Sciencemadness Discussion Board

substituting sulphate for metabisulphite

CHRIS25 - 11-4-2012 at 01:33

Hi, it's all about the pickle solution - not the stuff you put on your cheese sandwich. The stuff that strips oxidation layers from copper and silver.

I know that I can by the sodium metabisulphite and re-hydrate it in Water to get the sodium sulphite. BUT, to be completely precise I need the sodium sulphate,slightly more acidic and as I look at the formulas I see extra oxygen and more sulphur (I think, still learning).

Question: Would the sulphate In Practical terms be that much different than the sulphite do you think, it would simply be dissolved in distilled water and warmed up and kept warm.

Thankyou gentlemen.

weiming1998 - 11-4-2012 at 01:58

No, sulfate would not work. A sulfite ion is SO3(-2), a metabisulfite ion is S2O5(-2) and a sulfate ion is SO4(-2). That means the sulfur is in an oxidation state of +4 for sulfite and metabisulfite, but for sulfate, the sulfur would be in an oxidation state of +6. Since the +6 state for sulfur is the most stable state, the other states would try and achieve the S(+6) state. Take H2S, which has an oxidation state of -2, for example. It wants to achieve the +6, but it can't do all that in one step, so reduces oxygen (2H2S+O2===>2H2O+S2) in order to achieve the next higher oxidation state, 0 for elemental sulfur. Elemental sulfur also slowly reacts in nature, to form H2SO4 (2S+2H2O+3O2===>2H2SO4) which has the +6 oxidation state, and thus it is the most prevalent form in nature, in the form of sulfate salts.

The pickling ability of sulfites and metabisulfites comes from the desire to be in the +6 oxidation state, and so it reduces oxides, to metals, oxidizing itself to +6(example: 3Na2SO3+Fe2O3===>2Fe+Na2SO4). If you used a sulfate, nothing will happen, because it is already in the most stable oxidation state and will not take the oxygen away from the metal. So sulfates will not work in pickling metals.

CHRIS25 - 11-4-2012 at 02:18

An exhaustive explanation Welming, thankyou, I will always use this extra info and study what you are saying. Though I must say that I probably did not make myself so clear, the Sodium BISULPHATE is the main ingredient in pickling solutions. You said the sulphates would not work but you probably meant the sodium sulphate right? Hope so. But it is good to get to grips now with all this. Just re-read my post that was my fault - I said sulphate at the end, I meant Bisulphate, sorry

[Edited on 11-4-2012 by CHRIS25]

weiming1998 - 11-4-2012 at 02:52

Quote: Originally posted by CHRIS25  
An exhaustive explanation Welming, thankyou, I will always use this extra info and study what you are saying. Though I must say that I probably did not make myself so clear, the Sodium BISULPHATE is the main ingredient in pickling solutions. You said the sulphates would not work but you probably meant the sodium sulphate right? Hope so. But it is good to get to grips now with all this. Just re-read my post that was my fault - I said sulphate at the end, I meant Bisulphate, sorry

[Edited on 11-4-2012 by CHRIS25]


Oh, you meant bisulfate? And now I fully get what you meant. Thinking about it, sulfites aren't probably strong enough reducing agents to reduce oxides, so maybe I was wrong about the previous explanation.

Bisulfates pickle metals via low pH. If you think about it this way, sulfuric acid disassociates to ions this way: H2SO4===>H+ +HSO4-, then HSO4<===>H+ +SO4(-2). The double-headed arrow represent an equilibrium and a reversible reaction. When H2SO4 reacts with H2O, all of it forms H+ instantly and irreversibly(unless you remove the water), so we call it a strong acid. But the formed HSO4- is different. It disassociates only partially to H+ and HSO4-. This makes it a weak acid, although it disassociates in water more fully than most weak acids do. As it is an acid, lots of H+ ions are in solution. And these H+ ions react with the oxides on the metal (example: Fe2O3(s)+6(H)SO4-(aq)===>2Fe(+3)(aq)+3H2O+6SO4(-2)(aq).). Using this equation, we can say that the reaction between NaHSO4 and Fe2O3 is: 6NaHSO4+Fe2O3===>3Na2SO4+Fe2(SO4)3+3H2O. Sulfate, on the other hand, can't do that, because it doesn't release any H+ ions into the solution, and thus doesn't react with the oxide.

CHRIS25 - 11-4-2012 at 03:11

Ok, a lot to take in there, I actually copy what you write and put into text program as a sort of dictionary. Then I break it all down and try to apply your example to other things. But Thanks. This company is supplying me with metaBisulphite free to try which is why I asked. Otherwise I would use my citric acid granules and dissolve them, many metallurgists are doing this, but the big companies still supply the brand names which are bisulphates. Besides if chemistry was not something I enjoyed I would just be buying everything from the major jeweler and silver suppliers in Usa and UK. I just have this something which drives me to make it all myself and be more self-suffiicient, I like exploring.

Thanks Welming. have a good day.

[Edited on 11-4-2012 by CHRIS25]

woelen - 11-4-2012 at 03:13

Bisulfite is a much weaker acid than bisulfate and besides that, it has reducing properties, which bisulfate does not have. What matters here is the acidity. I do not think that bisulfite does the job, due to its weaker acidity.

CHRIS25 - 11-4-2012 at 03:26

Point taken Woelen. I can use the bisulphite on another patina application anyway if it does not work as a pickling.