Flammable Gas Elements

Substanees whieh reaet with water to liberate flammable gas, e.g. earbides (liberate aeetylene), alkali metals (hydrogen), organometallies (hydroearbons - see Table 6.8), and where the heat of reaetion is suffleient to ignite the gas. Thus metals whieh are less eleetronegative than hydrogen (see Table 6.10) will displaee this element from water or aleohols, albeit at different rates. 

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

Elemental hydrogen exists as a colorless, odorless, tasteless, diatomic gas with the lowest atomic weight and density of any known substance. This flammable gas melts at -259.14°C and boils at -252.8°C. 

Named by a French chemist, Lavoisier, hydrogen (H) is the first chemical element of the periodic table of elements with an atomic number of one. At standard temperature and pressure, hydrogen is a colourless, tasteless, odourless and easily flammable gas. With its atomic mass of 1.00797 g/mol, hydrogen is the lightest element. The British scientist, Henry Cavendish, was the first to identify H as a distinct element in 1766, publishing precise values for its specific weight and density (NHA, 2007). 

Consider the following information about the newly discovered element, vulcium, whose symbol is Vu. Vulcium is a solid at room temperature. It is easily cut by a penknife to reveal a shiny surface which tarnishes quite rapidly. It reacts violently with water, liberating a flammable gas and forms a solution with a pH of 13. When vulcium reacts with chlorine, it forms a white crystalline solid containing 29.5% chlorine. ... 

Cavendish discovered hydrogen in experiments like those that Boyle performed. He added iron metal to different acids and found that a flammable gas was produced. But Cavendish thought the flammable gas came from the iron and not from the acid. Chemists later showed that iron is an element and does not contain hydrogen or anything else. Therefore, the hydrogen in Cavendish s experiment came from the acid ... 

An astonishing thing about chemistry is that an element usually becomes completely unrecognizable when combined with another element. A highly flammable gas, hydrogen, combines with oxygen to form the chemical compound, water. 

Methane (METH-ane) is a colorless, odorless, tasteless, flammable gas that is less dense then air. It is the primary component of natural gas. Methane is the simplest of all hydrocarbons, organic compounds that contain carbon and hydrogen and no other elements. 

When a microscopic flammable gas mixture formed between neighboring droplets is coimected throughout the spray element, a premixed flame can propagate along this corridor and consume oxygen contained in the spray element. It should be noted that the flame propagation, which can be described by the conventional two-phase flow theories is restricted to this case. 

Gentry SJ, Walsh PT (1987) The theory of poisoning of catalytic flammable gas sensing elements. In Moseley PT, Tofield BC (eds) Solid state gas sensors. Adam Hilger, Bristol, Philadelphia, pp 32-50 Golovanov V, Pekna T, Kiv A, Litovchenko V, Korotcenkov G, Brinzari V, Comet A, Morante J (2005) The influence of stmctural factors on sensitivity of SnO -based gas sensors to CO in humid atmosphere. Ukr Phys J... 

To overcome the problems of gas-sensor failure due to poisoning of the elements, several manufacturers have used specific filter devices in front of the elements. For example, filters to remove alkyl lead compounds from petrol vapour have included paradichlorobenzene and fibrous silica (3). However, the most commonly used material is based on active charcoal. Such filters (4) effectively absorb many common poisons and inhibitors including silicones, alkyl lead compounds and halogen-containing species. However, along with the absorption of potentially harmful vapours, carbon filters will also absorb hydrocarbons. This effect in practice limits the use of such filters to detectors for C3 and lower hydrocarbons. They cannot be employed in general flammable gas detectors. 

The rapid growth in demand for catalytic flammable gas sensors to operate in hostile atmospheres (leaded petrol atmospheres, gas compressor stations, etc.) and areas where routine calibration and maintenance are difficult and costly (North Sea gas and oil platforms) has stimulated the search for sensing elements which are resistant to poisons. These devices generally take the form of a platinum wire coil embedded in a porous bead comprising active catalyst either dispersed throughout a porous substrate (5-7) or as discrete layers within the substrate (8). 

Two methods (15) of using these transducers can be identified. The first method, the most widely used in commercial gas detectors, is a non-isothermal method. In this case the temperature of the sensing element is allowed to rise as a result of chemical reaction at the catalyst surface and the rate of reaction, and hence the concentration of flammable gas is derived from the increase in temperature. In the second method the temperature of the sensing element is maintained constant during the reaction, and the reaction rate is obtained in terms of a difference in electrical power dissipation of the element under reacting and non-reacting conditions. 

In the non-isothermal method the catalytically active sensing element and a similar but catalytically inert element form two arms of a Wheatstone s bridge. Power is supplied to the circuit to heat the elements to their operating temperature, and the values of the fixed resistors selected such that the bridge balances in air. When flammable gas is passed over the elements, reaction takes place on the sensing element, imparting power to the element and thus... 

Phosphorus is the second element in Group 5A of the Periodic Table and, like nitrogen, has five electrons in its valence shell. Examples of trivalent phosphorus compounds are phosphine, PH3, and triphenylphosphine, PhjP. Phosphine is a highly toxic, flammable gas. Triphenylphosphine is a colorless, odorless solid. Because phosphorus is below nitrogen in the Periodic Table, phosphines are weaker bases than amines and good nucleophiles (Section 9.3E). Treatment of a phosphine with a methyl, primary, or secondary all l halide gives a phosphonium salt by an Sj 2 pathway. 

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