Autoclaving Media Versus Sterile Filter - (May/07/2008 )
You let it cool down at RT and keep it at 4 degree afterwards. We have common cold rooms in the institute. So it' s no problem to find a place for so many bottles:) And those bottles we only and only use for PBS.
The cap is only slightly loose just to balance the pressure inside and outside.
Well, something went wrong. After the PBS cooled to RT, there was a white precipitate at the bottom of each bottle. I am assuming it is due to excess evaporation. Can I just add some more water until the salts dissolve again? Is this a common ocurrence?
Hmmm. I don't think that this happened to us before. It' s the same thing with all liquid stuff you autoclave in glass or plastic. Of course, it will evaporate some but it should not be so much that it would precipitate.
If you think it is because of evaporation, you can screw it right after autoclaving. It should not harm I believe.
Are you sure if the bottles were really clean? (sorry, a bit stupid question)
Not a stupid question at all. The bottles were from a newly opened box from Fisher. I know we should probably have washed them before the first use, but I was anxious. Also, I figured since we were going to autoclave them anyway...Do you think this had something to do with the problem?
I don`t know 
Probably not.
Maybe something in your buffer? Is this the first time you are autoclaving the buffer?
I suppose there are many labs where PBS is autoclaved but I have never heard that it would be a problem.
Sorry, I cannot think of anything else.
You don't have magnesium in your "PBS" do you? Magnesium phosphate is insoluble and will precipitate out. People mean so many different things when they say PBS you can never tell.
Yes! It's actually called embryonic PBS (EPBS), and has magnesium chloride hexahydrate. Why does it form a precipitate after the autoclaving?
Hi,
just wonder how you sterile your PBS now? Back to sterile filtering?
Have you found out why magnesium would precipitate?
Cheers
I'm going to use the sterile filters. My hypothesis (although this is just based on my minimal knowledge of chemistry) is that when the water is heated during autoclaving, the water groups "protecting" the Mg come off, and allow the Mg and PO4 to form a salt and precipitate. Apparently, there are some more complicated protocols that involve autoclaving the Mg and PO4 salts in separate solutions and then combining afterwards, but that seems like more work (which is what I was trying to avoid in the first place). It turns out the sterile filters aren't all that expensive ($50 for a twelve-pack), anyway. Thanks, for the followup!
I normally autoclave PBS without Mg, and I have a 1000x Mg, Ca solution which is sterile filtered, and I add it before use.