Doc B
Hazard to Others
Posts: 107
Registered: 5-4-2012
Member Is Offline
Mood: No Mood
|
|
Physical properties of Caffiene
I was looking at a few different sources and have noticed what seems to me to be an anomaly in the quoted physical properties of anhydrous caffeine.
According to the referenced wiki page bellow the anomolus data is given as follows...
BP- 178`C,
MP- i) 227-228`C (ref .2)
ii) 235-238`C (ref .3)
Am I failing to see the obvious or is there an explanation beyond what seems to be askew? Not only are the two MP's being stated as up to nearly 5%
different but also the BP is as much as 60`C below the MP for the same pure anhydrous substance.
How can the solid substance boil before it melts to liquid? 
1. http://en.wikipedia.org/wiki/Caffeine
Which in turn references data from...
2. Caffeine (anhydrous). www.sigmaaldrich.com and
3. Susan Budavari, ed. (1996). The Merck Index (12th ed.). Whitehouse Station, NJ: Merck & Co., Inc.. p. 1674. (I still so want a copy!)
[Edited on 8-3-2013 by Doc B]
|
|
|
Dr.Bob
International Hazard
Posts: 2659
Registered: 26-1-2011
Location: USA - NC
Member Is Offline
Mood: No Mood
|
|
I see MP 234-236.5 in my Aldrich Catalog, which is well within range of the two other mps. The BP is likely someone mis-reading something, it will
likely decompose before boiling at STP. It might be a BP under a high vacuum, and someone forgot to show the vacuum used. My advisor used to
have a 5 pound bottle of it in his office, just in case he ran out of coffee, which he drank an insanely large amount of. I think we also extracted
it once or twice in labs I either took or TA'd at times, and got wildly varying quality of the extraction, due both to the wildly varying student
aptitude, as well as the fact the coffee is not a standardized substance and varies from different sources.
|
|
|
ScienceSquirrel
International Hazard
Posts: 1863
Registered: 18-6-2008
Location: Brittany
Member Is Offline
Mood: Dogs are pets but cats are little furry humans with four feet and self determination! 
|
|
Caffeine sublimes at 178 C.
The correct melting point is around 235 - 238 from a quick Google consensus of the Internet 
|
|
|
Maximus
Harmless
Posts: 8
Registered: 10-1-2013
Location: Roman Empire
Member Is Offline
Mood: Lying on a triclinium
|
|
As said, its called sublimation, my friend 
|
|
|
Doc B
Hazard to Others
Posts: 107
Registered: 5-4-2012
Member Is Offline
Mood: No Mood
|
|
Thats what my fitst impression was to Dr. Bob. However as mentioned there is approximate agreement with the referenced entries I listed.
Supersquirel, thank you for you google confirmation. Without decrease of appreciation I must appologise iin advanced as I may start ranting due to my
googleaphobia. Grrrrr google asks me! Sorry, it's started already. 
From the referenced and added supliments we are aware of the gooooooogle documented sublimation of caffeine at a BP of 178C, possibly with
decomposition in some instances. Now I do love reference books as much as the next mad scientist, (perhaps more because I have only Langs and still no
Merck or CRS to cuddle in bed.... if you're judging me you're not a mad scientist) but I never actually believe them to be such a wealth of resource
and gooooogle gospel to compromise the foundation of enquiery through the scientific method and reasoning.
As such while it also indicated that caffeine has a flexible MP but always higher than its BP at around 235ish, has anyone other than the good Dr. Bob
actually seen caffeine follow this data in RL?
[underbreath] or got of the computer stool and in to a lab [/underbreath]
Or is there many/any other pure chemicals that have a higher MP than BP?
[underbreath] watch em all run to google! [/underbreath]
Sure we all learnt what sublimation is chem101 before we started skipping lectures. But before I was cocky enough to swap my chem lectures for biology
study with that lovely german girl with raven hair... who's name escapes me... in
fact I may not have ever known it.... But I digressssssssss....
Hmmmhmmm
I vaguely recall that sublimation, BP, MP, etc having to do with phase transitions between states of matter. Yet I have zero recollection or
experimental observation of any substances having a higher MP than BP (sub.) Nor an explanation for its mechanism.
Now I did a crude test, because it's not for lack of googling or UTFSE that I ask about this trivial matter, but because it just makes no fucking
sense at all that it sublimes then melts after. Ok, ok.... calm down Doc.... But seriously! 
Crude and without measure though it was, my observations of a ~0.05g sample of pure anhydrous caffeine was placed on a glass microscope slide and
covered with a glass thimble. I applied heated to the underslide from an alcohol burner and watched to see if sublimate was captured on the thimble
before the powder turned to liquid...
It melted, then fumed...
 excellent, I might be mad but I'm not..... mad....
Never mind. Now I might have schrodinger'd it or elsewise ballzd it up because vox populi has it that google knows all and says blah blah blah but
anyone actually do an experiment or have a theory or explination on this shit? I could be missing more of my marbles than I have already lost and I
need the sciencey colored ones!
|
|
|
DraconicAcid
International Hazard
Posts: 4278
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
Well, it's not going to sublime *after* it melts- then it would just be evaporating.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
|
|
|
Doc B
Hazard to Others
Posts: 107
Registered: 5-4-2012
Member Is Offline
Mood: No Mood
|
|
Quite so but isn't sublimation essential a [solid<--->vapor] phase change, there by being no liquid state for sublimates?
|
|
|
Doc B
Hazard to Others
Posts: 107
Registered: 5-4-2012
Member Is Offline
Mood: No Mood
|
|
I am at a loss to explain my observations of [solid<->liquid<->vapour] phase change. I can redo and film the experiment but I can't upload
it (damn iPad, apple, google)
... unless....
That photobucket app I just downloaded might serve as means of creating an [url] code source....?
I'll be back soon.
|
|
|
Doc B
Hazard to Others
Posts: 107
Registered: 5-4-2012
Member Is Offline
Mood: No Mood
|
|
Oh there is some observation that I neglected to mention.
- The evolved condensate is not of accumulated fog type only
but also of particulate aerosol, the structure of which is therefor partially reminiscent of condensed sublimate
but there is a definite fog condensate as well, not typical of sublimates.
- Occasional observation of the experiment cooling down was that there was a very slight, slow and short evolution of gas after the liquid to solid
transition seemed to have completed. Being indicative of.... hmmmmn is there such a thing as a reverso sublimate?
|
|
|
DraconicAcid
International Hazard
Posts: 4278
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
Quote: Originally posted by Doc B  | | Quite so but isn't sublimation essential a [solid<--->vapor] phase change, there by being no liquid state for sublimates?
|
Not all substances will sublime completely before the melting point. Benzoic acid has a melting point of 122oC, but if you dry it in an oven at
100oC, some of it will sublime (you'll get nice feathery crystals growing on the cool spots of the oven, and a nasty smell in the air).
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
|
|
|
Dr.Bob
International Hazard
Posts: 2659
Registered: 26-1-2011
Location: USA - NC
Member Is Offline
Mood: No Mood
|
|
According to some old experimentals I found, caffeine does sublime, but only at low pressures, under a vacuum. They say that an aspirator will work,
but give low yields, so likely a higher vacuum will work better. Most sublimation will not work well at atmospheric pressure, as the materials tend
to oxidize or otherwise react with air when hot. And the rate of sublimation will depend on the vacuum as well as the cooling of the receiver, if it
is not cooled well, much of the caffeine will be lost to the vacuum or decomp. So if caffeine is heated at STP, it will likely melt/decompose
before much of it sublimes.
But under a good vacuum, with a well chilled receiver, it does sublime. I can't remember if the sublimation temperature changes with pressure, I
have some vague recollection that it does not, in theory, but the rate of sublimation does change with the temp and heating rate of the crude
material. But I don't remember my physical organic theory all that well.
|
|
|
kristofvagyok
National Hazard
Posts: 659
Registered: 6-4-2012
Location: Europe
Member Is Offline
Mood: No Mood
|
|
According to Reaxys database caffeine melts at 237 - 238 °C and sublimes at 89 °C on 15 Torr pressure.
I have a blog where I post my pictures from my work: http://labphoto.tumblr.com/
-Pictures from chemistry, check it out(:
"You can’t become a chemist and expect to live forever." |
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
