Definitive guide to pH measurement
Time for chemistry! Yes, let’s talk about our sweet love for elemental science – in a totally PG way, of course. Today we’ll have a session filled with fun and frolic focused on one of the key aspects of understanding compounds, pH. We’ll define it and discuss concepts such as measurement, calibration, and many more interesting topics. So let’s get to it!
Check out how Wikipedia defines pH:
pH /piːˈeɪtʃ/ Noun (acronym)
A figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acid and higher values more alkaline. The pH is equal to −log10 c, where c is the hydrogen ion concentration in moles per litre.
Looks like mumbo jumbo to the average reader, doesn’t it? Let’s break it down into simple language.
Nobody agrees on what the term pH originally meant. The French might say “puissance” or “pouvoir hydrogène,” the Germans would claim the p stands for “Potenz,” and in English you could go with “potential” or “power of hydrogen.” Of course all of this skips over the concept coming from a Danish chemist in a lab in Denmark!
Anyway, pH defines the strength of a substance – usually further defined as an acid or base – which has hydrogen ions in it. These ions combine with water molecules to form hydronium ions (H3O+):
H2O + H2O ⇔ H3O+ + OH–
The H3O+ concentration matches the H+ concentration, so you can say:
H2O ⇔ H+ + OH–
The hydrogen ions (H+) and hydroxyl ions (OH–) come from water splitting into cations and anions. And the strength of the hydrogen and hydroxyl ions define the nature of the substance – acid, base, or neutral.
Scale and the importance of pH balance
Now let’s look at the pH scale and see where substances fall according to their potencies.
Here we have the range of pH, with hydrogen ions on the left and hydroxyl ions on the right. For example, nitric acid will have a pH value of less than two, making it corrosive. Interestingly enough, lemon juice has a pH of 2.5 but isn’t corrosive. And Coke has a pH of 2.3, making it more acidic than lemon juice. Beware, Coke lovers!
Tap water with a pH below 7.0 could harm you if you drank it, not only from the acidity but also from metals dissolved from the pipes damaged by the acid. On the other hand, a pH above 8.50 could harm you too!
Now that we’ve had a fun intro, let’s check out an instrument called the pH meter. This device finds the hydrogen potential in liquids, using electrodes and some awesome electronics. Just immerse an electrode in a liquid, and you can get an electrical signal which tells you that liquid’s pH. Pretty cool, huh?
Now, temperature can change the measurement, so you’ll also find on a pH meter a connection where you can add a temp probe. In most cases, you can also read the temperature on the display.
An electrode with a measuring half cell and a reference half cell is one of the best tools to identify the pH of a solution. When you immerse the electrode, the two half cells create an electric potential proportional to the liquid’s pH. This difference gives us the basis for all modern pH measurements.
|pH measuring half cell||pH reference half cell|
|This bit has a glass membrane that, when exposed to liquid, forms a thin swelling layer that creates an ion exchange.
The glass both separates the inner and outer layers and connects them. The different surface potentials lead to a potential difference between the inside and the outside. To measure this difference, you need a zero current and a high-resistance voltmeter.
|This bit must create a stable electrical baseline over time, which makes it the most vulnerable and complex part of a pH electrode.
Unlike the pH-sensitive glass membrane, the reference half cell comes in direct contact with the medium via a diaphragm, gap, or hole. Therefore it needs a contact point made of a chemical-resistant material. Most today use silver wire coated with silver chloride and connect with the solution using an electrolyte.
Ways to extend the life of a pH electrode
An ion-selective electrode provides a quick and reliable way to read and control pH in a system. However, these electrodes don’t last long, so you have to replace them frequently. Let’s consider ways that will push these electrodes to their limits!
1 – Selection
You should start with good old fashioned research. Before you choose an electrode, you have to know its purpose, specs, and limits. Choosing the right model requires process data such as your measurement range, medium composition, and ion relativity. Factors like cleaning methods may also play a part.
2 – Regeneration
Consider this question: Why does a drop of ink in water spread completely through the water? We call this phenomenon diffusion. The ink molecules move from the most concentrated point (where the drop fell) to regions of lower concentration (the rest of the water) until it’s uniformly distributed. Electrodes also undergo diffusion, between their potassium chloride and contamination.
So what does this have to do with your electrode? Well, you can regeneratee it by reversing the diffusion process! Set your electrode in a solution of potassium chloride (3 moles per liter). Salts lost during operation will return and contaminants will leave. Ta da!
3 – Rotation
You should also use a rotation policy of two or more electrodes. Put one in use and the other(s) in a regeneration solution at the lab. When the time comes to replace the spent electrode, put it into the solution and install the regenerated electrode to continue your process.
Every component has a life cycle that depends on umpteen factors. However, these three tips can prolong the life of your electrodes, thereby saving money and improving accuracy!
A device has a range of precision, an ability to reproduce similar values under varying conditions. To keep within that range, you must calibrate.
Adjusting a pH electrode requires comparing its current measurement value to a reference value. You’ll usually find the reference value printed on your bottle of buffer solution.
You’ll need the following for calibration:
- Cleaning solution
- Distilled water
- Clean beakers
- Buffer solution
- Paper towels
- Examine: Check the electrode for contamination or damage. Then clean it thoroughly using the cleaning solution. If you find damage, either fix it or trash the electrode.
- Flush: Now use distilled water to rinse away particles or solvents, then paper towels to dry off the excess water.
- Immerse: Next fill a beaker with buffer solution and immerse the electrode.
- Calibrate: Start adjusting the electrode according to the vendor’s instructions. Old sensor may react sluggishly, so pay attention. When the value stabilizes, set the device to accept this calibration point.
- Rinse and repeat: Flush the sensor with distilled water again, then immerse it in another clean beaker with fresh buffer solution and check your calibration.
A pH sensor converts the signal it receives from an electrode into a pH value. Two types of sensing elements dominate the market, the conventional glass membrane and the ion-sensitive-field-effect transistor (ISFET). As for sensor materials, you have to choose from glass, plastic, or polyetheretherketone (PEEK). At first glance, you might go for the plastic or PEEK for their robustness, but vendors disagree and go for glass. Why?
Because despite the fragile nature of glass, it has more resistance to temperatures and chemicals than plastic or PEEK. Also, you can fix a glass membrane to a glass shaft with simple molding, which makes the attachment robust. Plus the glass shaft offers high physical stability because it avoids the inner glass tube for the wire brakes. This can become an issue with sensors longer than 120 millimeters. You can pick plastic if you need to save money, but keep in mind that it only works in less-demanding processes.
6 pH transmitters you must know
We reviewed some pH transmitters a while back that you’ll see again in a moment, and we wanted to pop in a few others for you to consider as well.
|This two-wire meter has a standard display with no backlight, along with a keypad near the display. The manual doesn’t say much about installation, but it looks easy to install. The TB82 measures with an accuracy of +/-0.01 pH and one degree Celsius. It only accepts analog sensors, but it’s the only entry on this list that you can use for ion-selective measurement.|
Endress+Hauser Liquiline M CM42
|This two-wire single-channel device reads for pH, conductivity, or oxygen using Memosens sensors. The display doesn’t have a backlight, but it gives messages in clear text and 14 languages, so this one can talk to a lot of people in the world. The scroll-wheel makes it particularly user-friendly. Installation for a wall looks easy, but you’ll need accessories for pipes or panels.|
KNICK Stratos Pro
|This transmitter works for analog and digital sensors. By exchanging the input card, you can switch between pH, dissolved oxygen, or conductivity. It also has a two-channel input card available except for Zone 1. In fact, it won’t support digital sensors in Zone 1 either. The device has an IP67 housing and a nice color display, although you’ll need the manual to translate abbreviations on the display.|
Mettler Toledo M400
|This multi-parameter meter can use analog or digital sensors for pH, conductivity, oxygen, or carbon dioxide. You’ll find the illuminated display easy to read and messages in clear text. Its IP66 housing will install on a wall or in a panel.
On the down side, this loop-powered device only has analog, FOUNDATION Fieldbus, and PROFIBUS PA available, and it only uses ISM digital sensors. Also, the manual is a hefty read.
WTW IQ Sensor Net DIQ/S 181
|This single-channel multi-parameter device only uses WTW IQ Sensor Net sensors and only works for wastewater. The backlit display has multiple languages and clear messages, making it pretty user-friendly.
It can measure pH, oxygen, conductivity, or turbidity, depending on the sensor you connect. It has two analog outputs and three relays but only one diagnostic, for glass breakage. On the other hand, it seems easy to install the device on a wall or a pipe. The manual is understandable, always a plus.
|This two-wire two-channel converter works for pH, conductivity, or oxygen. You can use analog sensors, but you’ll need to specify parameters or change input modules later. Although it claims two channels, you only have one output. Still, the touch display has clear text and 12 different languages.
If you choose the two-channel input card, then you can program it to set off an alarm if the measurements between the channels go beyond a preset value. You can use it in hazardous areas too.
Even the highest-quality devices degrade or develop issues over time, so you need to keep an eye out for common problems.
Factors such as humidity or wind can play merry cob with sensors, creating enough resistance on contacts to reduce signals. Fortunately, digital electrodes have small microprocessors that can shrug off environmental interference.
You should also recognize galvanic isolation. Basically, electrical potentials may occur between the grounding of the device, the control system, and the pipes where the measurements take place. These can interfere with the electrode’s tiny signal. You can check for this issue by taking a grab sample and measuring it with the same device used for the process. If the values differ, then you should look into getting a ground loop.
And of course temperature can affect measurements, but many users think that if their pH meters have automatic temperature compensation (ATC), then they can ignore this bit. Wrong! You can’t compensate for the temperature dependence of your sample unless you do some programming on your pH transmitter. Each sample will have its own dependence.
Analog versus digital
Now we end our lively session with a discussion on whether we should use analog or digital sensors.
|The sensor itself remains analog, whether your transmitter is digital or analog. So if you buy a digital transmitter thinking it will give your sensor a boost, then you just wasted money.
||A digital sensor needs a digital transmitter. You can’t install a digital sensor into your system unless your transmitter can accept a digital sensor.
Digital sensors offer multi-signal processing and improved reliability, though.
|Analog sensors store their corrections in the transmitter and ensure drift correction.||Digital sensors can store their own adjustment values. So you can adjust a sensor in a lab or workshop, then install it in the field. The instrument reads the values stored in the sensor and applies them to the process.|
|In an analog loop, you never risk signal loss between the sensor and the display. Either you have a measurement without signal loss or no measurement at all.||With digital analytical sensors, you receive 100 percent signal integrity, so they’ll either send you perfect signals or nothing, and nothing means you need to fix something.|
Selecting a device to measure the pH of your process requires careful study of the conditions of the process and the quality of the product. Generally we look for reliability, accuracy and reproducibility when choosing a meter. However, the right choice always depends on the user’s needs and resources.
Hope you learned something new today!