The pH scale is used to measure the acidity or the basicity of a solution where water is the solvent. Solutions with pH values that are lower than 7 are said to be acidic and those with pH values greater than 7 are basic in nature. Pure water (at a temperature of 25oC) has a pH of exactly 7, making it a neutral substance. The pH of a solution can be affected by several factors such as the common ion effect.
The pH of a solution is given by the negative logarithm (with the base 10) of the molar concentration of H+ ions in the solution. Therefore, the pH of a solution can be calculated using the following equation:
pH = -log[H+]
pH measurement is very important in several fields such as medicine, water treatment, and agronomy. The pH of seawater ranges from 7.4 to 8.4, playing an important role in the carbon cycle. However, evidence suggests that increased carbon dioxide emissions have led to ocean acidification (a continuous decrease in the pH of the Earth’s oceans).
The measurement of pH is complicated by the amphoteric nature of water. Since it dissociates into hydronium cations (H3O+) and hydroxide anions (OH–), water can behave as a weak acid and a weak base. The equilibrium reaction for the dissociation of water is given by:
2H2O ⇌ H3O+ + OH–
Note that the chemical equation for the equilibrium reaction is balanced, i.e. all the reactants and products have been assigned the appropriate stoichiometric coefficients. Click here to learn about balancing chemical equations. To learn more about the pH scale, subscribe to the BYJU’S Youtube channel and enable notifications.