nitroglycerin, a powerful explosive and an important ingredient of most forms of dynamite. It is also used with nitrocellulose in some propellants, especially for rockets and missiles, and medically it is employed as a vasodilator in the easing of cardiac pain.
Pure nitroglycerin is a colourless, oily, somewhat toxic liquid having a sweet, burning taste. It was first prepared in 1846 by the Italian chemist Ascanio Sobrero by adding glycerol to a mixture of concentrated nitric and sulfuric acids. The hazards involved in preparing large quantities of nitroglycerin have been greatly reduced by widespread adoption of continuous nitration processes.
Nitroglycerin, with the molecular formula C3H5(ONO2)3, has a high nitrogen content (18.5 percent) and contains sufficient oxygen atoms to oxidize the carbon and hydrogen atoms while nitrogen is being liberated, so that it is one of the most powerful explosives known. Detonation of nitroglycerin generates gases that would occupy more than 1,200 times the original volume at ordinary room temperature and pressure; moreover, the heat liberated raises the temperature to about 5,000 °C (9,000 °F). The overall effect is the instantaneous development of a pressure of 20,000 atmospheres; the resulting detonation wave moves at approximately 7,700 metres per second (more than 17,000 miles per hour). Nitroglycerin is extremely sensitive to shock and to rapid heating; it begins to decompose at 50–60 °C (122–140 °F) and explodes at 218 °C (424 °F).
The safe use of nitroglycerin as a blasting explosive became possible after the Swedish chemist Alfred B. Nobel developed dynamite in the 1860s by combining liquid nitroglycerin with an inert porous material such as charcoal or diatomaceous earth. Nitroglycerin plasticizes collodion (a form of nitrocellulose) to form blasting gelatin, a very powerful explosive. Nobel’s discovery of this action led to the development of ballistite, the first double-base propellant and a precursor of cordite.
A serious problem in the use of nitroglycerin results from its high freezing point (13 °C [55 °F]). This disadvantage is overcome by using mixtures of nitroglycerin with other polynitrates; for example, a mixture of nitroglycerin and ethylene glycol dinitrate freezes at −29 °C (−20 °F).