[rwooduk1]

Previously, I dissolved CaCO3 into solution using CO2 then used electrochemistry to raise the pH and precipitate CaCO3 on the surface of my electrode (see reaction scheme in LHS of table below).

I now want to do the same for BaSO4. Sulphur dioxide has similar speciation characteristics to CO2, so I thought it might be possible...



But I am confused what the reactions would be, the sulphur dioxide would be left bubbling into the solution overnight, so I'm predicting a pH of around 2 to 4. If I assume that the liquid becomes ##SO_{(2)(aq)}## I end up with ##BaSO_{3}## not ##BaSO_{4}##...



If I assume ##HSO_{3}^{-}##, or ##HS_{2}O_{5}^{-}##, or ##S_{2}O_{5}^{2-}## I get something else. In fact the reaction between sulphur dioxide and water is written as ##H_{2}SO_{3}##.

Am I comparing apples and oranges? Getting a little lost here. Can I use sulphur dioxide as I did the carbon dioxide?

Thanks for any help.

[chenbeier]

Probably  add some Peroxide for oxidation to convert sulfite to sulfate.

[Borek]

Looks like you might be mistaking sulfite and sulfate.

[rwooduk1]

Probably  add some Peroxide for oxidation to convert sulfite to sulfate.

Hmm interesting, yes, if I get barium sulfite i could just pour some hydrogen peroxide on it. Thanks.

Looks like you might be mistaking sulfite and sulfate.

Where? Wouldn't it depend on the pH? But even if I control the pH to control the sulphate species in solution I'm still not going to get ##BaSO_{4}## from the OH ions. Unlike the carbonate reaction, for sulphate I lose an oxgen when the pH decreases, so at the end I'm going to get ##BaSO_{3}## not ##BaSO_{4}##. Unless I've gone wrong somewhere or there are different species in solution. I would have to use sulphur trioxide to get my ##BaSO_{4}## back at the end, but does it speciate like the dioxide?

[chenbeier]

Compare the reaction Bariumhydroxide with CO₂ and SO₂.
In both cases you will have the main atom carbon or sulphur and 3 Oxygen. To get Sulfate you need on oxygen more. This can only obtained by oxidation. It is a redox reaction.
To change pH will not any effect.

[rwooduk1]

Compare the reaction Bariumhydroxide with CO₂ and SO₂.
In both cases you will have the main atom carbon or sulphur and 3 Oxygen. To get Sulfate you need on oxygen more. This can only obtained by oxidation. It is a redox reaction.
To change pH will not any effect.

Thank you chenbeier. If I wanted to use say hydrogen peroxide to oxidise the ##BaSO_{3}## to ##BaSO_{4}## on the electrode, what do you think would be the best way to do this? i.e. could it be done during the electrochemical process? Or would it only be done after the electrochemical reactions on the solid ##BaSO_{3}##?

Also I did look for sulphur trioxide speciation information and couldn't find anything, so I assume sulphur trioxide does not change species with pH in solution?

[chenbeier]

I think you have to add it to the solution.
Elektrode no need.