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Precipitation in NaOH on shelf


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#1 Andy Lane

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Posted 17 March 2009 - 07:49 AM

Hi

So, I went to my 5M NaOH today to make some miniprep solutions, and found it had a bunch of white precipitate in it. I made the solution myself around a year ago.

Does anyone have any idea what might have happened to it? Is it likely to be an actual precipitate, or is it some kind of contamination?

Thanks! Apologies in advance if this is a really dumb question.

Andy

#2 klinmed

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Posted 17 March 2009 - 10:15 AM

Hi

So, I went to my 5M NaOH today to make some miniprep solutions, and found it had a bunch of white precipitate in it. I made the solution myself around a year ago.

Does anyone have any idea what might have happened to it? Is it likely to be an actual precipitate, or is it some kind of contamination?

Thanks! Apologies in advance if this is a really dumb question.

Andy


The precipitate is sodium carbonate. This is formed by reaction of the NaOH with atmospheric carbon dioxide. In 5M NaOH, in contrast to in water, sodium carbonate is essentially insoluble.
If you use NaOH only to adjust the pH of solutions it is normally OK to just allow the precipitate to sink and use the supernatant NaOH. This will contain only low levels of carbonate but will be slightly more dilute NaOH than when it was fresh.
For alkaline lysis plasmid preps I would make up a fresh solution every few months.
By the way, it is best to store strong alkali in plastic (polypropylene) containers as they can attack glass resulting i sodium silicate contamination.

#3 Andy Lane

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Posted 17 March 2009 - 11:01 AM

Thanks for your answer! It was in a glass bottle; it may well have not been properly closed in the couple of weeks since I'd used it last. I'll make up more today in a polypropylene bottle.

#4 phage434

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Posted 17 March 2009 - 02:06 PM

Today is NaOH day. Everyone celebrates this joyous day by neutralizing and throwing away their old bottle of 5M NaOH and then remaking it. Thanks for reminding me of this important holiday, coincident with St. Patrick's day, every year. Make this part of YOUR holiday traditions! (Green dye is not recommended).




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