Introduction
In this article, I will touch upon everything I know about reference cells, also called half-cells. From the different types, their applications, factors that affect them, to calibration and maintenance, I will pour my heart and soul into making reference cells interesting again.
Reference Cells: What are they? Why do we use them?
Reference cells, half-cells, survey stick, portable reference electrode - they're all the same thing. A reference cell takes a known potential and measures it against the voltage potential of the structure you're trying to measure. They're a very inexpensive, critical tool for collecting test station readings, completing corrosion surveys, interference testing, etc. There's different types of reference cells, so you should always be aware of the application and environment the cell will be exposed in to determine the proper selection.
Different Types of Reference Cells: There's different types?!
The different types of common reference cells are the following:
Copper-Copper Sulfate Electrode (CSE): These are primarily used for buried metallic structures and freshwater environments. Personally, this is the only reference cell I have been exposed to while being a technician in West Virginia.
Silver-Silver Chloride Electrode (Ag/AgCl): Primarily used in seawater/brackish and concrete applications.
Saturated Calomel Electrode (SCE): A weird cell that is only used in laboratory settings.
Zinc Electrode: These can be used in soil or water applications.
Every reference cell is unique and must be thoroughly understood prior to just grabbing one to collect a reading. For instance, I would never used a CSE in saltwater applications, because the cell will actually be heavily contaminated an skew the results drastically. All of these cells possess a certain potential when measured to the standard hydrogen electrode (SHE). With that certain potential, you can actually compare one cell's readings and convert that same reading if you were using a different cell. To illustrate, if someone presented a reading of -0.400V using a ZINC electrode, don't freak out. Convert that voltage reading to the voltage reading using the reference cell you use. If the reading is still low, worry-some, or failing, then you can freak out.
Reference Cell Conversions: An Easy Way
Some people struggle with converting one cell's readings to another cell's reading. Personally, I think AMPP classes make this conversion even more difficult to complete in classes. I had the privilege of sharing CP1 and CP2 with a gentlemen (shoutout to Adam - you rock!) who gave me this formula; the formula helped me greatly.
X = E(m)1 + E(s)1 - E(s)2
X = The calculated number you're trying to obtain using the needed reference cell.
Voltage potential collected with the cell in use.
The standard voltage potential of that cell in used compared to a SHE. Check out AMPP's charts for this number.
The standard voltage potential of the needed reference cell.
This may seem a little confusing, but I personally think this formula is way easier to understand than typical AMPP explanations.
Factors Affecting Reference Cell Accuracy
There are some common factors that affect cells; however, these can easily be mitigated and controlled with proper techniques.
Temperature:
Cell potentials correlate directly with temperature fluctuations. A copper-copper sulfate cell has a temperature coefficient of about +0.5mV/degree F (we don't use Celsius here). Therefore, as temperatures rise, the readings will become more electropositive. This coefficient is small, but this variable could make or break compliance readings.
Try to keep half-cells in a room temperature environment prior to field use. I know this is tricky, and us technicians typically keep them in toolboxes. For best practice, put them in the cab for a while prior to use.
Always record daily temperatures when collecting readings.
Light Sensitivity:
Some cells, not all, can be reactive to light exposure. For instance, CSEs have been shown in studies to shift -0.50mV when exposed to direct sunlight at noon, compared to smaller shifts in shade. This happens because of a photochemical reaction involving the copper salts. Michael Roberts actually informed me of this one, so I had to share his wisdom.
A lot of half-cells come with a clear window on the side of the cell - allowing you to observe the moisture content inside the cell. I would recommend covering that with some electrical tape to mitigate light exposure. Keep in mind, CSEs are typically the ones readily affected by light exposure.
Electrolyte Concentration and Composition:
The content inside the reference cell directly affect their potentials. in a CSE, the goal is to keep the copper rod in tact and as uncontaminated as possible. Depending on the concentration used to saturate the rod, potentials can be skewed greatly and cause rapid depletion of the electrode.
At RCS, we always use a copper-copper sulfate solution from MCMiller to saturate copper-copper sulfate crystals inside our CSE for field use. This provides longevity for the cell and cuts back on the contamination fear.
Calibration - Critical:
A reference cell used for collecting potentials is useless if proper calibration isn't completed. There's different ways individuals calibrate their cells, so check with your company's SOP to ensure you're calibrating correctly.
For me, when I'm gearing up for a CIS, I compare my two survey cells to each other using a multimeter. I connect one cell in the positive port, one in the negative port, and I flip to DC mV. and touch the two porous tips together. I aim for less than one mV between the two of them. With this check, I have determined my cells are within 1mV of each other while collecting potentials in the survey. Everyone has different check and balances, so please check with your company first. I'm not the Corrosion God.
Maintenance
Along with daily calibrations, routine maintenance is just as critical for a proper functioning half-cell.
Always clean the porous tip prior to use. Remove excess mud or debris that has accumulated.
Attach the cover that comes with the half-cell at the end of its daily use. This allows the cell to remain saturated, avoids contamination, and saves you a mess of cleaning up a leaky cell.
Store in areas that keep the cell out of direct sunlight and harsh environments.
Check crystal and solution levels prior to use.
Reference cells are used throughout the cathodic protection world. Always be mindful and choose the right type, calibrate it, and take care of it. Ensuring these steps will create accurate results and a happy boss. Can't beat that.