Metal ions are generally known to be contained in the commercial lignosulfonate, and exert effects on the color of lignosulfonate. In this study, the kinds and quantities of some metal ions, and properties of chromophorics structure containing these metal ions were investigated by the use of commercial lignosulfonate purified by gel filtration. The quantities of metal ions were mesured with a atomic absorption spectrophotometer and calculated by calibration curve method.
The results obtained are summarized as follows:
(1) Iron, copper, manganese, chromium and some other metals are contained in lignosulfonate. Iron, presumably as ferric ion, influences most appreciably on the color of lignosulfonate among the metals used here.
(2) The higher molecular weight fraction of lignosulfonate contains the more quantity of iron. The quantity of iron, however, decreases by the treatment with IR 120 cation exchanger and becomes nearly constant regardless of the molecular weight of lignosulfonate. From these results, the iron contained in lignosulfonate is divided into two types, namely, a large amount of weak bonded iron and a small amount of strong bonded iron.
(3) Chromophoric structures which contain weak bonded iron, consist of chelates of iron-cathecol grounp and iron-free phenolic hydroxyl group. The constituent ratio of two chelate groups in lignosulfonate is concluded by comparison of visible light spectra of lignosulfonate and model compounds as follows:
cathecol structure: free phenolic hydroxyl group=1:5
It remains to be solved how strong bonded iron is present in lignosulfonate.
(4) Effect of iron on color of lignosulfonate is more prominent at the longer wave length than the shorter one.
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