ヘンダーソン-ハッセルバルヒ式

The pH of a buffered solution containing a conjugate acid-base pair may be calculated using the Henderson-Hasselbalch equation as an alternative to an ICE table. The Henderson-Hasselbalch equation is derived from the equilibrium constant expression for Ka. This expression can be rearranged to determine the hydronium ion concentration. If the negative log of both sides is taken, the negative logarithm of the hydronium ion concentration and the negative logarithm of the acid dissociation constant can be replaced by the pH and the pKa, respectively. This yields an equation where the pH of a buffer can be calculated by adding the pKa and the log of the equilibrium concentrations of a conjugate base over its weak acid. These equilibrium values can be replaced by the initial concentrations if the change in the  hydronium ion concentration, x, is less than the 5% of the initial concentrations of both the weak acid and the conjugate base. The Henderson-Hasselbalch equation also shows the ratio of base to acid needed to prepare a buffer at a specific pH. Similarly, the pH of a solution containing a weak base and its conjugate acid can be determined using this equation by calculating the pKa of the conjugate acid from the pKb using the formula: pKa plus pKb is equal to fourteen. The pH of a buffer containing 0.15 molar formic acid and 0.18 molar sodium formate can be determined using either the Henderson-Hasselbalch equation or the Ka for formic acid and the ICE table. However, the Henderson-Hasselbalch equation is a quicker method to calculate the pH when a reaction involves a conjugate acid-base pair and the change in hydronium concentration is small. The pKa is determined by taking the negative logarithm of the Ka for formic acid, which equals 3.75. When the initial concentrations of formic acid and formate are plugged into the equation, the pH value for the solution equals 3.83. This pH value can be used to determine the hydronium ion concentration, 1.5 × 10−4. As this value is less than 5% of 0.15 molar formic acid, the approximations needed to use the Henderson-Hasselbalch equation are valid.