Phosphoric acid with {HEAD ~ 5 to 20% concentration} is added drop by drop to {TAIL ~ the solution} while stirring 
Phosphoric acid with { HEAD ~ 5 to 20 % concentration } is added drop by drop to { TAIL ~ the solution } while stirring
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH