ACIDS and BASES

An Acid is a substance that releases protons when dissolved in water to form the hydronium ion H₃O⁺. The higher the concentration of H₃O⁺ the more acidic the solution.

A base is simply a substance which raises the concentration of hydroxyl (OH^-) ions. For example, NH₂ groups can generate OH^- ions by taking a taking a proton from water. The OH^- ion can then combine with H₃O⁺ to form 2 water molecules. This reduces the concentration of H₃O⁺ making the solution less acidic.

In a neutral solution, the [H⁺] = [OH^-] = 10^-7M. Because [H⁺] is small in an aqueous solution, [H⁺] is often expressed in terms of pH which is defined as the negative logarithm in base 10 of [H⁺]. Thus the pH of a neutral solution would be -log 10^-7 or 7. Thus an acidic solution is one where the pH is less than 7 and a basic solution is one where the pH is greater than 7.

Buffers are solutions that resist large changes in pH when small amounts of acid or base are added. Maximal buffering capacity occurs at a pH = to the pK_a. At a pH unit above or below the pK_a the buffering capacity is only 10%.

Henderson-Hasselback Equation expressed the relationship between pH and ratio of base to acid.

pH = pK_a + Log [(base)/(acid)

Thus could determine the pH for a solution of 0.1 M acetic acid and 0.2 M acetate ion where the pKa of acetic acid is 4.8.  pH = 4.8 _ log (0.2/0.1) = 4.8 + log 2.0 = 4.8 + 0.3 = 5.1