Henderson–Hasselbalch equation

In chemistry and biochemistry, the Henderson–Hasselbalch equation In chemistry and biochemistry, the Henderson–Hasselbalch equation can be used to estimate the pH of a buffer solution. The numerical value of the acid dissociation constant, Ka, of the acid is known or assumed. The pH is calculated for given values of the concentrations of the acid, HA and of a salt, MA, of its conjugate base, A-; for example, the solution may contain aceticacid and sodium acetate. Lawrence Joseph Henderson derived an equation with which the pH of a buffer solution may be calculated. Later, Karl Albert Hasselbalch re-expressed that formula in logarithmic terms, resulting in the Henderson–Hasselbalch equation. A simple buffer solution consists of a solution of an acid and a salt of the conjugate base of the acid. For example, the acid may be acetic acid and the salt may be sodium acetate.The Henderson–Hasselbalch equation relates the pH of a solution containing a mixture of the two components to the acid dissociation constant, Ka, and the concentrations of the species in solution. To derive the equation a number of simplifying assumptions have to be made. Assumption 1: The acid is monobasic and dissociates according to the equation It is understood that the symbol H+ stands for the hydrated hydronium ion. The Henderson–Hasselbalch equation can be applied to a polybasic acid only if its consecutive pK values differ by at least 3. Phosphoric acid is such an acid. Assumption 2. The self-ionization of water can be ignored. This assumption is not valid with pH values more than about 10. For such instances the mass-balance equation for hydrogen must be extended to take account of the self-ionization of water. and the pH will have to be found by solving the two mass-balance equations simultaneously for the two unknowns, and .