Aqueous solutions of acids have a sharp taste, turn litmus red, liberate CO 2 , form a metallic carbonate, and evolve hydrogen in reaction with certain metals, e.g., iron.
The 'strength' of an acid is measured by the value of its dissociation constant, a strong acid such as hydrochloric being substantially fully ionized in solution, and a weak acid such as formic being predominantly unionized.
Acids, and particularly the inorganic acids (because of their corrosiveness and low volatility), are harmful to paper and bookbinding materials. Their presence weakens the holding power of the individual links of the cellulose chains of paper, causing brittleness; results in corrosive effects in some inks; and weakens the fibers of leather. The source of acids in archival materials may be intrinsic or extrinsic. They may be present in the materials used in the manufacture of paper, adhesives, leather, etc., and may be left in intentionally, e.g., alum-rosin sizing; they may be introduced during manufacture and not sufficiently removed, e g., acids used in clearing and/or dyeing leather; or they may gain access during storage, e.g., sulfuric acid in paper or leather, resulting from the atmospheric pollutant, sulfur dioxide (SO 2 ). See also: ACID GASES . (72 , 195 , 198 , 306 )