How Is Acid Formed?

Definitions of Acids

• Bronsted and Lowry defined an acid as a species that donates a proton. In water, acid will dissociate to form a positive ion and a negative ion. A strong acid, such as hydrogen chloride, will strongly dissociate into a hydrogen ion and a chloride ion. The chloride ion is very stable, which means formation of the positively charged hydrogen ion, or proton, is favored: Hydrogen chloride donates its proton and the resultant aqueous solution is called hydrochloric acid. In this solution the positive ion is called the conjugate acid, and the negative ion the conjugate base.

Measuring Acid Strength by Dissociation

• Acid strength is commonly measured in two ways: the degree of dissociation, and the concentration of hydrogen ions in the solution. The degree of dissociation of an acidic solution is defined as the product of the concentrations of the conjugate acid with the conjugate base, divided by the concentration of the undissociated molecule. This is called the Ostwald dilution law, with ratio denoted by the letter "K". Acids with a large K are strong and those with a small K are weak.

Measuring Acid Strength by Hydrogen Ion Concentration

• Hydrogen ion concentration is a more common measure of acid strength. A strong acid will have a higher concentration of hydrogen ions than a weak acid, with hydrochloric acid having a concentration around 10^-1 moles per liter. A measure of pH is used to remove awkward negative numbers and is defined as the negative logarithm of hydrogen ion concentration. Therefore, hydrogen chloride has a pH of 1. Bases have much lower hydrogen ion concentrations, which sodium hydroxide having a pH of around 13.

The Importance of Organic Acids

• Organic acids are very important in industrial processes. Short-chained carboxylic acids such as acetic acid are formed by the oxidation of alcohols; these are important chemical intermediates. The sodium and potassium longer-chained acids such as stearic and oleic acid are used as soaps and detergents. Organic acids are also common in biological systems. Proteins are made from 20 amino acids which feature both a carboxylic and amine groups; while DNA is a long-chain acid with an organophosphoric backbone.