Sodium chlorate

Sodium chlorate is an inorganic compound with the chemical formula NaClO3. It is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 300 °C to release oxygen and leave sodium chloride. Several hundred million tons are produced annually, mainly for applications in bleaching Pulp to produce high brightness Paper.

Synthesis
Industrially, sodium chlorate is produced by the electrolysis of a hot sodium chloride solution:
NaCl + 3 H2O →  NaClO3 + 3 H2:

This reaction progresses in heat (at least 70 degrees Celsius), and controlled pH. In lower temperature or with high pH another reaction progresses:
2 NaCl + H2O → NaClO + NaCl + H2

The sodium chlorate process is not to be confused with the Chloralkali process, which is an industrial process for the electrolytic production of sodium hydroxide and chlorine gas.

It can also be synthesized by passing chlorine gas through a solution of sodium hydroxide. It is then purified by crystallization.

Uses
The main commercial use for sodium chlorate is for making chlorine dioxide (ClO2). The largest application of ClO2, which accounts for about 95% of the use of chlorate, is in bleaching of pulp.  All perchlorate compounds are produced industrially by the oxidation of solutions of sodium chlorate by electrolysis.

Herbicides
Sodium chlorate is used as a non-selective herbicide. It is considered phytotoxic to all green plant parts. It can also kill through root absorption.

Sodium chlorate may be used to control a variety of plants including morning glory, canada thistle, johnson grass, bamboo, Ragwort, and St John's wort. The herbicide is mainly used on non-crop land for spot treatment and for total vegetation control on areas including roadsides, fenceways, and ditches. Sodium chlorate is also used as a defoliant and desiccant for:

If used in combination with atrazine, it increases the persistence of the effect. If used in combination with 2,4-D, performance is improved. Sodium chlorate has a soil sterilant effect. Mixing with other herbicides in aqueous solution is possible to some extent, so long as they are not susceptible to oxidation.

The sale of sodium chlorate as a weedkiller was banned in the European Union in 2009, with existing stocks to be used within the following year.

Chemical oxygen generation
Chemical oxygen generators, such as those in commercial aircraft, provide emergency oxygen to passengers to protect them from drops in cabin pressure.  Oxygen is generated by high-temperature decomposition of sodium chlorate:
2 NaClO3  →  2 NaCl  +  3 O2

Heat required to initiate this reaction is generated by oxidation of a small amount of iron powder mixed with the sodium chlorate, and the reaction consumes less oxygen than is produced. Barium peroxide (BaO2) is used to absorb the chlorine that is a minor product in the decomposition.
An ignitor charge is activated by pulling on the emergency mask. Similarly, the Solidox welding system used pellets of sodium chlorate mixed with combustible fibers to generate oxygen.

Toxicity in humans
Due to its oxidative nature, sodium chlorate can be very toxic if ingested. The oxidative effect on hemoglobin leads to methaemoglobin formation, which is followed by denaturation of the globin protein and a cross-linking of erythrocyte membrane proteins with resultant damage to the membrane enzymes. This leads to increased permeability of the membrane, and severe hemolysis. The denaturation of hemoglobin overwhelms the capacity of the G6PD metabolic pathway. In addition, this enzyme is directly denatured by chlorate reducing its activity.

Therapy with ascorbic acid and methylene blue are frequently used in the treatment of methemoglobinemia. However, since methylene blue requires the presence of NADPH that requires normal functioning of G6PD system, it is less effective than in other conditions characterized by hemoglobin oxidation.

Acute severe hemolysis results, with multi-organ failure, including DIC and kidney failure. In addition there is a direct toxicity to the proximal renal tubule. The treatment will consist of exchange transfusion, peritoneal dialysis or hemodialysis.

Formulations 
Sodium chlorate comes in dust, spray and granule formulations. There is a risk of fire and explosion in dry mixtures with other substances, especially organic materials, and other herbicides, sulfur, phosphorus, powdered metals, and strong acids. In particular, when mixed with sugar, it has explosive properties. If accidentally mixed with one of these substances it should not be stored in dwellings.

Marketed formulations contain a fire retardant, but this has little effect if deliberately ignited. Most commercially available chlorate weedkillers contain approximately 53% sodium chlorate with the balance being a fire depressant such as sodium metaborate or ammonium phosphates.

Trade names
Sodium chlorate is the active ingredient in a variety of commercial herbicides. Some trade names for products containing sodium chlorate include Atlacide, Defol, De-Fol-Ate, Drop-Leaf, Fall, Harvest-Aid, Kusatol, Leafex, and Tumbleaf. The compound may be used in combination with other herbicides such as atrazine, 2,4-D, bromacil, diuron, and sodium metaborate.

Sodium chlorate was an extensively used weed killer within the EU, until 2009 when it was withdrawn after a decision made under terms of EU Regulations. Its use as a herbicide outside the EU remains unaffected, as does its use in other non-herbicidal applications, such as in the production of chlorine dioxide biocides and for pulp and paper bleaching.

Cultural references
Historian James Watson of Massey University in New Zealand wrote a widely reported article, "The Significance of Mr. Richard Buckley's Exploding Trousers" about accidents with sodium chlorate when used as a herbicide to control ragwort in the 1930s. This later won him an Ig Nobel Prize in 2005, and was the basis for the May 2006 "Exploding Pants" episode of MythBusters.

See also
Sodium chloride

References

Further reading
"Chlorate de potassium. Chlorate de sodium", Fiche toxicol. n° 217, Paris:Institut national de recherche et de sécurité, 2000. 4pp.

External links

International Chemical Safety Card 1117

Category:Chlorates
Category:Sodium compounds
Category:Desiccants
Category:Pyrotechnic oxidizers
Category:Oxidizing agents