Christy Abbas (00:01):
Hello everyone and welcome to our webinar today, pH analysis: Avoiding Potential Pitfalls presented by Katie Challis. I'm Christy Abbas and I will be your moderator today. Before we get started with the webinar, there's a few housekeeping details. The slide shows the council page, and this is where you'll see the slides for the presentation, Katie's information, survey questions and a list of resources available to you. There's also Q&A panel, so if you have a question during the webinar, please post your question there. Today's webinar is on pH analysis: avoiding potential pitfalls. pH is a seemingly simple measurement that can be performed accurately. However, due to tight proficiency testing criteria, it is one of the most failed tests in the yearly DMRQA study. This webinar will cover the basic chemistry of pH analysis, some of our product offerings for pH, and most importantly, recommendations for improving the accuracy of pH analysis. Our speaker today is Katie Challis. Katie is an applications chemistry analyst too at Waters ERA.
She graduated in 2020 with a PhD in environmental bio-geochemistry from the Colorado School of Minds. She has been working in laboratories for the past 11 years. Between graduating with her bachelor's degree from the University of Wisconsin lacrosse and returning to school for her PhD, Katie worked as a contractor with the EPA and with that, I'd like to turn the floor over to Katie. Thank you, Katie.
Katie Challis (01:46):
Thank you, Christy for that introduction. I appreciate it. Here's my presentation outline. First we're going to cover what is pH and why do we care a little bit about pH proficiency testing, then what products Waters ERA has for pH, and then finally the important stuff, pH analysis, measurement, calibration, probe care and troubleshooting. On the slide here, I show the hydronium ion, which is the acid associated with water, and I'll be calling it the hydrogen ion throughout this presentation. Just in general nomenclature, the hydroxide ions, the conjugate base of water, just wanted you to see both of those. I did also want to tell you a little story about how in grad school, I messed up some 30 pH measurements so badly that I had to measure other stuff about that water and recalculate the pH value using these complicated equations and software and those calculations took me two days to do. So after that, I learned how to do pH better, and that's what I'm going to share with you so that you don't end up in a bad situation like I did.
Okay, let's hop right into it. What is pH and then why does it matter, going to be the first two topics? So when I did a quick Google, I learned that pH denotes potential of hydrogen, and that makes sense because pH probes are potential metric probes that measure electrical potential between two different points. We're measuring potential when we measure pH. Personally, I always thought it was the second definition where the equation is actually equal to the negative log of the concentration of hydrogen ions in solution. So P is a mathematical mutation for a negative log, and H is the elemental symbol for hydrogen. So pH is basically the equation that we're using to calculate the concentration of ion. So pH is basically a unit in itself, which is why we have standard units as the unit for pH.
Just a quick note, pH, since the negative log scale, remember what logs are for a moment. I haven't used these very often recently. So a log of 10,000 is four. There's four zeros behind the one. If you take the negative log of 10,000, that would be negative four. So if you have a positive number from a negative log equation, you actually are in the decimals or less than one. So the negative log of 0.0001 is three.
So just a quick reminder. So when you're changing by one pH unit, you're actually changing the concentration by a log or an order of magnitude. So a couple of reminders everyone. pH 7 is considered neutral to where the concentration of hydrogen ion is 10 to the negative seven. That in solution usually is where the concentration of hydroxide ions is equal to the concentration of hydrogen ion. The two different charges kind of cancel each other out. That is why it is called a neutral pH. So that's kind of what pH is. All right, why does pH matter? Why do we care? Well, I personally care because of aquatic life. That's also why the EPA cares. So aquatic life has a pretty small range of pHs that it can survive in. If we look at this image, we see a large mouth bath. Large mouth bath, thrive best in a range of pH 6.5 to 8.5, right around neutral.
They can reproduce in pH is 5 to 10, but anywhere outside 5 to 10, they can only survive short periods of time. There's other things like freshwater snails that have a pH tolerance of six to nine, again, right around neutral and we want the waterways to stay as healthy as possible so we can continue to have fisheries and abundant biodiversity, which is just healthy for our planet. So we want to make sure we're not discharging water that will drastically impact the pH of all of this natural life and beautiful biodiversity that we see lake streams in wetlands. So we measure pH, we want to do it properly so that we can protect life.
What is pH proficiency testing and what are some criteria and examples of how it's used? So you've all probably done a proficiency test before. You know that it's to prove, you know how to measure pH in this example or copper in a different example. And our proficiency testing is pretty standardized. So the NELAC Institute, which is the National Environmental Laboratories Accreditation Conference, has put out guidelines for accredited laboratories on what range we can manufacture PT tests in for pH and what the acceptance limits are.
I don't get to decide what the acceptance limits are. ERA doesn't decide. It is all done by NELAC. So we manufacture pH for wastewater tests between 5 and 10 pH units. We can manufacture at 5 or 10 or anywhere in between, and the acceptance limits around that is going to be plus or minus 0.2 pH units. So for example, last year, DMRQA 42 had a pH made to you of 7.083.. Sorry 7.83 units with an acceptable range of 7.63 to 8.03. So anything outside of that range was evaluated as unacceptable. And those are actually pretty tight criteria. That's not a big range and it doesn't shift no matter where you move across the range. If I make a PT test at 9.9 pH units, the acceptance units would be 9.7 to 10.1, this is an example and same at the other end.
Okay, so for example, last year DMRQA had a failure rate of 6.9% in the pH measurement range. It happens, so if you are one of those labs that failed, you're not alone. 170 other five unacceptable values were reported out of 2537 reported values. Two years ago, DMRQA 41, we had a failure rate of 4.7. This is one of the most failed measurements in DMRQA. I think it's because of the tight limits, and so let's move ahead and try and figure out how we can help you pass this all the time in the future. So just to recap, pH proficiency testing is regulated by NELAC. The type criteria is plus or minus 0.2 pH units away from the manufactured two value, and you're not alone if you've failed.
Well, I'm going to wear a marketing hat for a little while and let you know what pH samples we have available from ERA. We have calibration buffers in pH 4, 4, and 10. We also have proficiency testing and quality control samples on the next slide. So as you can see, you have the regular pink, yellow and blue pH buffers that you can calibrate your probe width, and these are available from us in 500 ML, 1 L or cases of 6X500 ML. So those are your calibration buffers. We do have a table for calibrating at different temperatures on the bottom of the label here.
Our other products are the Proficiency Testing Products and our Quality Control Samples. So proficiency testing are the ones we were talking about that are blind samples that you take to prove you know how to measure. So we have a drinking water and wastewater PT test available every month as well as a DMRQA every year. We also have quality control products and that is essentially a PT testing product that has already gone through study and you will get a certificate of analysis when you order that, that has the made to value, the study mean, how well how many people measured that product, and then as long well as the acceptance criteria for proficiency testing and some information on quality control values that you might want to hit to make sure any unknown values are correct.
So I really do recommend having quality control products as part of your analysis because it really helps you figure out if you need to troubleshoot your calibration or anything else about your analysis. So yeah, so we have like I said on the previous side, the calibration buffers at pH 4, 7 and 10 and then PT and quality Control Products Available.
Here's the part that everybody actually wants to pay attention to, pH analysis, how to improve it, how to calibrate and some proper probe care and troubleshooting things at the end. I have two probes pictured on this slide. On the left is a gel probe and that means that the inside of that probe is not replaceable. You don't have a solution you can fill in there. So eventually this probe will go bad and you will have to replace it. The other probe is a flow through probe. This is like what I use currently. This has replaceable internal solution, might last a little longer, but of course eventually you will have to replace it as well.
Let's begin with what to check before you begin your pH analysis and then we'll follow with how to properly do your pH analysis. First, check over your equipment, your probe, is it connecting with your meter? Is there anything by me on the outside? Was the probe stored properly in the correct storage solution? This is important. If you have a flow through probe, are there crystals inside? If there are, you probably need to clean that out and change your solution. Is the internal solution high enough? You have to have enough pressure above the sample for the probe to work properly, so you need to fill that internal solution at least an inch above where your sample will stop. I recommend just going all the way up to the fill hole so that you definitely have enough solution present and then are all your solutions at the same temperature?
PH is impacted by temperature. This equation at the bottom, the ease of the electrode values that we're measuring the difference between. The pH is a concentration of hydrogen and then 2.3, F and R are all constants. T stands for temperature, which is the only other thing that changes and impacts pH. So you need to match the temperature of your solutions pretty closely. Is the fill hole open when you're doing measurements? That will be important because everything needs to be equilibrated with the atmosphere and it will just go quicker if you have that open. And then you should have automatic temperature correction turned on to account for even small variations in the pH temperature solution. This will help correct for that change in temperature, although it is not perfect. Just want to note that all PT tests are certified at 25 degrees Celsius, so you want to measure PT tests at close to 25 degrees Celsius as possible. I have this nice little image of a little bit more information on how inform the electrode works. I'm not going to cover that in detail, but let's move on to measurements.
When you're doing measurements, you got to pay attention to what your probe is made up of. So we have the pH sensing bulb and the reference junction and that fill solution needs to have the proper fill solution that covers the reference junction and then is over an inch above where your sample is stopping. So the reference junction needs to be covered by your sample, so when you put your probe into solution, you need to make sure that it's completely covered for the entire measurement. I just wanted to show that so you have an idea of how deep you need to put your probe into the solution there. If you have to take smaller volumes, there are different options for pH probes that you can buy. I'm not an expert in that, but just be aware. There's some that are flatter, some that are for smaller tubes, there's options out there.
So pH analysis, a few tips for here. You're going to want to calibrate the same day of a PT pH analysis. Don't use a calibration that's a week old. Some probes are capable of holding calibration for a week and that's fine if you approve and it's good. Make sure you're submerging the probe properly past the reference junction. Stir all samples at the same rate because if you stir at different rates, you might have slightly different values. You just want to have the conditions as similar as possible. Wait for a stable value, don't take a value that's drifting around and taking forever to settle. If that's happening, you might need to do some probe maintenance.
This I just learned when I was preparing for this talk. Do not wipe your pH probe completely dry with a tissue or a chem wipe. This causes issues on the charge and just takes a lot longer for your pH measurements to occur. So you do want to clean the probe in between samples, you might want to condition it with the neck sample. You can plot the probe with the tissue between samples, but you don't have to. It might just be easier to rinse it and kind of shake it off.
Now that we've checked our probe and know how to properly do a pH measurement, we are ready to calibrate our probe. When you're doing calibrations, you want to use fresh calibration solutions. Ones that are not expired, ones that have not been sitting out for a while, use fresh ones. This is really important. Also, we've found that it's best to only calibrate using two buffers, so we'll always use pH seven and then we'll bracket the pH of our sample with the other one. So if we know our sample's around five, we'll calibrate pH seven and pH four. For PT tests on unknown, you might need to test your sample with a little bit of pH paper to estimate which range it is and then calibrate so you don't have to recalibrate multiple times.
You do want to make sure it is bracketed though if you get a pH of five, but you've only calibrated between 7 and 10 recalibrate and reread that number. Also, when you're calibrating in the most meters, you have the option to edit the value that you're putting in and you want to match that up. So if your calibration solution is at 20 degrees Celsius, you want to look at our bottle or the certificate of another pH buffer and make sure you're putting in, oh I mean at temperature 20 degrees Celsius, the pH is actually 6.98, not seven for the seven buffer. Make sure you're doing that little correction, it helps a lot. And then do have the automatic temperature correction turned on your probe that's available? I forgot to mention earlier that you might need temperature probes in your solution as well to be able to do this properly. Here I show the graph of how temperature impacts the slope of the calibration curve. So yeah, so this is why I'm talking about temperature a lot because if we're at different points on that curve, we're going to get pretty different numbers.
An important step after you calibrate is to do some checks to make sure your calibration is good, everything's functioning well, and the first check is that the slope of your line should be close to 100%. In my book, it said 92 to 102% was accepted. I would say you might want to have a little narrow range on PT test days and have it between 96 and 101. You can usually see that displayed on your probe, you can see here that this day I had a 98%, which was great. You are going to also want to check your calibration by doing a quality control check.
That would be perhaps one of our QC samples like I mentioned earlier in the presentation. Or you can make your own Sodium Testraborate has a pH of 9.2 if you're doing a check between seven and 10. And then for us, we have a second source for pH for a buffer as a check. But again, our QC samples are available and when you're ordering QC samples, you can request, hey I want the pH value to be less than seven or more than seven if you need one for each part of your curve.
And yeah, so if it's not meeting that criteria, you're probably going to have to do some troubleshooting or probe maintenance.
So you want to calibrate with good solutions and then check that you're getting good values before you proceed to your unknown. You next need to think about how you're taking care of your probe. How do you store it? Is that being done properly? Are you doing maintenance on it? How often does that need to be done? And then I'll close a troubleshooting side. So everything about the probe, everything in this presentation actually should not override what the user manual of your particular probe says. I based all most of my research on the probe we have in house and then look through some user manuals for a couple other probes.
All of them agreed that you do not want to store your probe in DI water. You do not want to store your probe dry. But you do want to make sure you cover that bulb so it doesn't dry out in a storage solution. If you have a probe that dried out by chance, some gel probes do say it's kind of okay to store them with just a little bit of solution, although not completely dry. You can rehydrate a probe by stoking it in a storage solution for more than 30 minutes, but then you're going to have to calibrate and check that it's working before you would do any analysis with it.
All right, so probe cleaning, I already mentioned this once, but in between samples, don't wipe it off completely with a tissue, but blotting is okay. So if you have crystals that are accumulating inside your probe, you probably need to change the solution in there. You can remove the solution in the probe chamber just by dumping it out from the fill hole. My booklet says do not put any water into the probe because it could cause damage to the electrode in there. So you will want to fill the probe up with fill solution a little bit, shake it up, kind of try and break up those crystals. You can maybe run it under hot water if you need to kind of warm up and get those crystals dissolving and then dump it out again, do another condition and then fill the solution with Fill solution. Only ever put Fill solution inside of the chamber in the probe.
Of course, gel probes do not have a replacement solution. So once the gel goes bad, you essentially have to replace the probe and you'll tell that it's going bad by your calibration curve isn't passing, you're not getting good values, things are slowing down, things like that. If there's slime or other dirt on the outside of the probe that doesn't come off with just a simple rinse and stuff like that. You might want to clean the outside of your probe by soaking it in a dilute nitric or hydrochloric acid for 30 minutes. After you do that though, you're going to have to change the solution inside of the probe because of the interaction between the highly acidic solution. So just keep that in mind, but you can clean the outside of your probe.
Here's some more details on troubleshooting and when maybe you need to do some probe cleaning or maybe replace your co probe. So first you want to check the milli voltage reading at pH seven. pH seven millivolt should be right around zero, within plus or minus 30 millivolts of zero. It can be a little bit off because the reference value will change over time, and then you want to put it into your second buffer and see if you have a change of in milli voltage that's between 160 and 180. If that's not happening, first I would try cleaning your probe and maybe rehydrating it in storage solution and then try again. If that doesn't work, it might be time for a new probe. You might want to try make sure your meter corrections connections are correct, but probes do go bad. They're not infallible, they aren't good forever, there's something that do have to be replaced sometimes.
So yes, so make sure you are storing a probe properly in the correct solution. You are cleaning it when it needs to be cleaned. And then here's the little bit of troubleshooting information I had available. This brings us to my last slide, which is a recap of what we've talked about. So pH is a measure of hydrogen ions and solution on a negative log scale. Efficiency testing for pH has some pretty tight limits, so you need to be really accurate in your pH analysis. Hopefully these tips will help you be accurate in all your unknown samples and daily samples as well as your PT sample.
When you're doing pH measurements, you want to check everything, get things prepared before you start just doing your pH measurement. Again, temperature is the only other thing besides the high ion concentration hydrogen and solution that impacts pH. So you want to make sure temperature is pretty similar across your analysis and you want to calibrate likely daily for your analysis to be good. Of course, we need to take good care of our probe, so you want to make sure you're storing it properly. Sometimes you might just need to take the time to do a cleaning before analysis if you want good values, and you will sometimes just have to replace your probe if things go bad or you're not meeting the criteria that you've stood up for yourself. I hope this was helpful to everybody. I'm going to turn it back over to Christy.
Christy Abbas (27:08):
Thank you Katie for a great webinar. Lots of really good information and we appreciate it very much. We'd like to move now to the Q&A section of our webinar.
What should I store my pH probe in if I don't have storage solution?
You could store it in a pH 7 buffer with some KCL added in. The manual for the probe might suggest something; the manual for the probe Katie Challis uses suggested 200 grams KCL per liter of pH 7 buffer. However, it's best to check your manual for specific instructions. If you don't have anything else, a pH seven buffer might be the best bet, with the addition of some salt.
When taking pH in the field, there is going to be a temperature difference between the sample and the calibrations. Is that a problem?
The impact of this difference depends on how precise you want to be with your pH measurements. For high precision, you could calibrate your buffers at the same temperature as the water sample. If this isn't necessary, just calibrate normally and then take the temperature. Alternatively, you could let the water sit for a while to reach ambient temperature.
When will the DMRQA open?
The DMRQA is slated to open around mid-May, but the exact date is not confirmed yet.
How can I simply portray, explain to operators why it's important to calibrate with three buffers, spanning the range of acceptable pH, also to change the buffers daily and how the slope matters?
Calibrating with three buffers is necessary if you're unsure of the pH range of the sample. If you know the expected range, calibrating with two buffers is okay. Changing buffers daily is crucial because used buffers could have shifted pH either due to contamination or evaporation. The slope is important because it impacts the accuracy of your readings - a 90% slope gives a reading that's 90% of the actual value.
What is the correct bottle for materials to hold the buffer and the correct procedure for blotting electrodes?
Buffers can be stored in polypropylene bottles or glass. For blotting electrodes, either blot it gently with tissue or rinse it with DI water and shake it off. Another option is to condition it in one aliquot of your sample before moving to the aliquot where you take the final reading.
We use a basic gel probe for ease of maintenance, but find that it has an extremely slow response time to measure our wastewater effluent sample and that it needs replacement in less than a year. Any advice?
If your effluent samples are very clear, you might be dealing with low ion concentration. You might want to look for pH probes designed for low conductivity samples. Also, ensure you're storing and handling your probe correctly. If your readings take time to settle, you could standardize your procedure to take the reading at a set time. For example, always take a reading 1 min after the probe enter the water even if it isn't fully settled.
Even with the gel probes, do not wipe dry?
No, do not wipe gel probes completely dry as it may alter the charge at the end of the pH probe, making it take longer to get a steady reading.
Is cleaning using 0.1 molar acid advised for gel probes?
This is uncertain. While it might be possible to wipe the probe off with a 0.1 molar acid, it's best to check your manual before proceeding.
Do you prefer the buffer powder pillows or the premixed calibration solution?
The preference between buffer powder pillows and pre-mixed calibration solution is personal. Katie Challis prefers pre-mixed solutions, especially for DMRQA, as they have been checked and confirmed by an external party.
Do not store in DI water, but is it okay to rinse probe in between samples with DI?
Yes, it is okay to rinse probes in between samples with DI water. However, DI water should not be used for storing the probe.
Should I stabilize a drifting pH from a clean sample by adding saturated KCL solution?
Yes, adding a saturated KCL solution is recommended for stabilizing the pH of a very clean sample.
What is your opinion of the Hawk SL 2000 for pH measurement that uses chem keys?
Katie Challis does not have personal experience with the Hawk SL 2000 for pH measurement that uses chem keys, and therefore cannot provide an opinion.