What exactly do we mean when we use term titratable acid in renal physiology? Why is ammonium not considered a titratable acid?
Titratable acidity is a total amount of acid in the solution as determined by the titration using a standard solution of sodium hydroxide (titrant). In other words, it is the maximal capacity of the solution to "neutralise" base solution.
H$^+$ produced from CO$_2$ and H$_2$O is actively transported into the distal tubular lumen. Titratable acidity is the amount of the H+ which is buffered by bases present in the tubular lumen. Mostly by phosphate (the highest concentration).
When the urinary pH change other molecules can gain the ability to buffer H$^+$. For example creatinine (pKa about 5.0) or ketoacids (pKa about 4.8) may contribute to the titratable acidity.
Standard measurement of the titratable acidity with the standard solution of sodium hydroxide spans pH range up to 7.4. Therefore:
Ammonium is not measured as part of the titratable acidity because the high pK of ammonium means no H$^+$ is removed from NH4$^+$ during titration to a pH of 7.4.