I would like to ask for your advice on the chemical change of iron(iii) oxide during neutralization in a strong acid environment.

As you know, iron(iii) oxide is red powder using as a dye. Also, iron(iii) oxide is soluble in acid.

The experiment I did is as follows two cases.

Case 1.

(1) Add 140 g of phosphoric acid (58%), 120 g of sulfuric acid (98%), and 1 g of iron oxide and stir.

The color of the solution at this time is dark red.

(2)Slowly add water and ammonia gas to neutralize the mixture. Due to the exothermic reaction, the temperature rises by about 100 to 120 degrees or more. As the reaction proceeds, ammonium sulfate and ammonium phosphate are produced and changed into a slurry form. The water added in the middle of the reaction gives the slurry fluidity so that the neutralization reaction can occur sufficiently. At this time, the color of slurry gradually changes to white.

Case 2.

(1) Add 140 g of phosphoric acid (58%), 120 g of sulfuric acid (98%), 1 g of iron oxide, and 100mL of water and stir.

The difference between the two experiments is the initial water input.

(2) Add water and ammonia to neutralize. (Proceed in the same way as the above experiment)

Even when the reaction proceeded, the red color of iron oxide was maintained.

Another difference from the above experiment was that steam was actively generated.

A change in color (from red to white) suggests that a chemical reaction has taken place on the iron oxide (probably producing FeSO4).

I am curious about the reaction pathway and reaction conditions of iron oxide. I would also be grateful if you could recommend related books or papers.

Thank you all for reading this long text.