When zinc chloride, which may be taken as a representative of the bivalent group, is treated with sodium carbonate, a difficultly soluble carbonate is precipitated, since zinc hydroxide, like the remaining bivalent hydroxides, is a sufficiently strong base to form a fairly stable carbonate.[388] When ferric chloride, a representative of the trivalent group, is treated with a solution of sodium carbonate, ferric hydroxide, mixed with some basic ferric carbonate[389] Fe_{2}(OH)_{4}CO_{3}, is precipitated and carbon dioxide escapes (‹exp.›). The trivalent hydroxides are too weak bases[390] to form stable salts with so weak an acid as carbonic acid. 2 FeCl_{3} + 3 Na_{2}CO_{3} + 6 H_{2}O ⥂  2 Fe(OH)_{3} ↓ + 3 H_{2}CO_{3} + 6 NaCl 3 H_{2}CO_{3} ⇄ 3 H_{2}O + 3 CO_{2} ↑. Since the bivalent metal ions are precipitated by sodium carbonate as carbonates and the trivalent ones as hydroxides, the reagent, obviously, cannot be used to separate the two groups. But ‹barium carbonate is so little soluble in water that it will not precipitate manganous, zinc, nickel, cobalious and ferrous carbonates›[391] from solutions of their chlorides or nitrates. We have, for instance, ZnCl_{2} + BaCO_{3} ↓ ⥃ BaCl_{2} + ZnCO_{3}. Barium carbonate has, however, the same effect on ferric chloride (‹exp.›) and on the other chlorides of the trivalent group, as has sodium carbonate, ‹i.e.› it precipitates their ‹hydroxides›. By means of barium carbonate [p195] ‹we can, therefore, precipitate the hydroxides of the aluminium group without precipitating the ions of the zinc group.› The separation is carried out in a, practically, neutral medium (free carbonic acid in excess is evolved; barium carbonate alone, when treated with water, is slightly alkaline) and thus avoids the error of facilitating the precipitation of the bivalent metals in the shape of salts of the acidic forms of the trivalent metals, i.e. as aluminates, chromites, and so forth. Manganous salts are liable to oxidation to manganic salts, when exposed to the air, especially in alkaline, neutral or ‹slightly› acid solutions, and prolonged exposure of the barium carbonate mixture to the air may result in the precipitation of manganic hydroxide, Mn(OH)_{3}, with the other trivalent hydroxides. Provision is made for its detection in the systematic analysis.