I use Sodium persulfate to etch a copper board to manufacture home made Printed Circuit Board (PCB), and I use the same etching tank on the below photo which has no cover on the top. I wonder if it is okay to keep it like that at the room temperature or would it evaporate?
Regardless of evaporation, it is an oxidizer and unhealthy so it is recommended to keep it in a closed container.
With that being said, you most likely have a solution of sodium persulfate in water. Water will evaporate so again, yes, keep it closed.
Sodium persulphate solutions slowly decompose while stored. The decomposition generates gaseous oxygen. Make sure that there is a way for the gas to excape or the bottle might rupture. The storage temperature should be low. The decomposition rate of persulphate solutions depends strongly on temperature. For a 10 % solution of sodium persulphate 10% of the active material will decompose in about 30...40 hours at 50C temperature. At 25 degrees centigrade the same amount of persulphate is lost in 30 days.
If you need to store a persulphate solution please keep the temperature very low. Storing in less than 8 degrees of Centigrade will probably keep the solution in working condition for at least half a year or more. http://www.peroxychem.com/media/90826/AOD_Brochure_Persulfate.pdf
Storing PCB chemicals in the same fridge with food is not recommended.
