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