Ana Chem > Voltammetry > Flashcards

Voltammetry Flashcards

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1
Q

information of the anlayte is obtained by measuring current in electrochemical cell as a function of applied potential

A

Voltammetry

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2
Q

Voltammetry vs. amperometry

A

voltammetry measures current as a function of applied potential. While in amperometry, current is proprortional to concentration and its montored at fixed potential.

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3
Q

What are the application areas of voltammetry?

A
  1. Fundamental studies of oxidatiion and reduction of various media
  2. Adsorption processes on surface
  3. Electron transfer mechanism at chemically modified electrode surfaces.
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4
Q

polarized electrode

A

This the the condition in which voltage is applied in excess of that predicted by the nernst equation.

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5
Q

potentiometry

A

The potential of an electrochemical reaction is measured at current approches to zero and where polarization is not present.

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6
Q

voltammetry vs coulometry

A

In voltammetry, the measures are taken under complete polarization and consumption of the analyte is minimal.

In coulometry, there are masures taken to minimize or compensate fror the effects of polarization and is essentially converted into another state.

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7
Q

what does voltammetry refer to?

A

This is a general term given to a group of electrochemical methods.it measures current as a function of applied potential. Measures are taken under complete polarization and minimal consumption of analyte.

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8
Q

How did the field of voltammetry developed?

A

It came from Polarograph, which is a type of Voltammetry designed by a Czech chemist, Jaroslav Heyrovsky.

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9
Q

why is polarography different?

A

it differs from other types of voltammetry because of its working electrode that uses the unique Dropping Mercury Electrode (DME).

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10
Q

what is polarography?

A

based on a difffusion controlled analyte that travels to the surface of the DME. The surface of the DME is constantly renewed.

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11
Q

depolarization potential in polarograpy

A

Enables the identifaction of ions present in the solution, and by measuring the direct current their concentration is calculated.

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12
Q

What is the device that is usedin voltammetric measurements?

A

Potentiostat, which is capable of applying a controlled potential to a working electrode and measuring the current that passes as a result of electron transfer to species of interest.

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13
Q

Voltammetry Exitation signal

A

this is a graph of Exitation Signal, E, against Time, t.

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14
Q

voltammetry type in Linear scan

A

Polarography and hydrodynamic voltammetry

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15
Q

volteammetry type in Square wave

A

Square-wave voltammetry

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16
Q

voltammetry type in differential pulse

A

Differential-pulse polarography

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17
Q

what is the type of Voltammetry in Triangular exctation signal?

A

Cyclic Voltammetry

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18
Q

List down the Excitation signals in Voltammetry

A
  1. Linear scan
  2. square wave
  3. Differential pulse
  4. Triangular
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19
Q

Electrode system in Voltammetry

A
  1. Reference Electrode
  2. Working electrode
  3. Counter or Auxiliary electrode
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20
Q

list all information of Working Electrode (WE)

A
  1. An electrode where its potential depends of concentration
  2. Where the oxidation or reduction of the analyte occurs.
  3. conductors are small flat disks that are pressed into an rod made up of inert material like teflon or Kel-F
  4. conductors may be a noble metal such as platinum, gold, and diff. form of carbon. While semi-conductors are made of Tin, indium oxide or metal with a coat of merury film
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21
Q

list all information on reference electrode

A
  1. Has fixed potential
  2. the potential b/n WE and RE is controlled
  3. Easy to assemble and maintain, and commercially available.
  4. Should be as close as possible to the working electrode
  5. Commonly used RE in aqueous solution are Ag/AgCl and Calomel electrode.
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22
Q

Counter or Auxiliary Electrode (CE)

A
  1. Its purpose is to complete the circuit and allow charge to flow.
  2. The current in the cell passes form RE to CE
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23
Q

what is a supporting electrolyte?

A
  1. This is a salt added in excess to the analyte solution
  2. Uses Alkali metal salt because it does not react at the working eletrode at the potential being used
  3. This salt reduces the effects of migration and lowers the resistance of the solution.
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24
Q

How is the Potential range is picked in WE?

A

The potential range used in WE is varied based on the electrode material and the composition of the electrolyte in which it is immersed.

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25
causes of positive potential?
The positive potential limitations are caused by the large current that can be developped due to the oxidation of water to produce oxygen gas.
26
What are negative limitations?
These arise from the Reduction of water to produce hydrogen. For mercury electrodes, it can use relatively large negative potentials due to the high overvoltage of hydrogen.
27
what are the common types of commercial voltammetric electrodes?
a. Disk electrode b. hanging mercury drop electrode (HDME) c. Microelectrode d. Sandwhich-type flow electrode e. Dropping mercury Electrode (DME)
28
what are the parts of a current-voltage plots in voltammetry?
voltammetric wave, limiting current, and the half-wave potential
29
what is a voltammetric wave?
This is a sigmoidal-shaped wave that appears in current-voltage plots in voltammetry.
30
what is the limiting current in voltammetry?
This is the constant current beyond the steep rise. This occurs because the surface of the analyte falls to zero. The mass transfer rate is also zero, and there is a complete concentration polarization.
31
What is the relationship between the limiting current and the reactant concentration?
It is usually proportional. Its Eqn is I_L=kC
32
what is the potential at which the current is equal to one-half of the limiting current?
Half-wave potential, E_1/2
33
what the implications for a half-wave potential?
this is useful for identifications of components in a solution. After the correction for the reference electtrode potential, the half-wave potential is closely related to the standard potential for the half-reaction.
34
how to make reproducible limiting currents?
this can be achieved rapidly when either the analyte of the working electrode is in continuous or reproducible motion.
35
what are the ways of performing hydrdynamic voltammetry?
1. The solution is stirred vigorously while it is in contact with a fixed working electrode. The stirring can be done with an ordinary magnetic stirrer or by rotating the working electrode at a high speed.
36
what are the three mechanisms in which the reactants are brought to the surface of the elctrode?
1. Migration 2. convection 3. Diffusion
37
explain migration
migration is where the reactants are transported under the influence of electric field.
38
explain convection
convection is the transport of reactant to the surface of the electrode due to the result of the stirring action.
39
explain diffusion
Diffusion is the difference in concentration between the film of the liquid of the electrode surfance and the bulk of the solution.
40
what are the types of voltammetry?
1. Linear sweep voltammetry LEES 2. staircase voltammetry STARK 3. cyclic voltammetry PSYCH 4. square-wave voltammetry SQUAVE 5. stripping voltammetry STRIP 6. alternating current voltammetry ORCA 7. polarography POLAR 8. rotated elelctrode voltammetry ROTTED EEL 9. differential pulse voltammetry DILF PULL
41
a method where the current in WE is measure and the potential between WE and RE sweep linearly with time
Linear Sweep voltammetry
42
oxidation or reduction of species is registered as a peak or trough in the current signal at the potential at which the species begins to be oxidized
Linear Sweep Voltammetry
43
The Plot for LSV
linear regression : potential, E vs time for stirred solution : Current, uA vs applied potential, E_ appl
44
can identify unknown species and determine the concentration of solutions
LSV - from its plot of current vs E_appl: a. half-potential is used to identify the unkown species, and the height of the limiting current can determine the concentration b. the sensitivity of the graph can be increased by increasing the scan rate c. higher potential per second result in more oxidatio/reduction of a species at the surface of WE
45
Its current is measured at the end of each potential change, right before the next.
Staircase voltammetry
46
this is a derivative of linear sweep voltammetr. And the potential sweep is a series of stair case
staircase voltammetry
47
this is a powerful and popular electrochemical technique commonly employed to investigate the reduction and oxidation process of molecular species.
Cyclic voltammetry
48
what is the principle of CV?
this involves applying a potential wave to an electrochemical cell and measuring the resulting current, and the potential sweep linearly with time. this goes from the initial potential (Ei) to the final potential (Ef) and then back to the initial potential.
49
this method is used to study the electrochemical properties of an analyte. This is plotted as the Current at WE vs applied potential, E_appl
Cyclic voltammetry
50
In this method, the current flowing in the cell is measured as a function of the potential of the working electrode.
Polarography
51
the current is proportional to the concentration of the analyte. the WE is a dropping mercury electride, then the analyte is either reduced (most of the cases) or oxidized at the surface of the mercury drop
Polarography
52
what are the advantages of polarography?
1. surface area is reproducible 2. electrode can be renewed and eliminates poisoning effect 3. Mercury forms amalgam with many metals 4. the surface area can be calclated from the weigth of the drop
53
list down the disadvantages of polarography
1. capillary is very small and ths can be easily blocked 2. mercury is very toxic 3. surface area of each drop of mercury is never constant 4. it cannot be used at higher positive potential due to oxidation of mercury
54
where is polarography applied?
it can applied in qualitative or quantitative analysis, determining organic or inorganic compounds, estimation of dissolved oxygen and in the pharmaceutical field.
55
how is the dropping mercury electrode reproducible?
It constantly forms a fresh metallic by simply producing a new drop. This fresh reproducible surface is important because the currents measured in voltammmetry are quite sensitive to cleanliness and freedom from irregularities.
56
Give example of mercury electrodes
1. Hanging mercury drop electrode 2. Dropping mercury electrode 2. Static mercury electrode
57
hanging mercury drop electrode, HDME
an electode in which a drop of Hg is suspended from a capillary tube
58
dropping mercury electrode, DME
an electrode where successive drops of mercury from at the end of a capillary tube as a RESULT OF GRAVITY, which each drop Providing a fresh electrode surface
59
Static mercury electrode
an electrode in which a successive drops of mercury AS A RESULT OF MECHANICAL PLUNGER
60
what are the names of the components in each mercury electrode
1. HDME - micrometer assembly, capillary 2. DME - mercury reservior, capilliary 3. SME - mercury reservoir, solenoid. plunger, capillary
61
Why dropping mercury electrode
mercury yield reproducible current-potential data.
62
True of false: in DME, with any other electrode (such as Pt), the potential depends on the surface condition.
TRUE
63
True of false : Many reactions studied with the mercury electrode are oxidation in Dropping mercury electrode
FALSE majory of reaction studies were Reduction
64
what are the advantages of DME
1. at a PT surface, reduction of solvent is expected of many analyte species, esp. in acidic solution. 2. high overpotential for the H+ reducition at the mercury surface. hence, H+ reduction does not interfere with many reduction.
65
what are the problems with mercury electrode?
1. It is not good at performing oxidations. 2. Hg is oxidized mear + 0.25 V in a noncomplexing medium 3. Mercury is toxic and slightly volatile Mercury dissolve in zin
66
what is faradaic current?
A current produce due to redox reaction at the working and counter elctrodes
67
give the sign convention in current in voltammetry?
Cathodic - reduction - Positive Anodic - oxidation - Negative
68
This is any current due to oxidation or reduction reaction
Faradaic current
69
this current is due to oxidation
Anodic current
70
Current due to reduction
Cathodic current
71
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