check:Pg 83 in the following book.
http://books.google.ca/books?id=Xonl1qtZ3NkC&pg=PA82&lpg=PA82&dq=citric+acid+complex+barium+stability+constant&source=web&ots=6gb4o8ZCWR&sig=cX0_YBWFq_bn737cL1d35oYkqPw&hl=en&sa=X&oi=book_result&resnum=3&ct=result#PPA83,M1
generally speaking,high pH will be helpful to generate the deprotonated citrate,which should be the chelating reagent.