Sodium sesquicarbonate

Identifiers
CAS Number	533-96-0 Y
3D model (JSmol)	Interactive image
ECHA InfoCard	100.007.802
EC Number	208-580-9
PubChem CID	10791
UNII	Y1X815621J Y
CompTox Dashboard (EPA)	DTXSID6049646
InChI InChI=1S/2CH2O3.3Na/c2*2-1(3)4;;;/h2*(H2,2,3,4);;;/q;;3*+1/p-3 Key: WCTAGTRAWPDFQO-UHFFFAOYSA-K
SMILES C(=O)(O)[O-].C(=O)([O-])[O-].[Na+].[Na+].[Na+]
Properties
Chemical formula	Na3H(CO3)2·2H2O
Appearance	white, needle-like
Density	2.112 g/cm3 (dihydrate)
Solubility in water	dihydrate 13 g/100 mL (0 °C) 42 g/100 mL (100 °C)
Refractive index (nD)	1.5073 (dihydrate)
Structure
Crystal structure	monoclinic (dihydrate)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Sodium sesquicarbonate (systematic name: trisodium hydrogendicarbonate) Na₃H(CO₃)₂ is a double salt of sodium bicarbonate and sodium carbonate (NaHCO₃ · Na₂CO₃), 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.

The dihydrate, Na₃H(CO₃)₂ · 2H₂O, occurs in nature as the evaporite mineral trona.^[1]

Due to concerns about the toxicity of borax which was withdrawn as a cleaning and laundry product, sodium sesquicarbonate is sold in the European Union (EU) as "Borax substitute".^[2] It is also known as one of the E number food additives E500(iii).

Uses

Sodium sesquicarbonate is used in bath salts, swimming pools, as an alkalinity source for water treatment, and as a phosphate-free product replacing the trisodium phosphate for heavy duty cleaning.^[3][4]

Sodium sesquicarbonate is used in the conservation of copper and copper alloy artifacts that corrode due to contact with salt (called "bronze disease" due to its effect on bronze). The chloride from salt forms copper(I) chloride. In the presence of oxygen and water, even the small amount of moisture in the atmosphere, the cuprous chloride forms copper(II) chloride and hydrochloric acid, the latter of which dissolves the metal and forms more cuprous chloride in a self-sustaining reaction that leads to the entire destruction of the object. Treatment with sodium sesquicarbonate removes copper(II) chlorides from the corroded layer.^{[citation needed]}

It is also used as a precipitating water softener, which combines with hard water minerals (calcium- and magnesium-based minerals) to form an insoluble precipitate, removing these hardness minerals from the water.^[5] It is the carbonate moiety which forms the precipitate, the bicarbonate being included to moderate the material's alkalinity.