User:Lawrence Mayer M.D., M.S./sandbox

Magnesium citrate is a salt of magnesium ion and citrate ion. Citrate contains 3 carboxylate -COO- groups, each of which is a monoprotic Brønsted–Lowry base (proton acceptor). That makes citrate a triprotic Brønsted–Lowry base: i.e. it has 3 protonation sites, one for each carboxylate. Citrate thus has n = 3-m available, vacant protonation sites (i.e. can accept n = 3-m protons) in protonation state m = 0,1,2,3.

Each of the lowest 3 protonation states (m = 0,1,2) yields a different salt with different chemical properties, namely magnesium citrate tribasic, magnesium citrate dibasic, and magnesium citrate monobasic (see table below). Of these, magnesium citrate tribasic and magnesium citrate dibasic are by far the most studied and commercially important, with only a few references to magnesium citrate monobasic. Depending on context, "magnesium citrate" means any one of these 3 salts, a mixture of 2 or more, or a generic term for all 3.

The molecular formula for magnesium citrate (generic) is Mg2+x(H+m(COO-)3C3H4OH)y, where m = 0,1,2 is the citrate protonation state, and x and y, determined by electroneutrality, are the number of magnesium and citrate ions per salt molecule, respectively. This molecular formula is not unique to magnesium citrate but shared by isomers such as magnesium isocitrate.

At highest citrate protonation state m = 3, magnesium citrate dissociates into magnesium ion + citric acid. Since citric acid is a weak triprotic Brønsted–Lowry acid, and citrate salt, its conjugate base, is a weak triprotic Brønsted–Lowry base, the mixture of citric acid + citrate salt (e.g. magnesium citrate) makes a useful buffer.