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Gamry Potensiostat (FAS2) Instructions and other useful information General Info An electrochemical cell must consist of at least two electrodes and one electrolyte. An electrode may be considered to be an interface at which the mechanism of charge transfer changes between electronic (movement of electrons) and ionic movement of ions. An electrolyte is a medium through which charge transfer can take place by the movement of ions. In a cell used for electroanalytical measurements there are always three electrode functions (see below). The first of the three electrodes is the indicating electrode also known as the test or working (GREEN) electrode. This is the electrode at which the electrochemical phenomena being investigated takes place. The second functional electrode is the reference (WHITE) electrode. This is the electrode whose potential is constant enough that it can be taken as the reference standard against which the potentials of the other electrodes present in the cell can be measured. The final functional electrode is the auxiliary or counter (RED) electrode which serves as a source or sink for electrons so that current can be passed from the external circuit through the cell. In general, neither its true potential nor current is ever measured or known. Indicator electrodes 2 (Noble metal indicator electrodes) There are a number of noble metal electrodes currently available for voltammetric studies. In order of frequency of use, they are platinum, gold and silver followed occasionally by palladium, rhodium and iridium. Various polycrystalline forms including sheets, rods and wires are commercially available in high purity and the materials are readily machined into useful shapes. All of the noble metals have an over potential for hydrogen evolution. All of the noble metals adsorb hydrogen on their surfaces although gold does so to a lesser extent. Palladium adsorbs hydrogen into the bulk metal in appreciable quantities and is not recommended for use as a cathode in protic solvents. (Carbon indicator electrodes) As an inert electrode material, carbon is useful for both oxidation and reduction in both aqueous and non aqueous solutions. Only graphitic forms of carbon conduct and are therefore useful as electrode materials. Ordinary spectroscopic grade graphite rods can be used for work in which the surface area of the electrode does not need to be well defined. Other types of carbon electrode include the vitreous (glassy) carbon electrode and the carbon paste electrode. Reference electrodes The ideal reference electrode should posses the following properties; • • • • • it should be reversible and obey the Nernst equation with respect to some species in the electrolyte its potential should be stable with time its potential should return to the equilibrium potential after small currents are passed through the electrode if it is an electrode like the Ag/AgCl reference electrode, the solid phase must not be appreciably soluble in the electrolyte it should show low hysteresis with temperature cycling One of the commonest reference electrodes is the KCl saturated calomel half cell (SCE). A simple form of this electrode can be assembled by adding to a tube, mercury metal, a small amount of solid mercury (II) chloride, several grams of solid KCl and some distilled water. Connection to the external measuring circuit can be made by using a fine platinum wire dipping into the mercury pool. The potential of the SCE can be obtained from: 3 The principal shortcoming of the SCE as a reference is that the solubility of KCl changes substantially with temperature and therefore the cell potential has a relatively large temperature coefficient. Instrumentation (Photographs courtesy of Windsor Scientific, Slough, UK and BAS inc., USA) Modern electroanalytical measurements are normally performed with software driven potentiostats, two examples of which are shown above. Further details about the instruments are available from the manufacturers. Electrochemical Cell The normal material for cell construction is pyrex glass for reasons both of visibility and general chemical inertness. The size of the cell is variable and depends upon the volume, cost and degree of dilution of the sample being studied. If material availability and cost poses no problems, then 25-50cm3 cells can be used. With enzymes, cells with volumes of approximately 1cm3 are preferable because of the costs involved. Some electrochemical measurements can be 4 run in cells under an atmosphere of air. Oxygen however is electrochemically active and its solubility in water is sufficiently great that oxygen reduction can be a problem. As a consequence, most measurements are carried out under an inert atmosphere of either nitrogen or helium. The photograph shows a conventional three electrode cell as used in the authors laboratory showing the working electrode, reference electrode and auxiliary electrode. The cell lid is made from resistant PTFE plastic. A gas line for ebulliating the solution with nitrogen or helium is also evident. (Photograph courtesy of BAS inc., USA) 5 User Instructions Logon to “Potensiostat” on the logon computer in the center corridor Software Available: PHE200 EIS300 PV220 VFP600 Cyclic Voltammetry Click on the “Gamry Framework” icon Click on “Experiment” Move mouse over “A Physical Electrochemistry” Move mouse over and click on “5 Cyclic Voltammetry” Pstat: Leave FAS2 selected Test Identifier: Automatic label showing the type of test selected Output File: Pick your directory (or create one) under C:\Users\ NOTE: You MUST add a "*..DTA" filename extension for your data filenames Electrode Area: In cm2 Notes…: A place to identify your sample in the output file Initial E (V): Starting Potential (see description below for vs Eref and vs Eoc) Scan Limit 1 (V): First peak or maximum voltage Scan Limit 2 (V): First valley or minimum voltage Final E (V): Ending Potential Scan rate (mV/S): Potential ramping rate Step size (mV): Potential difference per step Cycles (#): I/E Range Mode: Auto or Fixed Max Current (mA): IRComp: None or PF or CI PF Corr (ohm): Equil.Time (s): Init Delay: Off or On Time(s) Stab. (mV/s Conditioning: Off or On Time(s) E(V) Advanced Pstat Setup: Off or On Electrode Setup: Off or On Then click “OK” button “Hardware Settings” box appears I/E Stability: Fast or Norm or Slow CA Speed: Fast or Norm or Med or Slow Vch Range: 30mV or 300mV or 3V or 30V Vch Filter: 300kHz or 1kHz or 5Hz or Auto Ich Filter: 300kHz or 1kHz or 5Hz or Auto 6 Then click “OK” button “Electrode Settings” box appears Electrode Type: None or Solid or DME or SMDE or HMDE or Rotating Purge/Stir Cell: Off or On Time(s) Quiet(s) Click on Chart icon (“Start Analysis”) to Analyze Data “File” “Open” Find your file Test Identifier The Identifier parameter is a string that is used as a name. It is written to the data file, so it can be used to identify the data in database or data manipulation programs. The Identifier string defaults a name derived from the technique's name. While this makes an acceptable curve label, it does not generate a unique descriptive label for a data set. The Identifier string is limited to 80 characters. It can include almost any normally printable character. Numbers, upper and lower case letters, and most normal punctuation characters including spaces are valid. Output File The default value of the Output File parameter is an abbreviation of the technique name with a ".DTA" filename extension. We recommend that you use a ".DTA" filename extension for your data filenames. The data analysis package assumes that all data files have ".DTA" extensions. NOTE: The software does not automatically append the ".DTA" filename extension. You must add it yourself. If the script is unable to open the file, an error message box, "Unable to Open File," is generated. Common causes for this type of problem include: • • • An invalid filename. The file is already open under a different Windows application. The disk is full. After you select OK in the error box, the script returns to the Setup box where a new filename can be entered. Electrode Area 7 The Electrode Area parameter is the surface area of the electrode (in cm²) exposed to the sample solution. If you do not wish to enter an area, leave this parameter at its default value of 1.0 cm². Notes: Describe your sample and number in series if applicable The Notes string is limited to 400 characters. It can include all printable characters including numbers, upper and lower case letters, and the most normal punctuation including spaces. TAB characters are not allowed in the Notes string. You can divide your Notes into lines using ENTER. Initial E The Initial E parameter is the starting potential of the scan segment. This potential can be selected in a versus Eoc or versus Eref. This potential is entered in Volts. Scan Limit 1 The Scan Limit 1 parameter is the first apex potential in a Cyclic voltammetry scan. This potential can be selected in a versus Eoc or versus Eref. This potential is entered in Volts. Final E The Final E parameter is the ending potential of the scan segment. This potential can be selected in a versus Eoc or versus Eref. This potential is entered in Volts. Scan Rate The Scan Rate parameter defines the speed of the potential sweep during data acquisition. The Scan Rate is entered in units of mV/sec. A practical bound on the Scan Rate is 1000 mV/sec. Higher Scan Rates may run, but can yield inaccurate data due to the inability of the software to acquire data points fast enough. The Scan Rate parameter when combined with the Step Size parameter determines time between data points and thus the data acquisition rate used in the experiment. Time (seconds/point) = [ Step Size (mV/point) ] / [ Scan Rate (mV/second) ] The maximum data acquisition rate is dependent on the speed of the computer, the configuration of Windows and the other software currently executing. As a guideline, you should avoid sample times below 100 µs. Note that for scans faster than 1 ms that the acquired data will only be displayed once the experiment has completed. This reduces the chance that the computer will limit the acquisition speed. 8 Step Size The Step Size parameter determines the spacing between the data points in mV. A typical Step Size setting is between 1 and 5 mV. The Step Size parameter combines with the scan range on any given cycle to determine the number of data points. # Points = [ Scan Range (mV) ] / [ Step Size (mV) ] The total number of data points must be less than 64000 for all cycles. The Step Size parameter also combines with the Scan Rate parameter to determine the time interval between the data points. Cycles The Cycles parameter controls the number of times the potential scan will be repeated during the experiment. Conceptually it is the number of times the potential will cycle from the Initial E setting to Scan Limit 1 to Scan Limit 2 to the Final E setting. I/E Range Mode The I/E Range Mode parameter controls the autorange state of the I/E converter. If Auto is selected, the I/E Range will be able to freely adjust based on measured currents. If Fixed is selected, the I/E Range will be fixed on a range which is able to measure the current entered in the Max Current parameter. For fast experiments, it is recommended that Fixed be used for the I/E Range Mode. This setting will prevent glitches in the current measurement as the I/E Range resistor is switched. Max Current The Max Current parameter controls the current measurement range when the I/E Range Mode is Fixed. When the I/E Range Mode is Auto, the Max Current parameter specifies the maximum expected starting current. You enter a Max Current value that is the largest current that you expect to see during the scans. From this information the software sets the current range used in the experiment. In order to use the most sensitive range that will not overload, the software will chose the current range based on a value that is 89% of the full scale current range. For example, when using a PC4/750, if a Max Current of 66 mA is input, the current range will be 75 mA. On the other hand, if a Max Current of 67 mA is entered, the 750 mA current range will be selected. NOTE: The Max Current parameter is a current not a current density. The electrode area is not used calculation of the current range to use. 9 If your current data looks very choppy and steppy, the problem could be a poorly selected current range. If you enter a Max Current value of 10 mA and the maximum current in your sweeps is only 100 A, . Theyou result a reisonly us ing 1/100th significant quantization error. Rerun the test entering a smaller Max Current in Setup. If your current data shows perfectly flat, horizontal regions, the current has most likely overloaded the potentiostat's current measurement circuits. Check that the value that you entered for the Max Current parameter is larger that the actual measured cell current. Try rerunning the test with a larger value for the Max Current. IR Comp The IR Comp parameter specifies the type of IR Compensation to be used during data acquisition. There are three possible settings for this parameter. None No IR Compensation is performed. PF Positive Feedback IR Compensation takes a user entered correction value (PF Corr) to correct for the uncompensated resistance. Because this value is entered at the beginning of the experiment, and is not measured after every point, this compensation method is suitable for fast experiments. CI Current Interrupt IR Compensation will be used. In this technique, a current interrupt measurement is after each point, and a determination of the uncompensated resistance is made based on the drop in the voltage measurement. This technique is not suitable for fast experiments, and Positive Feedback IR Compensation should be used instead. PF Corr The Positive Feedback Correction parameter is used during Positive Feedback IR Compensation. This parameter is where the value for the uncompensated resistance is entered in ohms. This resistance may already be known, or may need to be measured first. The Ru Estimation experiment gives a measured value for the uncompensated resistance. Equilibration Time The Equilibration Time corresponds to the amount of time the cell spends at the Initial E setting with the cell turned on. This allows the electrode and the solution time to equilibrate if needed. The current is not monitored during the equilibration time. The time is entered in seconds and must be an integer value. If it is not an integer value, it will be rounded to an integer value when the experiment executes. If you do not wish to have the cell 10 equilibrate at the Initial E setting, set the Equilibration Time to zero. The maximum setting for the Equilibration Time is longer than practically needed (>10^9 seconds). Initial Delay The Initial Delay phase of the experiment is the first step to occur in the experimental sequence. This phase of the experiment is used to stabilize the open circuit voltage of the sample prior to any applied signal and to measure that open circuit potential. The Initial Delay is turned On or Off with the Initial Delay parameter check box in the Setup dialog. The Initial Delay Time parameter is the time that the sample will be held at the open circuit prior to the scan. The delay may stop prior to the Initial Delay Time if the Stability criterion for Eoc is met. The units for Time are seconds. The minimum time is one second. The maximum time is 400,000 s (more than 4 days). Below 1000 seconds, the time resolution is 1 s. Between 1000 and 10,000 s, the resolution is 10 s and above 10,000 s it is 100 s. In many cases, you really do not want to delay for a fixed time. What you really want is to delay until Eoc stops drifting. The Stability parameter allows you to set a drift rate that you feel represents a stable Eoc. If the absolute value of the drift rate falls below the Stability parameter, the Initial Delay phase of the experiment ends immediately, disregarding the programmed Initial Delay Time. Enter a Stability setting of zero to assure that the delay will last for the full Time. The units of Stability are mV/s. A typical value is 0.05 mV/s. The upper limit in this parameter is 8 V/s, well above the range of practical stabilities with real cells. The lower limit of the Stability parameter is set by your patience. A stability of 0.01 mV/s means that a 1 mV drift takes 100s. The software will always take data long enough to resolve a 1 mV change in the potential at the requested drift rate. No open circuit voltage measurement will take place if the initial delay is turned off. In this case, the open circuit voltage is defaulted to 0.0 volts. Conditioning One of the first steps in the experimental sequence is the optional conditioning of the electrode. Conditioning is used to insure that the electrode has a known surface state at the start of the electrode. You may condition the electrode to remove an oxide film or to grow one. Conditioning can be turned On or Off with the Conditioning check box on the Setup menu. Conditioning is done potentiostatically at the Conditioning E for a known time, Conditioning Time. E is the potential applied during the conditioning phase of the experimental sequence. The conditioning potential has an allowed range of ±8 V. E is always specified versus the reference electrode. Time is the length of time that the sample is potentiostatically controlled at the Conditioning E. The units for Time are seconds. The minimum time is one second. The maximum time is 400,000 s 11 (more than 4 days). Below 1000 seconds, the time resolution is 1 s. Between 1000 and 10,000 s, the resolution is 10 s and above 10,000 s it is 100 s. Advanced Pstat Setup The Advanced Pstat Setup checkbox, if checked, will bring up the Hardware Settings dialog. This dialog is used to control specific aspects about your hardware. If you are not an advanced user, or simply wish to use the default hardware settings as specified in the scripts, just un-check this box. If however, you wish to specifically set some hardware items, check this box and you will be presented with further options upon pressing Ok. The Hardware Settings dialog will look similar to the picture depicted below. Electrode Setup The Electrode Setup checkbox, if checked, will bring up the Electrode Setup dialog. This dialog is used to control specific aspects about your electrode. If you are not an advanced user, or simply wish to use the default electrode settings, just un-check this box. If however, you need to specifically set the electrode type or stir/purge conditions, some hardware items, check this box and you will be presented with further options upon pressing Ok. The Electrode Setup dialog will look similar to the picture depicted below. The electrode types are: None Solid DME SMDE HMDE Rotating No special electrode Solid Type Electrode Dropping Mercury Electrode Static Mercury Drop Electrode Hanging Mercury Drop Electrode Rotating Disk Electrode If you select a Rotating electrode, you will be shown an additional setup dialog shown below. In this setup dialog you specify the rotation speed of the electrode. This speed is entered in Revolutions Per Minute (RPM). If you wish to have the rotation stop at the end of the experiment, select the checkbox to turn off rotation after the experiment. 12 * Cyclic Voltammetry Experimental Sequence 1. A Runner window is created by the Framework and the "Cyclic Voltammetry.EXP" script is run in this window. 2. The script creates the Setup dialog box which becomes the active window and accepts changes in the experimental parameters. This Setup box remembers the experimental settings from the last time this script was run. To restore the parameters to the values defined in the script, select the Default button. If the Advanced Pstat Setup is toggled to the on position a second Setup dialog box contain hardware configuration details will become the active window allowing the user to modify the hardware configuration used during the experiment. 3. The script next obtains the use of the potentiostat specified during Setup and opens the data file using the Output File name. If the potentiostat is in use or the file cannot be opened, the script returns you to the Setup dialog box. 4. The file header information is written to the data file. This information is written to the file prior to data acquisition. If the experiment is aborted, the output file contains only this information. This header information includes: a. Tags identifying possible analyses b. The current time and date c. A list of the Setup parameters and hardware configuration 5. If Initial Delay is on, then the cell is turned off and the specimen's Eoc is measured for the time specified as the Initial Delay time or until the potential stabilizes to a value less than the stability setting. A plot of potential versus time is always displayed. The last measured potential is recorded as Eoc. If Initial Delay is off, this step is skipped and Eoc is assumed to be 0.0 V vs. Ref. 6. The script conditions the electrode if Conditioning was specified in the Setup. Conditioning is done by applying a fixed potential for a defined time. A plot of current versus time is displayed during Conditioning. 7. Finally an actual scan occurs. The potential of the sample is set to Initial E and is held at that value for the Equil. Time. The potential is then swept from the Initial E to Scan Limit 1, then to Scan limit 2 and then to Final E. If Scan limit 2 equals Final E then the scan stops at that value. Current readings at fixed voltage intervals are taken during the sweep. The voltage interval between steps is defined as the Resolution in the Setup dialog box. If the number of Cycles exceeds one, the potential will repeat the sweep for n cycles. Note that if Initial E does not equal Final E the potential will jump from Final E to Initial E as each cycle is repeated. The sweep is actually a staircase ramp. The sample is potentiostatted at the Initial E, a 13 delay of one sample period occurs (sample period = 1/Scan Rate * Resolution), and a reading of the current is taken. The potential is then stepped by a few mV as defined by Resolution, a delay of one Sample Period occurs, and the next current reading is taken. Stepping the potential, delaying and acquiring data points continues until the potential equals the Final E. If Autoranging was selected in Setup at each point the current range is automatically switched to the optimal range for the measured cell current. If Positive Feedback IR Compensation has been selected all data is continuously corrected for IR drop. If Current Interrupt IR Compensation has been selected, each potential is corrected for the measured IR drop of the preceding point. A plot of I vs. E is displayed during the scan. 8. The data is written to the output file and the script cleans up and halts. Once the scan is over, the cell is turned off. The acquired data is written to the output file. The script then waits for you to select Skip. Once you do so, the script closes everything that's open, including the Runner window. * * Current and Voltage Definitions A current value of +1.2 mA can mean different things to workers in different areas of electrochemistry. To an analytical electrochemist it represents 1.2 mA of cathodic current. To a corrosion scientist it represents 1.2 mA of anodic current. In the PHE200's standard techniques we follow the analytical convention for current. Positive currents are cathodic, arising from a reduction at the electrode under test. This convention is the opposite of the current convention used in other Gamry application software packages such as the DC105, and EIS300. Potentials can also be a source of confusion. Throughout Gamry's software the equilibrium potential assumed by the electrode in the absence of electrical connections to the electrode is called the Open Circuit Potential, Eoc. In the PHE200, all potentials are specified or reported as the potential of the working electrode with respect to either the reference electrode or this open circuit potential. The former is always labeled as "vs Eref" and the later is labeled as "vs Eoc". The equations used to convert from one form of potential to the other are: E vs Eoc = ( E vs Eref) - Eoc E vs Eref = ( E vs Eoc) + Eoc Regardless off whether potentials are versus Eref or versus Eoc, one sign convention is used. The more positive a potential, the more anodic it is. 14 * Introduction Welcome to the Gamry Instruments, Inc. PHE200 Physical Electrochemistry package. This package is meant for researchers who are performing studies in the area of electrochemistry. Included in the package are techniques for performing linear sweep and cyclic voltammetry experiments, as well as chronopotentiometry, chronoamperometry, chronocoulometry, and controlled potential coulometry techniques. An experiment which determines uncompensated resistance (Ru) is also included. * References The following are references are useful for learning more about the techniques that are available in the PHE200. Certain parts of the text will refer you to specific references. Cyclic Voltammetry R. S. Nicholson, Anal. Chem., 37, 1351 (1965). Electrochemical Methods: Fundamental and Applications, Allen J. Bard and Larry R. Faulkner, John Wiley & Sons, New York (2000) pp. 226ff. ISBN 0-471-04372-9. R. S. Nicholson and I Shain, Anal. Chem., 36, 706 (1964), and Anal. Chem., 37, 178 (1965). Chronocoulometry Electrochemical Methods: Fundamental and Applications, Allen J. Bard and Larry R. Faulkner, John Wiley & Sons, New York (2000) pp. 210ff. ISBN 0-471-04372-9. Chronopotentiometry Electrochemical Methods: Fundamental and Applications, Allen J. Bard and Larry R. Faulkner, John Wiley & Sons, New York (2000) pp. 305ff. ISBN 0-471-04372-9. * Purpose Cyclic Voltammetry is used to study the mechanism, kinetics, and thermodynamics of chemical reactions. Both heterogeneous reactions occurring at the electrode surface, and homogeneous reactions in solution can be studied. 15 In the classical Cyclic Voltammetry triangle waveform, the potential is swept from an Initial E, to vertex E, and back to Final E, where Final E equals Initial E. An example of this applied waveform is shown below. Repeating this waveform for N times will perform N cycles of Cyclic Voltammetry. In the PHE200 we use the more generic double vertex triangular waveform shown below. This applied waveform allows the user to set a second vertex potential (Scan Limit 2 in the software) which could be more positive than the initial potential. Setting ScanLimit2 and Final E to equal the Initial E can perform the classically defined triangle waveform for cyclic voltammetry. Lets look at a simple example of a Cyclic Voltammetry experiment, using the classically defined triangular waveform. For the case of a simple one-electron transfer reaction, the resulting current vs. voltage plot will give the familiar "duck shape" waveform shown below. In these cases, the reversible potential for the electron transfer can be evaluated from the half-wave potential for the redox process. Electron transfer kinetics can also be studied by varying the scan rate of the applied potential and observing the increase in Ep ( Nicholson). An overall review of potential sweep voltammetry methods is covered in Chapter 6 Bard and Faulkner. In cases where the chemistry of the system is more complicated, cyclic voltammetry can be used to determine the mechanisms and kinetics involved. In their work in the 1960's, Nicholson and Shain published a series of articles that discussed the use of cyclic voltammetry to study chemical systems which included chemical reactions either proceeding or following the electron transfer seen in the cyclic voltammogram (Nicholson and Shain). The user is encouraged to review these works for a better understanding of the versatility of the cyclic voltammetry experiment. * Experiment Menu Commands A typical Experiment pull down menu is shown in the figure below. The items found on this menu are not the same in all systems. They depend on: • • • Which Gamry Instruments Windows based software systems you have installed. The order in which you installed your software systems. The most recently run experiments. 16 • Custom modifications that have been done to the menu. All of the commands in the Experiment menu directly or indirectly open a Runner window and then compile and execute an experimental script in that window. Typical Experiment Pull Down Menu The Experiment pull down menu is always divided into 3 sections. You use The bottom section of the menu to access standard techniques. The middle section of the menu to rerun a recently used script. The upper section of the menu to run a Named script. * Analysis Menu Commands A typical Analysis pull down menu is shown in in the figure below. The Analysis Pull Down Menu The Analysis menu is used to access the Gamry Echem Analyst. The menu is divided into two sections. The top section allows the user to start the Gamry Echem Analyst. The bottom section is a list of the most recently used (MRU) data files. The Start Analysis command is used to start the Gamry Echem Analyst. No data file will be opened. Clicking on the MRU list will start the Gamry Echem Analyst and open the data file selected from the MRU. * Gamry Framework: Library Routines Math functions Abs() Calculate the absolute value of a number Exp() Calculate ex of a number Index() Convert a quantity to an INDEX LineOpt() Find a line noise rejecting frequency Log() Calculate natural logarithm of a number Log10() Calculate base-10 log of a number Modulus Convert Real, Imaginary to Modulus Phase Convert Real, Imaginary to Phase Pow() Calculate number raised to a power Rand() Generate a random number 17 Real() Convert quantity to floating point Round() Round off a REAL to N decimal places Sqrt() Calculate square root of a number Trigonometry Functions ArcCos() Arc Cosine ArcSin() Arc Sine ArcTan() Arc Tangent Cos() Cosine Cosh() Hyperbolic Cosine DtoR() Convert Degrees to Radians NormalD() Normalize an angle to +/- 360 Degrees NormalR() Normalize an angle to +/- 2 RtoD() Convert Radians to Degrees Sin() Sine Sinh() Hyperbolic Sine Tan() Tangent Tanh() Hyperbolic Tangent Mathematical Classes class COMPLEX Complex number class String Functions Ascii() Convert a character to an Ascii number Char() Convert an Ascii number to a character IsAlnum() Check if character is alphanumeric IsAlpha() Check if character is letter IsDigit() Check if character is numeric IsLower() Check if character is lowercase letter IsPunct() Check if character is punctuation IsSpace() Check if character is white-space IsUpper() Check if character is uppercase letter IsXdigit() Check if character is hexadecimal digit StrCmp() Compare two strings StrGet() Get ith character in a string StrLen() Calculate length of string StrLwr() Convert string to lowercase StrSet() Set ith character in a string StrUpr() Convert string to uppercase Time functions DateStamp() Generate a string containing a date Time() Set a variable to the current time TimeStamp() Generate a string containing a time Statistical functions Mean() Calculate Mean of a data set StatsOne() Calculate Statistics on one data set StatsTwo() Calculate Statistics between two data sets Delay and Program Control functions Abort() Immediately abort a script Dawdle() Halt script until SKIP selected Execute() Launch a child script ExecWait() Launch a child script and wait for completion Pause() Pause script for a set number of seconds 18 Sleep() Tell a script to sleep until a specified time Suspend() Allows a user to continue or abort WinExec() Start up a Windows application Yield() Let other Windows programs gain control Output functions Print() Output to the current output file Printl() Output to file with automatic newline Sprint() Output an item to a STRING Video Display functions Stdout() Sends output to the STDOUT window StdoutActivate() Brings STDOUT window to foreground Error() Report a fatal error and terminate the run Headline() Update the headline above the data curve Notify() Update runner window status display Query() Ask operator a multiple choice question Setup() Open a dialog box for parameter input Warning() Warn operator - wait for OK to proceed Class and Object Manipulation functions ClassIndex() Numerical sorting of classes ClassName() Place class name in a STRING ClassNew() Dynamically create a class ClassAddISel() Dynamically add an instance variable ClassAddCSel() Dynamically add a class variable FindClassByName() Return a class given a STRING ObjectNew() Dynamically create a new object Setup Disk File functions SetupRestore() Recover an old setup on disk SetupSave() Save a setup on disk Miscellaneous functions Callin() Dynamically acquire DLL library function Config() Read an INI file value LoadLibrary() Dynamically load a DLL library SetConfig() Set an INI file value VectorCount() Determine size of a Vector VectorNew() Generate a new VECTOR Parameter Classes having Dialog license suitable for Setup class CHANNEL Used to setup multiplexed experiments class DLGSPACE Used to generate white space in Setup() class IQUANT An integer parameter for general use class LABEL A short string for general use class NOTES A multiline string too long to SAVE/RECALL class ONEPARAM Complex class- 1 TOGGLE, 1 QUANT class OUTPUT An output file class POTEN A potential with vs Eref & vs Eoc info. class QUANT A real parameter for general use class SELECTOR Radio button selection class STATIC String used for descriptive purposes class TOGGLE An on/off parameter for general use class TWOPARAM Complex class -1 TOGGLE, 2 QUANTS 19 Instrument Driver Classes class PSTAT A potentiostat class MUX An ECM8 Multiplexer Classes having Signal licenses used by curve functions class ICONST A constant current class VCONST A constant voltage signal Curve Classes class CGEN Generic curve class class CURVE A generic name for all this group's classes class IVT Generic IVT curve used for conditioning class OCV Curve to measure open circuit potential Miscellaneous Classes class DATACOL Hold data extracted from CURVE object class ARRAY Array class which can hold any object * Cyclic Voltammetry Experimental Sequence 1. A Runner window is created by the Framework and the "Cyclic Voltammetry.EXP" script is run in this window. 2. The script creates the Setup dialog box which becomes the active window and accepts changes in the experimental parameters. This Setup box remembers the experimental settings from the last time this script was run. To restore the parameters to the values defined in the script, select the Default button. If the Advanced Pstat Setup is toggled to the on position a second Setup dialog box contain hardware configuration details will become the active window allowing the user to modify the hardware configuration used during the experiment. 3. The script next obtains the use of the potentiostat specified during Setup and opens the data file using the Output File name. If the potentiostat is in use or the file cannot be opened, the script returns you to the Setup dialog box. 4. The file header information is written to the data file. This information is written to the file prior to data acquisition. If the experiment is aborted, the output file contains only this information. This header information includes: a. Tags identifying possible analyses b. The current time and date c. A list of the Setup parameters and hardware configuration 5. If Initial Delay is on, then the cell is turned off and the specimen's Eoc is measured for the time specified as the Initial Delay time or until the potential stabilizes to a value less than the stability setting. A plot of potential versus time is always displayed. The last measured potential is recorded as Eoc. If Initial Delay is off, this step is skipped and Eoc is assumed to be 0.0 V vs. Ref. 20 6. The script conditions the electrode if Conditioning was specified in the Setup. Conditioning is done by applying a fixed potential for a defined time. A plot of current versus time is displayed during Conditioning. 7. Finally an actual scan occurs. The potential of the sample is set to Initial E and is held at that value for the Equil. Time. The potential is then swept from the Initial E to Scan Limit 1, then to Scan limit 2 and then to Final E. If Scan limit 2 equals Final E then the scan stops at that value. Current readings at fixed voltage intervals are taken during the sweep. The voltage interval between steps is defined as the Resolution in the Setup dialog box. If the number of Cycles exceeds one, the potential will repeat the sweep for n cycles. Note that if Initial E does not equal Final E the potential will jump from Final E to Initial E as each cycle is repeated. The sweep is actually a staircase ramp. The sample is potentiostatted at the Initial E, a delay of one sample period occurs (sample period = 1/Scan Rate * Resolution), and a reading of the current is taken. The potential is then stepped by a few mV as defined by Resolution, a delay of one Sample Period occurs, and the next current reading is taken. Stepping the potential, delaying and acquiring data points continues until the potential equals the Final E. If Autoranging was selected in Setup at each point the current range is automatically switched to the optimal range for the measured cell current. If Positive Feedback IR Compensation has been selected all data is continuously corrected for IR drop. If Current Interrupt IR Compensation has been selected, each potential is corrected for the measured IR drop of the preceding point. A plot of I vs. E is displayed during the scan. 8. The data is written to the output file and the script cleans up and halts. Once the scan is over, the cell is turned off. The acquired data is written to the output file. The script then waits for you to select Skip. Once you do so, the script closes everything that's open, including the Runner window. 21 * PHE200 Analysis Overview The PHE200 uses the Gamry Echem Analyst to display, analyze, and print acquired data files. The Gamry Echem Analyst also allows the user to use experiment specific analysis commands, such as normalize current by scan rate in Cyclic Voltammetry experiments, by selecting those analysis commands from experiment specific pull down menus. The Gamry Echem Analyst uses Visual Basic for Applications (VBA) as a scripting language to control the graphing and manipulation of the experimental data. Gamry's standard VBA analysis scripts are accessible by the user, so that they can be modified to include special analysis routines that are customized to your laboratories needs. Please refer to the Gamry Echem Analyst section of Help for information on customizing analysis scripts. * Cyclic Voltammetry Analysis Overview The Cyclic Voltammetry analysis script allows the user to open Cyclic Voltammetry data files by selecting the data file from the File Open window. The data file will open, and a series of tabbed pages are created in a new window. These tabs will contain a graph of the data, experimental setup information, experimental notes, and hardware settings. The standard Cyclic Voltammetry analysis script shows a plot of current (I) vs. voltage (E) on the first page. Common analyses performed from this graph include background subtractions, peak find, and normalize by area or scan rate. Descriptions of the analysis commands in the Cyclic Voltammetry menu can be found in the Commands section of the PHE200 Data Analysis Help. * Cyclic Voltammetry Analysis Pages 22 The standard cyclic voltammetry analysis contains the following pages at startup: Chart The Chart page is the heart of the Cyclic Voltammetry analysis. It contains the data plotted in a Voltage versus Current format. Experimental Setup The Experimental Setup page contains a list of the parameters used to acquire the data presented on the Chart page. Experimental Notes The Experimental Notes are the notes entered by the user prior to the data acquisition. The Notes can be edited in the analysis, and saved with the changes to an analysis data file. Hardware Settings The Hardware Settings Page contains all of the potentiostat settings used during the run of the experiment. Additional Pages Which May Become Visible Integration This page contains a grid of integrated regions which were defined using the Integrate command. Quick Integration This page contains a grid of integration values based on different sections in visible traces based on the most recent use of the Quick Integrate command. Min/Max This page contains a grid of minimum and maximum values of each visible trace based on the most recent use of the Min/Max command. Peak Locations This page contains a grid of Peak locations and magnitudes based on the most recent use of the Peak Find command. * Integrate 23 The Integrate command is used to integrate the current to achieve a total charge value. This command requires that an portion of a specific curve be selected. The command will operate on only the active trace, and the results will be placed on a new Integrate page, assuming one does not already exist. In the case where this page already exists, the new information will append the old information. 1. The steps to use this command are as follows: 1. Make the trace on which to integrate a region the Active Trace. This can be done by right clicking on the trace and pressing Activate Trace. 2. Select a region on this trace using the Select Portion of Curve tool . To use this tool, left click with the mouse close to one bounding point of the region. Next, left click again near the second bounding point of the region. The region will become highlighted. If you select an improper range, no region may be displayed. To reselect a range, simply toggle the Select Portion of Curve tool, and try again. 3. Once a range is selected, press the Integrate command on the document menu. The algorithm will now begin determine the area of the region. 4. A new page will be added to the document, with a grid containing the integration information. If regions had been integrated previously, the new region information will simply append the existing information on the Integration page. The region information will also be displayed in the QuickView window at the bottom of the graph. You can hide the QuickView window by right clicking on the handle of the toolbar, and un-checking QuickView. 5. The baseline is either zero, or the Line object which is selected to be the baseline for the region. The Region Baselines command allows you to specify which line goes with which region. 6. Additional regions can be found by repeating steps 1 through 3. To clear regions you can use the Clear Regions command. Note that the results are presented both on their own page, and in the QuickView area at the bottom of the chart. You can hide the QuickView window by right clicking on the handle of the toolbar, and un-checking QuickView. * Quick Integrate The Quick Integrate command is used to integrate the current to achieve a total charge value. This command requires that an X Region be selected. The command will operate on all visible traces, and the results will be placed on a new Quick Integrate page, assuming one does not already exist. In the case where this page already exists, the new information will overwrite the old information. 24 The steps to use this command are as follows: 1. Select an X region using the Select X Region tool . To select a region using this tool, left click in the main chart body at one bounding edge of the region. Holding the mouse button down, drag the mouse to the second bounding edge, and release the left button. You will then have a highlighted region on your chart. 2. Once the region is selected, you may select the Quick Integrate command from the document menu. As soon as you select this command, the computer will begin to calculate the charge for the different sections of the curve. There can be up to three sections in a cyclic voltammetry curve. These sections are defined by the vertices of the applied voltage signal. In the case where you have a Vinit, a VLimit 1, and a VLimit2, you would have a 2 section curve. In the case where you have a Vinit, a VLimit1, a VLimit2, and a Vfinal, you would have a 3 section curve. The integrated current is listed by section. Note that the results are presented both on their own page, and in the QuickView area at the bottom of the chart. You can hide the QuickView window by right clicking on the handle of the toolbar, and un-checking QuickView. * Min/Max The Min/Max command is used to find the minimum and maximum values of the data plotted on the Y-Axis. This command requires that an X Region be selected. If you do not select a region, the search will be performed over the entire range of the data set. The command will operate on all visible traces, and the results will be placed on a new Min/Max page, assuming one does not already exist. In the case where this page already exists, the new information will overwrite the old information. The steps to use this command are as follows: 1. Select an X region using the Select X Region tool . To select a region using this tool, left click in the main chart body at one bounding edge of the region. Holding the mouse button down, drag the mouse to the second bounding edge, and release the left button. You will then have a highlighted region on your chart. 2. Once the region is selected, you may select the Min/Max command from the document menu. As soon as you select this command, the computer will begin to locate the minimum and maximum values for each visible trace. Note that the results are presented both on their own page, and in the QuickView area at 25 the bottom of the chart. You can hide the QuickView window by clicking the X on the sidebar of the QuickView. * Peak Find The Peak Find command is used to have the computer search a region of data for a localized peak or valley. The algorithm looks for a change in the slope of the specified region, which indicates a peak or a valley. The algorithm then searches in this area for the maximum or minimum value which it then takes as its peak. This algorithm is written in VBA and may be viewed or modified by the advanced user as necessary. The steps to use this command are as follows: 1. Make the trace on which to search for peaks the Active Trace. This can be done by right clicking on the trace and pressing Activate Trace. 2. Select a region on this trace using the Select Portion of Curve tool . To use this tool, left click with the mouse close to one bounding point of the region. Next, left click again near the second bounding point of the region. The region will become highlighted. If you select an improper region, no peaks may be found. To re-select a region, simply toggle the Select Portion of Curve tool, and try again. 3. Once a region is selected, press the Peak Find command on the document menu. The algorithm will now begin to search for peaks in the selected region. If no peaks are found, you will see a message box stating no peaks have been found. Try selecting another region. 4. If peaks are found, a new page will be added to the document, with a grid containing the peak information. If peaks had been found previously, the peak information will simply append the existing peak information on the peak page. The peak information will also be displayed in the QuickView window at the bottom of the graph. You can hide the QuickView window by clicking the X on the side-bar of the QuickView. 5. You can now display the peak on the chart if you wish by using the Mark Peaks tool . Selecting this tool will display the peak location, and the distance from the peak to the baseline. The baseline is either zero, or the Line object which is selected to be the baseline for the peak. The Peak Baselines command allows you to specify which line goes with which peak. 6. Additional peaks can be found by repeating steps 1 through 4. To clear peaks you can use the Clear Peaks command. * * 26 http://www.pineinst.com/echem/files/LMCBP-PROF1.pdf * http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l5html/cvr.html * http://employees.oneonta.edu/viningwj/ * http://www-biol.paisley.ac.uk/marco/Enzyme_Electrode/Chapter1/Ferrocene_animated_CV1.htm Animated Cyclic Voltammetry experiment This shows the cyclic voltammogram for the reversible oxidation (forward sweep) and reduction (reverse sweep) for hydroxy-ferrocene. In aqueous buffered electrolyte, hydroxy-ferrocene undergoes a simple outer sphere one-electron redox process according to the following scheme: The heterogeneous kinetics of this reaction are rapid so that over a wide range of sweep rates, the reaction is reversible. The important points to note with the above cyclic voltammogram are as 27 follows; • The ferrocene compound is initially present in the reduced state. The voltage scan starts at a potential negative of the Eo value for this couple and hence, there is no flow of current. • As the voltage approaches the Eo value (320mV) a positive current begins to flow indicating that the ferrocene molecule is being oxidised. The current continues to rise (exponentially). This is known as the kinetic region of the voltammogram. • As the voltage increases, the rate of reaction also increases until a point is reached when the process becomes limited by the mass transfer of ferrocene from the bulk to the electrode surface. The current then begins to fall and a peak is produced. The decay profile follows a t-1/2 temporal relationship. • As the voltage sweep is reversed (when the switching potential is reached), the oxidised material that is in the vicinity of the electrode is reduced resulting in a reduction peak of similar magnitude. • Because this is a reversible system, the peak separation is 57mV. * http://www.cartage.org.lb/en/themes/sciences/Chemistry/Electrochemis/Electrochemical/CyclicVoltammetry /CyclicVoltammetry.htm Cyclic Voltammetry Cyclic voltammetry (CV) is very similar to LSV. In this case the voltage is swept between two values (see below) at afixed rate, however now when the voltage reaches V2 the scan is reversed and the voltage is swept back to V1 28 A typical cyclic voltammogram recorded for a reversible single electrode transfer reaction is shown in below. Again the solution contains only a single electrochemical reactant The forward sweep produces an identical repsonse to that seen for the LSV experiment. When the scan is reversed we simply move back through the 2+ equilibrium positions gradually converting electrolysis product (F back to reactant 3+ (Fe ). The current flow is now from the solution species back to the electrode and so occurs in the opposite sense to the forward seep but otherwise the behaviour can be explained in an identical manner. For a reversible electrochemical reaction the CV recorded has certain well defined characteristics. I) The voltage separation between the current peaks is 29 II) The positions of peak voltage do not alter as a function of voltage scan rate III) The ratio of the peak currents is equal to one IV) The peak currents are proportional to the square root of the scan rate The influence of the voltage scan rate on the current for a reversible electron transfer can be seen below As with LSV the influence of scan rate is explained for a reversible electron transfer reaction in terms of the diffusion layer thickness. The CV for cases where the electron transfer is not reversible show considerably different behaviour from their reversible counterparts. The figure below shows the voltammogram for a quasi-reversible reaction for different values of the reduction and oxidation rate constants. 30 The first curve shows the case where both the oxidation and reduction rate constants are still fast, however, as the rate constants are lowered the curves shift to more reductive potentials. Again this may be rationalised interms of the equilibrium at the surface is no longer establishing so rapidly. In these cases the peak separation is no longer fixed but varies as a function of the scan rate. Similarly the peak current nolonger varies as a function of the square root of the scan rate. By analysing the variation of peak position as a function of scan rate it is possible to gain an estimate for the electron transfer rate constants. Information provided by: http://www.bath.ac.uk 31 Software Features Available Physical Electrochemistry 1. 2. 3. 4. 5. 6. 7. Chronoamperometry Chronocoulometry Chronopotentiometry Controlled Potential Coulometry Cyclic Voltammetry GetRu Linear Sweep Voltammetry Pulse Voltammetry 1. 2. 3. 4. 5. 6. 7. 8. 9. A. B. C. D. Sampled DC Voltammetry Differential Pulse Voltammetry Square Wave Voltammetry Normal Pulse Voltammetry Reverse Normal Pulse Voltammetry Sampled DC Stripping Voltammetry Differential Pulse Stripping Voltammetry Square Wave Stripping Voltammetry Normal Pulse Stripping Voltammetry Reverse Normal Pulse Stripping Voltammetry Potentiostatic Generic Pulse Galvanostatic Generic Pulse GetRu.exp Electrochemical Impedance 1. 2. 3. 4. 5. 6. 7. 8. Galvanostatic EIS Hybrid EIS Mott Schottky Multiplexed Potentiostatic EIS Potentiostatic EIS Single Frequency EIS Multiplexed Potentiostatic EIS Repeating Multiplexed Potentiostatic EIS Repeating Sequential 32