A novel depletion indicator (time watch) for removable substances containing acid or basic groups is disclosed, together with a novel method of use therefor. The removable substance of a predetermined concentration comes into contact with a suitable pH color indicator that can change color at a predetermined pH range within a much larger measurable pH range, which color change occurs as the concentration of the removable substance diminishes by passing through a permeable film containing both the removable substance and the pH color indicator.The color change occurs at a predetermined time, e.g., the same time that the active ingredient has diminished to the point it is determined to be inactive.