# pH

## Explanation

pH is a quantitative scale used in chemistry and biology to measure the acidity and basicity, or more generally, the strength of hydronium ions in aqueous solutions. Mathematically, it is the logarithmic measure of the hydronium ion concentration of the solution, generally expressed as pH = -log[H+]. At standard temperature and pressure, the concentration of hydronium ions and hydroxide ions in the pure water is equal and is around 1 × 10-7 mol/l. Using the mathematical formula, the pH will equate to 7 for pure water, which is considered as the neutral pH value. By this definition, if a solution has more hydronium ions than the hydroxide concentration, the solution is acidic and the pH value drops. Similarly, if the solution has less hydronium ions than the hydroxide concentration, the solution is basic and the pH value rises. In modern calculations of pH, the thermodynamic activity of the substances in the solution is also taken into account. The above formula becomes pH = -log[aH+], where a represents the thermodynamic activity of the chemical substance.

## Frequently Asked Questions

### Why does water have hydronium ions?

Hydronium and hydroxide ions occur naturally in the water due to the self-ionization process. In water, some of the molecules attract a hydrogen atom from the neighbouring water molecule and ionize themselves to form a hydronium ion, leaving a hydroxide ion.

 H2O + H2O ⇌ H3O+ + OH−

The self-ionization is due to the polar nature of the water molecule, which rarely leads to the ionization of two molecules in each other's vicinity. Thus pure water always has a certain number of hydronium ions and an equal number of hydroxide ions in it.

### How is the pH of a solution measured?

The pH of a solution can be measured by using a pH indicator like a litmus paper. A litmus paper dyed blue will change its colour to red when exposed to an acidic solution. Similarly, a litmus paper dyed red will change its colour to blue when exposed to a basic solution. The litmus will be purple when a neutral solution is used. For industrial and laboratory experiments, a pH meter is used.

### Why is pH important in biology?

One reason biologists follow pH is because it can have a tremendous influence on the activity of proteins, including enzymes. Since pH is a measure of the concentration of proteins [H+], it is conceptually a measure of the amount of 'floating charges'. The amino acid histidine readily adds or loses protons onto its side chain in the pH range normal for the blood and body. Thus slight changes in pH can alter the fraction of histidines that are positively charged vs. net neutral. One example where this is the oxygen carrying molecule hemoglobin. In the lungs, the pH is slightly higher than it is in the muscles and tissues; this results in a greater likelihood of certain histidines to be positively charged in the tissues, which in turn drives hemoglobin molecules toward their oxygen releasing state.