ascorbic acid is vitamin c with a low ph ie acidic whereas sodium carbonate is washing soda with a high ph ie alkaline. They can be mixed in an acid/base reaction to form sodium ascorbate when in the correct quantities will increase the ph ie reduce the acidity of the vitamin c making it easier on the stomach. A similar process can be done using sodium bicarbonate which is usually more likely considered to be food grade and reacts better with acids due to it's higher ph.
A similar process can be used with magnesium carbonate which is alkaline and usually a bright white powder which turns clear when it reacts with sufficient acid. This forms magnesium ascorbate a magnesium salt which is quite well absorbed providing magnesium and vitamin c which is gentle on the stomach. I use this myself. Magnesium carbonate is fairly inexpensive and provides about 28% magnesium by weight so works out fairly good value. Magnesium carbonate can be mixed with many different acids to enhance it's absorption for example malic acid to form magnesium malate.