Sciencemadness Discussion Board

Using sodium bisulphate instead of sulphuric acid

danieldavies - 10-12-2021 at 07:12

I have got a few litres of waste iron chloride solution. If i add sodium bisulphate to the solution, would i be able to distill Hydrochloric acid off leaving iron sulphate behind? Cheers

woelen - 10-12-2021 at 11:18

You would not have pure iron(III) sulfate, but a mix of sodium sulfate and iron(III) sulfate.
Besides that, I expect that you will not get a pure product at all. Either you will have a very acidic product, or you'll get some partially soluble basic product, which will contain iron(III), chloride, sulfate and hydroxide. What you get depends on how much NaHSO4 you add.

I would boil down the solution, without adding anything and go so far that you end with a solid, but not purely dry mass. This humid mass will be a basic chloride/oxide/hydroxide mix, with some water in it. This mix could be dissolved in a solution of NaHSO4 and then you could let this evaporate slowly. Probably you'll first get crystals of Na2SO4, and in a later stage you may get crystals of ferric sulfate. Still, I do not expect very pure material, but it will be better than directly boiling down the mix.

If the waste solution also contains other metal ions (e.g. copper), then purification will become even more difficult. I consider the purification of such a low grade waste as hardly useful, unless you want to do it for the purpose of getting experience with crystallization and purification of chemicals.

clearly_not_atara - 10-12-2021 at 11:56

If you use ammonium instead of sodium bisulfate, you can get a nice crystal from this solution, although it is not iron sulfate:

danieldavies - 10-12-2021 at 13:23

The purpose of this experiment is to recover the hydrochloric acid and discard the sodium/iron sulphate. I know sulphuric acid works as a catalyst but was wondering if sodium bisulphate can be used instead?

woelen - 10-12-2021 at 13:58

That's a totally different thing. Recovering the HCl certainly is possible. Just add solid NaHSO4 to the ferric chloride waste and heat. The NaHSO4 will 'melt' in its own water of crystallization (it usually comes as the 1-hydrate) and that makes a perfect highly concentrated mix of NaHSO4 and your ferric chloride waste. I would, before mixing the chemicals, first concentrate the ferric chloride solution to 40% or so. If you do that and then add the solid NaHSO4 you most likely will recover the HCl (together with water vapor). You may get concentrations of well over 10% of very pure hydrochloric acid.

With hydrated NaHSO4 and NaCl you also can get HCl, even in higher concentrations. Mix both solids and heat. You need to heat quite strongly (200 C or so). Use a heating mantle and a suitable flask and then you'll drive off quite some HCl and water vapor.

Bedlasky - 10-12-2021 at 22:13

Quote: Originally posted by woelen  
You would not have pure iron(III) sulfate, but a mix of sodium sulfate and iron(III) sulfate.

Ferric sulfate form with sodium sulfate double salt NaFe(SO4)2.12H2O.

Boffis - 11-12-2021 at 04:36

When hydrochloric acid was produced by the salt cake process the NaHSO4 and NaCl were fused together to give anhydrous sodium sulphate and HCl gas I am not sure it will work in the cold. The reaction would be an equilibrium but it may be possible to distil off the HCl and water so shifting the equilibrium but its going to be messy. It may also be possible to shift the equilibrium by another means. Sodium sulphate 10hydrate is less soluble NaHSO4 so if you dissolve the latter in hot ferric chloride and cool you may be able to get the sodium sulphate 10 hydrate to crystallise out +/- some ferric sulphate double salt. Separate the liquid phase and distil it. The residue can be recycled into the next batch and the distillate is your hydrochloric acid.

This process works with sodium bromide and NaHSO4 and there is in fact a YouTube video about this method by Nurdrage

I think that this method would work with chlorides too but how the presence of much iron in the system will affect things only experiments will tell. Please keep us informed.