The principal site of acidification of urine is

The cells of the proximal and distal tubules secrete hydrogen ions. Acidification also occurs in the collecting ducts. The reaction that is primarily responsible for H+ secretion in the proximal tubules is Na+-H+ exchange. Na is absorbed from the lumen of the tubule and H is excreted. Ref: guyton and hall textbook of medical physiology 12 edition page number:504,505,506