The unit used to express thiosulfate levels in fixer retention testing is measured in which unit?

• A. mg/cm2
• B. mg/cm3
• C. g/cm2
• D. μg/cm2

Answer: (A)

The measurement is about how much thiosulfate remains per unit area of film or processed surface. Expressing this as milligrams per square centimeter standardizes the amount of residue relative to the surface area involved, so results are comparable across different film sizes and processing setups. Using a concentration in the fixer's volume (mg/cm^3) would describe how concentrated the fixer is, not how much residue is retained on the film itself. A per-area unit like mg/cm^2 is the appropriate way to quantify fixer retention because it reflects surface contamination, which is what the test is assessing.