The self-ionization constant of water, Kw, changes with temperature, pressure, and ionic strength. Kw is also used as cologarithm pKw. If you are not aware, X

The self-ionization constant of water, Kw, changes with temperature, pressure, and ionic strength. Kw is also used as cologarithm pKw. If you are not aware, X = 10^(-pX), and pX = -log(X). So a lower pH is a greater concentration of [H+] (actually [H3O+]), a lower pOH is a greater concentration of [OH-], and a lower pKw is a greater self-ionization of water. pKw decreases with increasing temperature. It decreases with increasing pressure. And it decreases with increasing ionic strength until around 0.6-0.8, then goes back up. While those are held constant, a dropping pH does indicate acidification. pH + pOH = pKw . But if they are not constant, pOH can drop at the same rate as pH, because pKw is also dropping. As long as pH and pOH are both near pKw/2, the solution is neutral. The solution is only acidifying when pH is decreasing faster than pOH. If you become slightly less basic, your pOH goes up. Under normal circumstances, that means pH is going down by an equal amount. Either way you describe it in English, de-alkalinization usually yields the same mathematical results as acidification. Deviation from this rule, which holds at standard temperature and pressure, and constant ionic strength, requires additional explanation.