I tried making some MgCl2 by reacting solutions of MgSO4 and CaCl2. Upon decanting and evaporating the liquid at just above the boiling point, I
tried to melt some with a torch. It did not melt, but instead appeared to turn to MgO. I assumed that there was probably still quite a bit of MgSO4
present, since I didn't know the exact purity and composition (hydration) of the CaCl2. That would explain why the decomposition to the oxide
vs. melting.
So I tried again. I added excess NaOH to a MgSO4 solution. I let the Mg(OH)2 settle, and dumped off the top part of the liquid. I then added more
water, shook it up, and allowed it to settle again. I repeated the washing 8 times over a period of about 24 hours. I then slowly added HCl until
the precipitate just disappeared. I checked the pH with a pH meter and it was around 6. The amount of HCl I needed agreed very closely with my
calculations based on the original amount of MgSO4. So this time I was sure I had MgCl2 of reasonable purity. I evaporated the water as before, and
tried heating it with a torch. Still it would not melt. Instead it gave off a strong odor of chlorine, and again changed to MgO. I could tell this
because while the original substance dissolves easily in water, the heated residue is insoluable. I tried leaving some of the MgCl2 out in the air
and sure enough it does become wet. So I know I have MgCl2, not MgSO4. So why won't it melt? CRC lists melting point around 700C and there is
no mention of decomposition even up to its boiling point of 1400C.vulture - 6-6-2004 at 12:51
Hydrated MgCl2 decomposes to magnesiumoxychloride and HCl upon heating:
If you try to make pure Mg via electrolysis you should add some NH4Cl to prevent the mentioned hydrolysis.
Also I guess some KCl is used that you have a mixture of salts which has a lower melting point than the pure MgCl2
magnesia cements
Mr. Wizard - 6-6-2004 at 13:40
MgO and MgCl2 combine to make a great cement, stronger than regular cement. There are quite a few variations on it. Check this link. http://www.premierchemicals.com/corner/articles/cements.htm
It looks like the sulfate forms an analogous cement, which I'm going to read up on.Theoretic - 7-6-2004 at 07:22
"Hydrated MgCl2 decomposes to magnesiumoxychloride and HCl upon heating:
2MgCl2 + H2O ---> 2HCl + Mg2OCl2"
Also, magnesium chloride is oxidized by oxygen on heating to magnesium oxychloride and chlorine...hodges - 7-6-2004 at 14:03
Quote:
Originally posted by Theoretic
"Hydrated MgCl2 decomposes to magnesiumoxychloride and HCl upon heating:
2MgCl2 + H2O ---> 2HCl + Mg2OCl2"
Also, magnesium chloride is oxidized by oxygen on heating to magnesium oxychloride and chlorine...
Thanks for the replies.
Yeah, I tried re-crystalizing with addition of excess NH4Cl and I got the same result (i.e. no melting, and conversion to an insoluable powder after
driving off the NH4Cl). I have seen that the heat of formation of MgO is less than MgCl2 so I didn't expect oxidation to occur - but apparently
the oxychloride (which I don't have a HF value for) is higher still, at least at elevated temperatures.
