Destabilizing impurities

TATP is very sensitive to impurities. The primary destabilizing impurity present in the TATP synthesis is the residual sulfuric acid catalyst. TATP purity can be related to the extent of which the acid is removed or washed out of the final product. The TATP produced by the author is made in an exceedingly meticulous fashion and is as free of impurities as it can physically be. The common criminal or terrorist cannot be counted on to be as careful in his or her TATP preparations. 

We are, therefore, left with an explanation involving impurities. There are two possibilities (i) a stabilizing impurity that extends the achievable range of metastability is present only in some of the inclusions or (ii) a destabilizing impurity that reduces the achievable range of metastability is present in all experiments, except in some inclusions. 

In the next paper [160], Villain discussed the model in which the local impurities are to some extent treated in the same fashion as in the random field Ising model, and concluded, in agreement with earlier predictions for RFIM [165], that the commensurate, ordered phase is always unstable, so that the C-IC transition is destroyed by impurities as well. The argument of Villain, though presented only for the special case of 7 = 0, suggests that at finite temperatures the effects of impurities should be even stronger, due to the presence of strong statistical fluctuations in two-dimensional systems which further destabilize the commensurate phase. 

Decomposition due to contamination or contact with active surfaces. The rate of decomposition can be increased by the presence of soluble impurities and/or contact with active surfaces. High and low pH will also destabilize hydrogen peroxide. pH affects the activity of the catalytic impurities and the stabilizers which are present.47 Self-heating can rapidly accelerate the decomposition rate of destabilized hydrogen peroxide. Large amounts of oxygen and steam can be formed quickly (Table 1.4). 

Hydrogen peroxide, when supplied commercially, is usually stabilized with phosphates and tin(IV) materials. The tin compounds are effective at the product s natural pH via hydro-colloid formation, which adsorbs transition metals and reduces their catalytic activity. In the majority of cases, extra stabilization is not required when hydrogen peroxide or its derivatives are used in synthesis. Elevated temperatures and increased metal impurities all tend to destabilize peroxygens, and where such conditions are unavoidable, additional stabilizers may be employed, added either to the hydrogen peroxide or the reaction mixture separately. Stabilizer type falls into two categories seques-trants and radical scavengers. 

A-Nitro and acetyl-substituted 1,3,5,7-tetrazocanes are important compounds as explosives and propellants <1996CHEG-II(9)705>. In the syntheses of the nitro-substituted 1,3,5,7-tetrazocanes, their processing, and application, it is possible that they come into contact with ammonium nitrate, or they are directly mixed with this oxidant. Thermal reactivity of the nitro-substituted 1,3,5,7-tetrazocanes has been examined by means of nonisothermal differential thermal analysis <2005MI11>. It has been established that impurities of ammonium nitrate can destabilize some A-substituted 1,3,5,7-tetrazocanes and that this effect is due to acidolytic attack of nitric acid. 

Presence of impurities in excipients can have a dramatic influence on the safety, efficacy or stability of the drug product. Monomers or metal catalysts used during a polymerization process are toxic and can also destabilize the drug product if present in trace amounts. Due to safety concerns, the limit of vinyl chloride (monomer) in polyvinyl pyrrolidone is nmt 10 ppm, and for hydrazine (a side product of polymerization reaction) nmt 1 ppm. Monomeric ethylene oxide is highly toxic and can be present in ethoxylated excipients such as PEGs, ethoxylated fatty acids, etc. 

The reaction between residual Na" ions in HY and V impurities (such as V2O5) can lead to the formation of stable sodium vanadates [36]. Removal of these charge-compensating cations destabilizes the zeolite lattice. 

Each crystalline substance has a unique structure. Groups of compounds classified as isomorphous have similarities of lattice symmetry, but dimensions, and hence interionic forces, are different. Moreover, a particular substance can adopt alternative structures under changed conditions of temperature, pressure, crystallization conditions, presence of impurities, etc. Ordered packing, with symmetrical intracrystalline forces, appears to confer enhanced stability within the bulk solid so that decomposition processes usually occur at surfaces within a restricted reaction zone. Interfaces can be regarded variously as complex imperfections, zones of destabilizing strain, or (product) sites of catalytic activity. 

That the liquid hydrophobic oils formed above their respective melting temperatures are ineffective is a consequence of the metastability of the relevant pseudoemulsion films, as we have discussed elsewhere (see, e.g.. Section 3.3). In the case of the impure materials where a region of mixed solid and liquid phase exists, it seems likely that the solid component is able to destabilize those films. Similar effects have been described in oil-in-water emulsion systems where partial melting leads to instability of the oil-water-oil films due the presence of crystals at the oil-water surface [204-206]. We address the effect of particle destabilization of pseudoemulsion films on antifoam action in the following section. 

Decarburization of IHX alloys was examined under purposely designed impure helium atmospheres with a low carbon monoxide partial pressure (see Fig. 3.6) and various water vapor content. Fig. 3.9 shows cross-sections through AUoy 230 after exposure under decarburizing conditions at 950°C and 250 h. Under a very dry atmosphere, AUoy 230 shows no surface oxide. When the water vapor partial pressure is higher, fuUy porous surface oxide is formed. In aU cases, intergranular Cr-rich carbides have been totaUy dissolved and large W-rich primary carbides are partiaUy destabilized. 

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