An indicator chemical is a neutral substance added to a solution to reveal certain properties, such as pH level or the presence of residual fixer (hypo) in photographic processing. It changes colour when specific chemical conditions are met, allowing photographers and chemists to easily assess the composition of a solution.
In darkroom photography, indicator chemicals are particularly useful for checking that film or paper has been thoroughly fixed and washed, preventing image deterioration over time. Understanding how to use indicators helps photographers maintain consistent results and ensures the long-term stability of both black-and-white and colour photographs.