[pcm81]

Greetings all. It has been about 16 years since i took undergrad chemistry in college and am hoping some of you can help me understand couple things which i did not find online.

What i am doing is making a rust removal solution of phosphoric acid. In the true nature of a nerd I started with Sulphuric acid and am reacting it with Apatite:
Fluorapatite - Ca5(PO4)3F
Chlorapatite - Ca5(PO4)3Cl
Hydroxylapatite - Ca5(PO4)3OH

 to produce phosphoric acid. I was expecting to get liquid phosphoric acid and solid calcium sulphate precipitate. I did in fact get a white precipitate during the initial reaction, but after I added more water the white precipitate went away. Not sure what that precipitate was, I thought Calcium Sulphate is non-soluable in water. Any idea?


Also, have a more general question about reaction of acids with metals. I get the whole Ka and pKa distinction between strong and weak acids, but that does not explain why something like phosphoric acid would be better for rust removal than sulphuric acid. Or more specifically why certain acids do not react as aggressively with certain metals, while still eat away at rust. If it was all down to pH then higher concentration of weaker acid could attain the same pH as lower concentration of stronger acid; but what make one acid not eat away metal while the other acid chews right through it? As I recall it has something to do with electro negativity or some other electron potential, but I can't remember what it is called to look up a table of what acids will not attack which metals... Also, i guess certain acid + metal reaction can produce a protective layer on the metal, preventing further attack by the acid. Is this the reason for use of phosphoric acid in place of sulphuric acid for rust removal?

My end goal is to have a 5-galon bucket of rust remover that i can stick things into for a day or two and not worry about acid eating away at metal, while still eating away at rust. I added excess of apatite (2.2lbs) to 460g of sulphuric acid in a 5 gallon bucket. Initial reaction volume was ~1 gallon (added water. first 1L then 3 more L). I am hoping to keep excess of apatite at the bottom of the bucket and add sulphuric acid as the solution continues to deplete, but i do not want sulphuric acid to eat away at bare metal when i am derusting stuff..

Just trying to understand the relative reaction rates of this solution with different metals to gauge how long i can leave various metal parts (Steel, cast iron, brass, copper) in the solution before the solution starts to attack base metal rather than just rust.

Lack of insoluble Calcium Sulphate is puzzling me the most. But of-course calcium sulphate is insoluble in water and i have pH ~= 1 (according to chepie ph meter) solution of phosphoric and sulphuric acids in water.

Thanks ahead

[Borek]

Apatites (or, in general, calcium phosphate) are weakly soluble too, so getting double displacement can be impossible.

Removing rust is not only about just dissolving it in low pH, but also about complexing the Fe³⁺ cations so that they won't precipitate again.

[pcm81]

Apatites (or, in general, calcium phosphate) are weakly soluble too, so getting double displacement can be impossible.

Removing rust is not only about just dissolving it in low pH, but also about complexing the Fe³⁺ cations so that they won't precipitate again.

Does this mean that i should be concerned with concentration of sulphate left in the solution attacking bare metal since they did not become calcium sulphate? Or if i have Apatite in the bucket for a long time (weeks) it will take care of sulphates to the point that i should not worry about them?

Or was this just an overall bad idea and i should have started with phosphoric acid in the beginning. The nerd in me wants to start from the basics and have control over the mixture, rather than just buy off the shelf solutions.

[Borek]

Or was this just an overall bad idea and i should have started with phosphoric acid in the beginning.

Much better.

The nerd in me wants to start from the basics and have control over the mixture

Using phosphoric acid makes it much easier to know what is going on. Reaction between apatite and sulfuric acid produces rather complicated equilibrium, not easy to reliably predict its results.