Thiosulfate levels for archival film are measured in which units?

• A. mg/cm2
• B. mg/mL
• C. g/cm2
• D. ug/cm2

Answer: (A)

The amount of thiosulfate on archival film is expressed as mass per area because the concern is how much fixer residue remains on each unit area of the film’s surface. Measuring in milligrams per square centimeter directly ties to the film’s two‑dimensional surface and provides a practical, standard way to assess residue across different film sizes. This surface-based metric helps ensure archival quality, since too much thiosulfate on the emulsion can affect long-term image stability.

Using other units isn’t as appropriate here: mg/mL would describe a bath concentration rather than residue on the film, g/cm^2 implies a much larger quantity per area than is typical, and ug/cm^2 would be an impractically small scale for standard QC thresholds.