Lots of people have to test pH. In fact, pH is the 2nd most tested parameter
after temperature. However, pH testing requires the use of sensitive
equipment and a lot can can go wrong.. There is a lot of different advice out there
which adds to the challenge. We’ve put together this comprehensive video
to help everyone from hobbyists to labs test pH better. You don’t need to be a chemist but you do
need to know a bit about pH. In technical terms, pH, or potential of hydrogen,
is the hydrogen ion activity in a solution. It’s measured on a logarithmic scale of 0
to 14, with 7 being neutral because the activity of positively charged hydrogen ions and negatively
charged hydroxide ions is equal. At low pH values (from 1 to 6), the hydrogen
ion activity is greater so the solution is considered acidic. At high pH values (from 8 to 14), the hydroxide
ion activity is greater and the solution is considered basic. There are three important things to know. One, you are measuring tiny concentrations
which is why pH meters are delicate. Maintenance is key to accurate pH measurement
but there will be more on that later. Two, pH readings are affected by temperature
so you need to account for that. For example, a pH of 7 at 25C may read differently
at 20C. Three, pH is a logarithmic scale. This means that at reading of 7 pH has a 10
times higher concentration of H+ ions compared with a reading of 6 pH. Small differences in reading can matter a
lot. A pH measuring system consists of four main
parts: a pH sensing half-cell, a reference electrode half-cell, a special meter to display
your pH value, and your sample solution. Together, these form an electrochemical circuit
that measures and displays the pH of your sample. Every pH meter has an electrode or probe. Until the 1970s, it was common practice to
offer two half-cells separately, a glass pH sensor and a reference electrode cell. Today, it is more common to use a single combined
electrode that has both sensing and reference components. The combination pH electrode has a layer that
is sensitive to Hydrogen ion activity. Inside the electrode, there is a reference
cell. The electrode creates a current that measures
the charge on the glass tip compared with the reference solution. Keeping this bulb in good condition is vital. As a result of immersions in samples, a residue
can form that stop the ions from binding or slows the process down. The binding layer can also get damaged. It can not be repaired if it is damaged. Remember also that the electrode is a consumable. The binding layer will get less sensitive
over time and the electrode eventually will need to be replaced. This is true for all pH electrodes. The next part of the pH measuring system is
the meter itself. The meter processes the voltage from the electrochemical
cell and converts it to a meaningful measurement unit, pH. The final part of the measuring system, your
sample, is the connection between the sensing electrode and the reference electrode. This connection allows the entire measuring
circuit to work. pH electrodes come in a variety of designs
but they all have some key parts. When choosing a pH electrode, design considerations
include the type and shape of glass, junction material, and body material. First, the glass membrane. Hanna Instruments produces four different
glass types covering a vast number of pH applications. General purpose glass provides the greatest
response over the entire pH range and can be used for a wide variety of applications. Low temperature glass membranes have a lower
impedance and are suitable for samples at lower temperatures and lower conductivities. Designed for extended use at elevated temperatures
where glass impedance is known to decrease, high-temperature glass offers a higher resistance
making it possible to obtain accurate results with excellent response times
HF resistant glass is made for aggressive applications containing fluoride ions, which
dissolves glass. This glass composition will ensure a much
longer life for your pH electrode. Here’s the sensing bulb on a simple tester. Here’s what it looks like on a laboratory
grade electrode. The pH sensing bulb can come in a variety
of shapes, each shape serves a unique purpose. Spheric tips are recommended for general use
in aqueous solutions. The round bulb geometry is the most common
shape for a glass pH membrane and provides a wide surface area for a variety of liquid
samples to contact. Conic designs are ideal for direct penetration
into samples due to their pointed profile and geometric strength. These tips are well suited for samples ranging
from soils and gels to sauces, cheeses, and meats. A flat-tip geometry allows for a direct surface
measurement of a sample. These designs are ideal for measuring the
pH of skin, leather, or paper. The next aspect of the pH electrode is the
junction. The type of junction used in a pH electrode
is one of the most important design considerations when selecting the right sensor for your application. The junction is the electrical pathway between
the sample and the internal reference half-cell. This reference chamber contains an electrolyte
solution, which diffuses through the junction into the sample. Any clogging of this junction may result in
erratic and unstable readings. A porous ceramic frit is one of the most common
junctions available for standard laboratory applications. The ceramic material is easily fused with
the electrode glass and has a similar coefficient of expansion. A single electrode may contain a single, double,
or triple ceramic frit allowing for enhanced electrolyte flow. Polytetrafluoroethylene (PTFE) contains hydrophobic
properties, providing one of the most chemically resistant junctions available. It is commonly used in pH sensors for industrial
applications because of its chemical advantages and durability. A fiber wick junction, also known as a cloth
junction, allows for a quick refresh when readings are erratic or unstable. An open junction does not have a frit. The electrode uses an exposed hard gel as
the junction. This layer is silver free. The open junction has a high flow rate making
it ideal for samples that would clog the junction of standard pH electrodes Whether you are using a half-cell pair or
combination electrode, the body material is an important consideration. The body of a pH electrode can be made of
many different materials that may help to make pH measurements easier. The right body material will vary depending
on the testing environment, the sample type, and the frequency of use
Glass body electrodes are ideal for any type of laboratory application. Glass is resistant to a variety of chemicals,
is easy to clean, and transfers heat readily for a quicker reading. Polyetherimide is a high-performance, durable
plastic that offers excellent resistance against aggressive chemicals. Rugged and resilient, PEI electrodes are ideal
for environmental or industrial applications in the field or on the factory floor. Polyvinylidene fluoride is food grade plastic
that stands up to a variety of cleaning chemicals and solvents. It is durable and has a high resistance to
abrasion, mechanical strength, and resistance to fungal growth. Stainless steel is an extremely robust material
that can withstand a wide variety of aggressive chemicals in the harshest of applications. Connection and communication is your final
consideration when selecting an electrode. It’s important to understand that not all
electrodes connect to all meters. Some sensor connections are specific to a
meter, brand, or manufacturer. Be sure to take note of your requirements
when replacing or purchasing a meter and electrode. When choosing a pH meter, it is important
to consider the sampling points at which you will be testing. Testers are great for spot-checking or for
use by hobbyists as are the least expensive. A portable, handheld meter offers the mobility
to test pH anywhere and may have data logging features that make fieldwork easier. While the most expensive, a high-performance
benchtop meter is best suited for stationary lab use and most often offers advanced features
and higher accuracy than other types of pH meters. Ok. You understand a bit about pH, how a meter
and electrode work, and what to look for when choosing a meter for your testing. Now it is time to make sure that you have
everything that you need to start taking measurements. You should have your pH meter with your application
specific electrode. If you have a refillable electrode, make sure
that the electrolyte fill level is at least half an inch from the fill hole or closer. Check the electrolyte for contamination and
refill it with more electrolyte if necessary. You also need buffer solutions. Standard calibration buffers are 4.01 pH,
7.01 pH, and 10.01 pH. Depending on your sample, you may need different
buffers. When calibrating a pH electrode it is important
to choose buffers that bracket the expected reading. For example, if the expected reading is pH
5.2 then it would be best to calibrate to pH 4.01 and pH 7.01. If the expected reading is pH 3.2 then it
would be best to use a pH meter that allows for custom buffers. You’ll also need containers or beakers for
calibration buffers, rinsing the electrode, and waste. You also need deionized water (DI) water and
a rinse bottle. Some nice to haves are an electrode holder
and a magnetic stirrer. The electrode holder reduces the risk of breaking
your electrode. The magnetic stirrer keeps the sample stirring
to speed up response time, improve accuracy, and increase stability. Calibration is an important step to ensure
you are getting the most accurate readings. You should always calibrate After a long period
of disuse After rehydrating a dried-out electrode Or
After using a cleaning solution We recommend calibrating daily for best results. However, if high accuracy is not crucial to
your measurements, it is okay to calibrate once or twice a week. Here’s an example of a two point calibration
using a basic pH tester. For this tester, we will stir manually. The things you’ll need for this calibration
are clean beakers for your pH 4.01 and 7.01 buffers, a beaker for your rinse waste, A
DI water squeeze bottle, and of course, your pH tester. The first thing you want to do is rinse your
electrode with your DI water. Enter the calibration mode on your tester,
and place it into your pH 7.01 Buffer. Stirring gently helps the tester to stabilize
and get a more accurate reading. Once the reading stabilizes, rinse the tester
again, and place it into your pH 4.01 buffer. Once that reading stabilizes, notice how the
tester saves the calibration points, and returns to measurement mode. The next example we’ll show you is calibrating
a portable pH meter. This meter is a bit more advanced and typically
has a separate glass pH electrode. For this calibration, you’ll need your beakers
for clean buffers, your waste beaker, a DI water squeeze bottle, and your portable meter. First remove the protective cover from the
electrode bulb and rinse the bulb in DI water to remove any salt crystals left over from
the KCL Storage Solution. It is completely normal for salt to build
up around the base of the electrode cap. This electrode is refillable so you need to
make sure that you remove the fill cap so that the electrolyte flows out of the junction
to close the circuit. Now you’re ready to start calibrating. Enter calibration mode on your meter, rinse
the electrode, and then place the electrode into your pH 7.01 buffer. Stir gently and wait for a stable reading. Once the reading stabilizes, rinse the electrode
again, and place it into your pH 4.01 buffer. Remember to stir gently when doing this calibration. Once stable, this meter returns to measurement
mode and you would be ready to begin testing samples. Before we get into sample preparation, let’s
take a look at a typical lab set up using a benchtop pH meter. These meters offer high accuracy and dependability
and can typically read between the hundredths and thousandths place. We’ll do a 2 point calibration. The things you will need to do this calibration
are clean beakers for your calibration buffers, a waste beaker, a DI water squeeze bottle,
a magnetic stirrer, stir bars, and an electrode holder. The edge pH meter comes with an electrode
holder making this a great benchtop option. Fill two beakers with enough pH buffer solution
to cover the electrode junction. Add a stir bar to each beaker if you are using
a magnetic stirrer. First remove the protective cover from the
electrode bulb and rinse the bulb in DI water to remove any salt crystals left over from
the KCL Storage Solution. This electrode is refillable so you need to
make sure that you remove the fill cap so that the electrolyte flows out of the junction
to close the circuit. then place your pH 7 beaker on the stirrer. Start stirring. Make sure that you do not stir too aggressively
or have the magnetic stirrer set too high. The bulb and junction need to be fully submerged
with no air breaking the contact. Here’s an example of a stirrer set at the
right speed. Notice that there is no vortex. Enter calibration mode on your meter, and
lower the electrode into your pH 7.01 buffer. Once the reading is stable, it will prompt
you to press CONFIRM. Press confirm and the meter will save the
calibration point, and ask you for pH 4.01. Turn the magnetic stirrer off and rinse the
electrode over your waste beaker. Place the pH 4.01 beaker onto the stirrer
and begin stirring. Lower the electrode into the buffer and again,
wait for the reading to stabilize. Once stable, press confirm. This meter can do up to 5 calibration points. In between each calibration point, remember
to use your waste beaker and rinse the electrode with deionized water to prevent cross contamination
of buffers. To complete the two-point calibration, we’ll
press the CAL button again and it will save and store both calibration points. Once you’re done calibrating, give the electrode
a final rinse before measuring your sample. There are many different types of samples,
and some require preparation before measurement. In this video, we will be testing the pH of
regular tap water. In this case, all you need to be sure of is
that there is enough sample in the beaker to cover the reference junction on the electrode. You can check out some of our other videos
on the preparation of cheese samples, soil, wine and others. Ok, now we’re ready to measure our sample! We have a few setups The tester Gently insert the end of the tester in the
sample. Gently stir like this keeping the junction
submerged. The portable Gently insert the electrode making sure that
the electrode junction is submerged. The lab meter We have our sample in our beaker, with a stir
bar, on our magnetic stirrer, stirring without a vortex. With everything in place, slowly insert the
electrode into your sample and wait for a stable reading. It seems like a lot of steps to get that really
quick measurement, but I assure you, for accuracy and repeatability, you need to take these
steps. We aren’t quite done yet though. Once you’re done with taking measurements,
you need to clean and store your electrode. This not only ensures that your electrode
will continue to have a fast response time and provide accurate measurements, but it
also maintains a longer life for the electrode. To clean and store the electrode, rinse with
deionized water. We recommend using an appropriate cleaning
solution after rinsing to remove any materials that may be left on the electrode. Let it sit in the solution for 15 minutes
with the junction covered. After, fill your electrode cap with storage
solution and fasten it onto the electrode until you’re ready to measure again. If you have a tester like this, fill the cap. It is a good idea to store upright so that
the storage solution does not leak out. Remember, the electrode should always be fully
submerged in storage solution when not in use. That’s it. 9 critical steps to understanding pH, choosing
the right meter and electrode, and properly taking measurements. If you’re curious about how to test more complex
samples, subscribe to our channel to get updates as we introduce more videos about how to use
a pH meter. Until then, happy measuring! Don’t forget to like us on Facebook and Instagram,
and subscribe to the channel for more update on products and services from Hanna Instruments. See you soon.

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