[christina]

I had a lab where i had to crystalize some pure sulfanilamide out of some impure sulfanilamide.

When in the first step when I had to dissolve the sulfanilamide in some 95% ethanol and heat to boiling, I found that there were always some pieces remaining and I added a little more ethanol and even called the lab tech over and showed them how much I added and I assume it was fine since they walked away. Then I ran out of ethanol so I went to get more and the sample got dry and the solvent evaporated. The 2nd time I heated the sample with the ethanol newly added even More was refusing to dissolve and I gave up since there was only 20min left in the lab.


I heard from others doing the same lab but in another section that they had the same problem but they solved it after doing the lab again.

Can anyone help me out with why this happened?

[craken66]

why do you use just only ethanol? I think sulfanilamide is polar. so, it will dissolve at DMF.

[christina]

this response pertains to the response post...the lab instructions are to dissolve it in ethanol so that's what I did..I can't not follow the lab book and do my own experiment.

[alphahydroxy]

Welcome to the world of organic chemistry...!

Recrystallisations can be tricky business... Volume, temperature and surface area are all key to getting things to dissolve.

[pharmDoo]

i just did this lab in my Ochem. i used 95 percent ethyl alcohol with .75 g of impure sulfanilamide with a melting point around 150

first we had to heat the solvent on the hot plate just to barely boiling

then off the heat added about 1 ml of solvent to the Sulfanilamide

place on the hot plate and its going to boil fast
but you don't want to boil too fast where the solvent is evaporating faster than your putting it in.
or too slow where there is no simmering

basically you want a heavy simmer and continually stir and heat. make sure that the temp does not drop

add more solvent if it did not ALL dissolve

keep adding solvent about no more that 1 ml at a time. heat and frequently stir

it took me about 15- 20 ml of solvent to dissolve my .75 g of sulfanilamide

don't be TOO shy with the solvent

i read in my lab techniques not to add too much solvent, just to cover the crystal; but actually adding a little more solvent at a time speeds the process. (add enough solvent so the crystals don't get stuck on the side of the glass when you stir)

this process took me 10-15 mins

but the sulfanilamide should dissolve
hope this helped

try crushing the crystals with the back of your micro spatula against the flask

[Custos]

Okay, you know why you are doing the recrystallization right? It's to remove impurities from your sample. This happens in two ways. First you dissolve your compound in hot solvent. Insoluble impurities will remain insoluble regardless of how much ethanol you add, right? (think about if you had some sand in your sample). So you filter this while hot. Then the solution cools down and your purified compound crystallizes out of solution. The soluble impurities stay in solution. So when you collect your compound by vacuum filtration it should be pure.

What you may have been seeing was insoluble impurities in solution. They might have even been tile chips or something added to stop the solution from bumping. Everything doesn't have to dissolve -- so don't add DMF. If you do that your compound might not come back out of solution again.

Allowing the solution to boil dry is not a good thing. It's possible in doing this that some of the sulfanilamide decomposed adding more (possibly insoluble) impurities into the mixture.

Hope that helps.

[christina]

Thank you everyone for replying to this post of mine.
I now have more of a insight as to why it may have not dissolved for me.

After speaking with others I found that some had the same problem as me. For another lab student it took them 2hrs to dissolve the sulfanilamide.


Thanks again