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Topic: Calcium stability at high pH  (Read 2096 times)

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Offline f0n4x

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Calcium stability at high pH
« on: April 27, 2016, 10:48:44 PM »
Hey guys,
I've been working on a project with seawater and have been experimenting with trying to make a solution with about 2000 ppm of Ca2+ while maintaining pH Between 7 and 8. So far I haven't had much luck with any buffers at all. I get too much precipitate. I've used calcium chloride and calcium lactate but every time I try to add NaOH or NaHCO3 the solution gets precipitate. Does anyone have any recommendations for a different calcium source or buffer to get to the desired 2000 ppm while avoiding precipitates? Thanks.

Offline Borek

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Re: Calcium stability at high pH
« Reply #1 on: April 28, 2016, 03:11:03 AM »
2000 ppm Ca is 0.05 M. Ksp of Ca(OH)2 is 5.5×10-6. In 0.05 M CaCl2 solution the precipitate shouldn't show before pH 12 - unless there are other, interfering ions present.
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Offline Arkcon

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Re: Calcium stability at high pH
« Reply #2 on: April 28, 2016, 05:54:25 AM »
There is much about your application that's hard to understand.  You start with seawater being mentioned, then you don't mention it again.  Your concentration of 2000 ppm is quite a high concentration, roughly 2x to 5x of other ions in sea water, so you're no longer talking about seawater, but instead a concentrated brine.  Calcium lactate is a human dietary nutrient, its not particularly soluble, just reasonably well absorbed by the human GI tract, so I don't know why you picked that one over calcium chloride or calcium nitrate, unless you have to get your analytical reagents at the local drugstore.  You haven't mentioned what buffers you've tried.  And I don't understand how you call a solution of sodium carbonate and sodium hydroxide neutral.

Lucy, urm f0n4x:, you've got some 'splaning to do.
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