Data in the image below shows a variable DME reacting to the increase in acid concentration. It looks like it has a very fast increase from when the concentration is raised from 0 to 0.5 and then a rapid diminishing return.
I thought that a base 10 log function would be a good approximation for this based on the figure below (I admit, the pH scale was an inspiration even though the fact it is base 10 is not relevant here as far as I can tell).
This and other bases for log did not yield good fits. Any ideas about a good functional form to use?