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