[Atir]

Hello,
I was preparing a medium for fungi.
I wanted to prepare a solution of each 18mM CuSO4 and FeSO4. For that I used CuSO4 salt and FeSO4*7H2O and added the correct amount of MQ water. All was done at room temperature.

I got a greenish, transparent solution. We left it for a day, closed with parafilm.
For inoculation we filtered the solution by a 0.2µm filter.
The remaining solution was blue-greenish, the filter yellow.
What happened? How can I ensure I get a +/- neutral aqueous solution of Cu and Fe easily?
Thanks for your *delete me*

[Hunter2]

First iron-II and copper-II don't harm each other, but Iron-II is easily oxidised to iron-III this is your yellow residue probably.
You want to have neutral solution is not possible. Both salt decrease pH to about pH 4-5. Neutralisation would give precipitation of Cu(OH)₂ and Fe(OH)₂/Fe(OH)₃.

[Borek]

Boil the water to get rid of dissolved oxygen (which is what oxidizes Fe²⁺ to Fe³⁺).