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Environmental fate & pathways

Hydrolysis

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Description of key information

The substance rapidly breaks down on contact with water to form hydrochloric and phosphorus acid.  Phosphorus acid decomposes in air when heated yielding phosphoric acid.  

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Additional information

The hydrolysis of phosphorus trichloride has been extensively investigated. Early work conducted on the area showed that on reaction of phosphorus trichloride in water, hydrochloric acid and phosphonic acid were formed.

Further studies by Mitchell (1925) suggested that there were intermediate stages of hydrolysis. About 98% of the final acidity was attained within 1 minute. The remainder was developed with decreasing rapidly during the next 30 minutes. An intermediate formation of an oxychloride such as P(OH)2Cl would account for this observation. However, also observed was an enhanced reducing power which was further investigated. However, this can not be attributed to the intermediate oxychloride and it is suggested that a second form, P(OH)3 was formed at this point which does not immediately change to the oridinary form, HPO(OH)2.

As phosphorus acid and phosphonic acid are tautomeric molecules. Tautomerisation occured to give the predominant form, phosphonic acid (HPO(OH)2).

A second product formed from the hydrolysis of phosphorus trichloride is hydrochloric acid which formed in the ratio of 3:1 with regard to phosphonic acid. It is a much stronger acid than phosphonic acid (pKa -7.0 versus 2.0) so therefore all effects on pH shift are mainly caused by HCl. The environmental effects of the hydrolysis products are also mainly due to the formation of hydrochloric acid.