The calculation formula of the pH value is as follows: where [H] refers to the activity of hydrogen ions in the solution (sometimes also written as [H3O], hydronium ion activity), the unit is mole/liter.
In a dilute solution, the activity of hydrogen ions is approximately equal to the concentration of hydrogen ions, and the concentration of hydrogen ions can be used for approximate calculation. At standard temperature and pressure, an aqueous solution with pH=7 (such as: pure water) is neutral, because the product of the concentration of hydrogen ions and hydroxide ions naturally ionized by water at standard temperature and pressure (water ions product constant) is always 1×10^-14, and the concentration of both ions is 1×10^-7 mol/L.
A pH value less than 7 indicates that the concentration of H is greater than that of OH, so the solution is strongly acidic, while a pH value greater than 7 indicates that the concentration of H is less than that of OH, so the solution is strongly alkaline. Therefore, the lower the pH value, the stronger the acidity of the solution; the higher the pH value, the stronger the alkalinity of the solution.
Under the conditions of non-aqueous solution or non-standard temperature and pressure, pH=7 may not mean that the solution is neutral, which needs to be determined by calculating the ionization constant of the solvent under such conditions to determine the value of pH as neutral. For example, at a temperature of 373K (100°C), pH=6 is a neutral solution.
