A timing device for visually determining the passage of a preselected period of time including a redox indicator and a metal salt wherein the indicator undergoes an observable change in color which is related to the desired preselected period of time.
1. A timing device for visually determining the passage of a preselected period of time comprising:
a redox couple containing a redox indicator in combination with a reactable metal ion, said redox couple deposed within a matrix, said matrix being exposable to air such that over a period of time during exposure to air, the redox indicator changes color and thereby indicates the passage of a predetermined period of time.
2. The timing device of claim 1 wherein the matrix is a film-forming polymer.
3. The timing device of claim 2 wherein the film-forming polymer is a cellulose derivative.
4. The timing device of claim 2 wherein said matrix has a thickness based on a wet film of said polymer of from 5 to 50 mil.
5. The timing device of claim 1 wherein the matrix is attached to an adhesive tape and is adhered to a package containing a consumer product.
6. The timing device of claim 1 , wherein the redox indicator is selected from the group consisting of indigo carmine and methylene blue.
7. The timing device of claim 1 , wherein the metal ion is Sn 2+ .
8. A method for determining when a consumer product has reached an end to its useful shelflife comprising:
preparing a timing device comprising a redox couple containing a redox indicator and a reactable metal ion, said redox couple deposed within a matrix, said matrix being exposable to air;
attaching the timing device to an outside surface of the consumer product;
observing the timing device for color changes, which color changes coincide with the end of the useful shelflife of the said consumer product.