once it cools down unless stored correctly it will grab for moisture out of the air so it will take away everything you just heated for anyway...
Fairly conc is required but its not to bad, just the only prob is the dissociation of the last hydrogen from phosphoric takes a bit of energy, you may need to heat the mixture to continue the reaction after two substitutions.
1mol phosphoric (make sure your accounting for the dilution) with 3mol KI.
H3PO4 + KI --> KH2PO4 + HI
KH2PO4 + KI --> K2HPO4 + HI
Thats about as far as it may go, so heat a little once you add all of the KI to keep it moving to --
KH2PO4 + KI --> K3PO4 + HI
Just incase it stops bubbling once you add all the KI and pH still shows slight acidic.
Once all three subs are completed solution should be neutral.
-AC