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 leaves 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 concentrated sodium chloride solutions. All other processes are obsolete. 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. The overall reaction can be simplified to the equation:
First, chloride is oxidised to form intermediate hypochlorite, ClO−, which undergoes further oxidation to chlorate along two competing reaction paths: (1) Anodic chlorate formation at the boundary layer between the electrolyte and the anode, and (2) Autoxidation of hypochlorite in the bulk electrolyte. Under electrolysis hydrogen and sodium hydroxide are formed at the cathode and chloride ions are discharged at the anode (mixed metal oxide electrode is often used). The evolved chlorine does not escape as a gas but undergoes hydrolysis:
The hydrolysis of chlorine is considered to be fast. The formation of H+ ions should make the boundary layer at the anode strongly acidic and this is observed at low chloride concentrations. However, large concentrations of chloride, as they occur in industrial chlorate cells, shift the hydrolysis equilibrium to the left. At the boundary layer the concentration of H+ is not high enough to permit diffusion into the bulk electrolyte. Therefore hydrogen is transported away from the anode mostly as hypochlorous acid rather than H+. The hypochlorous acid dissociates in the bulk electrolyte where the pH is high and the hypochlorite ion diffuses back to the anode. More than two thirds of the hypochlorite is consumed by buffering before reaching the anode. The remainder is discharged at the anode to form chlorate and oxygen:
The autoxidation of hypochlorous acid in the bulk electrolyte proceeds according to the simplified overall equation:
It is preceded by the dissociation of a part of the hypochlorous acid involved:
The reaction requires a certain distance from the anode to occur to a significant degree, where the electrolyte is sufficiently buffered by the hydroxyl formed at the cathode. The hypochlorite then reacts with the rest of the acid:
In addition to anode distance the autoxidation also depends on temperature and pH. A typical cell operates at temperatures between 80 °C and 90 °C and at a pH of 6.1–6.4. Independent of the reaction route the discharge of 6 mol of chloride is required to yield 1 mol of chlorate. However, the anodic oxidation route requires 50% additional electric energy. Therefore, industrial cells are optimised to favour autoxidation. Chlorate formation at the anode is treated as a loss reaction and is minimised by design. Other loss reactions also decrease the current efficiency and must be suppressed in industrial systems. The main loss occurs by the back reduction of hypochlorite at the cathode. The reaction is suppressed by the addition of a small amount of dichromate (1–5 g/L) to the electrolyte. A porous film of chromium hydroxide is formed by cathodic deposition. The film impedes the diffusion of anions to the cathode, whereas the access of cations and their reduction is facilitated. The film stops growing on its own after it reaches a certain thickness.
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 other, less important chlorates are derived from sodium chlorate, usually by salt metathesis with the corresponding chloride. 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 citing health dangers, 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:
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.
Oxygenless combustion Sodium chlorate can be mixed with sucrose sugar to make a highly energetic fuel, similar to that of gunpowder, that burns in airtight spaces. This is the reaction:
However this sodium chlorate is mostly replaced by potassium chlorate.
… excerpt ends here. Continue reading the full article.
