Sciencemadness Discussion Board - phosphoric acid synthesis

phosphoric acid synthesis

jamit - 17-7-2011 at 01:54

Has anyone actually made any reasonably pure phosphoric acid using a phosphate salt like trisodium phosphate and sulfuric acid?

I check some search on "phosphoric acid" and the results seem inconclusive. There are topics on orthophosphoric acid and heating it to make metaphosphoric acid, etc.

Any help would be appreciated.

Magpie - 17-7-2011 at 07:20

http://www.sciencemadness.org/talk/viewthread.php?tid=2923&a...

jamit - 17-7-2011 at 09:06

Thanks Magpie!!

jamit - 19-7-2011 at 02:52

I'm still a little unclear even after reading the thread Magpie suggested. Can someone clarify?

After mixing trisodium phosphate with rooto sulfuric acid, you get sodium sulfate (Na2SO4) in solution (at room temp) and phosphoric acid (H3PO4).

Many note that sodium sulfate should be filtered but it hasn't ppted. Should i put it in the refrigerator and cool it and force out the ppt (4.7g/100ml at 0C)?

Then should i take the filtrate solution of H3PO4 and distilled it? Some say that I should vacuum evaporate? What is vacuum evaporation?

Can someone help me on this? thanks in advance.

[Edited on 19-7-2011 by jamit]

Magpie - 19-7-2011 at 18:53

Quote: Originally posted by jamit

I'm still a little unclear even after reading the thread Magpie suggested. Can someone clarify?

After mixing trisodium phosphate with rooto sulfuric acid, you get sodium sulfate (Na2SO4) in solution (at room temp) and phosphoric acid (H3PO4).

Many note that sodium sulfate should be filtered but it hasn't ppted. Should i put it in the refrigerator and cool it and force out the ppt (4.7g/100ml at 0C)?

What I used was Ca3(PO4)2 as indicated in my post, not Na3(PO4). This gives CaSO4 when sulfuric acid is added. This is insoluble and can be filtered out.

Quote: Originally posted by jamit

Then should i take the filtrate solution of H3PO4 and distilled it? Some say that I should vacuum evaporate? What is vacuum evaporation?

Yes, use vacuum evaporation (distillation). This is potentially dangerous due to flying glass from an implosion if your glassware is not vacuum capable or is defective. You should already be proficient in distillations before trying a vacuum distillation. Get a good organic lab text and read up on vacuum distillation beforehand.

If you don't want to make H3PO4 maybe you can get it at a brewing supplies store. My local supplier was selling it by the gallon the last time I noticed.

jamit - 19-7-2011 at 19:52

thanks Magpie!

I noticed that in your comment on the thread, that your filtered product was cloudy. How did you get rid of that cloudiness? Was it caused by calcium sulfate? And did your final product after vacuum evaporation produce a clear concentrated solution of phosphoric acid?

thanks in advance!

Magpie - 19-7-2011 at 21:41

The acid I show is the concentrated pot product. IIRC I didn't do anything more to it. So I don't know what the fine material is that caused the cloudiness. I have very little use for this acid so never worried about it anymore. If I wanted to clarify it now I would try my newly acquired glass frit filter funnel.

When I make a reagent I always have an end use in mind. If the contaminant won't affect the end use or intended purpose I don't worry about it. It's the same philosophy as why buy chemically pure when technical is good enough?

Edit: Product is in the pot, a concentrate.

[Edited on 20-7-2011 by Magpie]

Bitburger - 23-7-2011 at 04:56

Pure phosphoric acid is a pale pink coloured, waxy solid. :cool:

It is very hygroscopic. I obtained it once when I used dilute HCl as solvent for the reaction between calcium phosphate and sulphuric acid. After leaving the solid CaSO4.xH2O behind, the 5%HCl solution was evaporated at only 20°C but with a air humidity of 0% since it was freezing outside and when the surrounding air warms up it takes up the water vapour really easy...

So, no need for an expensive and hard to catch vacuum oilpump