Pure water contains, in addition to H2O molecules, a very small proportion of electrically charged ions formed by electrolytic dissociation, according to the equation
Where H + is a hydrogen ion, hydrion or proton, produced from a hydrogen atom by the loss of an electron and hence carrying positive charge; and OH - is a hydroxyl ion, which is negatively charged.
In pure water the two ions are present in equal numbers, since the water is electrically neutral, and at 250C. their concentration is 10-7 gm-ions per litre. It can be shown (from the law of mass action) that, in any dilute aqueous solution, the product of concentrations of the hydrogen ions and hydroxyl ions is constant and is equal to 10-14 gm-ions per litre. i.e.,
[H +] X [OH -] = 1.0 X 10-14
When an acid is added to water it introduces hydrogen ions. Hydrochloric acid, for example, introduces hydrogen ions and chloride ions through electrolytic dissociation
Consequently, hydroxyl ions in the water are reduced so that the product of the hydrogen ion concentration and hydroxyl ion concentration remains equal to 10-14. For example, if the concentrations of hydrogen ions is increased to 10-3, the concentration of hydroxyl ions must fall to 10-11. Similarly, alkalies such as caustic soda introduce hydroxyl ions.
and so cause a reduction in the number of hydrogen ions present in the water. Solutions are acid when the number of hydrogen ions exceeds the number of hydroxyl ions, and alkaline when the hydroxyl ions are in excess of the hydrogen ions. The degree of acidity or alkalinity depends entirely on the relative proportions of the two ions.
In pure water at 250C, the hydrogen ion concentration (H +) = 10-7gm-ions per litre, or –log10 (H+) = 7. The pH value of water is therefore 7, the relation between pH and hydrogen ion concentration being expressed by the equation –