Sulfur, molecular toxicity

It is found, in general, that if the relative toxicity of a number of poisons containing, for instance, a toxic sulfur atom is determined, the toxicity per gram atom of sulfur increases with the molecular weight of the sulfur compoimd, i.e., with the complexity of the chain or ring attached to the sulfur atom. This result would be expected from the dependence of molecular toxicity on the covering power of the poison. 

The degree to which normally nontoxic structures attached to a toxic sulfur atom may influence the molecular toxicity has been studied more systematically (Maxted and Evans, 36) by measuring the relative toxici-ties of a homologous series of alkyl thiols and sulfides in which simple hydrocarbon chains of varying lengths are linked to sulphur, a single chain of this nature being present in the thiols, and two in the sulfide. 

Fig. 11. Influence of molecular size on toxicity. The diagram, which refers to the relative toxicity of a series of ra-thiols and sulfides, shows the effect on the molecular toxicity of the attachment of one or of two normally nontoxic aliphatic chains, of increasing length, to a sulfur atom.

If, on the other hand, the two sulfur atoms, instead of occupying terminal positions, are adjacent to one another, little change in toxicity occurs thus, diethyl disulfide, (C2Hj)2S2, does not differ greatly in molecular toxicity from diethyl sulfide, (C2H6)2S, again in spite of its doubled sulfur content. 

Health and Safety Factors. Sulfur hexafluoride is a nonflammable, relatively unreactive gas that has been described as physiologically inert (54). The current OSHA standard maximum allowable concentration for human exposure in air is 6000 mg/m (1000 ppm) TWA (55). The Underwriters Laboratories classification is Toxicity Group VI. It should be noted, however, that breakdown products of SF, produced by electrical decomposition of the gas, are toxic. If SF is exposed to electrical arcing, provision should be made to absorb the toxic components by passing the gas over activated alumina, soda-lime, or molecular sieves (qv) (56). 

Thiuram Sulfides. These compounds, (8) and (9), are an important class of accelerator. Thiurams are produced by the oxidation of sodium dithiocarbamates. The di- and polysulfides can donate one or more atoms of sulfur from their molecular stmcture for vulcanization. The use of these compounds at relatively high levels with litde or no elemental sulfur provides articles with improved heat resistance. The short-chain (methyl and ethyl) thiurams and dithiocarbamates ate priced 2/kg. Producers have introduced ultra-accelerators based on longer-chain and branched-chain amines that are less volatile and less toxic. This development is also motivated by a desire to rninirnize airborne nitrosamines. 

Ethylenethiourea (ETU) is a toxic decomposition product/metabolite of alky-lenebis(dithiocarbamates). This compound could be generated during processing of treated crops at elevated temperature. Different chromatographic methods to determine the residue levels of ETU have been published. After extraction with methanol, clean-up on a Gas-Chrom S/alumina column and derivatization (alkylation) with bro-mobutane, ETU residues can be determined by GC with a flame photometric detector in the sulfur mode. Alternatively, ETU residues can also be determined by an HPLC method with UV detection at 240 nm or by liquid chromatography/mass spectrometry (LC/MS) or liquid chromatography/tandem mass spectrometry (LC/MS/MS) (molecular ion m/z 103). ... 

The very toxic sulfur(IV)-oxide, S02, is a bent molecule (C2v symmetry) that reacts as a reducing agent due to the higher stability of the oxidation state +VI for sulfur. S02 can be easily condensed to a colourless liquid (b.p. —10.0 °C) that has been shown to be a versatile solvent for several reactions, and below —75.5 °C a colourless molecular solid is formed. In the solid state, the S02 molecule shows a bond length SO of 1.429 A and an angle O-S-O of 119°.3... 

With regard to high-molecular-weight sulfur compounds, Maxted (36) demonstrated that at low temperatures the specific toxicity of sulfur increases with the size of the molecule. The molecule is adsorbed by means of sulfur atom anchorage, around which the free rotation of the carbon chain inhibits the adsorption of reactants on all of the adjacent surface. 

At low temperatures characteristic of conditions for hydrogenation of organic compounds, sulfur adsorption may occur associatively, in which case the toxicity of the poison is dependent upon molecular size, structure, and strength of adsorption. 

A further special area of propulsion systems is Chemical Thermal Propulsion (CTP). CTP is defined in contrast to STP (solar thermal propulsion) and NTP (nuclear thermal propulsion). In CTP, in a very exothermic chemical reaction in a closed system, heat but no pressure is generated since the products of the reaction are solid or liquid. The heat energy is then transferred to a liquid medium (the propellant) using a heat exchanger, which is responsible for the propulsion of for example, the torpedo. Suitable propellants are e.g. water (the torpedo can suck it in directly from its surroundsings) or H2 or He, due to their very low molecular or atomic masses. The basic principles of CTP can also be used in special heat generators. A good example for a chemical reaction which is suitable for CTP is the reaction of (non-toxic) SF6 (sulfur hexafluoride) with easily liquified lithium (m.p. 180 °C) ... 

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