When {HEAD ~ the pH} of {TAIL ~ the fixer solution} is below 3.5, formation of hydrogen sulfide gas can be a problem 
of
When { HEAD ~ the pH } of { TAIL ~ the fixer solution } is below 3.5 , formation of hydrogen sulfide gas can be a problem
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH