How Does A pH Probe Work?


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pH probes contain two electrodes (a sensor electrode and a reference electrode) that measure the hydrogen-ion activity in a solution. The exchange of ions generates a voltage that is measured by the pH meter converting the voltage into a readable pH value. 

The glass electrode was invented by Nobel Prize winner Fritz Haber in 1909, followed by the development of the modern electric pH meter in 1934 by Arnold Beckman, giving birth to pH probes that we use now to accurately measure pH in a variety of applications. 

pH (the potential of Hydrogen) measures the interaction between hydrogen and hydroxide ions within a water-based solution. A solution with a high amount of hydrogen-ion activity is an acid. In contrast, a solution with a high amount of hydroxide ion activity is a base. 

Using pH probes when measuring pH is important for a wide range of industries, which is why there are different pH sensors for different applications.

In this article, we will look at how a pH probe works, what a pH probe measures, and what components are found inside a pH probe. 

How Does A pH Probe Work?

Most people are familiar with litmus paper to measure a solution’s pH. Litmus paper turns a different color corresponding to the pH chart without considering the hydrogen ion content, however, a pH meter must measure the hydrogen ion concentration using a pH probe.

A pH meter works by measuring the electrical potential (voltage) produced by the solution being tested, using the potential difference to determine the pH. A solution that is acidic has more positively charged hydrogen ions than an alkaline solution, therefore, the solution has a greater electrical potential to produce an electrical current.

A pH meter has three components: the pH meter itself (moving-coil or digital meter), a reference pH electrode, and a pH probe that is inserted into the solution being tested. 

Most pH probes contain two electrodes inside the body: a measuring electrode (glass electrode) and a reference electrode. The glass electrode contains a reference electrolyte (usually potassium chloride), which has a neutral pH of 7, therefore, it contains a specific amount of hydrogen ions. 

The glass electrode works by measuring the difference in pH between the pH electrode and the solution being tested. The electrode does this by measuring the differences in the voltages of the hydrogen ions produced in both the electrode and the solution. This is easy to work out as we already know the pH value of the glass electrode composition. 

What Does A pH Probe Measure?

A pH meter uses a pH probe to measure the acidity or alkalinity (hydrogen-ion activity) of a solution, expressed as pH. The pH value of a solution is related to the hydrogen ion ratio inside the pH probe and the solution being tested. 

When you dip a pH probe into a solution, a number of hydrogen ions will move towards the glass electrode, replacing some metal ions inside, while some hydrogen ions will disperse into the solution being tested. This process of ion-swapping is known as ion exchange and is the main principle of how the glass electrode inside a pH probe works.

Ion exchange also occurs on the inside surface of the glass electrode. Because the potassium chloride inside the electrode and the solution being tested has different acidity, a difference in hydrogen-ion activity happens, resulting in a difference in electrical charges. When this happens, a potential difference appears between the sides of the glass electrode and the reference electrode producing a pH reading on the meter. 

Despite the meter measuring voltage, the digital display shows it as a pH measurement. It calculates this based on the voltage difference; the greater the voltage between the pH probe (electrodes) and the solution being tested, the greater the difference in hydrogen-ion activity. The more hydrogen-ion activity that happens, the lower the pH is, and vice versa. 

Before using a pH meter, it is important to calibrate it for accurate results and is an important part of general electrode maintenance. We recommend you follow our advice on pH probe calibration to always ensure your pH readings are reliable. If you are interested in how to calibrate a pH meter without a solution, you can find more information here

To understand how pH probes function in more depth, we need to look at how pH electrodes inside the pH probe work. 

How Do pH Electrodes Work?

pH electrodes come in a variety of types for both industrial and laboratory applications, however, they are mostly made of glass therefore they are easy to break and need to be hydrated. This is why every pH probe we ship comes in a plastic soaker bottle containing a pH probe storage solution. 

The first pH electrodes invented, consisted of a glass bulb that was filled with a strong electrolyte with a silver/silver chloride  (Ag/AgCl) half-cell inside and silver wire that came into contact with the external solution. 

The general workings of pH electrodes have not changed much, however with advances in technology combination electrodes and double junction electrodes are more commonly used. 

Combination Electrodes

Combination electrodes measure the two sides of a glass electrode and are the most common electrode found in pH probes. pH probes require electrodes to have a closed-circuit to measure the potentials which are done through the internal solutions (e.g. potassium chloride) of the electrode, the external solution being measured, and the pH meter. 

When the electrode is submerged into the test solution, it recognizes the positive charge of the hydrogen ions, measured in millivolts (mV), passing the signal to the internal electrode. The silver wire inside the pH probe passes the electrical signal to the electrode cable which is connected to the pH meter. 

Components Of A pH Probe

Today, most pH probes contain both a glass hydrogen ion-sensitive electrode and a reference electrode, known as combination electrodes, as mentioned above. 

A common pH electrode consists of several structures:

  • Electrode Body
  • Glass Membrane
  • Reference Electrode
  • Reference Electrolyte
  • Reference Junction

Electrode Body

The electrode body can either be plastic or glass, so when you hear the term “glass electrode” it is referring to the membrane material, not the external electrode body. 

Atlas scientific offers two main pH probe body types:

  • Lab grade pH probe: Extruded epoxy body giving it incredible resistance to strong acid and bases. 
  • Industrial pH probe: Ryton body making it chemically inert and virtually indestructible. 

Glass Membrane

The glass membrane is a pH-sensitive membrane filled with a buffer solution, located inside the glass electrode of the pH probe. 

The membrane allows the constant binding of hydrogen ions inside the membrane, while the outer part of the glass membrane is exposed to the sample being tested. It is the difference of hydrogen ions across the membrane that creates the voltage potential. 

The shape of the membrane can vary depending on the application. Some applications require a pH probe with a specialized membrane that can be used to pierce into semi-solid media like a spear tip pH probe

Spear Tip pH Probe

Reference Electrode

The reference electrode is needed to provide a constant electrical potential inside the pH probe to determine the pH.

The most common type of reference electrode contains silver/silver chloride (Ag/AgCl) because it is non-toxic to humans, therefore it can be used in a wide variety of applications including medical and food industries. 

Some industries still use reference electrodes containing mercury chloride (Hg/Hg2Cl2), however, due to the toxicity of mercury, it is not commonly used. 

The newest reference system to be developed contains iodine due to the fast response time and lower temperature sensitivity. An iodine reference system is commonly used when measuring the pH of tris buffers and protein solutions because it is a metal ion-free system; reference electrodes that contain metal ions such as Ag/AgCl can interact with solutions, clogging the reference junction. 

Reference Electrolyte

The reference electrolyte closes the electrical circuit inside the pH probe electrode. The electrolyte must be chemically neutral and have a good electrical conductivity to work correctly. 

Different diffusion rates of hydrogen ions in the electrolyte solution can create an electrical potential, creating problems when measuring the pH, therefore it is also important the electrolyte solution is mobile.

The most common reference electrolyte solution is potassium chloride (KCl), however, it is not recommended to use KCl when measuring the pH of solutions that have a low temperature as the KCl can crystallize inside the pH probe. 

Reference Junction

Also known as a diaphragm, the reference junction is a porous communication point that creates an electrical current between the reference electrode and the sample being tested. 

Different reference junctions have different discharge rates depending on the application you are working with. In applications where electrolyte flow reversal may occur, double junction electrodes are used to work under adverse conditions. An electrolyte flow reversal can happen when solutions being tested are extremely acidic or alkaline, or are under extreme pressure, or have an extremely high temperature. 

The double junction electrode protects the reference junction from contamination, as the solution being tested has to diffuse across both junctions before the pH value is recorded.

Is A pH Probe The Same As A pH Meter?

A pH probe is part of a pH meter, so technically, they are not the same, however, they cannot function without one another. 

A pH meter consists of three parts: a pH probe, a reference pH electrode, and the pH meter itself. It is the electrodes inside a pH probe that measures the pH of the solution. 

The pH probe records a voltage from the hydrogen ions, and it is the pH meter that converts the voltage into a readable pH value by measuring the difference between the internal electrode and the reference electrode in a pH probe. 

The pH probe is connected to the pH meter display panel to show the measured pH value. At Atlas Scientific, our pH probes can also be connected into a pipe, known as “mounting the probe inline”. 

Summing Up How A pH Probe Works

The working principle of a pH probe depends on the exchange of hydrogen ions that produce an electrical voltage. 

The glass electrode inside the pH probe measures the difference in pH between the pH electrode and the solution you wish to test. The electrode does this by measuring the differences in the voltages of the hydrogen ions produced in both the electrode and the solution. It is the pH meter that converts this voltage into a readable pH value.

If you have any questions regarding pH or are unsure which pH probe will best suit your needs, please do not hesitate to contact the world-class team at Atlas Scientific. 

pH Probes

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