The magnesium and calcium ions present in seawater ([mg2+] = 0.059 m and [ca2+] = 0.011 m) can be separated by selective precipitation with koh. what is the concentration of [mg2+] when ca(oh)2 starts to precipitate? ksp mg(oh)2 = 2.1 x 10-13 and the ksp ca(oh)2 = 4.7 x 10-6