All chemical reactions tend toward a state of chemical equilibrium.
The times necessary to reach equilibrium range from almost instantaneously to extremely long. A chemical reaction in a state of equilibrium can be characterized by an equilibrium constant, i.e. the numerical value of the mass-action expression. Strictly speaking, K is a constant only if activities are used. Ignoring the activity effect will account for some of the small variations in the K values. Rinse with deionized water and allow to drain.