Anhydrous hydrogen chloride, AHCl, is a colorless gas with a sharp, irritating odor. It is readily absorbed in water to form hydrochloric acid. It is very hydroscopic (attracts moisture) and in moist air, forms white fumes which are a mist of hydrochloric acid.
The boiling point of AHCl at one atmosphere pressure (14.7 psia, 760 mmHg) is -85°C (-121°F) and the vapor pressure of AHCl at ambient temperature of (20°C, 68°F) is 603 psia. Because of these extreme values, the pressure in equipment containing liquefied AHCl must be closely monitored and controlled.
Liquid AHCl has a high coefficient of thermal expansion. In designing systems, care should be taken to eliminate, in so far as possible, places where liquid AHCl and Liquid/gas AHCl mixtures can become completely sealed between two closed valves because pressure could build up resulting in a gasket or line failure. DO NOT ALLOW liquid AHCl to be trapped between closed valves – extremely high pressure can result.
More information on AHCl can be found in Pamphlet 99, Pamphlet 160, and Pamphlet 161.
AHCl will react with oxidizing agents to form chlorine gas. AHCl is nonflammable. However, in the presence of water, AHCl attacks most metals with release of flammable hydrogen gas. Potentially explosive mixtures of air and hydrogen could be formed in confined spaces or closed equipment and lines, and in equipment and lines after opening. Confined spaces should be well ventilated and equipment lines should be purged with inert gas or dry air until explosimeter tests show them to be free of hydrogen.
AHCl is a gas at atmospheric pressure. It is very irritating to the skin, eyes, and mucosal surfaces because of the rapid absorption by body moisture forming hydrochloric acid. It can cause serious burns. High or prolonged inhalation exposures may cause delayed pulmonary edema with cough, chest discomfort, and difficulty in breathing. Contact with vapor can damage the eyes. Prolonged overexposure can cause dental erosion. Ingestion may cause severe acid burns of the mouth, throat, esophagus, and stomach with burning pain of the mouth, throat, chest, and abdomen. Gross exposure may cause death.
The OSHA and ACGIH exposure limits for AHCl are 5 ppm or 7 mg/m3 and 7.5 mg/m3 TLV. These are the ceiling valves that should not be exceeded during any part of the work exposure.
The odor threshold is 1-5 ppm. 5-10 ppm will cause discomfort.
The exposure guidelines for HCL are: