In photographic processing, a preservative is a chemical added to developing solutions to prevent the rapid oxidation of the reducing agents that are responsible for forming the image. Sodium sulfite is a common example, helping to maintain the activity and stability of the developer over time.
Using a preservative ensures consistent development, prolongs the usable life of chemical solutions, and prevents degradation that can lead to uneven contrast or tonal shifts in black-and-white and colour photography. Proper use of preservatives is essential for reliable darkroom results and high-quality prints.