Nitroglycerine, stable form

There are two modifications of nitroglycerine, differing in freezing point and crystalline forms. With respect to crystal structure the form melting at the lower temperature represents a labile form (Fig. 6) which can be transformed spontaneously into the higher melting stable form of nitroglycerine (Fig. 7). 

The freezing and melting temperatures of these transformations have been established by Kast [14] who also observed that very pure nitroglycerine is transformed on freezing mainly into the labile crystals, whereas a less pure sample often crystallizes in the stable form.,... 

Hackel has extended Hibbert s observations concerning the influence of certain substances on the formation of nitroglycerine crystals of different forms. Thus, adding urethane, phenylurethane, collodion, nitrocotton, cyclonite, tetryl, dinitrobenzene, or centralites I and II promotes the formation of the labile form. Admixtures of siliceous earth, trinitrobenzene, TNT, favour the creation of the stable form. 

Hibbert [15] assumed that the labile form of nitroglycerine belongs to the triclinic system, while according to Flink [19] the stable form has orthorhombic bipyramidal crystals. 

On the basis of the dipole moment values and viscosity measurements of the two forms of nitroglycerine, de Kreuk [20] considered that the difference between the two forms is produced by rotational isomerism. According to this hypothesis the labile and stable forms would correspond to cis- and trans-isomers respectively. In a non-polar solvent the traits form predominates. In a polar solvent the content of the cis form increases and reaches a maximum in liquid nitroglycerine. 

The heat developed during the transformation of one form of nitroglycerine into the other is given on p. 46. The linear crystallization rate of nitroglycerine to form the stable modification at different temperatures was investigated by Nauckhoff [18] who found the following figures ... 

The specific heat of nitroglycerine was determined by Nauckhoff [18] as 0.356 cal/g and that of the crystalline substance (stable form) as 0.315 cal/g. For liquid nitroglycerine Belayev [45] published the value 0.4 cal/g. 

Dinitrodiglycol exists in two crystalline forms—a stable form melting at a temperature of +2°C and a labile one whose melting point is —10.9°C (thus resembling nitroglycerine). At 20°C it is an oily liquid of density 1.385 g/cm3, boiling and simultaneously decomposing at 160°C. Its refractive index is 1.4517 (at 20°C). 

Crystal line speed of stable nitroglycerine phase formed at different temperamre is listed in Table 5.28. 

The specific heat of nitroglycerine is 1.49 J/g. The specific heat of its crystal (stable form) is 0.315 cal/g. The specific heat of liquid nitroglycerine is 1.318 J/g. The crystallization heat (freezing) of nitroglycerine is listed in Table 5.42. 

Pure nitroglycerin is a stable Hquid at temperate conditions. It decomposes above 60°C to form nitric oxides which in turn catalyze further decomposition. Moisture increases the rate of decomposition under these conditions. Double- and multibase propellants containing nitroglycerin have substantially shorter stabiHty Hves at 65 and 80°C than do single-base propellants. The decomposition of nitroglycerin proceeds as... 

Dilution or simple mixing with a stable compound is sufficient to stabilise an unstable substance. In the case of a simple mixture with a neutral substance, this stabilisation process is called desensitisation . Thus hardeners such as benzoyl peroxide are nomially in the form of suspensions in heavy esters or oils. This peroxide is mixed with 30% of water by weight. Dynamite is nitroglycerine stabilised with the help of a neutral material. In all these cases, heat that is produced by the potential beginning of decomposition is absorbed by the inert substance. 

Nitroglycerine is toxic to handle, causes headaches, and yields toxic products on detonation. It is insoluble in water but readily dissolves in most organic solvents and in a large number of aromatic nitro compounds, and forms a gel with nitrocellulose. The acid-free product is very stable, but exceedingly sensitive to impact. Some of the properties of nitroglycerine are presented in Table 2.7. 

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