Argox (Newbee)
11-29-01 04:16
No 241890
      H3PO4 reaction mechanism  Bookmark   

The following reaction does not appear to be the only thing going on when potassium iodide and 75% orthophosphoric acid are heated:

KI + H3PO4(aq) => HI(aq) + KH2PO4

In fact this is definitely NOT the reaction mechanism, since a mole/mole mix of KI and H3PO4 plus addition of stoichiometric water should totally convert KI to HI(aq), but does not.  In fact, 105% H3PO4 plus KI produces HI(aq), when supposedly there is no water in this system. 

Observation indicates that ortho-phosphoric acid or potassium di-hydrogen phosphate must further decompose into another compound plus water, since the amount of 57% HI(aq) coming over at 127C is greater than the amount of water initially in the system, if one assumes that the only water is the 25%w/w contained in 75% ortho phosphoric acid.

If a real chemist can help with this reaction mechanism, I'll post results of SWIA's experiments with maximizing HI production from OTC ingredients, that's why this is posted in Methods. 

I'd like to hear from somebody who really understands the properties of phosphoric acid.