Posted 13 September 2016 - 07:46 AM
I am not for sure on this, but the only thing I can guess is that the magnesium threonate did not react and you formed magnesium bicarbonate at the end via the reaction of magnesium and the CO2 from the carbonated water. Because carbonated water, H2CO3, is in constant equilbrium with CO2 and H2O (as well as H2O in constant equilibrium with H+ and OH-) there is constantly CO2 and H2O in the system, until it eventually escapes. The residue disappearing probably means that what is remaining is magnesium threonate, magnesium bicarbonate and water.
Mg + CO2 --> MgCO3, magnesium carbonate
MgCO3 + H2CO3 --> Mg(HCO3)2, magnesium bicarbonate
Mg (aq) + 2 OH- (aq) --> Mg(OH)2 (s) also occurs, but
Mg(OH)2 + 2CO2 --> Mg(HCO3)2
(MgCO3 + H2O --> Mg(OH)2 + H2CO3 does not occur, unless heat is applied)
Magnesium bicarbonate exists only in aqueous solution, so there has to be some water still remaining. Drying the result would form MgCO3 via
Mg + HCO3- --> MgCO3 + CO2 + H2O,
And of course the magnesium threonate would still remain.
I don't know if this is all completely correct, but I don't think the magnesium threonate would separate into magnesium and threonic acid, as the acid dissociation constant for threonic acid is 3.4 while for carbonic acid it is 3.6, which is negligible of a difference for an acid-base reaction to occur. I also don't think the result would be magnesium threonate (bi)carbonate, but rather, a combination of MgT and Mg(HCO3)2. Anyone correct me if I am wrong, but this is my best guess.