Iron is affected by oxygen and water in the atmosphere to form a complex compound called rust. The composition of rust varies with the situation when it is formed, but iron rust contains ferric ions. The cotton fabric is stained with iron rust spots, forming defects, which must be removed. Oxalic acid can form iron oxalate anion chromium compound with trivalent iron ions. This anion complex is easily soluble in water, so iron rust can be washed away with oxalic acid. In addition, the iron rust spots on the nylon fabric can cause yellowing of the fabric, which can also be removed with oxalic acid.
When indigosol dyes and Naftol dyes are used for direct printing, nitrous acid is generated when the color is developed by sulfuric acid, which will make the Naftor AS sodium salt on the cloth a reddish brown precipitate. In order to eliminate nitrous acid, sometimes oxalic acid is added to the sulfuric acid color developing solution to replace reducing agents such as formic acid, urea or sulfuric acid. It can reduce nitrous acid to nitrogen and avoid HNO2 from causing AS to turn brown.
Note: Oxalic acid is easy to damage the fiber. After the cotton cloth is treated with oxalic acid, the oxalic acid remaining on the cotton fiber should be thoroughly washed with water. If not, whrn the cotton fiber is dried, the dilute oxalic acid solution will become concentrated acid, which will quickly damage the fibers.