Identification of Synthetic Organic Pigments by Characteristic Colour Reactions

Abstract

The purpose of this study was to develop a method for the identification of synthetic organic pigments on the basis of colour reactions with four different chemical reagents: concentrated sulphuric acid, concentrated nitric acid, a mixture of concentrated sulphuric acid and concentrated nitric acid and, finally, a solution of potassium iodate in concentrated sulphuric acid. Colour reactions for a large number of synthetic organic pigments in the form of pure powdered material were recorded. It is shown that the colour reactions observed are in overall agreement with the chemical grouping of pigments (for instance, naphthol AS pigments or quinacridone pigments), i.e., pigments belonging to the same pigment group yield similar colour reactions. This fact is the basis for the construction of flow charts for identification. Tests performed on a number of artists' colours containing pigments in various binding media show that the binding media do not noticeably affect the colour reactions observed, making the flow charts useful for the identification of actual artists' pigments. A fifth chemical reagent, namely a saturated solution of potassium hydroxide in 96% ethanol, was tested and abandoned as a result of its instability, which resulted in severe problems with reproducibility, especially for certain pigment groups, for example the acetoacetarylide pigments.