Base Dissociation Constant of Ammonia

Ammonia accepts a proton from water to give ammonium and hydroxide, with K_b = 1.77 × 10⁻⁵ — almost exactly the mirror image of acetic acid's K_a, which is why the two are so often taught side by side. A 0.10 M ammonia solution reaches pH 11.13, and pK_b = 4.75 pairs with pK_a(NH₄⁺) = 9.25 through the relation pK_a + pK_b = pK_w = 14.

That pK_a of 9.25 is the number that matters in the field. It fixes the split between free ammonia NH₃, which is toxic to fish at fractions of a milligram per litre, and ammonium NH₄⁺, which is comparatively harmless: at pH 7 only 0.6 % of total ammonia is free, at pH 8 it is 5.4 %, at pH 9 it is 36 %. A warm afternoon of algal photosynthesis can lift a pond's pH by a unit and turn a safe ammonia level into a lethal one without a single extra molecule entering the water.