Azo pigments lakes

The hue of a red azo pigment lake carrying sulfonic acid functions is determined to a considerable extent by the metal ion. In the series Na->-Ba->Sr->Ca->Mn the shift of hue from yellowish to bluish red increases in the order in which they are listed. The complex correlation between chemical constitution and color in pigment molecules poses a quantum mechanical challenge. This is complicated by interactions within the crystal lattice and by the contribution of intermolecular and... 

Likewise, the lightfastness of an azo pigment lake is controlled to a certain extent by the metal cation manganese salts are usually most lightfast. 

Red azo pigment lakes may be classified according to the coupling component. There are four industrially important groups. Pigments are based on the following coupling components ... 

In contrast to red azo pigment lakes, which have stimulated considerable technical interest, the corresponding yellow pigments are much less important (Sec. 2.3.1.2 2.3.4.1 Table 13). 

The development of azo pigment lakes was initiated by the discovery of Lithol Red by Julius (BASF) in 1899. Lithol Red, which is synthesized by an indirect diazotization procedure using 2-naphthylamine-l-sulfonic acid as a diazonium compound, was initially employed in the form of its calcium and barium salts, which were precipitated onto inorganic carrier materials. The pigment was used in its pure form after it became apparent that the carriers contribute very little to the application properties of the product. Lithol Red is one of the earliest colorant developed specifically for application as pigment. 

No X-ray crystallographic studies of azo pigment lakes have as yet been reported, but solid state high resolution 13C-NMR spectroscopy was used to demonstrate that P.R.57 1 also exists in the keto-hydrazone form [9]. 

Pigment synthesis follows the typical route to azo pigment lakes the aniline derivative or the aniline sulfonic acid is diazotized with sodium nitrite in an acidic medium (hydrochloric acid), followed by coupling on the Naphthol AS derivative, which is initially dissolved in an alkaline solution and then precipitated by adding inorganic or acetic acid. If a Naphthol AS sulfonic acid is used as a coupling component, it must be neutralized with alkali for dissolution and coupled directly. 

---