The aqueous solution of a mixture of a weak acid and its conjugate base and vice-versa is termed a buffer solution.
pH of buffer solution varies with the equilibrium constant of a weak acid and the ratio of weak base to weak acid in the solution.
The equilibrium constant of an acid determines the dissociation acid and its conjugate base.
Different acids have a different equilibrium constant so the amount of dissociation of ion varies.
The more ions in the solution the more acidic the buffer will be and so lower will be the pH.
The ratio of acid to its conjugate base also affects the solution of the buffer.
If the ratio of conjugate base is higher then more number of will be present in the buffer solution. so the solution will be basic and pH will be higher.
So, pH of a buffer solution varies with type of buffer.