Question: why is mgcl2 added to edta?
Answer: The EDTA removes magnesium ions, the MgCl2 adds them. You might want to watch out for using an EDTA containing buffer with His tagged proteins. The EDTA will chelate out the cobalt or nickel in the column used for His tag binding. due to disassociation constants, edta-mg will allow a trace of free mg in the solution.