3.2. Precipitation and preparation of solid dyestuff
A considerable part of the flavonoid dyes is able to form complexes with iron(II)-ions [19] and [20]. Preferred binding sites involved in iron chelation are the 3-hydroxy and 4-oxo groups or the 5-hydroxo and 4-oxo groups [18] and [19].
The formation of the iron complex is accompanied with a distinct change in colour. While the flavonoid extracts from Canadian Goldenrod appear yellow, the corresponding iron complexes exhibits dark olive colour [15]. The low solubility of the iron-flavonoid complexes can be used to precipitate a concentrated dye lake from the aqueous extract and to isolate a solid dyestuff product [14]. The use of iron-salt to precipitate the colourant is consistent with the later use of the material in dyeing processes, where iron mordants are applied. Also acceptable light fastness was reported for dyeings with use of Canadian Goldenrod and iron mordanting [8].
The production of the lake will be performed at the site of farming/harvesting, thus simple procedures were chosen, which can be handled by farmers without high investment.
After pH adjustment of the extract to pH 7.0 precipitation of the flavonoid-iron complex was initiated by addition of 25–200 ml L−1 of a solution containing 0.18 M (50 g L−1) iron(II)sulphate-heptahydrate. Dependent on the added volume of iron(II)sulphate solution the final concentration of iron(II)sulphate in the extract was between 4.39 mM (1.22 g L−1) and 30.0 mM (4.5 g L−1) iron(II)-salt.
The pH in the solution decreased with amount of added iron(II)-salt. At a concentration of 4.39 mM (1.2 g L−1) iron(II)sulphate a pH of 5.7–5.8 was measured in the mixture, while in presence of 30 mM (8.33 g L−1) iron(II)salt a pH of 4.37 was observed.
The precipitated extract gently was stirred for 2 h to grow particle size and then was allowed to settle overnight. The precipitate was collected by filtration through a paper filter and dried at 60 °C and weighted. In Table 1 the mass of precipitate collected as function of added iron(II)sulphate is shown for representative experiments.