The Stability of Acid, Basic, and Direct Dyes to Light and Water

The stability of twenty different commercial dyes to visible and ultraviolet light has been studied in water. The dyes were selected on the basis of their importance from three major dye classes including: basic, acid and direct. The results are compared to the degradation of two of the dyes in natural sunlight as they would exist in the environment the dyes degraded at least 10 times as fast in artificial light as in sunlight. Some previously identified degradation products of Basic Green 4 were confirmed and a mechanism leading to their formation is proposed. The over-all results show that most commercial colors are resistant to sunlight and water degradation and that it would take many weeks before appreciable dye degradation occurred in a natural acquatic environment.