Three Quick Tips To Increase your pH sensor selection Skills from Yokogawa Blog by Miranda de Bruijn
Industry Manual Repository
Join the AnalyzeDetectNetwork and Read This Manual and Hundreds of Others Like It! It's Free!
Three Quick Tips To Increase your pH sensor selection Skills
Recommended practices to extend your pH sensor lifetime 5/5: Sensor Selection
- Miranda de Bruijn
The lifetime of a pH sensor has a significant impact on the total costs of ownership and performance of
the entire pH measuring loop. Improving four key areas will lower costs and optimize process control and
overall plant efficiency. These four key areas are:
Maintenance & Storage,
In this 5/5 blog we will look closer at the key factor : Sensor Selection
What if the lifetime of your pH sensor is very short: only 2-3 weeks ? Besides that, you need to calibrate
it every day, and to make it even worse: clean it several times a day. Something definitely is going
wrong! And this situation will most likely influence your pH sensor lifetime, decrease reliable process
control and efficiency.
In this blog you will learn what how to optimize the lifetime of your pH sensor by looking into: proper
Selecting the correct pH sensor for each application plays an important role in optimizing the pH sensor
lifetime. It is important to know that selection will become easier once you understand two things: the
characteristics of the two pH sensor basic parts: pH sensor glass (1) and reference sensors (2), and
finally, the effects of temperature and pressure (3).
1. PH glass characteristics
First, let’s have a look at standard pH glass. The surface of the glass of a pH
sensor has pores caused by the structure of the glass. These tiny pockets are
perfect for H+ ions, which get stuck in them. Or as the industry calls it ‘the
sensor is selective for H+ ions’ (see figure 1).
For this reason “Ion Selective Electrodes” (ISE) exist i.e.: Sodium or Chloride ion
You might be wondering “Why is the above important to know ?” Let’s look at
an example: in high alkaline applications (>pH12) the H+ ion concentration is very
low. This is often done by dosing with Sodium Hydroxide (NaOH). This means
that the Na+ concentration is high. The Na+ ions will block these H+ “pores” in
the structure of the glass. Because these pores now have a Na+ instead of an H+
ion it will still see it as an H+ ion and register a lower pH than that it is in reality.
Figure 1: pH glass is porous
material. A special glass recipe is
used and these pores specifically
fir ‘H’ ions.
This is often referred to as the “Sodium Error” and occurs above a pH of 12.
The reading of the measurement will be typically 0.17 pH lower than it is in
reality (see figure 2). This is true for all pH sensors; not just the ones the
company I work for (Yokogawa) makes. It is the nature of pH glass and
Another factor is aging of the pH glass itself. This will cause the structure of
the glass to become uneven and will dissolve in the process over time. The
sensor will then become sensitive not only to H+ ions, but also to other ions
(see figure 3).
Knowing the glass characteristics of each sensor type will help to select the correct one for your
application and prevent unnecessary replacements of sensors in the future.
One of the most common questions we get is : “How will I know when to
Figure 2 When Na concentration is
high, the Na ions will block the pH
replace my pH sensor ?” The answer I normally give is usually quite
straightforward. During the calibration of the pH sensor the sensor will
probably start to respond slowly to pH changes and in addition the zero
offset will increase. This is a good indication that it is time to replace the
Another common question I received is : “What are the suggested limited
for the zero offset ?”. If you consider that ± 60 mV corresponds to a
difference in a 1 pH unit shift at 25°C, then if your sensor showed a zero
offset of 30 mV, the analyser would have to adjust for 0.5 pH difference. In
most cases a maximum difference of between 30-40mV is allowed before
the sensor needs to be changed (this is true for pH only or combined sensors).
Note that most analysers will warn you after a true two point calibration is completed when the zero
offset it out of limits. It is recommended to check the limits in the analyzer, because some instruments
are defaulted to alarm at 120mV and that is 2 pH units and not good practice.
For the company I work for (Yokogawa) there are two types of pH glass sensors available. In order to
minimize aging and corrosion of the glass it is important to select the
Figure 3 pH glass ages the structure of the
correct type. This will also help to extend the pH sensor lifetime.
glass changes uneven and the “pores”
sizes will change.
2. Reference sensor characteristics
The Reference sensor completes the electrical circuit of the
measuring loop and provides a stable reference potential.
Between 70-80% of the pH sensor failures are generally
caused by an issue with the reference junction. Selecting the
correct type of reference junction is therefore an important
step and will significantly increase the lifetime of the pH sensor.
The purpose of the junction is to maintain contact between the reference system in the electrode and
the process liquid.
Selecting the correct junction ensures the process liquid does not penetrate into the electrode. This is
something to be avoided as it causes poisoning, and it might even create an unstable liquid junction
The selection of the correct type of reference electrode junction, depends on the process conditions
under which this electrode has to do its job. There are several different types of junctions. Four of the
most commonly available junctions are :
Salt sensitive glass
Each of these junction materials have their own particular advantages and
Ceramic junctions have very large pores. This means the ceramic junction has to
be small. The added advantage is that the electrolyte inside the sensor will not
deplete very fast in applications for example with a high salt content. The
downside is that in dirty applications it is easily blocked, unless a flowing type sensor is used (cleans
itself). (See figure 4)
PTFE is well known for its fouling resistant properties. In applications where
there is a lot of fouling or scaling it will repel the dirt and keep the junction free.
The downside of having a PTFE junction are small pores, which only allows a
small flow out of the sensor. When measuring in solutions with a high
concentration of salts, this low flow can generate a potential difference across
the PTFE junction (see figure 5) generating an error in the pH measurement.
The Sleeve is a very special type of junction. There is one hole in the main glass
tube. Around the hole the glass is made deliberately uneven by lightly
sandblasting. This creates small pathways for the electrolyte to flow from the
sensor. Over this hole another sandblasted glass tube is placed (the sleeve). This
will ensure a steady small flow of electrolyte out of the sensor. This is especially
important in applications where the conductivity is low. The glass sleeve can be
lifted to clean the pathways, which is especially handy for very dirty applications
(see figure 6).
Salt sensitive glass is used in applications where there is a high salt
concentration. High concentrations always flow towards the lower
concentration and as a result the process will slowly ingress into the
reference sensor and poison it over time. When using a glass reference,
there is no open connection to the process and the sensor will not be
poisoned. The salt in the process will generate a potential difference across
the glass which is proportional to the salt concentration. As long as the
concentration is relatively stable (± 25%) this reference will eliminate the
poisoning and help provide a stable measurement.
To Flow or not to Flow?
Another question that you might be asking yourself is : “When shall I select a flowing type reference
sensor and when a non-flowing type ? “
The following two situations are the only times you would consider using a flowing reference:
o The electrolyte flowing out of the sensor will keep the junction clean.
o Everything flows from high to low. High salt concentration in the sensor, low in the
process. The electrolyte will flow fast out of the sensor.
o When using a non-flowing type sensor it will cause the sensor to deplete fast and fail.
3. The effect of temperature and pressure
Finally, I would like to discuss the effects of temperature and pressure. One of the most effective steps
to increase the pH sensor lifetime is to decrease the temperature. Each 10 degree C decrease will double
the lifetime of the sensor.
At higher temperatures the pH
sensitive glass will age quicker and
for the reference portion, the
pressure will increase inside the
sensor and cause the electrolyte to
deplete faster (see figure 6).
Figure 7 : Selection guide for
Yokogawa pH sensitive glass
Fluctuating process pressure and temperatures will cause a pump effect inside the reference electrode
sucking in the process liquid. This will significantly decrease the lifetime of the pH sensor.
Using a pH sensor with a bellow (always 300 mbar overpressure) inside the sensor will limit or even
illuminate the negative effects of pressure and temperature fluctuations.
The next time you have to select a pH sensor for you application hopefully you can use this blog as a
reference guide. I hope it will help you select the correct pH glass and reference type to improve your
pH sensor lifetime and you can limit or even eliminate the effects of temperature and pressure on
especially the reference sensor.
For more information on how to extend the lifetime of your pH sensor please check out our eBook here
or through this link :