Instruments4Chem

High Performance, Low Cost…

Potentiostat Calibration


If a linear potential ramp is imposed across a test resistor the measured current should also be a linear ramp. The purpose of the calibration procedure described here is to determine the slope and intercept of this ramp for each current range of the instrument. After following this procedure you will generate two files, Vcal.doc and Ical.doc so that the instrument will perform correctly. Once this is done, calibration only needs to be carried out occasionally.

Accurately measure the resistance of 3 test resistors with a multimeter. Nominal values of 3.9k, 39k and 390k are recommended for this calibration procedure.

To calibrate the potentiostat connect a test resistor between the potentiostat’s working and reference electrode leads and connect the counter electrode lead to the reference lead.

The calibration vi is called Potentiostat_Calibration. This vi performs a linear scan from an initial potential (StartV) to a final potential (EndV) with a step height of StepH mV. Each current range should be separately calibrated – the full scale current (FS Current) is set in the dialog box.

Method

The first step in the calibration procedure is to establish the potential limits of the potentiostat.
1) Enter a value of zero in the DAC_Set textbox and run the vi.
2) Using a multimeter, measure the potential difference between W and (R,C). Enter this value into the Vmin field.
3) Enter a value of 65535 in the DAC_Set textbox and run the vi.
4) Using a multimeter, measure the potential difference between W and (R,C). Enter this value into the Vmax field.
5) Enter the resistance value of the test resistor into the Resistor field.
6) Set StartV to -1.000 V, StepH to 1 mV and EndV to +1.000 V.
7) Set the desired full scale current and run the vi.

A potential scan will be performed and a straight line plot should appear. Superimposed on this plot will be a linear regression line. The slope, intercept and ADU’s/μA (ADU – analog-to-digital “units”) will be displayed in their respective fields.

(The #pts, #Bytes and RunTime fields are provided for information only – the number of points is the number of points in the scan, the number of bytes should be twice the previous value and Run Time is the total time in seconds for the scan and the subsequent data upload).

Check that the data is a good straight line. Check for saturation at either end of the scan – if the line has flat regions at both ends the current amplifier has become saturated, meaning that the resistor value is too small for the selected full scale current.

If necessary increase the value of the resistor. Too large a resistor value on a particular full scale current will result in a noisy calibration curve. When you are satisfied with the current vs voltage data record the values of the fit parameters.

8) Repeat steps 5-7 above for all current ranges, and enter the data into an Excel spreadsheet.
9) Enter the data collected in the above procedure into the two files : Ical.doc has the intercept and slope (ADU’s/μA) tab delimited, one pair per line. Vcal.doc has the minimum potential and the maximum potential entered as two separate lines.
10) These files need to be placed in the LabVIEW root directory for correct operation. The potentiostat vi requires the information to correctly set potential and measure currents in an electrochemical experiment.

Sample Calibration Results

Sample calibration data for a potentiostat after following the method outlined above.

Prior to using the potentiostat, data from the calibration needs to be stored in two files : Vcal.doc and Ical.doc. This data is used to set voltages and measure currents via the on-board DAC and ADC.