Disposal of ferric chloride etchant

Washing soda is the harmless of the two reagents, so it doesn't matter if your mixture contains some excess, unreacted soda.

Soda is cheap enough that you don't have to worry about optimizing your use to the absolute stoichiometric ratio.

You can use baking soda also (bicarbonate).

Put Sodium carbonate little by little to minimize frothing that leads to messy spills.

Keep adding it until no additional fizz. It might take hours... or even days, depending on the amount you have.

Heat really accelerates the reaction but needs to be temperature controlled and pumped and evenly spread for consistent etching. Room temp soak is just slow and simple, I think.

Disposal info