The pH value indicates whether a solution is acidic, neutral, or basic—mathematically defined as the negative common logarithm of the hydrogen ion concentration. Unlike our parameter overview page, this section focuses on the technology behind it: how a pH meter actually measures pH, and the mathematical principles behind the scale.
How does a pH meter work?
pH meters operate on the principle of potentiometry: A special glass electrode is sensitive to the concentration of hydrogen ions (H⁺) in a solution and thereby generates an electrical potential. This potential is measured relative to a reference electrode with a constant, known potential. The voltage difference between the two electrodes—typically in the millivolt range—is converted by the device into a pH value using the Nernst equation.
In most modern measuring devices, including Apera devices, the measuring and reference electrodes are combined into a single combination electrode. Since the relationship between voltage and pH value is temperature-dependent, most devices feature automatic temperature compensation (ATC), which automatically compensates for fluctuations in water temperature.
The further the pH value is from 7, the more acidic or basic it is. For example, a pH of 6 is described as slightly acidic, and a pH of 3 as strongly acidic.It is used in water analysis, hydroponics, and aquaculture.
Technical background of the pH value
What exactly does the pH value mean?
The “H” in pH is the chemical symbol forhydrogen. It is not clear what “p” stands for, but “pH” usually refers to the “power of hydrogen”. What does that mean?
Hydrogen, which is abbreviated to “pH”, actually refers to a hydrogen ion, which is written as H+. A hydrogen atom consists of a positively charged proton and a negatively charged electron and is therefore electrically neutral. A hydrogen ion is a hydrogen atom that has given up its (negatively charged) electron and thus consists of only one proton and is therefore positively charged overall (hence the “+” in H+).
Technical noteSometimes the hydrogen ion is written as hydronium (H3O+), as free H+ in solutions immediately combines with molecules.
The H+ are responsible for a solution being acidic. An acid is a compound that can transfer H+ to a reaction partner, essentially water. Let’s take hydrochloric acid (HCl) as an example, which splits into its components H+ and Cl- in water, thus adding H+ to the water. These H+ ensure that the water is now acidic. The higher the concentration of H+. the more acidic the water.
HCl -> H+ + Cl-
The H+ concentration can be expressed as moles per liter of water. One mole is the amount of substance in which 6.022x1023 particles. The pH value is related to the H+ concentration as follows:
(H+ concentration in mol/l) = 10-pH
At a pH value of 3, for example, the H+ concentration is 10-3 or 0.001 mol/l, at 6 the concentration is 10-6 mol/l.
Technical noteChemists may use slightly different definitions of pH, which take into account the non-ideal behavior of hydrogen ions or are formulated in other units (e.g. moles per kg of water) [1]. However, the explanation given here should make the significance of the pH value clear to most people.
What about neutral or alkaline solutions?
Water molecules are divided into two ions: hydrogen ions (H+) and hydroxide ions (OH-)
H20-> H+ + OH-
This reaction is reversible, so that H+ and OH- can also recombine to form water. At any given time, the concentration of H+ and OH- is very small in relation to the number of water molecules.
At a pH of 7, the concentrations of H+ and OH- are equal. For this reason, a pH of 7 is described as neutral. If the water has a pH below 7, the concentration of H+ is greater than the concentration of OH-, and the water is acidic. If the water has a pH greater than 7, the concentration of OH⁻ is greater than the concentration of H⁺, and the water is basic.
At a high pH value, i.e. in strongly alkaline solutions, the H+ concentration is very low. At 11 pH, for example, this is 10-11 mol/l. Conversely, the concentration of OH- is all the greater. The OH- concentration is given by
(concentration of OH- in mol/l) = 10pH-14
At 11 pH, the OH- concentration is therefore 1011-14 (=10-3) or 0.001 mol/l, which corresponds exactly to the H+ concentration at 3 pH.
The pH scale: “power of hydrogen”
The pH scale is a logarithmic scale, which means that a difference of one pH corresponds to a difference of a factor of ten in the H+ concentration. For example, the H+ concentration at 5 pH is ten times higher than at 6 pH, 100 times higher than at 7 pH, 1000 times higher than at 8 pH, etc. Differences between H+ concentrations are therefore represented bypowers of ten. The pH value therefore represents the“power of hydrogen“, so to speak. The pH value is usually defined as the negative decadic logarithm of the H+ concentration:
pH = -log10 [H+ concentration]
The OH- concentration behaves analogously. For example, the OH- concentration at 8 pH is ten times higher than at 7 pH, 100 times higher than at 6 pH, etc.
A common misconception is that the pH scale starts at zero and ends at 14. However, pH values lower than zero and higher than 14 are also possible. For example, concentrated acids used in laboratories or industry often have negative pH values. Even in nature, pH values as low as -3.6 have been observed [2].
Typical pH values of various liquids
The actual values may vary depending on the composition.
| Liquid | approximate pH range |
|---|---|
| Lemon juice | approx. 2 |
| Coffee | approx. 5 |
| Drinking water | approx. 6.5 to 9.5 |
| Pure water | approx. 7 |
| Seawater | approx. 8 |
| Soap solution | approx. 9 to 10 |
Frequently asked questions about the pH value
What is a neutral pH?
A pH of 7 is considered neutral at 25 °C; at this pH, the concentrations of H⁺ and OH⁻ ions are in equilibrium. At other temperatures, this point may shift slightly.
Why is the pH scale logarithmic?
Because the underlying hydrogen ion concentration itself varies over many orders of magnitude (from 1 mol/L in very acidic solutions to 10⁻¹⁴ mol/L in very basic solutions). A logarithmic scale makes it possible to represent this enormous range using manageable numbers. In practice, this is usually between 0 and 14, even though values beyond that range are theoretically possible.
How does the glass electrode work in a pH meter?
The thin glass membrane at the tip of the electrode is selectively permeable to hydrogen ions. When it comes into contact with a solution, an electric potential builds up across the membrane that is proportional to the H⁺ concentration. This potential is measured relative to the internal reference electrode and converted by the device into a pH value.
What does the accuracy specification (e.g., ±0.1 pH) mean for a measuring device?
It indicates the maximum deviation, under the worst-case scenario, between the displayed value and the actual pH value. A device with an accuracy of ±0.1 pH will display a value between 6.9 and 7.1 when the actual pH value is 7.0. Laboratory measuring devices often achieve an accuracy of ±0.01 pH.
Why isn’t pure water exactly neutral when CO₂ is dissolved from the air?
Carbon dioxide from the air dissolves in water and forms carbonic acid (H₂CO₃), which releases small amounts of H⁺ ions. As a result, the pH of distilled water exposed to air drops from 7.0 to values around 5.5–6.5 after some time.
References
[1] Langmuir, D. (1997). Aqueous environmental geochemistry. Upper Saddle River, N.J.: Prentice Hall.
[2] Nordstrom, D. K., Alpers, C. N., Ptacek, C. J., and Blowes, D. W. (2000) Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California. USGS Staff — Published Research. 479.