pH [Potential Hydrogen] is one of the units that indicate the properties of water. The pH is represented by a numerical value from 0 to 14, with pH 7 being neutral, pH less than 7 being acidic, _and pH greater being alkaline. PH 5.8 to 8.6 are set as standards for tap water, pools, river discharge, etc., _and some local governments have set standards for industrial water, etc.

The factor that determines the pH value is called “hydrogen ion concentration”, which is expressed by the rate at which hydrogen ions are ionized in an aqueous solution.
Ionization: An electrically neutral substance (H₂O) has a positive (H⁺) _or negative(OH^–) charge. A phenomenon that changes into an atom _or molecule with an electric charge.

For example, considering ordinary tap water [upper right image], water, that is, H₂Oexists as a very stable molecule due to the combination of hydrogen _and oxygen, but only one. Hydrogen ion H⁺ _and hydroxide ion OH^– exist in the form of ions. The balance between H⁺ _and OH^–determines the pH, _and when there are more hydrogen ions H⁺ it is acidic _and hydroxide. When there are more ions OH^–, it becomes alkaline.

As long as the water temperature is constant, the following relationship holds between H⁺ _and OH^– in any aqueous solution.
[H⁺]×[OH^–]＝Kw＝10^-14(= constant)

Kw is called the ionic product of water _or the dissociation constant of water, _and is always constant at 10^-14 Therefore, if the value of either hydrogen ion _or hydroxide ion is known, the other value is known. I also understand. In the actual pH measurement, only H⁺is measured _and used as a guideline for the pH value, _and the value is defined by the following formula.
pH＝-log₁₀[H⁺]

In the case of a neutral aqueous solution, [H⁺]＝[OH^–]=10^-7 so the pH value is- log ₁₀10^-7＝7, _and the hydrogen ion concentration is 10 ^-7mol/L, that is, 1×10^-7mol of hydrogen ions. Similarly, in the case of a pH 8 solution, the hydrogen ion becomes 1×10^-8mol, _and the hydrogen ion concentration increases _and decreases logarithmically with only one change in pH.