Acid Base Reactions

Acid-base reactions affect pH (the concentration of hydrogen ions in solution), which is a controlling factor in the type and rate of many other chemical reactions.

Acids dissociate in solution yielding hydrogen ions and anions according to the general reaction

The ionization is reversible. The anion (acting as a weak base) can recombine with the hydrogen ion to reform neutral HA. Both reactions occur continuously in solution, with the extent of ioniza-tion dependent on the strength of the acid. Strong acids, such as HCl, ionize completely in dilute aqueous solution. Thus a 0.01 molar (10-2 molar) solution has a pH of 2. Weak acids, such as acetic and other organic acids, ionize only slightly in solution and form solutions with pH from 4 to 6.

In the above example, the anion (A) functions as a base when it combines with a hydrogen ion. (By definition, any substance that combines with hydrogen ions is a base. Like strong acids, strong bases ionize completely in a dilute aqueous solution.) Thus NaOH dissolves in water to form hydroxide ions, which in turn function as a base when they combine with hydrogen ions to form water, as shown by the general equations

Strong acids (those that ionize completely in solution) are more likely to dissolve solids because charged particles such as hydrogen ions will interact more strongly with solids than will neutral particles. Weak acids do not readily donate hydrogen ions and consequently remain mostly in the neutral form. As a result, weak acids do not dissolve solids as readily as strong acids.

Strong bases (those that most readily extract hydrogen ions from solution) are also found predominantly in ionic forms and are similarly more reactive with solids than weak bases, which remain mostly in neutral form. The extent to which any base will extract hydrogen ions from solution depends on pH and the strength of the base.Acid-base reactions occur quickly. When the pH of a solution changes, acids and bases readily attain a new equilibrium between neutral and ionic forms. Because toxic organics almost always exist in very low concentrations and tend to be weak acids or weak bases, they have little, if any, influence on the pH of water. Acid-base equilibrium reactions involving hazardous organic compounds do not affect the toxicity of the waste and, as noted above, do not strongly influence pH.

When weak acids and bases ionize in wastestreams, pH is affected very little, but when strong acids and bases ionize in wastestreams, pH is affected dramatically. By definition, wastestreams having a pH < 2 (highly acidic) or a pH > 12.5 (strongly basic) are highly corrosive and are regulated as hazardous. Acid-base reactions can neutralize acidic or basic hazardous waste by raising or lowering its pH.

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