To account for this, Bronsted and Lowry redefined acids and bases an acid releases a hydrogen ion or proton (equivalent to H+) and a base accepts a hydrogen ion or proton ⁴. However, there are some substances that fit the operational definition (altering pH), without fitting the Arrhenius definition (releasing an ion). To further define these substances, Arrhenius determined in 1884 that an acid will release a hydrogen ion (H+) as it dissolves in water, and a base will release a hydroxyl ion (OH-) in water ⁴. In the same manner, a base is a substance that will increase the pH of water ⁴. Due to this influence, H+ and OH- are related to the basic definitions of acids and bases.Īcids and Bases Acid-base pairs can neutralize each other like H+ and OH- do in this equation.Īs an operational definition, an acid is a substance that will decrease pH when added to pure water. Thus the ions H+ and OH- are always paired – as the concentration of one increases, the other will decrease regardless of pH, the sum of the ions will always equal 10⁻¹⁴ M ². At a neutral pH of 7 (pure water), the concentration of both H+ ions and OH- ions is 10⁻⁷ M. The higher the H+ concentration, the lower the pH, and the higher the OH- concentration, the higher the pH. This determination is due to the effect of hydrogen ions (H+) and hydroxyl ions (OH-) on pH. For example, if a solution has a H+ concentration of 10 -3 M, the pH of the solution will be -log(10 -3), which equals 3. This is done by taking the negative logarithm of the H+ concentration (-log(H+)). The numerical value of pH is determined by the molar concentration of hydrogen ions (H+) ³. At a pH of 7, this decreases to 0.0000001 M. Thus at a pH of 0, H+ has a concentration of 1 M. The logarithmic scale of pH means that as pH increases, the H+ concentration will decrease by a power of 10. Likewise, when counting up above 7, each number is 10 times more basic than the previous number ². The logarithmic scale means that each number below 7 is 10 times more acidic than the previous number when counting down. The higher the number, the more basic it is. The lower the number, the more acidic the water is. Instead, it is a figure between 0 and 14 defining how acidic or basic a body of water is along a logarithmic scale ¹. This means that pH of water is not a physical parameter that can be measured as a concentration or in a quantity. PH is a determined value based on a defined scale, similar to temperature.