ACETYLENE

Acetylene
(Ethyne; welding gas)
CAS 74-86-2

HC CH

Colorless gas
bp -84 °C (sublimes), mp -82 °C
Slightly soluble in water (0.106 g/100 mL)

Odorless, although garlic-like or "gassy" odor often detectable because of trace impurities

0.91 (air = 1.0)

3.04 X 10 ⁴ mmHg (~40 atmospheres) at 16.8 °C

-18 °C

305 °C

LC₅₀ inhal (rat) simple asphyxiant (>500,000 ppm)

TLV-TWA (ACGIH) simple asphyxiant

Extremely flammable gas; simple asphyxiant.

Acetylene is relatively nontoxic and has been used as an anesthetic. Inhalation of acetylene can be hazardous because of its action as a simple asphyxiant. Concentrations of about 10% in air cause slight intoxication, and levels of 20% in air may produce headaches and labored breathing. At higher concentrations (33% and above), acetylene acts as a narcotic, causing unconsciousness in 7 min or less, with rapid and full recovery normally seen on removal from exposure of less than several hours. Concentrations of acetylene above 50% in air can cause death by asphyxiation within 5 min. Commercially available acetylene may contain highly toxic impurities, including phosphine, arsine, and hydrogen sulfide; the presence of these impurities must be considered in setting acceptable exposure levels to acetylene. For example, the concentration of acetylene containing 95 ppm of phosphine impurity (which has a TLV of 0.3 ppm) should not exceed 3160 ppm to stay within the TLV for phosphine.

There is no evidence that acetylene is a human carcinogen or reproductive toxin.

Acetylene is a highly flammable gas and forms explosive mixtures with air over an unusually wide range of concentrations (2 to 80%). Acetylene can polymerize exothermically, leading to deflagration. With a very high positive free energy of formation, acetylene
is thermodynamically unstable and is sensitive to shock and pressure. Its stability is enhanced by the presence of small amounts of other compounds such as methane, and acetylene in cylinders is relatively safe to handle because it is dissolved in acetone. Acetylene fires can be fought with carbon dioxide, dry chemical, and halon extinguishers; firefighting is greatly facilitated by shutting off the gas supply.

Acetylene forms highly unstable acetylides with many metals, including copper, brass, mercury, potassium, silver, and gold. The dry acetylides are sensitive, powerful explosives. Acetylene may react violently with fluorine and other halogens (chlorine, bromine, iodine) and forms explosive compounds on contact with nitric acid.

Acetylene should be handled in the laboratory using the "basic prudent practices" described in Chapter 5.C, supplemented by the additional precautions for dealing with extremely flammable substances (Chapter 5.F) and compressed gases (Chapter 5.H). In particular, acetylene should be used only in well-ventilated areas free of ignition sources. Acetylene is supplied in specially designed steel cylinders containing acetone and an inert material. More than 1 liter of the gas should never be stored in other containers. Brass or copper tubing, valves, or fittings should never be allowed to come in contact with acetylene. If acetylene must be purified, it should be passed through concentrated H₂ SO₄and NaOH (do not use activated carbon).

If large amounts of this compound are inhaled, move the person to fresh air and seek medical attention at once.

In the event of an acetylene leak, shut down and remove all ignition sources and ventilate the area at once to prevent flammable mixtures from forming. Carefully remove cylinders with slow leaks to remote outdoor locations. Limit access to an affected area. Respiratory protection may be necessary in the event of a large release or a leak in a confined area.

Excess acetylene should be returned to the vendor for disposal; disposal should not be attempted in the laboratory. Excess acetylene should be vented from reaction flasks, tubing, etc., rather than scrubbed with strong base to avoid the formation of acetylides.

The information in this LCSS has been compiled by a committee of the National Research Council from literature sources and Material Safety Data Sheets and is believed to be accurate as of July 1994. This summary is intended for use by trained laboratory personnel in conjunction with the NRC report Prudent Practices in the Laboratory: Handling and Disposal of Chemicals. This LCSS presents a concise summary of safety information that should be adequate for most laboratory uses of the title substance, but in some cases it may be advisable to consult more comprehensive references. This information should not be used as a guide to the nonlaboratory use of this chemical.

Copyright 1995 National Academy of Sciences. All rights reserved.