Hi. My question comes from the following: I was studying the precipitation of phosphates and EDX clearly shows the elements forming the phosphates. it also shows Mg, that were the ions most probably to come along with phosphate ion. However even being present in low concentrations, calcium was expected to be present as well. don't know if it may be usefull but In my solution Ca2+ amount for 0.003 mol while Mg2+ accounts for 0.021 mol. In the EDX report Mg2+ shows to be 9.50 Weight% or 8.18 Atomic%. My question is can calcium be present in precipitate but in such small concentration it doesn't show on EDX? or at least a small peak would be expected if it was present?
I think you explained it already:
In solution you have 0.003 ml Ca2+ and 0.021 mol Mg2+.
In the precipitate you have almost exclusively Mg2+. This is not unusual, since the preference of phosphate is far higher for Mg2+ than for Ca2+.
Thus, the residual Ca2+ remains in solution. You can evaporate the solution and do an EDX and you will see it.
Furthermore, if there are traces of Ca2+ in your precipitate it might escape the EDX experiment. As any experiment there is a limit of detection. Ask your operator about this.
Automated software may drop the display of results if concentration is below the limit of detection or determination (