| Abstract: | SUMMARIES. 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.
CONCLUSION. This study has confirmed that colour reactions with the four test reagents - concentrated H2SO4, concentrated HNO3 a mixture of concentrated H2SO4 and concen¬trated HNO3, and a solution of KIO3 in concentrated H2SO4 — are similar for pigments within pigment groups. Several exceptions do, however, occur and are included in the results. Generally, binding media do not affect the colour reactions observed. The few observed discrepancies can most likely be attributed to the possible presence of more than one pigment in the artists' colours or the delay of colour reaction caused by the pro¬tective properties of the binding media on pigment grains. Flow charts for the identification of synthetic organic pigments were developed on the basis of the colour reac¬tions obtained and tested successfully in the correct identi¬fication of six unknown yellow, orange and red pigments. Keeping the above notes about cured paint in mind, the flow charts are recommended as a useful tool for the simple and non-time-consuming identification of syn¬thetic organic pigments in general. |
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