Hydrogen cyanide

CAS RN: 74-90-8

Volatilization

Volatilization is expected to be the dominant fate process for hydrogen cyanide(1). A pKa of 9.2(2) indicates hydrogen cyanide will exist partially in the anion form at pH values of 5 to 9(SRC). At pH <9.2, most of the free cyanide should exist as hydrogen cyanide, a volatile form of cyanide. Wide variations in the rate of volatilization are expected since this process is affected by a number of parameters including temperature, pH, wind speed, and cyanide concentration(1). The Henry's Law constant for the neutral species is 1.33X10-4 atm-cu m/mole(3). This Henry's Law constant indicates that hydrogen cyanide is expected to volatilize from water surfaces(4). Based on this Henry's Law constant, the volatilization half-life from a model river (1 m deep, flowing 1 m/sec, wind velocity of 3 m/sec)(4) is estimated as 5 hours(SRC). The volatilization half-life from a model lake (1 m deep, flowing 0.05 m/sec, wind velocity of 0.5 m/sec)(4) is estimated as 3 days(SRC). Hydrogen cyanide's Henry's Law constant indicates that volatilization from moist soil surfaces may occur(SRC). Hydrogen cyanide is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 742 mm Hg(5).
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