Chloropicrin

Chloropicrin, also known as PS, is a chemical compound currently used as a broad-spectrum antimicrobial, fungicide, herbicide, insecticide, and nematicide. Its chemical structural formula is Cl3CNO2.

Discovery
Chloropicrin was first discovered in 1848 by a Scottish chemist John Stenhouse. He prepared it by the reaction of a chlorinating agent with picric acid:
 * HOC6H2(NO2)3 + 11 NaOCl → 3 Cl3CNO2 + 3 Na2CO3 + 3 NaOH + 2 NaCl

Because of the precursor he used, Stenhouse named the compound chloropicrin, although the two compounds are structurally dissimilar.

Properties
Chloropicrin’s chemical formula is CCl3NO2 and its molecular weight is 164.38 grams/mole. Pure chloropicrin is a colorless liquid, with a boiling point of 112 °C. Chloropicrin is highly soluble in water with solubility of 2,000 milligrams per liter at 25 °C. It is volatile, with a vapor pressure of 23.2 millimeters of mercury (mm Hg) at 25 °C; the corresponding Henry’s Law Constant is 0.00251 atmosphere-cubic meter per mole. The octanol-water partition coefficient (Kow) of chloropicrin is estimated to be 269. Its soil adsorption coefficient (Koc; normalized to soil organic matter content) is 25 cm3/g.

Use
In agriculture, chloropicrin is injected into soil prior to planting a crop in order to clean the soil of a broad spectrum of fungi, microbes, insects, and other harmful pests. It is commonly used as a stand alone treatment or in combination / co-formulation with methyl bromide and 1,3-Dichloropropene.

Safety
At the national level, chloropicrin is regulated by the United States Environmental Protection Agency as a restricted use pesticide. The EPA has approved chloropicrin as safe for use by farmers nationwide. The distribution and use of chloropicrin is available only to licensed professionals and specially certified growers who are trained in its proper and safe use. In the US, occupational exposure limits have been set at 0.1 ppm over an eight-hour time-weighted average.

Agriculture
In 2008 the US EPA re-approved chloropicrin as safe for use in agricultural settings, stating that treatments "can provide benefits to both food consumers and growers. For consumers, it means more fresh fruits and vegetables can be cheaply produced domestically year-round because several severe pest problems can be efficiently controlled." To ensure chloropicrin is used safely, the EPA requires a strict set of protections for handlers, workers, and persons living and working in and around farmland during treatments. EPA protections were increased in both 2011 and 2012, reducing fumigant exposures and significantly improving safety. Protections include the training of certified applicators supervising pesticide application, the use of buffer zones, posting before and during pesticide application, fumigant management plans, and compliance assistance and assurance measures.

High Concentrations
Unlike its use in agriculture, in unregulated settings chloropicrin can be harmful to humans. Chloropicrin can be absorbed systemically through inhalation, ingestion, and the skin. At high concentrations it is severely irritating to the lungs, eyes, and skin. In World War I German forces used concentrated chloropicrin against Allied forces as a tear gas. While not as lethal as other chemical weapons, it caused vomiting and forced Allied soldiers to remove their masks to vomit, exposing them to other, more toxic chemical gases used as weapons during the war.

Preparation
Chloropicrin is manufactured by the reaction of nitromethane with sodium hypochlorite:
 * H3CNO2 + 3 NaOCl → Cl3CNO2 + 3 NaOH