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Author: Subject: Mg(OH)2 impurity
soma
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[*] posted on 29-6-2018 at 06:56
Mg(OH)2 impurity


After reacting MgSO4 with NaOH and thoroughly washing with water, I then reacted the Mg(OH)2 with ascorbic acid. The liquid magnesium ascorbate was slightly cloudy.

I'm wondering what the impurity could be. There was enough water to completely dissolve the ascorbate and the pH was around 6.2 so there shouldn't have been any Mg(OH)2 unreacted.
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happyfooddance
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[*] posted on 29-6-2018 at 08:02


When you neutralize a base (esp. one which isn't very soluble) with a weak acid, it usually takes a little heat and a fair bit of time to reach equilibrium. I make magnesium citrate from the carbonate and citric acid on the regular and I have this problem.


Edit: Even when neutralizing CaCO3 with HCl, one needs to use an excess of acid and a fair bit of time/stirring to ensure complete conversion of the carbonate. I have had solutions stirring with a pH of 3, for an hour or more, that still have traces of carbonate! Believe it or not...

[Edited on 6-29-2018 by happyfooddance]
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Boffis
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[*] posted on 29-6-2018 at 09:30


You may have a little magnesium carbonate in the initial precipitiate due to the presence of a little carbonate in the sodium hydroxide. Magnesium carbonate MgCO3 is actually rather difficult and slow to dissolve in weak acids even when very finely divided and ascorbic acid is a rather weak acid. Add a little filter aid such as cellulose or diatomaceous earth and filter or just heat a little as happfooddance suggests.
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soma
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[*] posted on 1-7-2018 at 03:40


I've used MgCO3 in the past to make the ascorbate and the only problem was that there was carbonation of the ascorbate that slowly evaporated and so the pH would go too high. But the solution was always clear. I also used MgO which also gave a clear solution although it would take maybe 20 minutes to complete. I blend the ascorbic and carbonate, oxide, or hydroxide in a blender and then let it sit.
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