Explain Calomel Reference Electrode and its equation

A

Consist of mercury in contact with a solution that is saturated with mercury (I) chloride (calomel) and contains a known concentration of potassium chloride.

Hg2Cl2 (s) + 2 e- <=> 2 Hg (l) + 2 Cl- (aq)

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8
Q

Explain Silver/Silver Chloride Reference Electrode and its equation

A

Silver electrode is immersed in a solution of potassium chloride that is saturated with silver chloride

AgCl(s) + e- <=> Ag (s) + Cl-

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9
Q

Advantage and disadvantage of silver-silver chloride electrodes vs calomel electrodes

A

silver-silver chloride electrodes can be used at temperatures greater than 60°C

mercury (II) ions can react with fewer sample components than silver ions which can react with proteins

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10
Q

At 25°C, the potential of the saturated calomel electrode versus the standard hydrogen electrode is ____. For the saturated silver/silver chloride electrode, it is ____.

A

calomel: 0.244 V (0.241 V in other sources)
Ag | AgCl: 0.199 V (0.197 V in other sources)

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11
Q

Define indicator electrode

A

Electrode that responds rapidly and reproducibly to variations in the activity/concentration of an analyte ion or molecule

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12
Q

Define electrodes of the first kind

A

Pure metal electrode that responds to its cation in the solution within a single reaction

Cu2+ (aq) + 2 e- <=> Cu(s)

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13
Q

Why electrodes of the first kind are not widely used for potentiometric determinations?

A
  1. Not very selective and specific
  2. Many metal electrodes can only be used in neutral or basic solutions (dissolved in presence of acids)
  3. Other metals are so easily to oxidized (deaerated solution is a must)
  4. Harder metals do not provide reproducible potentials
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14
Q

Define electrode of the second kind

A

Metal electrode that is used to determine the concentration of an anion that forms a precipitate or a stable complex with the cation of the electrode metal

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15
Q

Define electrode of the third kind

A

Metal electrode at which at certain circumstances can be made to respond to a different cation (involves secondary redox couple)

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16
Q

Define inert metallic indicator

A

Inert metal electrode (Au, Pt, Pd) which served as indicator electrode for redox systems

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17
Q

Define membrane indicator electrode

A

Also called p-ion electrodes (data is usually presented as p-functions) or ion-selective electrode (ISE)

Consists of thin membrane separating 2 solutions of different ions concentration

(2 solutions of different hydrogen concentrations for the determination of pH)

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18
Q

Two setups of glass pH electrodes

A
  1. Glass electrode (indicator) and SCE (reference) immersed in a solution of unknown pH
  2. Combination probe consisting of both an indicator glass electrode and a silver/silver chloride reference
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19
Q

Explain boundary potential

A

Develops when two side of the membrane are immersed in solutions having different ion concentration (e.g., H+ ions; outer and inner concentration)

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20
Q

Explain asymmetry potential

A

Develops when the concentration of the solution on both side of the membrane is the same and arises from dissimilarities between inner and outer surface of the membrane.

(usually due to imperfection in the manufacturing)

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21
Q

Explain glass indicator potential

A

Potential with the contribution of boundary, asymmetry, and internal reference electrode potential

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22
Q

What should be the slope in pH calibration curves?

A

0.0592 V

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23
Q

Differentiate Acid and Alkaline Error

A

Alkaline error - measured pH is low due to the presence of alkali ions at solutions with pH 10 to 12 or greater (glass surface become responsive to not only hydrogen ions but also alkali metal ions)

Acid error - measured pH is high when the pH of the solution is less than 0.5 due to saturation effect that occurs when all surface sites on glass are occupied with H+ ions.

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24
Q

Define solid state (crystalline) membrane electrodes

A

Has a membrane that consists of either a polycrystalline inorganic salt or a single crystal of an inorganic salt

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25
Q

Define liquid-based membrane electrodes

A

Has a hydrophobic liquid membrane impregnated with a hydrophobic ion exchanger that is selective for analyte ion

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26
Q

Define permeable membrane electrodes or gas sensing electrode

A

consists of a porous/permeable membrane that will selectively interact with specific ions in a solution

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27
Q

Explain enzyme electrode

A

A potentiometric biosensor that immobilizes an enzyme at the surface of a potentiometric electrode that will react selectively with analyte.

The analyte’s reaction with the enzyme produces a product whose concentration is monitored by the potentiometric electrode

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28
Q

Explain the titration curves (4) in potentiometric titration

A
  1. Normal titration - sigmoidal curve (pH vs volume); equivalence point at sudden change in pH
  2. First derivative - prominent peak (∆pH/∆V vs volume); equivalence point at prominent peak
  3. Second derivative - rational function {[∆(∆pH/∆V)]/∆volume vs volume}; equivalence point at the center of two curves
  4. Gran plot - volume of base x [H+] vs volume of base; equivalence point at the straight line after the slope
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29
Q

How to get the first derivative plot given the volume and pH

A

x values: get the average of the two consecutive volumes
y values: change in pH / change in volume

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30
Q

How to get the second derivative plot given the volume and pH?

A

x values: get the average of the two consecutive volumes
y values: [∆(∆pH/∆V)]/∆volume

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31
Q

Define electrolysis

A

Process in which a chemical reaction is forced to occur at an electrode by posed voltage

(forcing a nonspontaneous reaction to happen by applying voltage)

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32
Q

Define overpotential and its three factors

A

Additional voltage required to overcome the effects of kinetic barrier. Difference between theoretical and actual potential

  1. Ohmic Overpotential (IR)
  2. Activation Overpotential (AO)
  3. Concentration Overpotential (CP)
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33
Q

Define ohmic overpotential and formula

A

voltage needed to overcome internal resistance of the cell

E ohmic = IR
(obey Ohm’s law)

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34
Q

Define activation overpotential

A

voltage required to overcome the activation energy of an electrode reaction

35
Q

Define concentration overpotential

A

voltage required to overcome the concentration gradient (occurs when the concentration of the electroactive species near an electrode is not the same as its concentration in the solution)

36
Q

Define the parts of three-electrode cell

A
  1. Working Electrode/ Generator Electrode
    - electrolysis (redox) takes place
  2. Auxiliary Electrode/ Counter Electrode
    - same material of working electrode; part of the other half-cell
  3. Reference Electrode
    - constant potential electrode
37
Q

Differentiate between galvanostat/amperostat and potentiostat

A

galvanostat/amperostat - control current
potentiostat - control potential (voltage)

38
Q

Three measuring devices used in electrolysis

A
  1. Electrometer - measures charge consumed by analyte
  2. Ammeter - measures current
  3. Potentiometer - measures potential (voltage) without drawing a current or altering cell’s composition
39
Q

Define coulometry

A

Measures the amount of electrolyzed substance (oxidation of a species is changed) by measuring the total electric charge (q) that passed through the cell

40
Q

Define Faraday’s Law of Electrolysis

A

the amount of substance deposited or liberated at an electrode is directly proportional to the quantity of electricity passed through the solution

41
Q

Charge of electron = ___
Faraday’s constant = ____

A

Charge of electron = 1.6022 x 10^-19
Faraday’s constant = 96,485 C/mol e-

42
Q

Define the types of coulometric methods

A
  1. Controlled-Potential (potentiostatic) coulometry
    - voltage is constant
  2. Controlled-Current (amperostatic) coulometry
    - current is constant
43
Q

Fundamental requirement for both coulometric methods

A

100% current efficiency whereas all electrons participate in the electrochemical process
(more current will be taken -> over-estimate of analyte if not)

44
Q

The current is a function of ____ and the total charge during electrolysis is the _____ in potentiometric coulometry

A

The current is a function of time and the total charge during electrolysis is the area under the curve

45
Q

Potentiometric coulometry is carried out in __ (3)

A
  1. small-volume electrochemical cells
  2. using electrodes with large surface area
  3. high stirring rates
46
Q

In amperostatic coulometry, the quantity of charge required to attain the end point is calculated from _____

A

the magnitude of the current and the time of its passage

q = It
area under the curve (square/rectangle graph)

47
Q

How to maintain 100% current efficiency?

A

Mediator is used such as Ce4+

  1. Mediators are redox-active species -> rapid electron transfer
  2. Selective of the analyte
48
Q

Define electrogravimetry

A

Analyte is quantitatively deposited as a solid on the cathode or anode and the mass of the electrode directly measures the amount of analyte

49
Q

Define voltammetry

A

Electrochemical technique to measure the current response of a system as a function of applied potential (under conditions that encourage polarization of a working microelectrode) to provide information about the analyte

50
Q

____ of material is electrolytically reduced or less commonly oxidized in voltammetry

A

Only a sample portion

51
Q

General concept of voltammetry

A

Sample -> excitation process -> (signa) -> detector/transducer/sensor -> (voltage) -> readout

52
Q

What happens during excitation process in voltammetry?

A

voltage is applied to analyte; appreciate current is measured

53
Q

What is the role of detector/transducer/sensor in voltammetry?

A

current is transformed to voltage by electronics

54
Q

Readout device can view current as a function of ____ (4)

A
  1. analyte concentration
  2. how fast analyte move to electrode surface
  3. rate of electron transfer to sample
  4. voltage time
55
Q

Describe the three-electrode system in voltammetry

A
  1. Working Electrode
    - where redox occur (Pt, Au, C, Hg)
  2. Auxiliary Electrode
    - conducts current to working electrode (coil of Pt, large in area)
  3. Reference Electrode
    - established (constant) potential
56
Q

What is a Quasi Reference Electrode (QRE)?

A

a wire at which some (unknown) redox process occurs in solution

57
Q

Advantages of Mercury Working Electrode (3)

A
  1. Easily renewable
  2. Large potential window
  3. Reversibly reduced to amalgams
58
Q

Differentiate types of Hg Electrodes (4)

A
  1. Hanging Mercury Drop Electrode (HDME)
    - drop of Hg is suspended from a capillary tube
  2. Dropping Mercury Electrode (DME)
    - successive drops of Hg form at the end of a capillary tube as a result of gravity
  3. Static Mercury Drop Electrode (SME)
    - successive drops of Hg form at the end of capillary tube as a result of mechanical plunger
  4. Mercury Film Electrode
    - metallic solution of mercury with another metal
59
Q

Differentiate currents in voltammetry (5)

A
  1. Faradaic Current - from redox rxns, working electrode, and auxiliary electrode
  2. Cathodic Current - faradaic current; due to reduction of analyte; postive sign
  3. Anodic Current - faradaic current; due to oxidation rxn at working electrode; negative sign
  4. Residual Current - current that flows in the cell even without electroactive species (analyte)
  5. Non-Faradaic Current/ Changing Current
    - type of residual current that arises from processes that do not involve electron transfer between analyte and electrode
    - causes the flow of residual current
60
Q

Describe the axes of voltammogram

A

x-axis: applied potential
y-axis: current

61
Q

Differentiate limiting current, peak current, and half wave potential

A

Limiting current - plateau in voltammogram
Peak current - highest point in voltammogram
Half-wave potential - 1/2 of limiting current

62
Q

Differentiate the three ways of mass transport

A
  1. Migration - Movement of charged species in response to electric field
  2. Convection - Occurs when we mechanically mix the solution, carrying reactants toward the electrode and removing products from the electrode
  3. Diffusion - Occurs whenever the concentration of an iron or molecule at the surface of the electrode is different from that in bulk solution
63
Q

Migration should be ____ by _____

A

Should be minimized by adding excess inactive supporting electrolyte

64
Q

Convection is enhanced by ____ with ____

A

enhanced by stirring the solution with stir bar

65
Q

In diffusion, the distance from electrode increases with ____

A

increasing time

66
Q

Father of electroanalytical chemistry

A

Jaroslav Heyrovský

67
Q

Describe polarography

A

Cell current is measured in an unstirred solution during electrolysis at various potentials using a dropper mercury electrode (DME)

68
Q

In polarography, the applied voltage is gradually ____ typically by going to a more positive E so _____ is observed

A

increased; a small residual current

69
Q

In polarography, when the voltage becomes great enough, ____ occurs at the ____ causing a _____

A

reduction; working electrode; current

70
Q

In polarography, the electrode is rapidly saturated, so the ____ production is limited due to the ____ to the ___ electrode

A

current; diffusion of analyte to the small electrode

71
Q

In polarography, the reduced species alters the surface of the mercury electrode. The mercury surface is renewed by _____ – providing a fresh
surface which results in an oscillation of the
data as it is collected

A

‘knocking off’ a drop

72
Q

Difference of Hydrodynamic Voltammetry with normal polarography

A

DME is replaced with a solid electrode and the solution is mechanically stirred during the analysis, either using a stir bar or by rotating the electrode

73
Q

Define pulse polarography

A

Type of voltammetry that uses a series of short, square-wave pulses of potential (improved sensitivity and resolution)

74
Q

Stripping voltammetry involves two processes. Define each

A
  1. Deposition: Analyte ions are preconcentrated at working electrode surface by applying constant potential that is more negative than the reduction potential of analyte either by reduction or oxidation
  2. Stripping: rapid oxidation or reduction to strip the products back into the electrolyte
75
Q

In stripping voltammetry, the current response is proportional to ____

A

its concentration

76
Q

Describe anodic stripping methid

A
  1. Cathodic deposition - applying negative potential, reduction of analyte cation, converted to insoluble form (deposited), stir
  2. Stop stirring to equilibrate
  3. Stripping - linear sweep, re-oxidation of deposited analyte, released back into solution

tl;dr
deposition - reduction
stripping - oxidation

77
Q

Describe cyclic voltammetry

A

The current response of a small stationary electrode in an unstirred solution is excited by a triangular voltage waveform

Linear voltage ramp from t0 to t1 is reversed back to the initial value at t2 (forms a triangle) that is repeated many times

78
Q

Differentiate US and IUPAC convention of triangular waveform

A
  1. US
    - High to low potentials from left to right
    - Reduction (above) and Oxidation (below)
    - Intersects at right side (low potential)
    - left to right movement
  2. IUPAC
    - Low to high potentials from left to right
    - Reduction (below) and Oxidation (above)
    - Intersects at left side (low potential)
    - right to left movement
79
Q

Differentiate Ilkovic and Randles-Sevcik Equation

A
  1. Ilkovic - Relationship between the analyte’s concentration and the limiting current
  2. Randles-Sevcik - Describes the effect of scan rate on the peak current
80
Q

Describe amperometry

A

Constant potential is applied to the working electrode and the response current is measured as a function of time

81
Q

used to measure dissolved oxygen by amperometry

A

Clark Oxygen Electrode

82
Q

In amperometric titration, the measured current at a suitable applied potential is evaluated as a function of the ____

A

added titrant

83
Q

In amperometric titration, the ___ should be able to generate a current while _____ should never generate a current at the potential being applied to the system

A

analyte and/or titrant - generate current
products - never generate current

84
Q

Describe different plot of current vs volume of titrant in amperometric titration

A
  1. Decreasing curve with plateau at the end
    - only analyte produces a current
  2. Increasing curve with plateau at the start
    - only titrant produces a current
  3. Parabolic curve (letter U or V)
    - both analyte and titrant produce a current

Analytical Chemistry (12 decks)

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