Limiting pH in humans is:

The amount of acid secreted depends upon the subsequent events in the tubular urine. The maximal H+ gradient against which the transpo mechanisms can secrete in humans corresponds to a urine pH of about 4.5; that is, an H+ concentration in the urine that is 1000 times the concentration in plasma. pH 4.5 is thus the limiting pH. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 40.