Abstract
A porous anodic oxide film formed on Al was developed as a new pH-sensitive film. A porous anodic oxide film was immersed in an aqueous solution containing a pH-indicator reagent and boiled to seal it in the film. Litmus, bromthymol blue (BTB), and Congo red were used as pH-indicator reagents. The film, containing litmus, changed color reversibly within 1 s as a function of the test solution pH. The film showed superior durability in the solution at pH of 1−12 because of a stable boehmite layer covering the surface within that range. From the response of color change as a function of sealing time, it was inferred that the color change occurred mostly at the film’s surface region. For example, the film containing BTB prepared with long sealing time appeared to be green in an alkaline solution because of mixing of blue at the surface and yellow inside the film. Optimization of film thickness, the choice of pH-indicator reagent, and sealing conditions are important to prepare a highly pH-sensitive film.
