Preparation
Chlorine dioxide is a highly endothermic compound that can decompose extremely violently when separated from diluting substances. As a result, preparation methods that involve producing solutions of it without going through a gas phase stage are often preferred. Arranging handling in a safe manner is essential.
In the laboratory, ClO2 is prepared by oxidation of sodium chlorite:
- 2 NaClO2 + Cl2 → 2 ClO2 + 2 NaCl
Over 95% of the chlorine dioxide produced in the world today is made from sodium chlorate and is used for pulp bleaching. It is produced with high efficiency by reducing sodium chlorate in a strong acid solution with a suitable reducing agent such as methanol, hydrogen peroxide, hydrochloric acid or sulfur dioxide. Modern technologies are based on methanol or hydrogen peroxide, as these chemistries allows the best economy and do not co-produce elemental chlorine. The overall reaction can be written;
Chlorate + Acid + reducing agent → Chlorine Dioxide + By-products
The reaction of sodium chlorate with hydrochloric acid in a single reactor is believed to proceed via the following pathway:
- HClO3 + HCl → HClO2 + HOCl
- HClO3 + HClO2 → 2 ClO2 + Cl2 + 2 H2O
- HOCl + HCl → Cl2 + H2O
The commercially more important production route uses methanol as the reducing agent and sulfuric acid for the acidity. Two advantages by not using the chloride-based processes are that there is no formation of elemental chlorine, and that sodium sulfate, a valuable chemical for the pulp mill, is a side-product. These methanol-based processes provide high efficiency and can be made very safe.
A much smaller, but important, market for chlorine dioxide is for use as a disinfectant. Since 1999 a growing proportion of the chlorine dioxide made globally for water treatment and other small-scale applications has been made using the chlorate, hydrogen peroxide and sulfuric acid method, which can produce a chlorine-free product at high efficiency. Traditionally, chlorine dioxide for disinfection applications has been made by one of three methods using sodium chlorite or the sodium chlorite - hypochlorite method:
- 2 NaClO2 + 2 HCl + NaOCl → 2 ClO2 + 3 NaCl + H2O
or the sodium chlorite - hydrochloric acid method:
- 5 NaClO2 + 4 HCl → 5 NaCl + 4 ClO2 + 2 H2O
All three sodium chlorite chemistries can produce chlorine dioxide with high chlorite conversion yield, but unlike the other processes the chlorite-HCl method produces completely chlorine-free chlorine dioxide but suffers from the requirement of 25% more chlorite to produce an equivalent amount of chlorine dioxide. Alternatively, hydrogen peroxide may efficiently be used also in small scale applications.
Very pure chlorine dioxide can also be produced by electrolysis of a chlorite solution:
- 2 NaClO2 + 2 H2O → 2 ClO2 + 2 NaOH + H2
High purity chlorine dioxide gas (7.7% in air or nitrogen) can be produced by the Gas:Solid method, which reacts dilute chlorine gas with solid sodium chlorite.
- 2 NaClO2 + Cl2 → 2 ClO2 + 2 NaCl
These processes and several slight variations have been reviewed.
Read more about this topic: Chlorine Dioxide
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