Sodium sesquicarbonate

Sodium sesquicarbonate (Systematic name trisodium hydrogendicarbonate) Na3H(CO3)2, which is a double salt of sodium bicarbonate and sodium carbonate, and has a needle-like crystal structure. However, the term is also applied to an equimolar mixture of those two salts, with whatever water of hydration the sodium carbonate includes, supplied as a powder.

Uses
Sodium sesquicarbonate is used in the desalination of copper objects to avoid the loss of surface colour, however, it's a slow process.

It is also used as a precipitating water softener, which combine with hardness minerals (calcium and magnesium based) to form an insoluble precipitate, removing these from the water. It is the carbonate moiety which forms the precipitate, the bicarbonate being included to moderate the material's alkalinity.