"' Ferric oxalate "', also known as "'iron ( III ) oxalate "', is a chemical compound composed of ferric ions and oxalate ligands; it may also be regarded as the ferric salt of oxalic acid.
22.
This is because the hydrogen sulphide produced from thiosulphate, which acts as a source of sulphur and creates a reduced oxygen tension in which " Vibrio " can grow due to its facultative anaerobic nature, combines with ferric ions from ferric citrate to produce ferric sulphide, which is black.
23.
The gradual darkening of the ink is due to the oxidation of the iron ions from ferrous ( Fe 2 + ) to ferric ( Fe 3 + ) state by atmospheric oxygen . ( For that reason, the liquid ink had to be stored in a well-stoppered bottle, and often became unusable after a time . ) The ferric ions react with the tannic acid or some derived compound ( possibly gallic acid or pyrogallol ) to form a polymeric organometallic compound.
