A colorimetric method to quantify acetic acid emissions from cellulose acetate motion picture films

Vinegar Syndrome (VS) is the typical chemical decay process resulting from the hydrolytic deacetylation of the cellulose acetate polymer, which leads to the emission of acetic acid. In archives storing cellulose acetate motion picture films, acetic acid detection strips are widely used to monitor VS progression; however, the interpretation of their colour changes remains qualitative and subjective, potentially leading to errors in the estimation of acetic acid concentrations and, consequently, of the VS stage. This study proposes an objective and quantitative method to assess strip colour changes and relate them to the acetic acid concentrations released by films affected by VS. The two types of detection strips most widely used in film archives worldwide were investigated. In laboratory experiments, strip colour was measured by spectrophotocolorimetry before and after exposure to known acetic acid concentrations, using the hue angle value as the metric. The same approach was then applied at two national film archives to test its feasibility. In the case studies, strip colour was interpreted together with metadata relating to the inspected film reels in order to explore which intrinsic and extrinsic factors may influence VS progression. The laboratory results demonstrate a sigmoidal relationship between hue angle values and acetic acid concentrations for both strip types, while revealing differences in their sensitivity, particularly in the autocatalytic concentration range of VS. The case-study results suggest that reel-specific manufacturing features and environmental and use history may influence single-reel VS progression more strongly than the current storage environment. The proposed method can be routinely implemented by archival staff in less than five minutes per film reel, including both strip colorimetric measurements and transcription of the resulting parameters into a database. This allows the creation of a traceable record of this information over time, which can be integrated with other routinely collected data.