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Author: Hans Lohninger, Wikipedia
Sodium sulfate is an important compound of sodium. When anhydrous, it is a white crystalline solid of formula Na2SO4. The decahydrate, Na2SO410H2O, is known as Glauber's salt. Sodium sulfate is mainly used for the manufacture of detergents and in the Kraft process of paper pulping, though it has many other uses. About half of the world's production is from the natural mineral form of the decahydrate (mirabilite), and half from by-products of chemical processes.
Physical and chemical propertiesSodium sulfate is chemically very stable - it does not decompose, even if heated, and it does not react with oxidizing or reducing agents at normal temperatures. At high temperatures, it can be reduced to sodium sulfide. It is a neutral salt, with a pH of 7 when dissolved in water, because it is derived from a strong acid (sulfuric acid) and a strong base (sodium hydroxide).
In aqueous solution, some reactions are possible. Sodium sulfate reacts with an equivalent amount of sulfuric acid to give an equilibrium concentration of acid salts, such as sodium hydrogen sulfate:
Na2SO4(aq) + H2SO4(aq) 2 NaHSO4(aq)
Na2SO4(aq) + BaCl2(aq) 2 NaCl(aq) + BaSO4(s)
Sodium sulfate has unusual solubility characteristics in water, as shown in the graph at the right. Its solubility rises more than tenfold between 0 °C to 32.4 °C, where it reaches a maximum of 49.7 g Na2SO4 per 100 g water. At this point the solubility curve suddenly changes, and the solubility becomes almost independent of temperature. If sodium chloride is added, the solubility is markedly lower. Such changes provide the basis for the use of sodium sulfate in passive solar heating systems, as well is in the preparation and purification of sodium sulfate.
This nonconformity can be explained in terms of hydration, since 32.4 °C corresponds with the temperature at which the crystalline decahydrate (Glauber's salt) changes to give a sulfate liquid phase and an anhydrous solid phase.
OccurrenceAbout half of the world's production of the decahydrate (Glauber's salt) is from the natural mineral form mirabilite - found in lake beds in southern Saskatchewan, for example. In 1990, Mexico and Spain were the world's main producers of natural sodium sulfate (each around 500 000 tons), with Russia, USA and Canada also important (around 350 000 tons each).
Anhydrous sodium sulfate occurs in arid environments as the mineral thenardite, which is less common than mirabilite. It slowly turns to mirabilite in damp air.
ManufactureAbout half of the world's sodium sulfate comes from natural sources (see above), while the other half is produced as a by-product of other processes. The most important of these is the production of hydrochloric acid from sodium chloride (salt) and sulfuric acid (the Mannheim process), in which case the Na2SO4 is known as salt cake:
2 NaCl + H2SO4 Na2SO4 + 2 HCl
Alternatively, it can be produced from sulfur dioxide using the Hargreaves process:
4 NaCl + 2 SO2 + O2 + 2 H2O 2 Na2SO4 + 4 HCl
In the US and UK, one of the largest sources of synthetic Na2SO4 is as a by-product of sodium dichromate manufacture. There is also a myriad of processes where leftover sulfuric acid is neutralised by sodium hydroxide, giving sodium sulfate as a by-product. This method is also the most convenient laboratory preparation.
2 NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2 H2O(l)
Bulk sodium sulfate is usually purified via the decahydrate form, since the anhydrous form tends to attract iron compounds and organic compounds. The anhydrous form is easily produced from the hydrated form by gentle warming.
UsesIn 1995, bulk sodium sulfate sold for around $70 per tonne in the US, making it a very cheap material. Probably the largest use for sodium sulfate today is as a filler in powdered home laundry detergents. Total consumption of Na2SO4 in Europe was around 1.6 million tons in 2001, of which 80% was used for detergents. However this use is waning, as domestic consumers switch to liquid detergents which do not include sodium sulfate.
Another major use for Na2SO4, particularly in the US, is in the Kraft process for the manufacture of wood pulp. Organics present in the "black liquor" from this process are burnt to produce heat, needed to drive the reduction of sodium sulfate to sodium sulfide. However this process is being replaced to some extent by newer processes; use of Na2SO4 in the US pulp industry declined from 980 000 tons in 1970 to only 210 000 tons in 1990.
The glass industry also provides another significant application for sodium sulfate, consuming around 30 000 tons in the US in 1990 (4% of total US consumption). It is used as a "fining agent", to help remove small air bubbles from molten glass. It also fluxes the glass, and prevents scum formation of the glass melt during refining.
Sodium sulfate is important in the manufacture of textiles, particularly in Japan. It helps in "levelling", reducing negative charges on fibres so that dyes can penetrate evenly. Unlike the alternative sodium chloride, it does not corrode the stainless steel vessels used in dyeing.
Glauber's salt, the decahydrate, was formerly used as a laxative. It has also been proposed for heat storage in passive solar heating systems. This takes advantage of the unusual solubility properties (see above), and the high heat of crystallisation (78.2 kJ/mol). Other uses for sodium sulfate include frosting windows, in carpet fresheners, starch manufacture and as an additive to cattle feed. In the laboratory, anhydrous sodium sulfate is widely used as an inert drying agent for organic solutions; Na2SO4 is added to the solution until the crystals no longer clump together.
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