Choosing the right pH meter and electrode for the job
There is so much choice out there when it comes to pH meters and electrodes that it can be overwhelming when you need to select something to suit your application. Some pH meters come with a built in electrode and others come with a separate electrode than can be replaced and switched out. Here we break down the main differences with between pH meters and the different types of electrode out there.
Types of pH meter available:
There are a number of different types of pH meter available, with different pros and cons summarised below.
Stick/pen/pocket type pH meters
Pocket pH meters are small, portable pH meters, and like their name suggests usually compact enough to just pop into your pocket. This type of pH meter usually has simple operation and you may not even be able to replace the electrode (in which case the whole thing will need replacing).
Generally this type of meter has a one or two button interface making them great for the beginner or those just those who require a quick and simple instrument. This type of meter will still require correct storage and maintenance to prolong is useable life like any other pH meter.
Handheld pH meters are portable, rugged units that usually will have a separate pH electrode plugged it. Other types can be all in one units such as the Myron Ultrameter. They are designed to be used in the lab but can often be taken out and about.
Bench top pH meters
Benchtop pH meters are larger units designed for use in the lab. They usually come with an electrode holder so you can set up your equipment. They are ideal if you must follow a set protocol with measuring every time or regularly use the equipment.
If you are looking at measuring another parameter (such as conductivity, redox, and dissolved oxygen) it may be worth considering a multi-parameter instrument which are available in numerous combinations. These meters can be single channel or multi-channel, having the ability to measure only one parameter at the same time or multiple parameters, respectively. Single channel instruments will need to be recalibrated when you switch electrode to change your measurement parameter.
There are hundreds of different electrodes available to fit all number of applications. With this variety, it can be very overwhelming and confusing when it comes to finding the right one to fit your application. The are a number of design features that can change between which we explain below. Finding the most suitable electrode is a case of finding the most suitable combination of design features for your application.
Most pH electrodes are combination electrodes, meaning they consist of a reference half-cell and a sensing half-cell.
- The reference electrode is not sensitive to H+ ions and provides a stable reference potential. It contains a Ag/AgCl reference electrode wire. Reference electrode solution (usually KCl saturated with AgCl will flow from inside the electrode body and out of the junction giving constant contact between the sample and the inner reference solution providing a stable potential. The junction is what separates the internal reference electrode from the sample itself.
- The sensing portion of the electrode contains a AgCl covered silver wire contained within a H+-sensitive glass membrane. This wire along with the reference wire complete a circuit resulting in your pH reading. Within the glass membrane is an internal buffer solution with a constant H+ concentration and therefore a constant pH. When in solution, a gel layer forms on the internal and external side of the glass membrane. Ions can move through the gel layers. In acidic solutions, the H+ concentration of your sample is high so H+ ions move into the gel layer giving it a more positive charge. Conversely, in alkaline solutions, the low H+ concentration of the sample means H+ ions will move out of the gel layer and give a negative charge. This charge is compared to the constant internal buffer.
Junction Type & Design
Internal reference electrode solution will slowly leak out from the junction into the sample. If the sample contains certain reactive substances, this can form a precipitate with the electrode solution, blocking the reference junction. To help combat this, some electrodes contain an additional internal junction. They are known as double junction.
A double junction electrode has an additional salt bridge between the reference electrode and the electrode fill solution creating second chamber with silver-free electolyte. This helps to prevent reactions between your sample and silver-containing reference electrode solution.
Electrode junctions can have a number of different designs which can also help prevent or reduct clogging:
Porous ceramic - this is the most common type found in electrodes and suitable for general applications. Not suitable for dirty or viscous samples and the junction is easily blocked.
PTFE - this is the junction often found on flat electrodes. They have a good level of chemical resistance. The junction has a larger surface area so is less susceptible to clogging.
Liquid junction – has a renewable liquid junction which offers performance and speed as the electrode can effectively be returned to new at the push of a button.
Open junction – the reference electrolyte is usually a solid gel. Ideal for samples that normally clog up the junction, such as solid and semi solid samples (pastes, gels).
The body of the electrode can be made from a number of different materials. Considerations to make when selecting what body it has include the temperature of your sample, the type of enivronment you will be using it in and the chemical composition of your sample (is it compatible with the body material?).
Glass body - glass has a broad resistance to chemicals including corrosive solvents. They are also very easy to clean after use. The have a high maximum temperature limit of around 100ºC. However, glass is considerably less rugged than other materials so may not be the best option in outdoor-based or more rugged applications.
Plastic (epoxy) body - plastic bodies represent a far more durable electrode. You will need to check chemical resistance against your sample especially if it is highly acidic or alkaline. The maximium temperature limit for most is around 80ºC.
Glass Membrane Shape
pH electrodes come with membranes in a variety of different shapes, designed to enhance response and readings across different sample types.
- Spherical - the most common shape seen in glass electrodes. This type of bulb is recommended for use with most general aqueous liquid samples, and also for low temperature samples as the bulb shape resists contractions. The shape however makes it quite easy to damage if you are a little more heavy-handed with it.
- Hemispherical - this tip shape makes it good for smaller sample volumes and due to the design, it is stronger than the spherical tips.
- Flat – the flat shape allows for direct surface contact with the sample. This is ideal for taking surface readings and also for very thick samples as they often have a larger junction area which means more resistance to clogging.
- Spear (& conical) - the pointed shape of this tip makes it suitable for penetrating into solid and semi solid or slurry samples. Sometimes they come with a metal cutting tip which is ideal for cutting into tougher samples, for example meat.
- Cylindrical – this shape has a large surface area, making it highly sensitive and enhancing its functionality.
- Micro – The small size of this tip makes it ideal for use in narrow tubes where a normal tip wouldn’t fit
Electrode Selector Guide
We have also produced a handy table guide for a few of our electrodes in, according to application.
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