ferric pyrogallate

Ingredients

ferric chloride, FeCl3, gallic acid, sodium silicate, ferrous chloride, Fe(II)Cl2

Preparation Steps

Original Recipe Text

The pigment structure was modeled by Wunderlich (1991) by reacting ferric chloride and gallic acid within a sodium silicate gel (p. 40). Later researchers (Feller and Cheetham, 2006) synthesized the precipitate from ferrous chloride to test the validity of the three-dimensional octahedral model (p. 43).

Evidence

"Wunderlich created black crystals for analysis by reacting ferric chloride, FeCl3 and gallic acid in a gel of sodium silicate in 1991 (Banik). These black crystals were analyzed using Mössbauer and X-ray absorption near edge structure (XANES) spectroscopy."

Remarks

The pigment is characterized as a three-dimensional polymer structure forming a slightly distorted octahedral high spin complex (p. 41). The coordination involves iron ions and oxygen atoms within the conjugate bases of gallic acid. Specific bond lengths were identified: oxygen atoms in positions three and five bond to iron at 2.000 Å, while the oxygen in position four (para-position) has a length of 2.028 Å (p. 42). The iron-oxygen bond length within the carboxylate moiety is 2.006 Å (p. 42). This structure is described as an insoluble precipitate that provides the ink its 'brilliant color' (p. 43).