According to Chembuddy, the formula for theoretical buffer capacity for a monoprotic buffer system is as follows:
where Cbuf is the total concentration of buffer and Kw is the water ionization constant.
However, I have conducted a lab in which I mix C4H6O4 (tartaric acid; diprotic) with its double salt NaKC4H4O4, to create a 1.0 M potassium sodium tartrate buffer solution where there are two simultaneous buffer systems. Also, pKa1 and pKa2 of tartaric acid are quite similar (around 1.6 difference), so I believe that this alternative equation that provides the theoretical buffer capacity for a system with multiple buffers:
will not be correct in my case.
Is this right? If so, is there a proper way to theoretically calculate the buffer capacity of the composite system?