###### Abstract
The second step of the degradation involves the acetic acid produced in the first step: a sufficiently high concentration of acetic acid will start an autocatalytic breakage process of the glucose-acetate ester bond. This process will become autocatalytic, resulting in a rapid degradation of the film once threshold concentrations of acetic acid are reached [10; 11; 12]. If the cellulose acetate film is put in a storage where there is already a high concentration of acid vapors (i.e., the presence of already degraded cellulose acetate films), the degradation process can immediately move to the second phase, with a very high degradation speed. It is thus very important to frequently monitor both the degradation state of the cinematographic films and their possible acetic acid contamination due to storage near already degraded films. The current recognized technique to measure acetic acid produced during film degradation is by using A-D strips. These are small paper strips, soaked with an acid-base indicator, which shift in color from blue, through shades of green and finally yellow in the presence of increasing amount of acetic acid [13; 14]. Although these strips allow a rough evaluation of the degradation state of the film, they depend strongly on the operator interpretation (acetic acid concentration is calculated by comparing A-D strip color to a provided reference) and make it difficult to monitor the ongoing conditions of very large film collections. In this work, we present an alternative, innovative solution for monitoring the conditions of cellulose-based films by quantifying the acetic acid released during degradation. A metal oxide semiconductor (MOS) sensors duplex has been assembled, testing different commercially available resistive MOS sensors, optimizing the sensor choice in order to create a sensing duplex capable of selectively quantifying acetic acid coming from cellulose acetate cinematographic films degradation. MOS-type sensors were chosen since numerous literature sources reports them as having low limit of detection, being quite selective for their target gas(es) and having extremely low energy and space requirements comparedto other detection tools having similar analytical properties [15; 16; 17]. These last points are of particular interest for the case discussed here, since the developed sensing solution aims to be integrated directly into old cinematographic film boxes in order to provide real-time estimation of the undergoing degradation. The sensor duplex was tested first against known concentrations of acetic acid and with the main secondary released vapor, as found with SPME-GC/MS measurements, and finally with real cinematographic film samples. The results were validated by comparison with those obtained employing the official degradation level determination system for cellulose acetate cinematographic films, A-D strips. Finally, software was written to automatically acquire data from the sensing array, display them both in numerical and graphical form, and perform calculations to determine the acetic acid concentration regardless of interferens. The obtained system can thus remotely monitor the degradation level of cellulose acetate-based films by quantifying the acetic acid present in the film box, allowing for sensitive (30 ppb detection limit vs. 500 ppb of A-D strips), fast (10-min incubation in the box vs. 24+ hours with A-D strips) and safe (no exposure of human operator to gasses accumulated in the box during storage) cinematographic films monitoring and preservation.