[Sanskrit]

Our instructor had us do a lab where we had to take a base in a beaker and convert it into the salt form with Sulfuric Acid. Seemed simple enough. With a stir bar going, my group started drop by drop adding 1:5 SulfuricAcid:IPA to the pure solvent with the organics, and sure enough the sulfate salt immediately dropped out, though we figured adding a generous amount of Sulfuric Acid(conc.) wouldn't hurt as to convert all of the organics, but suddenly during the addition the solution turned from yellow to dark red and the Salt vanished.

Attempts to filter did not yield any salt whatsoever. We then basified the solution and re-extracted the organics with DCM but class was ended so we stored our extracted portion.

Our teacher would not tell us why the excess sulfuric acid made the salt 'disappear' back into the solution, nor why it turned the solution red, she wanted us to think about it she said.


Me and my group have thought about this plenty and I can't quite figure it out. Why would sulfuric acid in a solvent(with presumably no water) make a sulfate salt soluble in that solvent which it was previously insoluble in? Something to do with the PH maybe, but how, what mechanism is at work here?

Edit:

To clarify, there was about 300ml of solvent with organics. The difference between the Salt disappearing and being visible in the solution was a mere 5-10ml of sulfuric acid. Why do I find it hard to believe such a small addition could drastically alter the solubility of all salts in the solution? Am I missing something obvious here? If so, is there a way to calculate how much acid is needed based on the volume of the solution when the amount of organics is unknown?

[408]

what was the salt?

[Sanskrit]

Your guess is as good as mine. Some sort of Sulfate it would seem.

[Borek]

we had to take a base

What base?

(IMHO that's what 408 asked about, you just took it too literally).

[Sanskrit]

The only thing we knew about the solution was there was 300ml of solvent with an organic base in it that converts into sulfate form when coming into contact with sulfuric acid. We were instructed to mix the Sulfuric Acid:IPA 1:5 parts in to get all the base to go into salt form, then we were supposed to extract it, but it seems too much Sulfuric Acid for some reason made the salt soluble making it impossible to filter out.

I did some thinking on it, is it just that acidity of a solution can change the solubility of a salt? in spite of the presence of very little water? This is the only thing I can reason but I haven't encountered this rule before.

[orgopete]

This is what I think happened. Your organic base was an amine, probably an aniline in which someone worked out conditions to form an insoluble bisulfate. As sulfuric acid was being added, the bisulfate began to precipitate. When all of the base was consumed, the pH of the solution began to change withe the addition of a "generous amount" of sulfuric acid. I'm assuming the precipitate formed this gummy material in the bottom of the flask.

What was not mentioned was the amount of sulfuric added/needed at the start and how much was the generous amount. With some materials, it may not require very large quantities of impurities to lower the melting points. Other materials can seem rather more impervious. I'm guessing your material was more sensitive to the excess sulfuric acid. I wasn't in the lab, but I'm guessing you altered the cookie recipe and now you are asking why the cookies taste funny. 

[kriggy]

Couldnt it be some reaction with H₂SO₄? Like some dehydrogenation or sulfonation?.. You could have degraded your base to something more soluble.

To clarify, there was about 300ml of solvent with organics. The difference between the Salt disappearing and being visible in the solution was a mere 5-10ml of sulfuric acid. Why do I find it hard to believe such a small addition could drastically alter the solubility of all salts in the solution? Am I missing something obvious here? If so, is there a way to calculate how much acid is needed based on the volume of the solution when the amount of organics is unknown?

No if you dont know the concentration of that solution. COuld be highly concentrated or diluted, who knows? The only way is to slowly add the acid and look carefuly if more precipitate is forming or not.

[Sanskrit]

This is what I think happened. Your organic base was an amine, probably an aniline in which someone worked out conditions to form an insoluble bisulfate. As sulfuric acid was being added, the bisulfate began to precipitate. When all of the base was consumed, the pH of the solution began to change withe the addition of a "generous amount" of sulfuric acid. I'm assuming the precipitate formed this gummy material in the bottom of the flask.
 

It did not form any gummy precipitate.

The solution was yellow before the addition. When adding the Sulfuric acid the solution got less and less yellow as the salt precipitated, after a point of adding the sulfuric acid, salt stops precipitating and the reaction rapidly turns into a dark red color, at which point the salt immediately disappears.

Today, basifying the mixture in an Acid/Base extraction allowed the organic substance to be extracted in the alcohol  and then turned back into a salt by and stopping sulfuric addition before the solution turns red, then filtered out.

Me and my partners are unsure whether or not we lost some of the organic substance because even in the acid/base extraction, the aqueous layer appeared to have a salty concentration at the bottom. We are not sure if this is just the NaOH(Lye) we used to to basify it, or something else. As far as I am aware the lye should have been soluble in the water.

So, some of the organic substance was recovered in sulfate form the second time, but is it possible that the sulfate form reacted with the sulfuric acid (in the first over addition) to form some salt molecule that could not be fixed with an acid base extraction? Or was that just lye that I was staring at in the bottom of the Aq Layer after attempting to extract the base? The lye was totally dissolved in water before we added it to the RXN to basify it, so I am not sure what that could be at the bottom.