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Phosphoric Acid

Posted by Mark & Linda Mahlum  
Mark & Linda Mahlum December 03, 2009 03:03PM
I'm thinking of purchasing a gallon of phosphoric acid to experiment with the cleaning of some yellowed quartz on fluorite epimorphs. Where can I obtain that amount? What concentration? What dilution rate? Is it ok to do the cleaning in a crock pot set on warm? Any suggestions or advice would be appreciated.

Mark Mahlum
Mark & Linda Mahlum December 11, 2009 03:58PM
Doesn't anyone use phosphoric acid?

Jolyon & Katya Ralph December 11, 2009 04:26PM
I use phosphoric acid to remove carbonate from Fluorite, it works pretty well at this. Don't leave it in too long or you'll get calcium phosphate crystallizing out on your specimens!

I've not used it for cleaning iron stains off quartz or fluorite though - not sure how effective it will be but it's reasonably safe to work with so try warming it up a bit.

Ian Merkel December 11, 2009 05:38PM

I have not been able to locate phosphoric acid. If you find a distributor, please let me know.

Jared Freiburg December 11, 2009 05:42PM
I use phosphoric etch from home depot (in the paint department) with a little added citric acid. Seems to work pretty good for Fe staining and removing carbonate.
Anonymous User December 14, 2009 01:37AM

Since recently I had been using Phosphoric acid very often to clean off carbonate from Galena.The reason why I stopped was that Citric can do the same(only much more difficultly) and it neutralizes much easier.

I don't see why one should use Phosphoric acid unless he/she wants to remove carbonate from a specific mineral.I find it very hard to neutralize the acid and this process takes longer than cleaning per se.

I buy Phosphoric acid (around 70% ?) from a local chemical supplier shop.Go to your local University of Chemistry and ask them for their supplier.

Phosphoric acid is great for removing oxidation!

hayat malik December 31, 2009 01:41AM
NH December 31, 2009 02:48AM
The best choice is sodium dithionite, often sold as "super iron out" in some countries. This will remove iron stains without damaging most other minerals.

Since mica and aquamarine are pretty chemically inert, you can try an acid to remove the iron oxides as well. Oxalic acid (sold as wood bleach) and phosphoric acid (sold as rust remover) can both remove iron stains. Note that with both of these, if there is calcite present in the coating, it can form precipitates that are difficult to remove.

You can refer to Rock Currier's quartz cleaning guide for instructions on using the chemicals.

The only problem I can foresee is if the iron oxide is cementing the minerals together (removing it could make the piece fall apart) or if there is a clay coating (not easily removed by chemical methods).
Mark & Linda Mahlum January 23, 2010 07:03PM
Just a follow up. I bought phosphoric acid from Home Depot (around 35%, I think) and it has performed well. Numerous crystals and specimens in my possession had a yellow stain following immersion in muriatic acid that resisted removal by any other means. The phosphoric has removed those stains.

Walter Kellogg March 10, 2010 02:21AM
I used to use the Works Tub and Tile Cleaner which until a few years ago was a very dilute form of Phosphoric Acid. It was the best thing I ever found for cleaning rust stains off Fluorite without damaging the luster of the Fluorite. I used this on many, many, Cave-In-Rock pieces when they used to be plentiful. Because of the weakness of the solution, I would leave them soak for a week or more. It also seemed very effective with Blanchard fluorites. It is also especially nice for removing tarnish on Sulfides - especially Pyrite and Arsenopyrite. I found it useless for heavily tarnished Galena (nor has anything else worked). I was disappointed when they changed the formula - probably an EPA thing to keep phosphorous out of the water system. I am pleased to hear it is sold by Home Depot and will be doing some experimenting.
Anonymous User March 13, 2010 01:25PM
Hi all!

Just to mention that Super Iron Out is a brand with S.Dithionite as an active compound.I've heard it contains Citric Acid as well.

You won't have any problem using it to clean the Aquamarine with Mica,but when cleaning a Calcite you will.I always use clean compounds and not brands.S.Dithionite is neutral and will clean even Carbonates without causing any damage.

All the best!
Keith Wood March 13, 2010 03:14PM
The bright yellow stains from oxalic acid can be converted to soluble rust stains by dipping them quickly in bleach. Then try fresh oxalic acid again and it will work. I discovered this by accident. I think the yellow stains are iron oxalate - very insoluble. Bleach probably breaks up the oxalate ion and the iron reverts to iron oxide/hydroxide, which can then be removed with new oxalic acid.

Edited 1 time(s). Last edit at 03/13/2010 03:15PM by Keith Wood.
Jason Herrmann April 11, 2010 07:39AM
You can get reagent grade Phosphoric from VWR Scientific. Won't be cheap, but it will do the job. The web address is
John Lichtenberger April 11, 2010 09:11PM
I purchase Regency Professional Multi purpose Delimer from a restaurant supply store in Louisville. It's ~ 33% H3PO4, and diluted 5:1 does a pretty good job.
John Duck April 11, 2010 09:55PM

Add sodium bicarbonate to Super Ironout solution to adjust pH to neutral or basic and you will be able to clean calcite without damage. You can adjust the pH up to about pH 10 with sodium bicarbonate. However carbonates of zinc and copper will dissolve if you allow the solution to be too basic.
Peter Haas April 15, 2010 09:39PM
Many iron-bearing sulphides will not survive the treatment when the solution is too strongly alkaline. Chalcopyrite will show visible signs of alteration at pH > 8 !
Alfredo Petrov April 15, 2010 10:42PM
What the hell do restaurants use phosphoric acid for? Now I'm worried!
Ronald John Gyllenhammer April 15, 2010 11:54PM
Hi Alfredo,

Presumably as a cleaner, maybe lime scale, etc?

Alex Homenuke April 16, 2010 12:14AM
CLR (in Canda,at least) and some kettle cleaners and other lime scale removers are dilute phophoric acid, usually with an organic aditive that should keep the phosphates in solution.
I have used CLR to remove calcite from the Franklin rhodonites and it leaves the rhodonite shiny, not dull like hydrochloric acid. Works well for cleaning copper without making it "new penny pink"
John Attard May 10, 2010 07:04AM
I have been able to buy 60% Phosphoric acid from a professional agricultural supply place where they use it to formulate phosphate fertilizer. The liquid is colorless and quite inexpensive. They asked me how many gallons I want, so I bought one gallon in a clean container I provided.

Hydrochloric acid is best to remove iron ( when its use is OK) but it often causes an apparently never-ending bleed of yellow ferric chloride to come out of the slightest of pores in minerals after the acid has been rinsed off. Use of phosphoric acid at this point enables the slight amount of residual iron to be complexed by phosphate into something almost colorless. The only better complexing agent is fluoride but it is usually unnecessary.
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