Fulminates, decompositions

Gamer and Hailes [462] postulated a chain branching reaction in the decomposition of mercury fulminate, since the values of n( 10—20) were larger than could be considered consistent with power law equation [eqn. (2)] obedience. If the rate of nucleation is constant (0 = 1 for the generation of a new nuclei at a large number of sites, N0) and there is a constant rate of branching of existing nuclei (ftB), the nucleation law is... 

The decomposition kinetics of mercury fulminate [725] are significantly influenced by ageing, pre-irradiation and crushing these additional features of reaction facilitated interpretation of the observations and, in particular, the role of intergranular material in salt breakdown. Following a slow evolution of gas ( 0.1%) during the induction period, the accelerator process for the fresh salt obeyed the exponential law [eqn. (8)] when a < 0.35. The induction period for the aged salt was somewhat shorter and here the acceleratory process obeyed the cube law [eqn. (2), n = 3] and E = 113 kj mole-1. 

Singh and Palkar [726] identified an initial deceleratory reaction in the decomposition of silver fulminate. This obeyed first-order kinetics (E = 27 kJ mole-1) and overlapped with the acceleratory period of the main reaction, which obeyed the power law [eqn. (2), n = 2] with E = 119 kj mole-1. The mechanism proposed included the suggestion that two-dimensional growth of nuclei involved electron transfer from anion to metal. 

Boddington and Iqbal [727] have interpreted kinetic data for the slow thermal and photochemical decompositions of Hg, Ag, Na and T1 fulminates with due regard for the physical data available. The reactions are complex some rate studies were complicated by self-heating and the kinetic behaviour of the Na and T1 salts is not described in detail. It was concluded that electron transfer was involved in the decomposition of the ionic solids (i.e. Na+ and Tl+ salts), whereas the rate-controlling process during breakdown of the more covalent compounds (Hg and Ag salts) was probably bond rupture. 

Explosions involving flammable gases, vapours and dusts are discussed in Chapter 5. In addition, certain chemicals may explode as a result of violent self-reaction or decomposition when subjected to mechanical shock, friction, heat, light or catalytic contaminants. Substances containing the atomic groupings listed in Table 6.7 are known from experience to be thermodynamically unstable, or explosive. They include acetylides and acetylenic compounds, particular nitrogen compounds, e.g. azides and fulminates, peroxy compounds and vinyl compounds. These unstable moieties can be classified further as in Table 6.8 for peroxides. Table 6.9 lists a selection of potentially explosive compounds. 

Mercury fulminate, readily formed by interaction of mercury(II) nitrate, nitric acid and ethanol, is endothermic (AH°f (s) +267.7 kJ/mol, 0.94 kJ/g) and was a very widely used detonator. It may be initiated when dry by flame, heat, impact, friction or intense radiation. Contact with sulfuric acid causes explosion [1], The effects of impurities on the preparation and decomposition of the salt have been described [2],... 

An unstable powerful oxidant, it explodes between 40 and 70°C, or on friction or impact, sensitivity being as great as that of mercury fulminate [1], Detonation occurs at 95°C, and under vacuum explosive decomposition occurs above 10°C [2], See Potassium permanganate Sulfuric acid... 

Thermal decomposition of pure explosives such as primary explosives lead azide, lead styphnate, mercury fulminate etc. [35], monomethylamine nitrate [36] and explosive mixtures RDX + HMX mixtures [37]. 

Singh [1] has examined infra-red spectra of mercuric, silver and lead fulminates, and Beck [2] those of sodium and potassium fulminates. The maxima 2147 and 1225 cm-1 were found to be characteristic of asymmetric and symmetric vibrations of the O—N—C group, respectively. The maximum 1181 cm-1 was assigned to the bending frequency of the same group [1]. Beck also found that the transient formation of an isomeric ion eN=C—O can occur on thermal decomposition of fulminates. 

Decomposition of methylnitrolic acid into fulminic acid and nitrous acid... 

Mercury fulminate also dissolves in many solutions of various salts, but in some of them (e.g. potassium iodide, sodium thiosulphate) it undergoes rapid decomposition. 

As previously stated, mercury fulminate is hydrolysed by heating in water in boiling water hydrolysis is very rapid. Farmer [31] noticed that on heating with water under pressure, mercury fulminate undergoes decomposition to metallic mercury. Marked decomposition also takes place on heating or standing for long periods at room temperature in an aqueous solution of ammonia or potassium... 

On boiling the solution takes on a violet colour. Dissolved in an aqueous solution of ammonia, fulminate decomposes even after 12 hr. On boiling a pyridine solution of fulminate, complete decomposition occurs. 

Mercury fulminate is relatively resistant to the action of dilute acids, in particular to that of nitric acid, but concentrated acids cause decomposition. Thus, under the influence of nitric acid decomposition occurs with evolution of NO, CO, acetic acid and mercuric nitrate. Under the influence of concentrated hydrochloric acid free fulminic acid is evolved (with an odour resembling that of hydrogen cyanide) as well as the decomposition products hydroxylamine hydrochloride, formic acid, mercuric chloride (Carstanjen and Ehrenberg [32] Scholl [33]). Mercury fulminate explodes on direct contact with concentrated sulphuric acid. 

Mercury fulminate undergoes rapid decomposition by the action of ammonium sulphide to form mercuric sulphide. The fulminate dissolves in sodium thiosulphate, according to the reaction ... 

Chemical stability and behaviour at higb temperatures. Mercury fulminate undergoes marked thermal decomposition even at 50°C. Rathsburg [37] found that a sample of the technical product stored at 50-60°C for 6 months in a dry atmo-... 

Fig. 30. Comparison of the rate of decomposition of mercury fulminate and other primary explosives at 75°C, according to Wallbaum [38].

Hess and Dietl [39] found that 0.5 g samples of fulminate at 90-95°C undergo partial decomposition to form substances with reduced explosive properties after 35-40 hr they also showed that after 75-100 hr a brown-yellow powder of low inflammability is produced. 

Aqueous solutions of organic acids such as formic, acetic, and oxalic, decompose mercury fulminate, forming the corresponding mercuric salts. On the other hand, the action of dilute inorganic acids involves decomposition with formation of C02. 

Farmer quotes the following figures for the time required for the production of 5 cm3 of gas by heating mercury fulminate (this corresponds to the decomposition of 11% of substance) ... 

Farmer s experiments were repeated and extended by Garner and Hailes [41]. They examined the behaviour of mercury fulminate at about 100°C and came to the conclusion that during the initial induction period, decomposition is accompanied by a slow evolution of gas at a constant velocity (linear decomposition). At the end of this phase the main decomposition period begins with an increased rate of gas evolution. The authors noticed that if the fulminate is finely ground, rapid evolution of gas begins at once, without any initial period. 

A number of later authors, e.g. Prout and Tompkins [42], Vaughan and Phillips [43] have confirmed that the thermal decomposition of mercury fulminate is a chain reaction. 

Fio. 31. Influence of various methods of treatment on the thermal decomposition of mercury fulminate, according to Bartlett, Tompkins and Young [45]. /4—pie-irradiated, B—crushed, C—aged. 

Curve A represents the decomposition of mercury fulminate irradiated with ultra-violet rays, curve B the decomposition of ground mercury fulminate, and curve C the decomposition of ordinary (freshly-prepared) mercury fulminate. 

Singh [24] noticed that when heated for a few minutes at a temperature nearing that of immediate decomposition mercury fulminate crystals undergo decomposi-... 

When crystals of mercury fulminate are heated at lower temperatures the decomposition reaction is localized mainly around lattice defects such as growth marks on the surface of crystals or points where dislocations emerge at the surface (Fig. 32(c)). 

The admixture of various substances acts in different ways bn the decomposition rate of fulminate inorganic acids accelerate decomposition organic acids... 

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