This is our definitive guide to pH meters, the pH scale, and everything you need to know about pH measurement.
What does pH stand for?
This is how Wikipedia defines pH:
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.
But what does pH stand for? Nobody really knows but are some explanations. 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.”
What do pH levels measure?
To put it simply, the pH level of a substance is a measure of how acidic or basic (the opposite of acidic) it is. The pH level of a substance corresponds to its strength, meaning how many hydrogen or hydroxyl ions it contains. 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.
The pH 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 level 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!
How to measure pH levels with a pH meter
A pH meter 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.
Now, the 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.
pH Sensor for water applications
A pH 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.
Types of pH electrodes
|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 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.
How 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. Luckily, we have a detailed guide about extending the life of pH electrodes.
pH meter Calibration
A device has a range of precision, and 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 pH meter calibration:
- Cleaning solution
- Distilled water
- Clean beakers
- Buffer solution
- Paper towels
If you’d like to know more, we have a detailed guide on how to calibrate a pH meter.
What is a buffer solution?
Replacing pH electrodes is inevitable. However, these three tips can make the most out of our electrode friends before they conclude their mission on our planet.
When we talk about buffer solutions, we usually mean liquids which don’t vary much in pH value when you add acids or bases to them. To achieve this stability, a buffer solution itself must have a balanced mix of acid and base.
Now a pH calibration buffer solution must have an accuracy of +/-0.01 at 25 degrees Celsius. Usually, buffer solutions range in pH from 1.68 to 10.1. They must also follow ISO standards and NIST traceability policy.
Last but not least, buffer solutions can have different colours to identify their pH values. Blue indicates a basic solution, red acidic, and yellow neutral.
A pH sensor converts the signal it receives from an electrode into a vale on the pH scale. 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 moulding, 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 millimetres. 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 analogue 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 analogue 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 colour display, although you’ll need the manual to translate abbreviations on the display.|
Mettler Toledo M400
|This multi-parameter meter can use analogue 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 downside, this loop-powered device only has analogue, 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 analogue 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 analogue 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.
Common errors in pH measurement
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. Check out our article on common errors in pH measurement if you’d like to know more.
Analog versus digital pH measurement
Now we end our session with a discussion on whether we should use pH meters with analogue or digital pH sensors.
Analogue pH sensor
Digital pH sensor
|The pH sensor itself remains analogue, whether your transmitter is digital or analogue. So if you buy a digital transmitter thinking it will give your sensor a boost, then you just wasted money.
||A digital pH 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.
|Analogue pH sensors store their corrections in the transmitter and ensure drift correction.||Digital pH 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 analogue loop, you never risk signal loss between the pH sensor and the display. Either you have a measurement without signal loss or no measurement at all.||With digital analytical pH 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.|
Last thoughts about pH measurement
Selecting a pH meter for 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 pH meter. However, the right choice always depends on the user’s needs and resources.
Having any more questions on pH measurement or pH meters, you can get in touch with our engineers!