Calibration of a Gold-Coated Quartz Crystal
The purpose of this Application Note is to demonstrate a simple experiment to reduce Cu2+ onto a gold electrode and then oxidize Cu back off. We then use the result to calculate a calibration factor for the crystal. Conversely, we show you how to calculate the molar mass of the species deposited using a calibration factor.
Each crystal has a theoretical calibration factor; however, under typical experimental conditions, these calibration factors vary slightly. We use Cu2+ to calculate a calibration factor which could then be used in subsequent experiments on the same crystal.
Cu2+ is reduced in a two-electron reaction:
CuSO4(aq) + 2 e- Cu(s) + SO42-(aq)
Our goal is to reduce Cu2+ onto a gold electrode, then use the decrease in frequency and the charge passed to calculate a calibration factor.
A solution of CuSO4 (5–10 mM) in 1 M H2SO4 was prepared and placed into the PTFE cell containing a 10 MHz Au-coated quartz crystal. No air/solution interface was present in the cell. The crystal’s electroactive area was 0.209 cm2 and the area of overlap was 0.205 cm2.
The eQCM 10M™ was connected to the cell using the supplied cell cable. A Gamry Instruments Reference 600+ potentiostat was connected to the working face of the crystal using a stacking banana plug-to-stacking pin cable. Cell setup was completed with a Pt counter electrode and an Ag|AgCl reference electrode.
Upon starting Gamry Instruments Resonator™ software, the nominal frequency of the crystal, 10 MHz, was entered in the Center Freq. field along with a Freq. Width of 50 kHz and a Freq. Step of 0.2 Hz. Clicking the Single Scan button resulted in the spectrum shown in Figure 1. Next, the green cursors that appeared on the spectrum were moved closer to the resonant frequencies and the Start button was clicked to trigger continuous data acquisition.
Figure 1. Resonator screenshot after entering initial parameters and clicking the Single Scan button.
The potentiostat was set up by selecting Cyclic Voltammetry from the Technique drop-down menu and clicking the Setup button. A setup screen for cyclic voltammetry appeared and parameters were entered as shown below:
|Initial E (V):||0.050|
|Scan Limit 1 (V):||–0.250|
|Scan Limit 2 (V):||0.300|
|Final E (V):||0.050|
|Scan Rate (mV/s):||50|
|Step Size (mV):||2|
|I/E Range Mode:||Fixed|
|Max Current (mA):||30|
After the potentiostat was set up, the OK button was clicked (which closed the setup window), followed by the Run button.
A screenshot of the potentiostat panel in Resonator during acquisition is shown in Figure 2. Note that the top plot shows both resonant frequencies for the entire time that the QCM has been acquiring data, while the bottom plot shows current and voltage data. It is also possible to show current and voltage versus time by selecting Time in the Display Graph drop-down menu.
Figure 2 Screenshot of Resonator during acquisition.
The first plot in Echem Analyst is a plot of the change in current and frequency versus voltage (Fig. 3), and the second plot is change in frequency versus charge (Fig. 4).
Figure 3. Echem Analyst showing change in current and frequency versus voltage for the five cycles.
Figure 4. Echem Analyst showing change in frequency versus charge for the five cycles.
Use the Curve Selector button to plot data a variety of ways and also to show or hide specific curves. The deposition portion of the curve was selected by clicking on the Select Portion of the Curve using the Mouse tool button A linear fit was then calculated by choosing the Linear Fit option under the Common Tools menu. The Quick View pane at the bottom of the window in Figure 5 gives the slope of the linear fit as 376 kHz/C.
Figure 5. Echem Analyst showing the linear fit of change in frequency
versus charge in the Quick Viewpane at the bottom of the window.
The calibration factor for the crystal is then calculated according to the equation
where F is the Faraday Constant, EA is the electroactive area, MMCu is the molar mass of Cu, and n is the number of electrons. The 106 is used to convert from grams to micrograms. Cf is calculated to be 232 Hz cm2/µg. This is approximately 3% different than the theoretical value of 226 Hz cm2/µg.
If you already know your calibration factor accurately you can determine the molar mass of the mobile species (deposited, in this instance) during your experiment:
Use the Curve Selector to plot change in mass (∆M) versus charge as shown in Figure 6.
Figure 6. Echem Analyst showing the linear fit for change in mass versus charge
in the Quick View pane at the bottom of the window.
Echem Analyst uses the calibration factor entered into Resonator software, along with the electroactive area, to calculate the mass. The slope of the line, after performing a second linear fit, for the deposition is –347.6 µg/C. The molar mass of the Cu2+ can be calculated using the equation
where F and n are as described previously. In this instance the molar mass was calculated to be 67.1 g/mol.
The eQCM 10M is shipped with the Gamry Instruments Resonator software, Gamry Echem Analyst software, a Quick Start Guide, a Hardware Operator’s Manual (CD), a Software Operator’s Manual (CD), one EQCM cell, one AC Power Adapter, one USB interface cable, one BNC cable, one potentiostat interface cable, and 5 Au-coated quartz crystals (5 MHz).
The eQCM 10M is protected by a two-year factory service warranty.
The eQCM 10M must be connected to a computer with a Gamry Instruments potentiostat and a Physical Electrochemistry software license for incorporation and combination of QCM and potentiostat data into Echem Analyst. Microsoft® Windows® 7 or higher is required.
|Frequency range||1–10 MHz|
|Frequency resolution||0.02 Hz|
|Operating temperature range||0–45°C|
|Relative humidity||maximum 90% non-condensing|
|Storage and shipping temperature||–25 to 75°C|
|Dimensions||175 × 115 × 80 mm|
|AC power adapter||100–264 VAC, 47–63 Hz|
|Quartz-crystal microbalance||12 VDC, 25 W|