Which concentration of ammonium sulfate works best to separate phosphatase from other proteins?

Answer 1

It depends on the phosphatase.
You need to look inthe literature for your specific protein and organism and if it is not avaliable then you need to set up an experiment with different concentraions of ammonium sulphate and find out yourself.
However you should be careful since this is a general method and for sure you will have more proteins in your sample staying in the same phase as your phosphatase, especially if you are using a cell lysate. It is just a first step of purification. You would also be careful as some enzymes tend to lose some of their activity during this process; if your enzyme is sensitive to ammonium sulphate you might need to find a different precipitation method (e.g. pH-based)