There are some videos about how to do this on Youtube, but they lack some details along with dosage calculations.
Magnesium Carbonate (MgCO3) – archaic name magnesia alba
This form of magnesium seems very easy on the gut, and seems to have all the same effects one would expect from other forms of magnesium. It is simply a mixture of Milk of Magnesia and Carbonated Water. Some of the videos online going over how to make it don’t account for the various water bottle sizes and therefore, they don’t account for the variation in the amount of Carbon Dioxide for which the Milk of Magnesia to react. It isn’t the end of the world if it doesn’t fully react because the sediment just settles on the bottom of the bottle. Unfortunately, it tastes much more nasty with unreacted Milk of Magnesia in the bottle.
I find that in one 25.3 ounce carbonated spring water I can only fit about two tablespoons and one teaspoon of milk of magnesia. A little more than that will leave unreacted milk of magnesia sediment. Which is gross and can make you have diarrhea. Some videos claim they use 3 tablespoons, but I think they’re using bigger bottles which therefore have more Carbon Dioxide.
I get my generic Milk of Magnesia from a grocery store chain (Winn-Dixie) here in Louisiana. Make sure you check the inactive ingredients to verify Sodium Hypochlorite isn’t listed as that is a form of chlorine which can negatively affect the thyroid.
If you don’t believe this reaction takes place, it is actually commonly used in “high purity industrial routes include a path through magnesium bicarbonate: combining magnesium hydroxide and carbon dioxide. A slurry of magnesium hydroxide is treated with 3.5 to 5 atm of carbon dioxide below 50 °C, giving the soluble bicarbonate, then vacuum drying the filtrate, which returns half of the carbon dioxide as well as water.” The chilled nature of the Carbon Dioxide seems important. That is why you should first refrigerate your water for at least 12 hours getting its temperature to 4°C. This seems to work better than room temperature carbonated water.
The reaction is as follows:
Mg(OH)2 + 2 CO2 → Mg(HCO3)2
Mg(HCO3)2 → MgCO3 + CO2 + H2O
I don’t recommend microwaving or heating up this solution because it will cause dissolution.
Here is a calculation table so you know how much elemental magnesium you’re getting for various serving sizes. I like Gerolsteiner water, but that doesn’t mean you have to use it. Recommend any of the glass spring waters that are carbonated.
Supplemental Magnesium Carbonate may affect intracellular potassium levels. So be sure to pay attention to your potassium intake as well. It’s a balance thing. “Magnesium also plays a key role in the active transport of calcium and potassium ions across cell membranes, a process that is important for nerve impulse conduction, muscle contraction, vasomotor tone and normal heart rhythm.”