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making/extracting Phosphoric acid from miricle gro

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Borek:

--- Quote from: NickBlack on February 24, 2023, 07:12:20 PM ---I guess it's inappropriate to call a solution with H+ protons and phosphate- ions if those aren't the only ions, and it's no longer acidic, phosphoric acid?
--- End quote ---

Yes, no chemist would call it that.


--- Quote ---since sodium ions are very greedy (reactive)
--- End quote ---

Quite the opposite, they are mostly inert and it is very difficult to make them react with anything.


--- Quote ---adding sodium hydroxide, (Na+ protons and OH- ions) it should still produce sodium phosphate salts shouldn't it? (since once they form, they should drop out of solution, no longer being dissolved) shouldn't it?

--- End quote ---

Whether the salt crashes out from the solution depends on its solubility. Sodium salts tend to be well soluble, so they will stay dissolve quite long.

But I believe there is something else that you are missing. Imagine you prepare a solution by adding 1 mole of NaCl and 1 mole of KBr to 1L of water. Both salts dissolve and dissociate. You have a solution containing 1 mole of Na+, 1 mole of K+, 1 mole of Cl- and 1 mole of Br-. Is it different from solution prepared by dissolving 1 mole of NaBr and and 1 mole of KCl? Does it still make sense to speak any of these four salts being present?

Your solution contains - between other things - PO43- (actually mostly HPO42- and H22PO4-, which is another thing that complicates the situation), some H+ (every water solution does), some K+, some Na+ - does it make sense to speak about phosphoric acid, sodium phosphate and potassium phosphate?

Sideshow:
If you want phosphoric acid - and to save yourself the hassle of trying to extract it from fertilizer (do-able but a PITA) - go and look for a phosphoric acid rust converter at the local hardware. Most of them are simply orthophosphoric acid, occasionally with a few other traces - dyes, maybe a co-solvent like MGBE, or a trace of chromium, but all in acid form and usually at 30 - 40% w/v H3PO4.

Simple enough to concentrate by distillation - bits and pieces can either be fractionally distilled off or removed by complexation or precipitation.

And, further to Borek's comments :

What you have in Miracle Gro is a complex fertilizer - fully soluble and with certain chelates present, as you've already noted.

Separation therefore becomes a problem of removing as many compounds as you can that aren't the material you're looking for ... so ... (and this process IS NOT for the faint-hearted)

Ammonia can be driven off by the addition of a strong base and boiling to reduce volume until there's no odour of ammonia.

You may get away with it using potassium carbonate, which has the added advantage of, potentially, forming insoluble carbonates for the transition metals that can be filtered off.

The resulting solution will then contain residual sodium and potassium salts of  phosphate,  chloride, free  carbonate,  molybdate and urea (you might have removed some of the urea via hydrolysis if your pH was high enough when you boiled the solution). Allow to cool.

Mix with calcium oxide or calcium hydroxide - this should form a mixture of calcium phosphate (insoluble) and calcium sulphate (insoluble) and maybe a little calcium chloride (soluble). Filter and retain the insolubles - rinse with DI water.

Collect the resulting precipitate and treat with concentrated sulphuric acid (98% qs) to convert calcium to calcium sulphate (insoluble) and free phosphoric acid. Filter and collect the crude Phos acid (H3PO4) - concentration and purification is left as an exercise for the student.

Or go to the first line of this post (recommended).

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