I think Dima may have been onto something with the Na2SO4 explanation.
Experiments with order of addition (to be published in the new
crystallography journal inaugurated 4/1/2009) show that using Bob Cudney's
recommendation (water -> buffers/salts -> PEG) yields a perfectly clear
solution. But, when mixed differently, precipitates form even after sitting
at 60degC for ~30min, suggesting that this solution is supersaturated in
Na2SO4. Is this inference false?
(Wikipedia:)
Na2SO4 Solubility in water:
04.76 g/100 ml (0 °C) ==> ~335 mM
42.7 g/100 ml (100 °C) ==> ~3000 mM
For interest, this compound has some nice names:
Thenardite (mineral)
Glauber's salt (decahydrate)
Sal mirabilis (decahydrate)
Mirabilite (decahydrate)
*******************************************
Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
Dallos Laboratory
F. Searle 1-240
2240 Campus Drive
Evanston IL 60208
lab: 847.491.2438
cel: 773.608.9185
email: j-kell...@northwestern.edu
*******************************************
----- Original Message -----
From: "Shao-Yang Ku" <s...@embl-hamburg.de>
To: "Jacob Keller" <j-kell...@md.northwestern.edu>
Sent: Wednesday, April 22, 2009 7:29 AM
Subject: Re: [ccp4bb] Screening Condition Precipitates
Hi,
I have not made the solution myself, but all the compounds listed are
highly soluble. Lithium sulfate can dissolve at a concentration as
high as 3M, and PEG400 is inert. None of the compound will be
protonated at pH 4.5. My guess is that some of your chemicals in the
lab are not what you expect them to be.
SY
Quoting "Jacob Keller" <j-kell...@md.northwestern.edu>:
Hi All,
there is a condition in the JCSG+ screen (0.2M Li2SO4, 0.1M NaOAc pH 4.5,
50.0% v/v PEG-400) which forms heavy precipitate when made here in the
lab. I assume that the order of addition of compounds might make a
difference, but doesn't this suggest that there is a high probabilty for
salt crystals under these conditions? How stable can this solution be? An
anecdotes/advice appreciated.
I assume that the PEG makes Li2SO4 fall out of solution, or perhaps it is
the pH?
Jacob Keller
*******************************************
Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
Dallos Laboratory
F. Searle 1-240
2240 Campus Drive
Evanston IL 60208
lab: 847.491.2438
cel: 773.608.9185
email: j-kell...@northwestern.edu
*******************************************
