Which chemical serves as a preservative to prolong the longevity of developer and fixer solutions?

Sodium sulfite is the chemical that serves as a preservative in both developer and fixer solutions. Its main function is to prevent the oxidation of the chemicals in these solutions, which can lead to a decrease in effectiveness and a shorter lifespan. By inhibiting oxidation, sodium sulfite helps maintain the activity of the developer, ensuring that it can effectively reduce silver halide crystals to form the image. It also helps preserve the fixer by preventing the premature breakdown of the fixing agents.

The other chemicals listed do not function as preservatives in this context. While sodium hyposulfate is commonly associated with fixing agents, it does not serve the same preservative role as sodium sulfite. Sodium nitrate is primarily used as a fertilizer and does not have applications in photographic chemistry as a preservative. Potassium alum, on the other hand, is used in various applications including as a coagulant in water treatment and for preserving some types of food, but it is not utilized as a preservative in developer or fixer solutions. Thus, sodium sulfite is the correct choice for prolonging the longevity of developer and fixer solutions in dental radiography.