Extension of Henderson-Hasselbalch

When does the Henderson-Hasselbalch equation break down and what would be the correct equation to follow?

I read "The Henderson–Hasselbalch Equation: Its History and Limitations" (J. Chem. Educ., 2001, 78 (11), p 1499) and it seems to state that I need to solve to iteratively solve [H+] = Ka{MA – [H+] + [OH ]}/{MB + [H+] – [OH ]} (

What is that equation called? I would like to look up examples of it being solved. Thanks