Preservative in zinc sulfate eye drops

I need to make next preparation: Zinc Sulfate 250 mg Boric acid Q.S. Aqua purificata AD 100 ml DT 10 ml Preferably in eye drops Benzalkonium chloride and Na2EDTA is used as a preservative. Here it is Na2EDTA incompatible with zinc sulfate as it would complexate the zinc. My question is which preservative I use best in this eyedrop? Can benzalkonium chloride be used without Na2EDTA or I use the best phenyl mercuriborate. However, In my course of medicine, I found that phenylmercuriborate is incompatible with metals. And in the Martindale I found that benzalkonium chloride should not be used with sulfates.