# PH

##### Article

August 13, 2022

The pH is a measure of the acidity (also called acidity) of an aqueous solution. The pH of a neutral aqueous solution is around 7 at room temperature. Acidic solutions have a pH lower than 7, and therefore a high acidity. Basic solutions have a pH higher than 7 and thus a low acidity. The concept of pH was introduced in 1909 by Søren Sørensen as PH in his publication Enzymstudien II: Uber die Messung und die Bedeutung der Wasserstoffionenkonzentration bei enzymitairen Prozessen, drawing on earlier research by Svante Arrhenius. The exact meaning of the 'p' is not known. Explanations range from potentia hydrogenii or pondus hydrogenii to Potenz (German) or puissance (French) which in all languages ​​means "power" with regard to exponentiation, because the pH scale is after all logarithmic. Another explanation is that the designation stands for the concentration in the test solution (Cp) and the reference solution (Cq) with the fairly common combination p and q.

## General description

The pH is equal to the opposite of the logarithm (base 10) of the concentration of H3O+ (the hydroxonium ion). The unit of concentration here is mol/litre. Because these concentrations can become very small, it is more convenient to work with a logarithmic scale. In formula form, pH is defined as follows: p huh log [ huh 3 O + ] {\displaystyle \mathrm {pH} -\log \left[\mathrm {H_{3}O^{+}} \right]} Formally, pH is defined as the opposite of the logarithm of hydrogen ion activity: p huh log ( [ huh 3 O + ] ) {\displaystyle \mathrm {pH} -\log \left(\gamma \left[\mathrm {H_{3}O^{+}} \right]\right)} The factor γ indicates the activity coefficient and the product γ[H3O+] indicates the chemical activity of the hydroxonium ions; the activity coefficient has a value between 0 and 1 (1.00 for pure water, diluted or contaminated: less than 1). In normal circumstances the factor γ is approximately equal to 1. However, in cold seawater, for example, the factors temperature and dissolved salts do indeed play an important role (and γ < 1): neutral pH is not equal to 7.

## The autoprotolysis equilibrium and pH scale

In water or in an aqueous solution, a small part of the water molecules is present in the form of ions (autoprotolysis). Two H2O molecules then form a positive H3O+ ion and a negative OH− ion. The solubility product of the two ions in water is 10−14 mol/l (at 22 °C), i.e. for any aqueous solution, the product of the concentration of OH− and the concentration of H3O+ is always equal to 10 −14 mol/l. Assuming pure water, so much water has been split into ions that both the concentration of H3O+ and the concentration of OH− are equal to 1:10 000 000 10−7. The pH of this according to the formula is therefore: log 10 7 ( 7 ) log 10