This broad definition of acids and bases includes many substances that are not usuallyĬonsidered acidic or basic. ![]() In aqueous systems, the addition or removal of hydrogen ions is best understood in terms of the Brönsted−Lowry concept of acids and bases, propounded in 1923.Ī Brönsted−Lowry acid is defined as a substance that can donate a proton (H+) conversely, a Brönsted−Lowry base is a substance that can accept a proton.Ī proton donor (i.e., an acid) and its corresponding proton acceptor (i.e., a base) make up a conjugate (coniungereL = to join together) acid−base pair. (credit: modification of work by Edward Stevens) Brönsted−Lowry concept of Acids and Bases Solutions having a pH lesser than 7 are acidic, i.e., the concentration of H+ is greater than that of OH−.Ĭonversely, solutions having a pH more than 7 are basic or alkaline, i.e., denote an excess of OH− over H+ The pH scale measures hydrogen ions’ (H +) concentration in a solution. The pH scale runs from 1 to 14 neutrality being at pH 7.0. The range of the pH scale is from 0 to 14. For each molecule of water dissociated, there is one H+ and one OH−, each one neutralizing the other.Ī neutral solution, such as water, where the number of hydrogen ions is balanced by the same number of hydroxyl ions, has a pH of 7.0. Water is a neutral solution because each molecule contains one H+ and one OH−. The alkalinity of a solution is dependent upon the number of hydroxyl ions present. The number of free hydrogen ions is a measure of its acidity rather than an indication of the type of molecule from which the hydrogen ions originated. Strong acids dissociate more freely than weak acids hydrochloric acid, for example, dissociates freely into H+ and Cl− whereas carbonic acid, a weak acid, dissociates much less freely into H+ and CO3–. The hydrogen ion concentration is a measure, therefore, of the amount of dissociated acid rather than of the an amount of acid present. All acids do not ionize completely when dissolved in water, i.e., all the molecules of acid do not ionize and exist in the solution as electrically-charged particles. ![]() The number of hydrogen ions present in a solution is a measure of the acidity of the solution. ![]()
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