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Sodium chlorate

Sodium chlorate (Na Cl O₃) is an oxidizing agent. It is mostly used to produce chlorine dioxide for bleaching paper pulp, but is also used as a herbicide and to prepare other chlorates. Production in the United States is about 441,000 tons per year.

When pure, it is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 250 °C to release oxygen and leave sodium chloride.

Synthesis

Industrially, sodium chlorate is synthesised from the electrolysis of hot sodium chloride solution in a mixed electrode tank:

NaCl + 3H₂O → NaClO₃ + 3H₂

It can also be synthesised by passing chlorine gas to a hot sodium hydroxide solution. It is then purified by crystallization.

Uses

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:

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 roadsides, fenceways, ditches and suchlike.

Sodium chlorate is also used as a defoliant and desiccant for:

cotton,
safflower,
corn,
flax,
peppers,
soybeans,
grain sorghum,
southern peas,
dry beans,
rice and
sunflowers.

If used in combination with atrazine, it increases the persistence of the effect. If used in combination with 2,4-D, it improves performance of the material. 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.

Chemical oxygen generation

Chemical oxygen generator for example in commercial aircraft provide emergency oxygen to passengers to protect them from drops in cabin pressure by catalytic decomposition of sodium chlorate. The catalyst is normally some iron powder and barium peroxide (Ba O₂) is used to absorbe the chlorine which is a minor product in the decomposition.

Sodium chlorate is used in some aircraft as a source of supplemental oxygen. Iron powder is mixed with sodium chlorate and ignited by a charge activated by pulling on the emergency mask. The reaction produces more oxygen than is required for combustion.

Due to its oxidative nature, it 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 enzymatic pathway - in addition, this enzyme is directly denatured by chlorate reducing its activity. Therapy with ascorbic acid and methylene blue may be effective, 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 renal failure.

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, that is other herbicides, sulfur, phosphorus, powdered metals, strong acids. Particularly when mixed with sugar it has explosive properties.

Marketed formulations contain a fire depressant, 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

The active ingredient sodium chlorate is found 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. In the United Kingdom there are many brands of Sodium Chlorate Weedkillers. Two well known ones are Doff and Wilkinsons. In Italy it is found under the Zapi brand, sold as "Grasskiller Liquid".

Cultural References

In the May 2006 Exploding Pants episode of MythBusters, Sodium Chlorate was used as the secret ingredient to explore the story of 1931 New Zealand farmers, such as Richard Buckley, whose trousers exploded or caught on fire, leading to a number of deaths.
James Watson of Massey University in New Zealand won an Ig Nobel Prize for agricultural history in October 2005 for his research into the exploding trousers of the early 1930s in New Zealand.