RESUMEN. CONCLUSIONES.

            En este blog he intentado recopilar, con mayor o menor fortuna, una serie de textos, páginas webs, artículos, videos, presentaciones… relacionados con un método electroanalítico avanzado: la VOLTAMPEROMETRÍA.

            La electroanalítica es una rama de la química analítica que hace uso de la medición de magnitudes eléctricas para establecer la concentración de una determinada sustancia.  Los métodos electroanalíticos los podemos dividir en varias categorías dependiendo  de qué aspectos de la célula son controlados y cuales se miden. Las tres principales categorías son: la Potenciometría (se determina la concentración de una especie electroactiva en una disolución  empleando un electrodo de referencia, un electrodo de trabajo y un potenciómetro), Coulombimetría ( se determina  la cantidad de materia transformada en una reacción de electrólisis midiendo la cantidad de electricidad consumida o producida), y Voltamperometría (la información sobre un analito se obtiene midiendo la corriente cuando se modifica el potencial).

            La Voltamperometría  engloba un grupo de métodos electroanalíticos en los cuales la información sobre el analito se obtiene a partir de medidas de la intensidad de corriente en función del potencial aplicado,  obtenidas en condiciones que favorecen la polarización de un electrodo indicador o de trabajo. Se basa en la medida de una intensidad de corriente que se desarrolla en una celda electroquímica en condiciones de polarización total de concentración; ocurre con un consumo mínimo del analito.

            La Voltamperometría se desarrolló a partir de la Polarografía. La Polarografía , que  todavía es una rama importante de la Voltamperometría, difiere de los otros tipos de ésta en que  el electrodo de trabajo es un electrodo de gotas de mercurio.

            Es utilizada ampliamente para el estudio de procesos de oxidación y reducción  en diversos medios,  procesos de adsorción sobre superficies y mecanismos de transferencia de electrones en superficies de electrodos químicamente modificados. Las investigaciones sobre nuevos alcances, como métodos de incremento de potencial, voltamperometría de barrido de potencial, voltamperometría de corriente alterna con detección de fase (CA), métodos hidrodinámicos y voltamperometría de redisolución demuestran la utilidad de los nuevos métodos voltamperométricos.

            Es un método analítico muy poderoso y está entre las técnicas analíticas más sensibles; se usa de manera rutinaria para la determinación de sustancias electroactivas en niveles de concentración por debajo de las partes por millón. Es posible tener tiempos de análisis de segundos. La posibilidad de determinar simultáneamente varios analitos en un solo barrido es frecuente en los procedimientos voltamperométricos.

            En Voltamperometría se le aplica a una celda electroquímica, que contiene un microelectrodo, una señal de excitación que es un potencial variable. La señal de excitación provoca una respuesta de intensidad de corriente característica, en la cual se basa el método.

Métodos electroquímicos de análisis

Using High-Speed Chronoamperometry with Local Dopamine Application to Assess Dopamine Transporter Function

http://www.ncbi.nlm.nih.gov/books/NBK2574/

Chronoamperometry

Synonyms: Potential Step Amperometry

Related Techniques: Double Potential Step Chronoamperometry (DPSCA)

Brief Description: Chronoamperometry is a technique where the potential of the working electrode is stepped for a specified period of time. Current is plotted as a function of time.

http://www.voltammetry.net/pine/aftermath/echem/chronoamperometry

Chronoamperometry With a Planar Solid Electrode

https://docs.google.com/open?id=0B_AiJQrarX6yVks3NWswbGFYMmM

Más CRONOAMPEROMETRÍA

https://docs.google.com/open?id=0B_AiJQrarX6yZE1wZXNZSDYzeGs
http://en.wikipedia.org/wiki/Chronoamperometry
http://www.asdlib.org/onlineArticles/ecourseware/Kelly_Potentiometry/PDF-6-Chronoamp.pdf

Chronoamperometry/Chronocoulometry

Introduction

Chronoamperometry (CA) and chronocoulometry (CC) have the same potential wave form - the potential step - which is one of the simplest potential wave forms. As shown below (F1), the potential is changed instantaneously from the Initial Potential to the First Step Potential, and it is held at this value for the First Step Time. This is a single potential step experiment. In a double potential step experiment, the potential is changed to the Second Step Potential after the First Step Time, and it is then held at this value for the Second Step Time. In CA, the current is monitored as a function of time, whereas in CC, the charge is monitored as a function of time. It is important to note that the basic potential step experiment on the epsilon is CA; that is, during the experiment, the current is recorded as a function of time. However, after the experiment, the data can also be displayed as charge as a function of time (the charge is calculated by integrating the current). Hence, chronocoulometry data can be obtained. CA is a standard technique on the epsilon.
CA is different from other constant potential techniques (constant potential electrolysis (CPE) and DC potential amperometry (DCA)) in that the time scale of CA is shorter (milliseconds and seconds) than those of BE and DCA (seconds and minutes).

Figure 1. Potential wave form for chronoamperometry and chronocoulometry.

Setting Up a Chronoamperometry/Chronocoulometry Experiment

As shown above, five parameters are used in the epsilon software to define the potential wave form for CA.

1. Initial Potential
2. First Step Potential (E)
3. Second Step Potential (E)
4. First Step Time
5. Second Step Time

The values of these parameters are set using the Change Parameter dialog box (F2) in either the Experiment menu or the pop-up menu.

Figure 2. Change Parameters dialog box for chronoamperometry/chronocoulometry.

1. Potential values are entered in mV, and time values in ms or s (select using Time Units).
2. If the Apply Open Circuit Potential for Initial E box is checked, then the open circuit potential will automatically be measured and used as the Initial Potential.
3. If the Run - External Trigger box is checked, the experiment is started from an external device using the Start In back-panel connection.
4. When the experiment is started, the cell is held at the Initial Potential for the number of seconds defined by the Quiet Time.
5. The experiment can be run on a hanging mercury drop electrode (i.e., a single drop is used for the entire experiment) using a BASi CGME by selecting CGME SMDE Mode from Cell Stand / Accessories in the Setup / Manual Settings (I/O) dialog box.
6. A rotating disk experiment can be run using a BASi RDE-2 by selecting RDE-2 from Cell Stand / Accessories in the Setup / Manual Settings (I/O) dialog box and entering the required Rotation Rate under RDE2 Rotation in the Cell dialog box.
7. There are two gain stages for the current-to-voltage converter. The default values of these stages that are used for a given current Full Scale value are determined by the software. However, they can be adjusted manually using the Filter / F.S. dialog box. This dialog box is also used to change the analog Noise Filter Value settings from the default values set by the software.
8. The default condition of the cell is that the cell is On (i.e., the electronics are connected to the electrodes) during the experiment, and is Off between experiments. However, the potential can be switched On between experiments using the Cell dialog box. HOWEVER, THIS OPTION SHOULD BE USED WITH CAUTION SINCE CONNECTING OR DISCONNECTING THE ELECTRODES WHEN THE CELL IS ON CAN RESULT IN DAMAGE TO THE POTENTIOSTAT, THE CELL, AND/OR THE USER!
9. A series of identical experiments on the same cell can be programmed using the MR (Multi-Run) option.
10. Clicking Exit will exit the dialog box without saving any changes made to the parameter values. Any changes can be saved by clicking Apply before exiting.
11. Range of allowed parameter values:
    • Potential = -3275 mV - +3275 mV
    • Quiet Time = 0 - 100 s
    • Step Time = 1 - 65 s OR 1 - 16000 ms
    • Maximum # of points in a step = 1000, 2000, 4000, 8000, 16000
    • The Sample Interval is determined by the Step Time and the Maximum # of points in a step, and can only be adjusted by the user indirectly through these latter two parameters. The relationship between these parameters is shown by the equationSample Interval = Step Time/Maximum # of points
      However, it should be noted that only certain values are allowed for each of these parameters, as is shown in the table below:
      

      Max. # of points	1000	2000	4000	8000	16000
      Sample Interval	Maximum Step Time (/ms)
      20 ms	20	40	80	160	320
      50 ms	40	81	162	325	650
      100 ms	100	200	400	800	1600
      200 ms	200	400	800	1600	3200
      500 ms	406	812	1625	3250	6500
      1 ms	1000	2000	4000	8000	16000
      2 ms	2000	4000	8000	16000
      5 ms	4062	8125
      10 ms	10000

12. Once the parameters have been set, the experiment can be started by clicking Run (either in this dialog box, in the Experiment menu, in the pop-up menu, on the Tool Bar, or using the F5 key).

Analysis of the Current Response

Let us consider the effect of a single potential step on the reaction R = O + e^-. At potentials well negative of the redox potential (E_nr), there is no net conversion of R to O, whereas at potentials well positive of the redox potential (E_d), the rate of the reaction is diffusion-controlled (i.e., molecules of R are electrolyzed as soon as they arrive at the electrode surface). In most potential step experiments, E_nr is the Initial Potential, and E_d is the First Step Potential. The advantage of using these two potentials is that any effects of slow heterogeneous electron transfer kinetics are eliminated. In double potential step experiments, (E_nr) is often used as the Second Step Potential.
Figure 3. Concentration profiles for a single potential step experiment.It is instructive to consider the concentration profiles of O and R following the potential step (F3). Initially, only R is present in solution (a). After the potential step, the concentration of R at the electrode surface decreases to zero, and hence a concentration gradient is set up between the interfacial region and the bulk solution (b). As molecules of R diffuse down this concentration gradient to the electrode surface (and are converted to O), a diffusion layer (i.e., a region of the solution in which the concentration of R has been depleted) is formed. The width of this layer increases with increasing time (b-d). There is also a net diffusion of O molecules away from the electrode surface.
Since the current is directly proportional to the rate of electrolysis, the current response to a potential step is a current 'spike' (due to initial electrolysis of species at the electrode surface) followed by time-dependent decay (F4) (due to diffusion of molecules to the electrode surface).

Figure 4. Current-time response for a double-potential step chronoamperometry experiment.
For a diffusion-controlled current, the current-time (i-t) curve is described by the Cottrell equation:
i = nFACD^½p^-½t ^-½

where:

n = number of electrons transferred/molecule F = Faraday's constant (96,500 C mol-1) A = electrode area (cm2) D = diffusion coefficient (cm2 s-1) C = concentration (mol cm-3)

The charge-time (Q-t) (the Anson equation) is obtained by integrating the Cottrell equation, and can be displayed in the epsilon software by selecting Q vs T from Select Graph in the pop-up menu (the original i vs. t plot can be recovered by selecting Original from Select Graph in the pop-up menu). A typical charge-time plot is shown in F5:
Q = 2nFACD^½p^-½t ^½

Figure 5. Charge-time response for a double-potential step chronocoulometry/chronoamperometry experiment.
Charge is the integral of current, so the response for CC increases with time, whereas that for CA decreases. Since the latter parts of the signal response must be used for data analysis (the finite rise time of the potentiostat invalidates the early time points), the larger signal response at the latter parts for CC makes this the more favorable potential step technique for many applications (in addition, integration decreases the noise level).
The analysis of CA and CC data is discussed elsewhere.

http://www.basinc.com/mans/EC_epsilon/Techniques/ChronoI/ca.html

Cronoamperometría

http://www.fing.edu.uy/iq/cursos/ipeq/teorico/2007/6-9.pdf

Differential pulse Voltammetry at Microelectrodes

https://docs.google.com/open?id=0B_AiJQrarX6ycEsyR0labWxWZGc

Estudio electroquímico y cromatográfico de complejos de inclusión estrona/ciclodextrina
https://docs.google.com/open?id=0B_AiJQrarX6ycnJaTzlrUmN1Q3c

Determination of Trace Metals by Differential Pulse Voltammetry at Chitosan Modified Electrodes

https://docs.google.com/open?id=0B_AiJQrarX6yNXhiRzJEWVl3dm8

Differential pulse voltammetry

http://en.wikipedia.org/wiki/Differential_pulse_voltammetry
http://www.voltammetry.net/pine/aftermath/echem/differential_pulse_voltammetry

Avances en la detección simultánea de dopamina, ácido ascórbico y serotonina mediante voltamperometría cíclica y voltamperometría de pulso diferencial empleando electrodos de puntas de lapicero modificados con nanotubos

https://docs.google.com/open?id=0B_AiJQrarX6yN2t6MWlQNlNvQ1E

Pulse Voltammetric Techniques

Introduction

The basis of all pulse techniques is the difference in the rate of the decay of the charging and the faradaic currents following a potential step (or "pulse"). The charging current decays exponentially, whereas the faradaic current (for a diffusion-controlled current) decays as a function of 1/(time)^½; that is, the rate of decay of the charging current is considerably faster than the decay of the faradaic current. The charging current is negligible at a time of 5R_uC_dl after the potential step (R_uC_dl is the time constant for the electrochemical cell, and ranges from µs to ms). Therefore, after this time, the measured current consists solely of the faradaic current; that is, measuring the current at the end of a potential pulse allows discrimination between the faradaic and charging currents.
The important parameters for pulse techniques are as follows:

1. Pulse amplitude is the height of the potential pulse. This may or may not be constant depending upon the technique.
2. Pulse width is the duration of the potential pulse.
3. Sample period is the time at the end of the pulse during which the current is measured.
4. For some pulse techniques, the pulse period or drop time must also be specified. This parameter defines the time required for one potential cycle, and is particularly significant for polarography (i.e., pulse experiments using a mercury drop electrode), where this time corresponds to the lifetime of each drop (i.e., a new drop is dispensed at the start of the drop time, and is knocked off once the current has been measured at the end of the drop time - note that the end of the drop time coincides with the end of the pulse width).

A number of different pulse techniques are available on the epsilon, which differ in their potential pulse wave forms, the number of sampling points, and whether a solid electrode (voltammetry) or a mercury drop electrode (polarography) is used. These are listed below. The discrimination against the charging current that is inherent in these techniques leads to lower detection limits (when compared to linear sweep techniques), which makes these techniques suitable for quantitative analysis.
Sampled Current Polarography
Normal Pulse Voltammetry/Polarography
Differential Pulse Voltammetry/Polarography
Square Wave Voltammetry

Sampled Current Polarography

Sampled current polarograph (SCP) is a modification of the classical DC polarography experiment, and was designed to reduce the effect of the changing surface area of the mercury drop electrode. The potential wave form is shown in F1 and the Change Parameters dialog box is shown in F2. The potential is varied in a series of steps, with the current sampled at the end of each step.
Figure 1. Potential wave form for sampled current polarography.

Figure 2. Change Parameters dialog box for sampled current polarography.

1. All potential values are entered in mV, and the Step Width is entered in ms.
2. If the Apply Open Circuit Potential for Initial E box is checked, then the open circuit potential will automatically be measured and used as the Initial Potential.
3. If the Run - External Trigger box is checked, the experiment is started from an external device using the Start In back-panel connection.
4. The Pulse Type must be specified when using a mercury electrode (CGME SMDE Mode selected in the Cell Stand dialog box). If Voltammetry is selected, the whole experiment is performed on a single mercury drop (after the Pre Run Drops); if Polarography is selected, a new drop is used for each data point.
5. The change in the applied potential for each step is defined by Step E.
6. The Scan Rate cannot be directly changed by the user, and is determined by Step E x 1/Sample Width.
7. Three options are available for the Sample Period:
   • The current is measured once at the end of the Step Width (1 Point)
   • The current is measured multiple times in 1 ms at the end of the Step Width, and averaged (1 mSecond)
   • The current is measured multiple times over 1 line cycle at the end of the Step Width, and averaged (1 Line Period). The time required for 1 line cycle is the reciprocal of the line frequency (16.7 ms for 60 Hz, and 20 ms for 50 Hz). The line frequency is selected in the Setup / Manual Settings (I/O) dialog box.
   Generally speaking, increasing the Sample Period increases the signal-to-noise ratio. However, the 1 Line Period option may not be possible for short Step Width values.
8. If the Apply Open Circuit Potential for Initial E box is checked, then the open circuit potential will automatically be measured and used as the Initial Potential.
9. When the experiment is started, the cell is held at the Initial Potential for the number of seconds defined by the Quiet Time.
10. There are two gain stages for the current-to-voltage converter. The default values of these stages that are used for a given current Full Scale value are determined by the software. However, they can be adjusted manually using the Filter / F.S. dialog box. This dialog box is also used to change the analog Noise Filter Value settings from the default values set by the software.
11. The default condition of the cell is that the cell is On (i.e., the electronics are connected to the electrodes) during the experiment, and is Off between experiments. However, the potential can be switched On between experiments using the Cell dialog box. HOWEVER, THIS OPTION SHOULD BE USED WITH CAUTION SINCE CONNECTING OR DISCONNECTING THE ELECTRODES WHEN THE CELL IS ON CAN RESULT IN DAMAGE TO THE POTENTIOSTAT, THE CELL, AND/OR THE USER!
12. A series of identical experiments on the same cell can be programmed using the MR (Multi-Run) option.
13. Clicking Exit will exit the dialog box without saving any changes made to the parameter values. Any changes can be saved by clicking Apply before exiting.
14. Range of allowed parameter values:
    • Potential = -3275 - +3275 mV
    • Step E = 1 - 40 mV
    • Step Width = 100 - 6550 ms (Polarography); 4 - 6550 ms (Voltammetry)
    • Quiet Time = 0 - 100 s
15. Once the parameters have been set, the experiment can be started by clicking Run (either in this dialog box, in the Experiment menu, in the pop-up menu, on the Tool Bar, or using the F5 key).

The current response for SCP is shown in F3. The limiting current (i_d) is given by the Ilkovic equation:

i_d = 708nCD^1/2m^2/3t^1/6

where:

n = number of electrons transferred/molecule C = concentration (mol cm-3) D = diffusion coefficient (cm2 s-1) m = mercury flow rate (mg s-1) t = sampling interval (s)

Figure 3. A typical sampled current polarogram.

Normal Pulse Voltammetry/Polarography

The potential wave form for normal pulse voltammetry/polarography (NPV/P) is shown in F4 and the Change Parameters dialog box is shown in F5. The potential wave form consists of a series of pulses of increasing amplitude, with the potential returning to the initial value after each pulse.
Figure 4. Potential wave form for normal pulse voltammetry.

Figure 5. Change Parameters dialog box for normal pulse voltammetry.

1. All potential values are entered in mV, and the Pulse Width and Pulse Period are entered in ms.
2. If the Apply Open Circuit Potential for Initial E box is checked, then the open circuit potential will automatically be measured and used as the Initial Potential.
3. If the Run - External Trigger box is checked, the experiment is started from an external device using the Start In back-panel connection.
4. The Pulse Type must be specified when using a mercury electrode (CGME SMDE Mode selected in the Cell Stand dialog box). If Voltammetry is selected, the whole experiment is performed on a single mercury drop (after the Pre Run Drops); if Polarography is selected, a new drop is used for each data point.
5. The amplitude of the initial potential pulse, and the incremental increases in amplitude for subsequent pulses is defined by Step E.
6. The Scan Rate cannot be directly changed by the user, and is determined by Step E x 1/Pulse Period.
7. The Pulse Period must be at least twice the Pulse Width.
8. Three options are available for the Sample Period:
   • The current is measured once at the end of the Pulse Width (1 Point)
   • The current is measured multiple times in 1 ms at the end of the Pulse Width, and averaged (1 mSecond)
   • The current is measured multiple times over 1 line cycle at the end of the Pulse Width, and averaged (1 Line Period). The time required for 1 line cycle is the reciprocal of the line frequency (16.7 ms for 60 Hz, and 20 ms for 50 Hz). The line frequency is selected in the Setup / Manual Settings (I/O) dialog box.
   Generally speaking, increasing the Sample Period increases the signal-to-noise ratio. However, the 1 Line Period option may not be possible for short Pulse Width values.
9. If the Apply Open Circuit Potential for Initial E box is checked, then the open circuit potential will automatically be measured and used as the Initial Potential.
10. When the experiment is started, the cell is held at the Initial Potential for the number of seconds defined by the Quiet Time.
11. There are two gain stages for the current-to-voltage converter. The default values of these stages that are used for a given current Full Scale value are determined by the software. However, they can be adjusted manually using the Filter / F.S. dialog box. This dialog box is also used to change the analog Noise Filter Value settings from the default values set by the software.
12. The default condition of the cell is that the cell is On (i.e., the electronics are connected to the electrodes) during the experiment, and is Off between experiments. However, the potential can be switched On between experiments using the Cell dialog box. HOWEVER, THIS OPTION SHOULD BE USED WITH CAUTION SINCE CONNECTING OR DISCONNECTING THE ELECTRODES WHEN THE CELL IS ON CAN RESULT IN DAMAGE TO THE POTENTIOSTAT, THE CELL, AND/OR THE USER!
13. A series of identical experiments on the same cell can be programmed using the MR (Multi-Run) option.
14. Clicking Exit will exit the dialog box without saving any changes made to the parameter values. Any changes can be saved by clicking Apply before exiting.
15. Range of allowed parameter values:
    • Potential = -3000 - +3000 mV
    • Step E = 1 - 40 mV
    • Pulse Width = 3 - 2000 ms
    • Step Width = 100 - 6550 ms (Polarography); 4 - 6550 ms (Voltammetry)
    • Quiet Time = 0 - 100 s
16. Once the parameters have been set, the experiment can be started by clicking Run (either in this dialog box, in the Experiment menu, in the pop-up menu, on the Tool Bar, or using the F5 key).

Consider a reduction. If the Initial Potential is well positive of the redox potential, the application of small amplitude pulses does not cause any faradaic reactions, hence there is no current response. When the pulse amplitude is sufficiently large that the pulse potential is close to the redox potential, there is a faradaic reaction is response to the potential pulse (assuming moderately fast electron transfer kinetics), and the magnitude of this current may depend on both the rate of diffusion and the rate of electron transfer. When the pulsed potentials are sufficiently negative of the redox potential that the electron transfer reaction occurs rapidly, the faradaic current depends only on the rate of diffusion; that is, a limiting current has been attained. The sigmoidal shape typically observed for NPV/P (F6) is similar to the shape of the current-potential curve obtained in the classical polarography experiment, which gives rise to the name of "normal" for this technique.

Figure 6. A typical normal pulse voltammogram.

Differential Pulse Voltammetry/Polarography

The potential wave form for differential pulse voltammetry/polarography (DPV/P) is shown in F7 and the Change Parameters dialog box is shown in F7. The potential wave form consists of small pulses (of constant amplitude) superimposed upon a staircase wave form. Unlike NPV, the current is sampled twice in each Pulse Period (once before the pulse, and at the end of the pulse), and the difference between these two current values is recorded and displayed.

Figure 7. Potential wave form for differential pulse voltammetry.

Figure 8. Change Parameters dialog box for differential pulse voltammetry.

1. All potential values are entered in mV, and the Pulse Width and Pulse Period are entered in ms.
2. If the Apply Open Circuit Potential for Initial E box is checked, then the open circuit potential will automatically be measured and used as the Initial Potential.
3. If the Run - External Trigger box is checked, the experiment is started from an external device using the Start In back-panel connection.
4. The Pulse Type must be specified when using a mercury electrode (CGME SMDE Mode selected in the Cell Stand dialog box). If Voltammetry is selected, the whole experiment is performed on a single mercury drop (after the Pre Run Drops); if Polarography is selected, a new drop is used for each data point.
5. The amplitude of the potential pulse is defined by Pulse Amplitude, and the height of the staircase wave form is defined by Step E.
6. The Scan Rate cannot be directly changed by the user, and is determined by Step E x 1/Pulse Period.
7. The Pulse Period must be at least twice the Pulse Width.
8. Three options are available for the Sample Period:
   • The current is measured once at the end of the Pulse Width (1 Point)
   • The current is measured multiple times in 1 ms at the end of the Pulse Width, and averaged (1 mSecond)
   • The current is measured multiple times over 1 line cycle at the end of the Pulse Width, and averaged (1 Line Period). The time required for 1 line cycle is the reciprocal of the line frequency (16.7 ms for 60 Hz, and 20 ms for 50 Hz). The line frequency is selected in the Setup / Manual Settings (I/O) dialog box.
   Generally speaking, increasing the Sample Period increases the signal-to-noise ratio. However, the 1 Line Period option may not be possible for short Pulse Width values.
9. If the Apply Open Circuit Potential for Initial E box is checked, then the open circuit potential will automatically be measured and used as the Initial Potential.
10. When the experiment is started, the cell is held at the Initial Potential for the number of seconds defined by the Quiet Time.
11. There are two gain stages for the current-to-voltage converter. The default values of these stages that are used for a given current Full Scale value are determined by the software. However, they can be adjusted manually using the Filter / F.S. dialog box. This dialog box is also used to change the analog Noise Filter Value settings from the default values set by the software.
12. The default condition of the cell is that the cell is On (i.e., the electronics are connected to the electrodes) during the experiment, and is Off between experiments. However, the potential can be switched On between experiments using the Cell dialog box. HOWEVER, THIS OPTION SHOULD BE USED WITH CAUTION SINCE CONNECTING OR DISCONNECTING THE ELECTRODES WHEN THE CELL IS ON CAN RESULT IN DAMAGE TO THE POTENTIOSTAT, THE CELL, AND/OR THE USER!
13. A series of identical experiments on the same cell can be programmed using the MR (Multi-Run) option.
14. Clicking Exit will exit the dialog box without saving any changes made to the parameter values. Any changes can be saved by clicking Apply before exiting.
15. Range of allowed parameter values:
    • Potential = -3000 - +3000 mV
    • Step E = 1 - 40 mV
    • Pulse Amplitude = 5 - 250 mV.
    • Pulse Width = 3 - 1000 ms
    • Step Width = 100 - 6550 ms (Polarography); 4 - 6550 ms (Voltammetry)
    • Quiet Time = 0 - 100 s
16. Once the parameters have been set, the experiment can be started by clicking Run (either in this dialog box, in the Experiment menu, in the pop-up menu, on the Tool Bar, or using the F5 key).

Consider a reduction. At potentials well positive of the redox potential, there is no faradaic reaction in repsonse to the pulse, so the difference current is zero. At potential around the redox potential, the difference current reaches a maximum, and decreases to zero as the current becomes diffusion-controlled. The current response is therefore a symmetric peak (F9).

Figure 9. A typical differential pulse voltammogram.

Square Wave Voltammetry

The potential wave form for square wave voltammetry (SWV) is shown in F10 and the Change Parameters dialog box is shown in F11. The potential wave form consists of a square wave of constant amplitude superimposed on a staircase wave form. The current is measured at the end of each half-cycle, and the current measured on the reverse half-cycle (i_r) is subtracted from the current measured on the forward half-cycle (i_f). This difference current (i_f - i_r) is displayed as a function of the applied potential.

Figure 10. Potential wave form for square wave voltammetry.

Figure 11. Change Parameters dialog box for square wave voltammetry.

1. All potential values are entered in mV.
2. If the Apply Open Circuit Potential for Initial E box is checked, then the open circuit potential will automatically be measured and used as the Initial Potential.
3. If the Run - External Trigger box is checked, the experiment is started from an external device using the Start In back-panel connection.
4. The step height of the staircase wave form and the potential resolution is defined by Step E.
5. The pulse width is the length of each half-cycle, which is determined by the 1/S.W. Frequency.
6. The Scan Rate cannot be directly changed by the user, and is determined by Step E x S.W. Frequency..
7. Three options are available for the Sample Period (note the options requiring longer measurement times are not available at higher frequencies):
   • The current is measured once at the end of each half-cycle (1 Point - maximum S.W. Frequency = 2000 Hz)
   • The current is measured multiple times in 1 ms at the end of each half-cycle, and averaged (1 mSecond - maximum S.W. Frequency = 125 Hz)
   • The current is measured multiple times over 1 line cycle at the end of the Pulse Width, and averaged (1 Line Period). The time required for 1 line cycle is the reciprocal of the line frequency (16.7 ms for 60 Hz, and 20 ms for 50 Hz). The line frequency is selected in the Setup / Manual Settings (I/O) dialog box.
   Generally speaking, increasing the Sample Period increases the signal-to-noise ratio.
8. If the Apply Open Circuit Potential for Initial E box is checked, then the open circuit potential will automatically be measured and used as the Initial Potential.
9. When the experiment is started, the cell is held at the Initial Potential for the number of seconds defined by the Quiet Time.
10. The experiment can be run on a hanging mercury drop electrode (i.e., a single drop is used for the entire experiment) using a BASi CGME by selecting CGME SMDE Mode from Cell Stand / Accessories in the Setup / Manual Settings (I/O) dialog box.
11. There are two gain stages for the current-to-voltage converter. The default values of these stages that are used for a given current Full Scale value are determined by the software. However, they can be adjusted manually using the Filter / F.S. dialog box. This dialog box is also used to change the analog Noise Filter Value settings from the default values set by the software.
12. The default condition of the cell is that the cell is On (i.e., the electronics are connected to the electrodes) during the experiment, and is Off between experiments. However, the potential can be switched On between experiments using the Cell dialog box. HOWEVER, THIS OPTION SHOULD BE USED WITH CAUTION SINCE CONNECTING OR DISCONNECTING THE ELECTRODES WHEN THE CELL IS ON CAN RESULT IN DAMAGE TO THE POTENTIOSTAT, THE CELL, AND/OR THE USER!
13. A series of identical experiments on the same cell can be programmed using the MR (Multi-Run) option.
14. Clicking Exit will exit the dialog box without saving any changes made to the parameter values. Any changes can be saved by clicking Apply before exiting.
15. Range of allowed parameter values:
    • Potential = -3000 - +3000 mV
    • Step E = 1 - 40 mV
    • S.W. Amplitude = 1 - 250 mV
    • S.W. Frequency = 1 - 2000 Hz
    • Quiet Time = 0 - 100 s
16. Once the parameters have been set, the experiment can be started by clicking Run (either in this dialog box, in the Experiment menu, in the pop-up menu, on the Tool Bar, or using the F5 key).

There are two advantages to measuring the difference current. First, it increases the discrimination against the charging current, since any residual charging current is subtracted out. Second, the shape of the current response is a symmetric peak (F12), rather than the sigmoidal curve typically found for normal pulse voltammetry. If we consider a reduction, then at potential well positive of the redox potential, both the forward and reverse currents are zero, so the difference current is also zero. At potentials well negative of the redox potential, the current is diffusion-controlled, and the potential pulse has no effect; hence, the forward and reverse currents are equal, and the difference current is again zero. The largest difference between the forward and reverse currents (and hence the largest current response) is at the redox potential.

Figure 12. A typical square wave voltammogram.


http://www.basinc.com/mans/EC_epsilon/Techniques/Pulse/pulse.html

Application of normal pulse voltammetry to the kinetic study of formic acid oxidation on a carbon supported Pd electrocatalyst

https://docs.google.com/open?id=0B_AiJQrarX6yZUNJVUdQdjdzWDg

Voltamperometría de Pulso Normal

http://www.basinc.com/mans/EC_epsilon/Techniques/Pulse/pulse.html

Rotating Disk Electrode Voltammetry Applied to the Kinetics of Uptake and Ef¯ux in Wild-Type and Mutant Catecholamine Transporters

https://docs.google.com/open?id=0B_AiJQrarX6yYVVFWnNpdmg4cTg

Hydrodynamic Voltammetry

Hydrodynamic devices use convection to enhance the rate of mass transport to the electrode and can offer advantages over techniques which operate in stagnant solution. The addition of convection to the cell usually results in increased current and sensitivity in comparison to voltammetric measurements performed in stagnant solution. Also the introduction of convection (usually in a manner that is predictable) helps to remove the small random contribution from natural convection which can complicate measurements performed in stagnant solution. Finally, it is possible to vary the rate of reaction at the electrode surface by altering the convection rate in the solution and this can be usefully exploited in mechanistic analysis and electroanalytical applications. In the discussion below a range of traditional and recent developments in the field of hydrodynamic techniques and their potential applications are outlined.

  

Hydrodynamics

There are two main approaches of introducing convection into the electrochemical cell. First the electrode can be held in a fixed position and solution is flowed over the surface by an applied force (usually a pressure). Second the electrode can be designed to move which acts to mix the solution via convection. The introduced (forced) convection is generally made to be considerably stronger than any natural convection processes and therefore the influence of natural convection becomes insignificant on the electrolysis reaction. To allow quantitative analysis it is vital that the forced convection introduced to the cell is predictable. The cell and experimental conditions are therefore designed so that the solution flow within the cell becomes laminar. You will recall from the mass transport section (link) that in this case solution flows in a well defined and non mixing way. The the two figures below show the extremes of flow behaviour through a pipe. First turbulent conditions where the flow is essentially random and unpredictable and second the well defined Laminar flow conditions.

The transition between laminar and turbulent flow can be predicted using the concept of the Reynolds number. When laminar flow prevails the fluid dynamics within the cell can be predicted by the Navier-Stokes equations  and these allow the calculation of the velocity throughout the device. Once these have been predicted the concentration of the reagents and the relationship between the current and the transport rate within the cell can then be calculated.

  

Hydrodynamic techniques

In this section we take a historical look at the development of hydrodynamic devices and the voltammetric behaviour of the different techniques. The following techniques are covered:

• The Dropping Mercury Electrode
• The Rotating Disc and Ring Disc Electrode
• The Wall Jet Electrode
• The Channel Electrode
• The Confluence Reactor 

  

The Dropping Mercury Electrode

The first of the hydrodynamic techniques developed was the Dropping Mercury Electrode (DME). In this arrangement a fine capillary is connected to a reservoir of mercury. The cell is designed so that mercury is allowed to flow down the capillary at a controlled rate and out into the solution.

Electrical contact to the mercury is made in the reservoir and a reference and counter electrode are sited in the electrolyte solution. Voltammetry can performed in an identical manner to that described earlier, however, now the electrode is constantly changing area and so a linear sweep voltammogram will exhibit significantly different behaviour. The current can be seen to go through a series of peaks and troughs as the voltage is swept. As the mercury drop grows the surface area accessible to the electrolyte also increases and consequently the current increases. However, at some point the Mercury drops from the tip leaving a small new area in contact with the electrolyte, consequently the current drops rapidly at this point before gradually increasing as the drop grows again. This behaviour is repeated throughout the scan. Clearly this process is not something that is easily predicted and the fluid dynamics are still not fully understood for this particular device. However, this technique proved very popular due to the ability to continually refresh the electrode surface during the experiment.

  

The Rotating Disc and Ring Disc Electrode

Due to the limitations of the DME a new device was developed called the Rotating Disc Electrode (RDE). In this arrangement solution is brought to the surface by a Teflon disc which rotates in solution. A schematic of the cell including the working electrode is shown below. The working electrode (typically Platinum or Gold) is embedded in the top face of the Teflon shield.

When operated at rotation speeds of around 60 Hz (cycles per second) the flow profile to the electrode is laminar and so can be predicted mathematically. The figure below shows the type of flow profile that is developed when a circular object is rotated in solution and how this brings fresh reactant to the surface

The act of rotation drags material to the electrode surface where it can react. Providing the rotation speed is kept within the limits that laminar flow is maintained then the mass transport equation is given by

where the x dimension is the distance normal to the electrode surface. It is apparent that the mass transport equation is now dominated by both diffusion and convection and both these terms effect the concentration of reagent close to the electrode surface. Therefore to predict the current for this type of electrode we must solve this subject to the reactions occurring at the electrode.

A typical voltammetric measurement used with the rotating disc and other hydrodynamic systems detailed below is linear sweep voltammetry. The figure below shows a set of current voltage curves recorded for a reversible on electron transfer reaction and different rotation speeds. The scan rate used was 1 mV s^-1 (compared to perhaps 20 mV s^-1 for conventional cyclic voltammetry) and as can be seen the total current flowing depends upon the rotation speed used

This can be understood by returning to the concept of the 'diffusion layer' thickness controlling the flux of material to the surface. As the rotation speed is increased the distance that material can diffuse from the surface before being removed by convection is decreased. This results in a higher flux of material to the surface at higher rotation speeds. The mass transport limited current arises from the fact that the system reaches a steady state and so the current reaches a plateau once the equilibrium at the surface is driven to the products side.

We can analyse the variation of the mass transport limited current as a function of the rotation speed. This was first solved mathematically by Levich who showed the following relationship between the current and the rotation speed, for a reversible electron transfer reaction.

Therefore by plotting i_L vs the square root of the rotation speed (ω) for a set of experimental data a straight will be observed if the reaction is reversible. Plots such as that shown below can be employed to analyse whether reactions involve adsorption/desorption steps chemical reactions or slow electron transfer steps.

Hydrodynamic systems lend themselves particularly to the investigation of mechanistic processes. This is because a steady supply of reactant is fed to the electrode and the ability to vary the mass transport rate provide information regarding the process under investigation. Let us use the ECE reaction as an example. In this case it is easier to reduce the product (C) from the chemical reaction that the starting material (A). A typical voltammogram for such reaction is shown below.

The current recorded in the experiment is shown as the solid line. The current that would be recorded without chemical reaction is shown as a dashed line on the figure. Analysis of this type of process is usually performed using the following ratio

N_eff can vary between 1 and 2. When no chemical reaction occurs the total current flowing is the same as that for the (A) to (B) step so N_eff is 1. When all of the product from the reduction of (A) is converted to (C) then N_eff becomes 2 since (C) is then reduced at the electrode as well. The figure below shows how the quantity N_eff varies as a function of rotation speed.

As can be seen N_eff drops as the rotation speed increases. This occurs because as the rotation speed increases the product (B) from electrolysis is removed from the electrode more rapidly and therefore has less chance to react to form (C). Consequently it is possible to outrun the reaction kinetics by increasing the convection rate. Analysis of the shape of the curve in the figure can reveal the rate constant for the chemical reaction.

  

The Wall Jet Electrode

The Wall Jet Electrode (WJE) is an interesting alternative to the RDE. The velocity profiles were first examined (under laminar flow conditions) by Glauert who was working on the effects of jets directed towards a flat plane (for applications in vertical take off aircraft!). A fine nozel is sited within a large container of electrolyte and positioned directly above a disc working electrode. Solution is pumped through the nozel (under laminar flow conditions) and impinges on the surface containing the electrode. The reagent then flows from the surface creating a complex but predictable flow pattern. Like the rotating disc it is possible to obtain an expression for the transport of material to the electrode WJE and this can then be used to predict how the mass transport limited current would vary as a function of the solution flow rate. The WJE has found many applications in the area of Electroanalytical Chemistry.

  

The Channel Electrode

Like the WJE above in this device the electrode is maintained in a specific position and solution pumped through the cell. In the channel electrode (ChE) the electrode is embedded into one wall of a rectangular duct and solution pumped over the surface. 

In this cell it is the volume flow rate (V_f) that controls the convection to the surface and the corresponding Levich equation for a reversible electron transfer is predicted by

h,d,w,and x_e are constants for the cell size and V_f is the volume flow rate. It is possible to obtain such an expression because when the solution flow rate is controlled appropriately it is possible for a parabolic laminar profile to develop This is mathematically well defined and consequently allows quantitative analysis.

  

The Confluence Reactor

The above devices have allowed a range of (electro)chemical process to be examined, however, they each have limitations when reactions of interest involve bringing together different starting materials which are highly reactive. Consequently more recently new devices have been developed which permit the introduction of two or more species into a cell at a predetermined point and under well defined transport conditions. The confluence reactor (CR) shown below is constructed of two separate rectangular ducts which are independently supplied with different reagents. At some point the separation between the two ducts finishes and the separate streams can then mix. The cell is designed so that laminar flow conditions prevail and so the concentration of reagents can be computed. A detector electrode is sited in one wall and is used to sense the products as they are generated.

University of Cambridge.

http://www.ceb.cam.ac.uk/pages/hydrodynamic-voltammetry.html

Determination of mercury at the rotating gold electrode by anodic stripping voltammetry

https://docs.google.com/open?id=0B_AiJQrarX6yLU5SdHY3MFZnVXM

Cyclic Voltammetry and RRDE Studies on the Electrochemical Behavior of Azetidinone Ester

https://docs.google.com/open?id=0B_AiJQrarX6yVXppclJjMHRDOTg

Voltamperometría de Electrodo Rotatorio

Una técnica hidrodinámica en la que el electrodo de trabajo, usualmente un electrodo de disco rotatorio (RDE) or el electrodo de disco-anillo rotatorio (RRDE), rota a muy alta velocidad. Esta técnica es útil para estudiar la cinética y los mecanismos de reaccion electroquímicos para una semirreacción.

Polarography(Amrita University)

PowerPoint Polarografía

https://docs.google.com/presentation/d/1eYYqXezVyYma0kiQg-HuFiHOu7qodey9cGibibU5NPc/edit

JAROSLAV HEYROVSKY AND POLAROGRAPHY

http://electrochem.cwru.edu/encycl/art-p03-polarography.htm

Más definiciones de Polarografía

http://clasesusanita.blogspot.com.es/2010/01/polarografia.html

http://www.husseinfahmy.com/HMF/HF_BOOK.pdf

Presentación PowerPoint
https://docs.google.com/presentation/d/1uMP4PFvFs5_ecQAWUPOJjT2stzC_1yr2eUKjDPB6pn8/edit

Métodos voltamperométricos. Polarografía.
http://ocw.usal.es/ciencias-experimentales/analisis-aplicado-a-la-ingenieria-quimica/contenidos/course_files/Tema_9.pdf

LA POLAROGRAFIA EN LOS FENOMENOS FISICO-QUIMICOS COMO METODO DE ANALISIS ELECTROQUÍMICO

https://docs.google.com/open?id=0B_AiJQrarX6yYjYxUlBLblJ0Y2s

Polarografía

La Polarografía es una subclase de voltamperometría donde el electrodo de trabajo es un electrodo de gota de mercurio (DME), útil por su amplio rango catódica y su superficie renovable. Fue inventado por Jaroslav Heyrovský.

http://es.wikipedia.org/wiki/Polarograf%C3%ADa

Determination of Triazines in Hemodialysis Saline Solutions by Adsorptive Stripping Voltammetry after Extraction in Acetonitrile

https://docs.google.com/open?id=0B_AiJQrarX6yYUQwZmpoei1aZUU