The color of acidic water

Why is it white sometimes and yellow other times?

Acidic water generally refers to the sulfur-containing wastewater from the tops of distillation columns, stripping columns, and other similar units in various atmospheric and vacuum distillation as well as hydrogenation plants; it contains substances such as Fe, Cl, S, and NH3. The inside of the pipeline is generally in an oxygen-free state; after corrosion, the metal is converted into Fe2+ (ferrous ions). Ferrous ions have a very light color, and at concentrations of just a few ppm they are almost invisible, which is why the solution appears colorless. When water is injected with oxygen, part of the iron is converted into Fe3+ (iron ions); iron ions are yellow in color, so the solution appears yellow ； When sulfur-containing wastewater contains a large amount of FeS precipitate, since FeS reacts easily with oxygen (in the anhydrous state, FeS can ignite spontaneously in air), oxidation reactions occur more readily to produce Fe3+ (iron ions), resulting in a yellow color.

A yellow color simply indicates an abundance of trivalent iron ions; it does not mean that there is significant corrosion. To determine the extent of corrosion, it is necessary to rely on various analytical data and conditions

I think the yellowing of the color of acidic raw water is mainly caused by corrosion inhibitors, ammonia addition, alkali addition, or substances containing ammonia nitrogen. Generally, iron ions are controlled at 5 ppm. I have seen that acidic water remains colorless even when there are dozens of iron ions present. However, the concentrated water from the acidic water stripping reflux tank is yellow, mainly due to the presence of ammonia-containing substances.

After the acidic water was released, it turned black after a short while

What color was it when it was first released?

Transparent; it will oxidize to a dark bluish color in a short while.