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