FAQ - General

Compliance voltage is the voltage available at the counter electrode that can be used to force current to flow and still maintain control of the working electrode voltage.  The compliance should be specified in conjunction with a current value: "20V @ 200mA" or "20V @ full rated current". An amplifier's output voltage decreases at higher currents due to the output impedance of the amplifier.

The voltage at the counter electrode is needed to force the electrochemical reaction at the counter electrode to proceed and to overcome the iR drop through the bulk of the electrolyte solution. Compliance voltage can become important when the currents are high, or when the conductivity of the solution is low (dilute electrolytes), or when a high resistance sintered glass or Vycor frit has been used to isolate the counter electrode.

In practice, depending on the electrolyte and cell design, the potentiostats useable limits may be defined by either its maximum current specification or its compliance voltage limit. In either case, if the limits are exceeded the potentiostat will be unable to properly control the voltage of the working electrode.
Example for testing compliance voltage: Construct a “dummy cell” with the following resistors, and connect the leads of the potentiostat as shown:

 

This is representative of a cell with electrolyte of low conductivity. As the reference electrode is generally placed closer to the working/sense (the junction across which voltage is controlled and/or measured depending on potentiostatic or galvanostatic control, respectively), there is less impedance than between the counter and working/sense.  Request the potentiostat to apply a 5V potential (i.e., a Chronoamperometry experiment), and you will see a current of 5mA current (as expected).

One of the most common calls received concerning actual experiments is that the data collected appears "noisy" or that it contains current oscillations. This phenomenon is most often caused by: 1) reference electrode, 2) improper grounding of equipment, 3) highly capacitive cells (test specimens), and 4) environmental pick-up.

Reference Electrode

The most common source of noise or oscillation problems results from reference electrode issues.  Princeton Applied Research potentiostats/galvanostats utilize a differential electrometer to measure the voltage, which requires input not only from the reference electrode, but also from the working electrode. One of the benefits of using a differential electrometer is that during periods of overload of the current, the potential control of the cell is maintained. A downside to a differential electrometer design is that when the reference impedance becomes large (on the order of 50,000 Ω or more), this increased impedance coupled with the stray capacitance of the reference circuit slows down the negative feedback (stability generating) side of the electrometer's operational amplifier. The positive feedback (stability destroying) side of the electrometer is not similarly slowed, causing oscillation of the potentiostat to result.

Most of the time, this problem can be addressed by examining your reference electrode. Clogged Vycor frits, trapped air bubbles in the electrode, use of a double junction bridge tube (aka luggin capillary), and high solution resistance in the bridge tube are a few of the causes of increased impedance in the reference. One quick test to determine if it could be a reference problem is to disconnect the reference lead from the reference electrode and connect the reference lead to the counter electrode along with the counter lead.  In this two-terminal connection, the counter is serving as both the auxiliary electrode and as a pseudo-reference. If the noise is eliminated by making these connection changes, the reference electrode should be examined. 

The following are recommended procedures for examining/solving problems related to the reference electrode:

• Lower the reference electrode impedance. Make sure that your frit is in good condition and replace as needed. Avoid highly resistive solutions in the bridge tube or luggin.
• Increase the systems I/E stability by applying one of the I/E filters (if available on your system). These filters place a 10 nF capacitor in parallel with the current range measuring resistor to reduce the bandwidth of the I/E converter, thus minimizing the phase shift and potential to oscillate.
• If the problem results from increased solution resistance that cannot be avoided, place a 100 nF capacitor between the counter and reference electrodes. This has the effect of slowing down the potential controlling op amp (summing amplifier) and does not allow the potential control to outrace the feedback response.
• If the experiment is a corrosion type experiment, try placing a 0.1 - 1 µF capacitor between the working and ground leads. This will allow the high frequency ac signal causing the problem to bypass the current measuring resistors. This will tend to slow down the potentiostats current measurement and would eliminate oscillations.  Note: This solution is no good for impedance measurements.
• For impedance measurements, use a platinum wire in parallel with the reference electrode that is normally used.  One end of the platinum wire should terminate close to the tip of the reference electrode or bridge tube, which ever is closest to the specimen. The other end of the wire is connected to the reference electrode lead, with a 0.1 - 1 µF capacitor in series with the wire.  This allows the high frequency component of the signal to bypass the reference (if a 1 µF capacitor is used, anything over 2 KHz will be shunted) while the DC component will be passed through the reference.

These are a few of the suggestions that have been offered to improve the response of a differential electrometer in a cell environment which produces a high reference impedance.

Improper Grounding

When interconnecting instrumentation with other instruments and computers, it is vital that all the chassis be at the same ground plane. Believe it or not, most ground connections in outlets are not at the same potential due to differing lengths in the wiring within the walls and several hundred millivolts potential difference can exist between outlets on opposite sides of a room. If two instruments are connected together with a signal carrying cable, with each instrument plugged into a different wall outlet, a current may flow between the two instruments proportional to the potential difference and the reistance of the cable shield. This ac voltage and current developed across the shield can eventually be added to the signal you're trying to measure. How can this be avoided?

All power cords from all instruments in an experiment should be connected to the same wall outlet. This includes the potentiostat, computer, other computer attachments (Note: printers have been identified as contributing to noise problems because they are often overlooked and plugged into an outlet across the room), rotating electrodes, etc. If necessary, use an industrial type multi-outlet strip, plug all instruments into it and in turn plug it into the wall outlet. The power cords should be routed together, away from all signal bearing cables and instruments not in the system.  Another approach is to reduce the amount of current flow through signal circuit braids by providing additional paths. This is often called the "strap-them-together approach". The easiest way to do this is to use a large sized (#10 or larger) copper wire, making direct connections between all the chassis in the experiment, following the shortest possible path.

Highly Capacitive Cells

All potentiostats can become unstable when connected to capactive cells. The faster the response of the potentiostat, the more pronounced the problem. For instance, a Princeton Applied Research Model 283 potentiostat/galvanostat will begin to oscillate when performing a routine corrosion experiment, the ASTM G5 (test of 430 SS in sulfuric acid), once the specimen reaches the "passive" region where a stable oxide layer has formed. This oxide layer creates a capacitive situation, adding phase shift to the potentiostat's already phase-shifted feedback signal not to mention acting as an antenna for noise. For a system with the bandwidth of a 283, this can cause the system to go into oscillations in this region of the curve.

Fortunately, the easiest fix for this particular event is to apply the I/E filters (see the second bullet in the reference electrode section above), reducing the bandwidth of the I/E converter. Also, placing a capacitor (100 nF) between the reference and counter electrode leads (see the third bullet in the reference electrode section above) can have the effect of slowing down the potential controlling op amp (summing amplifier) and does not allow the potential control to outrace the feedback response.

Environmental Pick-Up

Power lines and cords, computer monitors, stirring motors, and fluorescent lighting can all be sources of noise, particularly when operating at low currents. The most practical way to shield your experiment is by construction of a Faraday Shield from a fine wire mesh, sufficiently large to accommodate the experiment cell.  Seams should be soldered and cables from the experiment should enter and exit at one point through a circular hole no larger than necessary. The shield should be grounded to the common system ground, that is, the black ground lead on your cell cable or connection at the potentiostat. As an alternative, a grounded metal fume hood can provide good shielding and should be tried before constructing any special shielding.

One problem in particular, as of late, is the new mandatory energy efficient lighting causing excessive current noise in potentiostat/galvanostat systems. The new standard is the T-8 lamps which use less energy, are quieter, do not contain PCBs, provide truer color lighting, and operate on an electronic ballast. The electronic ballast operates at a much higher frequency, 20-25 kHz, than the electromagnetic design (which operated at 60 Hz) of the T-12 ballasts.  This higher frequency results in less electronic energy being needed to provide the same level of illumination as the T-12s. Unfortunately, it is the electronic ballast that is probably causing the problem with noise pick-up in our systems. The easiest way to determine if your noise problem is from your lights is to simply turn them off while performing an experiment to see if the noise is reduced or eliminated.

In addition to eliminating the source, you can apply I/E and low pass filters to reduce the effects of the environment on your potentiostat and its response.

The open circuit (oc) potential is the potential of the working electrode relative to the reference electrode when no potential or current is being applied to the cell. When a potential is applied relative to oc, the system measures the open circuit potential before turning on the cell, then applies the potential relative to that measurement.  For example, if the initial potential is set to +100 mV vs. oc in the software and the measured open circuit potential is +300 mV, then the initial potential will be set to +400 mV.

Ecorr is the corrosion potential of a specimen in a given electrolyte. When a specimen is placed in a cell, the open circuit potential can be immediately measured. This is not Ecorr, however. It takes a certain amount of time, dependent upon the sample and electrolyte, for a specimen to reach a stable corrosion potential. Corrosion measurements should be made only after the stable corrosion potential has been reached. Ecorr is an open circuit measurement but Eoc doesn't necessarily mean Ecorr has been reached.

You can easily test your reference electrode prior to use to determine whether the reference potential is correct:

1. Place a freshly filled reference electrode of the same type (i.e. SCE) in a small beaker containing a saturated solution of potassium chloride.  You must assume that the fresh electrode is at the correct potential, thus it is advisable to always have a "standard" reference electrode on hand, one that is never used in experiments, always stored in its filling solution, and solely used to compare with other reference electrodes.
2. Place the reference electrode (i.e. SCE) to be tested in the same beaker as the fresh reference electrode.
3. With a digital voltmeter, measure the potential difference between the two reference electrodes.  If you do not have a DVM, you can use the potentiostat as a DVM, with the reference to be tested hooked up as the working electrode and the fresh reference connected to the reference lead.  Read the OC potential.
4. If you measure more than 40 mV potential difference (or less depending on your accuracy requirements) between the two reference electrodes, we recommend you renew the filling solution and replace the Vycor frit (the frit should be replaced periodically anyway).

It is best to store your reference electrode with the tip immersed in filling solution.  This will help prevent damage to the porous Vycor frit, which must remain wetted, and extend the lifetime of your reference electrode

Listed below are the references electrodes sold by Princeton Applied Research and their proper filling solutions. For your convenience, part numbers are provided for all of the filling solutions available. For certain electrodes, filling solutions are not offered. This is because the solution is easily made by the user (and costing less than the shipping charges if we shipped it), custom made by the user, or should be freshly made by the user.

The frits are held in place with Teflon heat-shrink tubing (HST) which comes with the frits.  You can remove the old frit by cutting the tubing and sliding it off the electrode tip.  This is a good time to replace/replenish the filling solution if needed.  You will need to cut a new piece of the HST so that it fits onto the glass tip of the electrode and extends far enough above it so as to cover 3/4's of the new frit.  At this point, the frit fits loosely in the tubing and will need to be held inverted until sealed.  A heat gun is most commonly used to seal (shrink) the tubing, although careful use with an open flame can also be utilized (hold it near, not in the flame).  Hold the electrode so that the frit/tubing are in the heat until the tubing shrinks enough to hold the frit in place, then begin to rotate the frit in the heat until a good, tight seal is achieved (the HST will change from translucent to transparent as it shrinks/seals).

Strong basic solutions (pH ≥ 10) will dissolve Vycor® frits.  We carry polyethylene frits that are able to withstand higher pH values.  These frits come in a package of 5 with heat-shrink tubing (G0194).  When replacing these frits, take care not to apply too much direct heat, as they can melt if over heated.

There are several possibilities.  A reference electrode bridge tube may suffice to separate the aqueous reference electrode from the non-aqueous solution.  However, this could lead to increased solution resistance between the working and reference electrodes by adding a junction potential.  An alternative is to use a non-aqueous reference electrode, preferably one where the electrolyte is the same as the analyte.  Acetonitrile can be used for most general purposes.  We offer a non-aqueous reference electrode (K0103) consisting of a silver wire immersed in acetonitrile with 0.1 M AgNO₃.  This electrode can be used in applications requiring non-aqueous supporting electrolytes such as benzene, methanol, acetonitrile and DMF. 

Listed below are the potentiostat Cell Cables currently sold by Princeton Applied Research. For your convenience, the Potentiostat Model number and corresponding Cell Cable part number are provided.  Your Cell Cable should be replaced if any / all of its connectors are oxidized, the cable has been visibly damaged (shielding or insulation missing or cut), or if tests on an external Dummy Cell yield unexpected results while the internal Dummy Cell tests on the potentiostat yield results as expected.  With regards to this last point, the cable in question should also be removed from the potentiostat and each of its leads checked for proper continuity with a Digital Volt Meter.
NOTE:  If you do not see your Potentiostat Model / Cable # listed below, it is because we no longer manufacture cables for it.