The Henrys law constants mentioned so far do not consider any chemical equilibria in the aqueous phase. This type is called the intrinsic, or physical, Henrys law constant. For example, the intrinsic Henry's law solubility constant of formaldehyde can be defined as
In aqueous solution, formaldehyde is almost completely hydrated:
The total concentration of dissolved formaldehyde is
Taking this equilibrium into account, an effective Henry's law constant  can be defined as
For acids and bases, the effective Henrys law constant is not a useful quantity because it depends on the pH of the solution. In order to obtain a pH-independent constant, the product of the intrinsic Henrys law constant  and the acidity constant  is often used for strong acids like hydrochloric acid (HCl):
Although  is usually also called a Henrys law constant, it is a different quantity and it has different units than .