Are you making errors when measuring pH?
Proper technique in pH measurement can seem overwhelming when you are first starting to take measurements. Questions like, How often should I calibrate? and When should I replace my electrode? are common for anyone starting a pH measurement process.
The team at Hanna has seen a lot of good (and bad) pH measurement practices throughout the company's 39 years of experience, so we’ve compiled a list of the top 10 mistakes in pH measurement, as well as advice on what to do instead.
Here are the top 10 mistakes that our technical staff see most often.
- Storing the Electrode Dry
- Wiping the Sensing Glass
- Storing the Electrode in DI Water
- Not Cleaning the Electrode
- Calibration Errors
- Improper Electrode Selection
- Not Loosening or Removing the Fill Hole Cap
- Low Electrolyte Fill Level
- Inadequate Probe Submersion
- Using an Old or Expired Electrode
Mistake #1: Storing the Electrode Dry
Improper electrode storage is something we see come up from time to time. Occasionally we’ll encounter an electrode stored without the use of a storage solution. Typically, the rationale behind this is that the dry storage will keep the electrode inert and viable longer.
What is actually happening is that the electrode is drying out, possibly permanently, if kept dry too long. A pH electrode’s sensing glass is composed of three discrete glass layers: a hydrated outer glass gel layer, a dry middle layer, and a hydrated inner layer. The hydrated layers are responsible for giving the electrode the sensitivity needed to detect changes in pH.
This leads to drifting pH values, slow response times, and incorrect values. Fortunately, in most cases, you can revive a dried out electrode by submerging the bulb and junction in pH storage solution for at least an hour. After that you can calibrate the electrode and get back to testing.
Mistake #2: Wiping the Sensing Glass
We get it. You want to make sure your pH sensing glass is nice and clean for the next measurement, so you thoroughly wipe your electrode with a paper towel. However, this can cause more problems than a little bit of residual buffer would.
In order to understand the problem, it is important to know how the pH electrode works.
The electrode sends a voltage to your meter that is based on the pH of the sample it is submerged in. Wiping the pH glass can produce a static charge (think of it like rubbing a balloon on the carpet, then using it to make your hair stand up).
The static charge interferes with the voltage reading of the electrode. When the voltage reading is wrong, the pH value that the voltage is interpreted as is thrown off too. In addition to this, the hydrated layer of glass that you have worked to develop by proper storage is interrupted.
Instead of wiping the electrode sensing glass, simply rinse the electrode with distilled or deionized water. If necessary, you may blot the electrode with a lint-free paper towel (e.g. Kimwipes®) to remove excess moisture, but be extra careful not to rub the surface of the glass.
Mistake #3: Storing the Electrode in DI Water
Using pure water (such as deionized, distilled, or reverse osmosis) is also a major mistake when storing your pH electrode. This happens most often when someone runs out of storage solution but wants to keep the electrode hydrated. This, however, creates a bigger problem.
Deionized water contains virtually no ions. The pH electrode is full of ions, both in the filling solution and in the hydrated portion of the pH sensing glass. So when an electrode is submerged in a solution that has no ions, the ions in the electrode will want to move out into the solution in an attempt to establish an equilibrium. Over time, most of the ions will leave the electrode, rendering it useless. The glass will also degrade much faster, leading to shorter electrode lifespans.
If you ever encounter an electrode stored in deionized or distilled water, immediately remove it. If the electrode is refillable, replace the fill solution. Once the fill solution is changed, store the electrode in the storage solution and calibrate it.
Hanna Tip: It’s always best practice to use storage solution for storage. This will keep your electrode hydrated and ionized for best results.
Mistake #4: Not Cleaning the Electrode
We’ve seen a lot of interesting applications since we first released our pHep back in the 1980's. With its launch, pH measurement was opened up to applications beyond the laboratory as a variety of samples and substances were being measured, from soil to wine and everything in between.
With so many different samples, it makes sense to have cleaning solutions developed specifically for these applications, because cleaning is just as important as calibration when it comes to attaining accurate pH measurements.
This is because deposits that form on the electrode coat the sensing glass, like fats and oils in foods. As a result, you will be measuring the deposits and the sample, rather than just the sample. A slow response time can also happen with dirty electrodes. You might even record the value when it appears stable, but in reality it's drifting very slowly to the real value. This can happen even if the electrode appears clean; a very thin coating of oil or scale may be present, but not visible.
The best way to clean the electrode is to use a specially formulated cleaning solution for pH electrodes. Even better would be to use one that is developed for the application you are using the electrode for. For example, cleaning solutions are available that are ideal for removing wine deposits/stains from electrodes. This way, you can be sure that residues are completely removed from the electrode.
Mistake #5: Calibration Errors
Calibration is definitely one of the most common processes we get asked about. Frequency (or infrequency) of calibration is a major concern. We also have a lot of questions about which buffers to use for specific applications. Sometimes the frustration is so real, that users stop calibrating altogether. Fortunately, all of the questions and frustrations can be answered by understanding how calibration works.
All pH electrodes rely on a principle known as the Nernst equation (above). The Nernst equation takes a voltage (mV) reading and correlates it to ion concentration (the pH). This correlation forms a straight line.
For pH electrodes, the theoretical mV value at pH 7 is 0mV (neutral) and the slope of the line is 59.16mV. This means that in theory the electrode will change its output by 59.16mV for every pH unit you go (e.g. pH 6 to pH 7 would be 59.16mV/pH unit). All of this is in theory since electrodes will change their slope and offset as they age.
In reality, the electrode might behave slightly different than the theoretical behavior (e.g. 58.2 mV slope and 8mV offset). Calibration compensates for this by determining the actual slope and offset of your electrode by using known buffers and updating the algorithm in the meter accordingly.
Hanna Tip: For best results, you should make sure you are calibrating using buffers that bracket your sample. pH 7 buffer should always be included to obtain the offset (neutral) point. This means that if your sample is pH 8.6, then pH 7 and pH 10 buffers should be used.
The frequency of calibration ultimately depends on how accurate you like your numbers. Daily calibration is ideal; however, we get that calibration takes some extra time out of what might be a busy schedule.
If you can tolerate a little bit of error in your measurement, daily calibration is not completely necessary but is still highly recommended!
Mistake #6: Improper Electrode Selection
All pH electrodes are not created equal. Even with the best technique you still may not be getting the best measurements. This is because some electrodes are better suited to certain applications than others. Using the less-than-ideal electrode can result in a longer response time and a shorter electrode lifespan.
Consider the standard pH electrode. It is typically glass-bodied with a large spherical bulb at the end which makes up the sensing glass portion. There is usually a small ceramic junction that allows electrolyte flow from the reference portion of the electrode. This electrode is functional for a wide variety of applications, but not ideal for all samples.
Problems arise when the pH is measured in samples that are semisolid, solid, or have solids suspended in the solution, like wine, wastewater, and food. Samples low in ions can also pose problems with response time and stability, like drinking water.
In these cases, it's best to use an electrode that is especially suited for these specific kinds of samples. Conical sensing tips with open junctions allow direct measurement of solid and semisolid samples, eliminating the need to make a slurry. Electrodes with multiple ceramic junctions allow electrolyte to diffuse into the sample faster, allowing more stability in pH measurements of samples with low-conductivity.
Ensuring that your electrode is best suited for the job is crucial to a good measurement!
Mistake #7: Not Loosening or Removing the Fill Hole Cap
The fill hole screw cap seems like such a minor detail in the whole construction of a refillable pH electrode. After all, it just prevents the electrolyte from drying out, right? Yes, but if it is screwed on as tight as when you first receive the electrode, you might be in for some problems.
Most modern pH electrodes are technically two electrodes in one: a sensing electrode and a reference electrode. The reference electrode requires a slow but steady flow of electrolyte out of the electrode and into the solution.
When the electrode fill hole cap is screwed on tightly, electrolyte cannot easily diffuse out of the electrode and into the solution. This phenomenon is just like covering one end of a straw with your finger; even if there’s a hole in the bottom, liquid won’t escape as long as your finger is covering the other end. This results in an erratic reading that might never stabilize in a reasonable amount of time.
Fortunately, the fix for this mistake is simply loosening or removing the fill hole cap. It’s that easy!
Mistake #8: Low Electrolyte Fill Level
Refillable electrodes allow you to replenish the electrolyte in the reference compartment once it begins to run low. However, if you do not replenish the electrolyte from time to time, your pH measurements can be impacted.
Erratic electrode response is the most common problem with inadequate electrolyte levels.
Electrolyte flow from the reference junction permits the completion of the measuring cell. This ultimately allows you to take the mV value from the pH electrode and convert it to an adequate pH value. Ensure that your electrode is replenished and functional by maintaining the fill solution level less than a half-inch from the fill hole cap.
Mistake #9: Inadequate Probe Submersion
It’s easy to think that as long as the pH electrode is touching the sample, then the value that appears on the screen is going to be the pH. The reality is that the pH sensing portion and reference junction needs to be completely immersed in order to properly function.
Let’s go back to the reference electrode and sensing electrode. A pH sensor works because the sensing glass interacts with the sample and produces a voltage that gets compared with the reference electrode (which is stable in all samples). Without one of these portions in complete contact with the sample, the measuring system is incomplete, leading to erroneous values.
Submersion problems are easily corrected by adding enough sample to submerge both the junction and sensing glass.
The position of the reference junction changes based on the electrode design, so be sure to check the manual to determine where the junction is located.
Mistake #10: Using an Old or Expired Electrode
Just like any piece of equipment, pH electrodes need to be replaced from time to time as part of regular maintenance. As electrodes age, the sensing portion of the glass will break down and become less responsive than it was when it was new. Eventually, your electrode will stop responding adequately to changes in pH.
There are some numbers associated with properly functioning electrodes. Slope and offset are familiar metrics that you can use to measure your electrode’s functionality. These numbers can be determined during calibration.
Offset is simply the mV reading in pH 7 buffer and slope is the mV change per pH unit. On many meters, these values can be viewed automatically through the Good Laboratory Practice (GLP) screen. Functional electrodes have a slope between 85-105% of the ideal value. The offset should be ±30mV.
Sometimes, despite all of your best efforts, the electrode still does not perform like you would like it to. If the electrode is old, it might just be time to replace it.
Although this seems like a lot of steps to keep track of in order to take measurements, many Hanna Instruments meters like the HI5221 Research Grade pH meter offer CAL Check™. Hanna's CAL Check compares electrode slope and offset data from calibrations made over time. It immediately identifies potential problems with the electrode and/or buffers using built-in diagnostics.
These diagnostics will alert you to any possible errors associated with dirty electrodes and contaminated buffers, as well as determine overall electrode condition after each calibration. CAL Check takes the guess work out of pH calibration, allowing you to be confident that your electrode is in good working condition and ready to take accurate measurements.