What is the Difference Between pH and pKa?

The main difference between pH and pKa lies in their definitions and what they represent in the context of acidity and basicity. Here are the key distinctions:

1. pH:

• Represents the acidity or basicity of a system, specifically the concentration of hydronium ions (H₃O⁺) in an aqueous solution.
• Is calculated using the logarithmic formula: $$pH = -\log[\text{H}_3\text{O}^+]$$.
• A lower pH indicates acidity, while a higher pH indicates basicity.

1. pKa:

• Represents the acid dissociation constant of a compound, specifically the negative logarithm of the acid dissociation constant (Ka).
• Is calculated using the logarithmic formula: $$pKa = -\log{Ka}$$.
• A lower pKa value indicates a stronger acid, while a higher pKa value indicates a weaker acid.

Another key difference is that pH is a measure of the hydronium ion concentration in a particular aqueous solution, while pKa is an inherent property of a compound or functional group. Additionally, pH and pKa values are not equal unless at the half equivalence point when the moles of the weak acid are equal to the moles of the conjugate base.

Comparative Table: pH vs pKa

The main difference between pH and pKa is that pH is a measure of the acidity or alkalinity of a solution, while pKa is used to show the strength of an acid. Here is a table summarizing their differences:

In summary, pH measures the acidity or alkalinity of a solution, while pKa indicates the strength of an acid. pH and pKa are related in buffer solutions through the Henderson-Hasselbalch equation.