The alternate forms of the equations are possible because of the rules of logs, one of which states that taking the inverse of a fraction within a log reverses the sign of the log.
In cases where polyprotic acids are titrated, the titration curve has a different shape. When a base such as is added to the beginning solution of a titration, ions are produced. As an acidic solution is added, the sulfate ions react with the hydronium ions to form . Eventually, the ions react with hydronium ions to form sulfuric acid. The titration curve reflects these different stages of the reaction. In this titration, two sudden changes of pH are reflected in the curve. The first pH change reflects the presence of ions; the second change reflects the presence of
. In applying the Henderson-Hasselbach equation to this titration, the pKa of is used in the area of the first equivalence point, while the of must be used after the first equivalence point.
