PR 9 – naphthol red AS

Inexpensive, widely used in printing inks but very fugitive

Manufacturer/ product code:  labtek
Chemical Description: bromoeosine
Color: red
Color Index: PR 9
Date:  from 1922

PR 9 – Naphthol Red AS is a synthetic organic pigment belonging to the monoazo naphthol AS family, a group of pigments developed in the early 20th century and widely used for their strong, warm red hues and relatively good opacity. PR 9 is characterized by a bright, slightly bluish to neutral red, occupying a chromatic space between scarlet and crimson, with a clean and direct color appearance.

Chemically, PR 9 is formed by coupling a diazonium salt with a naphthol AS component, producing an azo (-N=N-) chromophore stabilized by the naphthol structure. Compared to earlier azo pigments such as Toluidine Red (PR 3), Naphthol Red AS generally exhibits improved lightfastness and solvent resistance, though it still falls short of the performance offered by modern high-performance pigments like quinacridones and pyrroles.

In practical applications, PR 9 offers good opacity and moderate to strong tinting strength, making it suitable for printing inks, coatings, plastics, and rubber. In artists’ materials, it has been used in oil and alkyd paints, where its covering power and strong masstone are appreciated. However, tints and mixtures containing white are more susceptible to fading, particularly under prolonged light exposure.

From a historical and analytical perspective, PR 9 is significant as a transitional pigment, bridging the gap between early, highly fugitive azo reds and later, more stable organic pigments. Its identification in artworks and industrial objects can assist in dating materials from the mid-20th century. As an organic pigment, it is invisible to XRF analysis and requires spectroscopic or chromatographic techniques for confident identification.