Hey I just got some PH down from the hydroponics store, as I was not patient enough to order a phosphoric acid solution, and on the front of the container it says 30% H3PO4 which is phosphoric acid. On the back though it says it contains Soluble Phosphate P2O5) at 21.5% derived from Phosphoric Acid.

P2O5 is Phosphorus Pentoxide. Is that usually in phosphoric acid solutions? I want to acidify my bark with this but I don't want to use anything that may alter the experiment.

Edit: Here is the MSDS: http://sunlightsupply.s3.../product/721870_MSDS.pdf It says it only has phosphoric acid at 30%.

The GENERAL HYDROPONICS brand of PH down has phosphoric acid, citric acid and mono ammonium phosphate and it is yellow so don't buy that one if you are getting phosphoric acid at the store.

So I think I figured it out: P2O5 once dissolved in water becomes H3PO4

It is usually safest to use pure reagents... patience is a virtue. See if you can find the MSDS for that specific brand that you have there.