In photographic emulsion manufacture, nitric acid plays a role in the preparation of silver nitrate - the fundamental light sensitive silver salt from which all silver halide photographic emulsions are derived. Silver nitrate is produced by dissolving metallic silver in nitric acid, a reaction that converts the silver metal into silver ions in solution that can then be reacted with halide salts to precipitate the silver halide crystals that form the basis of the photographic emulsion. The purity and concentration of the nitric acid used in this process is critically important to the quality and consistency of the resulting silver nitrate and the emulsion produced from it.
In photographic toning and bleaching applications, dilute nitric acid solutions have been used as components of certain formulations to control the pH of the working solution, to activate or moderate the action of other chemical components, or to serve as a mild bleaching agent for silver images. Its strong oxidising properties make nitric acid an effective bleaching agent capable of attacking and dissolving metallic silver, which is exploited in certain intensification and reduction processes where controlled removal or conversion of silver image density is required.
The handling of nitric acid in any photographic or chemical context demands the most stringent attention to health and safety precautions, as it is among the most hazardous chemicals encountered in photographic chemistry. Concentrated nitric acid is intensely corrosive to all biological tissue, causing severe and deep chemical burns on contact with skin, eyes, or mucous membranes that can be slow to heal and may cause permanent damage. It reacts violently with organic materials, many metals, and a wide range of other chemicals, and produces highly toxic nitrogen dioxide gas - a reddish brown fume with a sharp, acrid odour - both during its own decomposition and when it reacts with metals and organic materials. Inhalation of nitrogen dioxide fumes even at relatively low concentrations can cause serious pulmonary damage, and exposure to high concentrations can be rapidly fatal.
Nitric acid must always be handled in a well ventilated environment or fume hood, using appropriate personal protective equipment including acid resistant gloves, eye protection or face shield, and protective clothing. It should be stored in tightly sealed, chemically resistant containers away from organic materials, flammables, and incompatible chemicals, and any spills must be dealt with immediately using appropriate neutralizing agents and protective equipment. The disposal of nitric acid waste must comply with applicable hazardous waste regulations, and it should never be disposed of through normal drainage without appropriate neutralization and dilution.