in the interest of... well... me, a test was performed to determine which yield better results, gassing or titration. less than expected results has become a trend lately, and has cast a dark shadow over the lab. now, before you yell NOOB!, drying and gassing are not new experiences hereabouts, and proper yields have been accomplished regularly in previous attempts. investigations were made, apparatus cleaned and replaced, but to no avail; the spate of poor gassing yields continued. so, this experiment was attempted to decide if methodology should be changed. titration has never been attempted before, and it was decided this was a perfect opportunity to try it out.
PREFACE: the Al/Hg run in question was run as per
Post 343969
(RoundBottom: "Os/BS AlHg MeAm Amination writeup", Methods Discourse), and at least 50g was expected from this particular attempt. upon final drying of toluene, the volume of toluene was measured (about 650mL), and split into 2 equal halves.
the first half was gassed immediately, as per usual, and a disappointing 16.1g was recovered where ~25g was expected. the toluene was gassed and vacuum filtered 6x, until, on the 6th gassing, there was no more precipitate. the standard H2SO4 on HCl generator with inline drying tube (filled with indicating dri-rite) was used.
the second half was left overnight and attended to the next afternoon. a 5% HCl solution was made (using muriatic acid, 31.45%), and slowly dripped into the beaker containing the toluene while the toluene was stirring on HIGH. the pH was checked after every eyedropper squirt (~1mL of 5% HCl). after many, many, many! squirts (got bored, lost count after about 30) the pH measured 3.5/4. the contents of the beaker were placed in a 500mL sep funnel, and allowed to separate. the bottom, aqueous layer (which had turned dark blue, possibly from the pH strip indicator?) was drained into a clean pyrex dish (the toluene was kept to make sure nothing was lost) and left to slowly evaporate the H2O. after a couple days the water had (mostly) evaporated, and a large, flat crystal sheet was on the bottom of the dish. it still had a dark blueish hue. this was scraped off, broken up and put in boiling acetone for 20 minutes, then filtered again. after drying, 25.3g clean salt was recovered.
even for a first attempt, titration seems to have won a new convert. a difference of almost 10g was achieved, and a whole lot of recovered toluene is going to be titrated for hidden remainders.
gassing stinks, is tedious, dirty, and can be dangerous. it also requires anhydrous conditions, which, if not achieved, may scuttle your yields. gassing's main advantage is it is quick.
titration has the advantage of doing away with anhydrous conditions, but it takes a while to evaporate the water and get crystals.
if you have the opportunity, give this experiment a try. the results may surprise you.
next up, sassy distillation vs. crystalization.
Now with 12% more Bottom!
Nymphomania is not a disease, its a goal! (Methadist)