Calcul des changements de pH dans une solution tampon

First, use stoichiometric calculations to determine how the concentrations of the weak acid and conjugate base change when strong acid or base is added. Second, apply an equilibrium calculation using the henderson hasselbalch equation calculating buffers or an ICE table to find the new pH. This two-step approach accounts for both the neutralization reaction and the resulting equilibrium state.

When the concentrations of a weak acid and its conjugate base are equal, the ratio term in the Henderson-Hasselbalch equation equals one, and the logarithm of one is zero. This means pH = pKa + 0, so pH equals pKa. This relationship holds regardless of the absolute concentrations, as long as they are equal.

Adding strong acid to a buffer causes a small pH decrease because the conjugate base ions neutralize the added H+ ions, consuming buffer capacity. In contrast, adding the same amount of strong acid to pure water causes a dramatic pH drop. The buffer's ability to resist pH change demonstrates its effectiveness at maintaining relatively stable pH within its buffering capacity.

When strong base is added, the hydroxide ions react with the weak acid component, decreasing its concentration while increasing the conjugate base concentration. For example, adding 0.1 moles of NaOH to a buffer decreases the weak acid by 0.1 moles and increases the conjugate base by 0.1 moles. These new concentrations are then used to calculate the resulting pH.

The Henderson-Hasselbalch equation is valid when buffer component concentrations are high relative to the weak acid's Ka value and when concentration changes are less than 5%. Under these conditions, the approximation that equilibrium concentrations equal initial concentrations minus the change is accurate, making the equation reliable for quick pH calculations.

First, determine that the added H+ ions are neutralized by the conjugate base, decreasing its concentration by 0.2 moles and increasing the weak acid concentration by 0.2 moles. Then substitute these new concentrations into the Henderson-Hasselbalch equation to calculate the final pH. The resulting pH will be lower than the initial pH but still relatively stable due to buffering.

Buffer capacity determines how much acid or base a buffer can neutralize before significant pH change occurs. Solutions with higher concentrations of weak acid and conjugate base have greater capacity to resist pH changes. When buffer capacity is exceeded, pH changes become more dramatic, and the buffer's protective effect diminishes.