Nitrates phase changes

Sohd ammonium nitrate occurs in five different crystalline forms (19) (Table 6) detectable by time—temperature cooling curves. Because all phase changes involve either shrinkage or expansion of the crystals, there can be a considerable effect on the physical condition of the sohd material. This is particularly tme of the 32.3°C transition point which is so close to normal storage temperature during hot weather. 

Ammonium nitrate undergoes phase changes at 32° and 83°C and melts at 170°. It is not normally considered an explosive when pure, although under suitable conditions it can be made to detonate. When mixed with small amounts of organic matter it becomes much more sensitive and several serious explosions have occurred with such mixtures. The limit of organic matter so far allowed in the U.K. is 0-05%, but in some countries 0 1% is accepted. 

These measurements clearly indicate that relatively large changes in the distributions of various components between the acid and hydrocarbon phases occur during the course of a batch nitration. Such changes can have important effects on the rate of nitration. 

Ammonium nitrate-containing pyrotechnic gas-generating formulations may suffer from phase changes in a crystalline structure associated with volumetric expansion during temperature cycling. Potassium nitrate and cupric oxide have a synergistic effect in stabilizing ammonium ni-... 

The enormous differences in temperature one can create depend (disregarding energy dissipation) on heat output, rate of reactioo, and on the physical properties of the ingredients and the products. Of these factors, the phases of the products—solid, liquid, or gaseous —are undoubtedly the most important. We need only think of a certain amount of thermite compared with the same amount of an ammonium nitrate propellant, both of identical heat output per unit weight, to appreciate the extremes of behavior due to phase.changes. 

Rates of nitration determined over a range of temperatures in two-phase dispersions have been used to calculate energies of activation from 59—75 kj/mol (14—18 kcal/mol). Such energies of activation must be considered as only apparent, since the tme kinetic rate constants, NO2 concentrations, and interfacial area all change as temperature is increased. 

Anionic and neutral polymers are usually analyzed successfully on Syn-Chropak GPC columns because they have minimal interaction with the appropriate mobile-phase selection however, cationic polymers adsorb to these columns, often irreversibly. Mobile-phase selection for hydrophilic polymers is similar to that for proteins but the solubilities are of primary importance. Organic solvents can be added to the mobile phase to increase solubility. In polymer analysis, ionic strength and pH can change the shape of the solute from mostly linear to globular therefore, it is very important to use the same conditions during calibration and analysis of unknowns (8). Many mobile phases have been used, but 0.05-0.2 M sodium sulfate or sodium nitrate is common. 

Mobile-phase selection for cationic polymers is similar to that for the other polymers in that ionic strength and pH can change the shape of the solute from linear to globular (9). Mobile phases are often low pH e.g., 0.1% trifluo-roacetic acid, including 0.2 M sodium chloride, has been used successfully for polyvinylpyridines. Sodium nitrate can be substituted for the chloride to avoid corrosive effects. Some salt must be included so that ion exclusion does not occur (3). 

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