I mixed up a stock solution using Zinc sulphate Monohydrate (from Alpha chemical) to dose zinc. I mixed two batches already and have had the same thing happen i.e. the solution turning slightly brown tint with what looks like a flaky precipitate laying on the bottom on the glass container the stock solution is stored in. I use one gram of the zinc compound dissolved into 250 mL of DI water. The first time I thought I cross contaminated it with another additive. I mixed a second batch and after 2 weeks have the same problem. I use the same eye dropper but rinse it in between the Iodine and Manganese that I also dose daily.