What is the pH value?

Many people are aware that the pH value is related to acids and bases and that it is important for e.g. water quality, food, aquariums or even plants. The exact meaning will be explained in the course of this article.

In simple terms, the pH value indicates how acidic or basic a solution or liquid is. Here, solutions with a pH value of 7 are referred to as neutral, with a value below 7 pH as acidic, and with a value above 7 pH as basic (or alkaline).

pH scale

The further away the pH value is from 7, the more acidic or basic it is. For example, at 6 pH one speaks of weakly acidic and at 3 pH of strongly acidic.

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” is usually referred to as “power of hydrogen”. What does that mean now?

The hydrogen abbreviated in “pH” actually means a hydrogen ion, which is written as H+. A hydrogen atom consists of a positively charged proton and a negatively charged electron and is thus 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 note : Sometimes the hydrogen ion is written as hydronium (H3O+), because free H+ in solutions combine immediately with molecules.

The H+ are responsible for a solution being acidic. An acid is a compound that can transfer H+ to a reactant, essentially water. As an example, let’s take hydrochloric acid (HCl), which splits into its components H+ and Cl- in water, 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 containing 6.022x1023 particles. The pH is related to the H+ concentration as follows:

(H+ concentration in mol/l) = 10-pH

For example, at a pH of 3, the H+ concentration is 10-3 or 0.001 mol/l; at 6, the concentration is 10-6 mol/l.

Technical note : Chemists may use slightly different definitions of pH that take into account the non-ideal behavior of hydrogen ions or are formulated in other units (e.g., mol per kg of water) [1]. However, the explanation presented here should make the importance of pH clear to most people.

What about neutral or basic solutions?

Water molecules divide into two ions: Hydrogen ions (H+) and hydroxide ions (OH-).

H20 -> H+ + OH-

This reaction is reversible, so H+ and OH- can also recombine to form water. At any given time, the concentration of H+ and OH- is very small relative to the number of water molecules.

At a pH of 7, the concentrations of H+ and OH- are equal. For this reason, 7 pH is referred to 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, i.e. in strongly basic solutions, the H+ concentration is very low. At 11 pH, for example, this is 10-11 mol/l. Conversely, the greater the concentration of OH-. The OH- concentration is given by:

(concentration of OH- in mol/l) = 10pH-14

Thus, at 11 pH, the OH- concentration is1011-14 (=10-3) or 0.001 mol/l, which is exactly equal 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, and so on. Differences between H+ concentrations are thus represented by powers of ten or in English“power of ten“. Therefore, the pH value 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. Thus, the OH- concentration at 8 pH is ten times higher than at 7 pH, 100 times higher than at 6 pH, and so on.

A common misconception is that the pH scale starts at zero and ends at 14. In fact, however, pH values less than zero and greater 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].


[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.