Ph control when gassing product

Friendlycat · Tue May 31, 2005 5:59 am

Is there a good and bad way to monitor Ph when gassing freebase with HCl in acetone?
If someone wants to gas once until neutral, how is it done efficiently without going too acid. Do you need a Ph probe? How do you do it?

Kind regards,
FC

Bumblebee · Tue May 31, 2005 6:33 am

I don´t know if it is possible to monitor the ph correctly in time. I guess that it takes some time for the takeup of HCl in the acetone. so if your ph-meter shows ph 7 and you stop gassing the ph will fall to maybe 3 or 2,5 , in the acetone... or it rises again to 8 or 9 because some more freebase got converted

I did gassing step by step... gas it, filter with buchnerfunnel, gas it again, filter it, gas it, filter and so on until no more chrystals are coming out of solution.

you can try gassing dry acetone, so that it takes up HCl, make it superdry again and use this for titrating your freebase...

java · Consumer

I recall a solution we used to use in the chem labs to indicate acidic or basic solution when titrating......you might want to try that for a visual indication when the solution is acidic.....java

........read about it here,
http://chemistry.about.com/library/weekly/aa112201a.htm

loki · guinea pig

is it safe to put anhydrous HCl into acetone anyway? I mean, isn't there a significant risk of causing some kind of polymerisation or something with the acetone. Or does that only apply to aldehyde/ketone mixtures?

Vitriodor · Wed Jun 01, 2005 4:51 am

slacker · Mon Jun 06, 2005 8:15 am

im relatively new to this and am by no means an expert in chemistry so forgive my stupidity, but is ph control really relevant? i mean, if one is trying to precipitate something you could just gas it until no more hcl salt precipitates right? i might be missing the point.

bio · Working Bee

If the Ph goes too low the amine usually oils out and more impurities are also carried through due to the excess HCl.

............gas it until no more hcl salt precipitates right?................

This works sure if done correctly but it is very easy to overshoot the Ph. So make sure the solvent and amine is dry, dry, dry. Use at least 7 or so times solvent volume and take 3 or 4 filterings. If everything is done well the crystals form instantly the gas is introduced even sometimes before the bubbler is put into the liquid.

It's easier though and more fool proof to just titrate with aqueous HCl in Hexane, toluene, benzene etc. 2 or 3 times and evaporate. Just takes a little longer.

slacker · Mon Jun 06, 2005 8:00 pm

ok, i see what you mean. i guess i did kinda miss the point earlier. i always just quit after the crystals started looking "ugly". now i know exactly why they got ugly. thanks for the chem lesson.

Friendlycat · Tue Jun 07, 2005 5:35 am

"Use at least 7 or so times solvent volume"

Swim uses a ratio of 4:1, solvent to freebase. Is there an advantage useing more solvent?

abacus · Sun Jun 12, 2005 5:41 pm

mmm, this reminds me of an old place i used to visit.

first off, if you pre gas acetone with HCL gas it will turn slightly yellow in about 24 hours so use it relatively quickly.

If you mix your freebase with acetone there may be a risk of forming an imine, ie amine plus ketone make imine.

Gassing in acetone can work well, if you make sure your solvent volume is high, otherwise you heat up the low bp solvent too much. Gas until you get a good crop, filter, gas again. If you gas until acetone acidic, you get colour changes often from yellow to red which is not good, you can see and smell the changes so with a bit of practice you soon learn and dont really need to check ph IMO.

also, oiling out has little to do with PH and more to do with a reaction between the impurities and hcl gas (depending on what amine)

Friendlycat · Tue Jun 14, 2005 4:11 am

Thank you, your advice has been assimilated.
"the old place"

you mean the "hive" newbee forum?

regards
FC

64bandil · Busy Bee

Once again I must advice you guys to use the azeotroptical salt formation technique. All of the above problems are avoided. No need to dry anything, no imines formed, no overgassing, and LOTS of crystals. I have never failed crystallizing an amine this way...

I believe I have posted the details regarding the method somewhere around here...

Regards
Bandil

mk-1 · Wed Jun 15, 2005 9:16 pm

can u please point me dear bandil to this procedure? it interests me Very Happy

64bandil · Busy Bee

Hmmm... I can't seem to find it either. If anyone finds the original post. Here's a quick'n'dirty writeup I found somewhere else:

With the somewhat pure freebase amine at hand... Start by weighting the substance, followed by the addition of equimolar amounts of hydrochloric acid.Adding 10X the solution volume of xylol and azeotropically distilling the water / xylol away. Once the xylol / amine.HCl is pure, the temperature will rise quite a bit. Xylol binds water to a greater extent than toluene, which makes it more suitable for drying stuff. After the solvent has been stripped, some acetone (ether og ethylacetate will also do) is added and the whole mess cooled in the freezer. This will leave glister white crystals of the hydrochloride in near quantitative yields.

Recrystallize as you please afterwards. I've succeded in gassing once, and I've tried quite some times. This method allows quite some sloppines - and the amine will still crystallize. No need to really dry anything - just distill and it'll get as dried up as an 80 year old woman!

Regards
Bandil

mk-1 · Wed Jun 15, 2005 10:20 pm

bandil, im guessing the hydrochloric acid should not be concentrated much. which molar concentrations do u suggest?