Vapours and gases

Metal ions are most eommonly measured using atomie absorption speetrometry. In this teehnique 

Here a ehemieal reaetion produees a moleeule with eleetrons in an exeited state. Upon deeay to the ground state the liberated radiation is deteeted. One sueh example is the reaetion between ozone and nitrie oxide to form nitrogen dioxide emitting radiation in the near infra-red in the 0.5-3/x region. The teehnique finds use for measuring nitrie oxide in ambient air or staek emissions. 

Use is made of eolour ehanges resulting from reaetion of pollutant and ehemieal reagents eolour intensity indieates eoneentration of pollutant in the sample. Reaetion ean take plaee in solution or on solid supports in tubes or on paper strips, e.g. litmus or indieator paper. Quantitative assessment of eolour formation ean also be determined using visible speetroseopy. Instruments are ealibrated 

In electrochemical cells sample oxidation produces an electric current proportional to the concentration of test substance. Sometimes interferences by other contaminants can be problematic and in general the method is poorer than IR. Portable and static instruments based on this method are available for specific chemicals, e.g. carbon monoxide, chlorine, hydrogen sulphide. 

Coulometry measures the amount of cunent flowing dirough a solution in an electrochemical oxidation or reduction reaction and is capable of measuring at ppm or even ppb levels of reactive gases. Thus a sample of ambient air is drawn through an electrolyte in a cell and the required amount of reactant is generated at the electrode. This technique tends to be non-specific, but selectivity can be enhanced by adjustment of pH and electrolyte composition, and by incorporation of filters to remove interfering species. 

Metal ions are most commonly measured using atomic absorption spectrometry. In this technique 

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Active solids are widely used as adsorbents of gases and vapours, and the specific surface is the most important parameter for characterizing their... 

S. Brunauer, The Adsorption of Gases and Vapours , Oxford University Press (1945). 

Oil in BOCB or MOCB This decomposes into vapourized and dissociated hydrocarbon, which in turn ionizes into H2 and other gases and vapours. Ht constitutes around 70% of all the gases and vapours produced. 

Refer to the general arrangement of this breaker in Figure 19.4. In this device the moving contacts make and break in an oil bath. When the arc is formed during an interruption, the oil becomes decomposed due to excessive heat, and produces a few gases and vapours such as H2... 

LEL (lower explosive, OR FLAMMABLE, LIMIT) The minimum eoneentration of a gas, vapour, mist or dust in air at a given pressure and temperature that will propagate a flame when exposed to an effieient ignition souree. Generally expressed as % by volume for gases and vapours, and as mg/m for mists or dusts. 

Table 4.1 Densities of some toxic gases and vapours relative to air at 20°C ...

Simple asphyxiant. Some gases and vapours present at high concentrations act as asphyxiants by reducing the oxygen content of air. Many of these are odourless and colourless. Many also pose a fire or explosion risk, often at values below which asphyxiation can occur. (Although capable of asphyxiation, they are not considered to be substances hazardous to health under COSHH.)... 

Liquids and solids do not burn as such, but on exposure to heat vaporize or undergo thermal degradation to liberate flammable gases and vapours which burn. Some chemicals undergo spontaneous combustion (see page 214). 

The density of a vapour or gas at eonstant pressure is proportional to its relative moleeular mass and inversely proportional to temperature. Sinee most gases and vapours have relative moleeular masses greater than air (exeeptions inelude hydrogen, methane and ammonia), the vapours slump and spread or aeeumulate at low levels. The greater the vapour density, the greater the tendeney for this to oeeur. Gases or vapours whieh are less dense than air ean, however, spread at low level when eold (e.g. release of ammonia refrigerant). Table 6.1 ineludes vapour density values. 

Analyses of gases and vapours tend to utilize the teehniques deseribed on page 308. Many of these methods were traditionally limited to laboratory analyses but some portable instruments are now available for, e.g., gas ehromatography (Table 10.16) and non-dispersive infra-red speetrometry (Table 10.17). 

Protocol for assessing the performance of a pumped sampler for gases and vapours... 

Respirators for gases and vapours eomprise a faeepieee and a eontainer filled with a speeifie adsorbent for the eontaminant. Care must be taken to seleet the eoiTeet type. More than one eanister ean be attaehed. 

However, the transport of a penetrant in glassy polymers is much more complex. It has been clearly shown 21.23.33> gases and vapours sorb in glassy polymers... 

Experimental values of diffusivities are given in Table 10.2 for a number of gases and vapours in air at 298K and atmospheric pressure. The table also includes values of the Schmidt number Sc, the ratio of the kinematic viscosity (fx/p) to the diffusivity (D) for very low concentrations of the diffusing gas or vapour. The importance of the Schmidt number in problems involving mass transfer is discussed in Chapter 12. 

Table 10.2. Diffusivities (diffusion coefficients) of gases and vapours in air at 298 K and atmospheric...

For an ideal gas, the total molar concentration Cj is constant at a given total pressure P and temperature T. This approximation holds quite well for real gases and vapours, except at high pressures. For a liquid however, CT may show considerable variations as the concentrations of the components change and, in practice, the total mass concentration (density p of the mixture) is much more nearly constant. Thus for a mixture of ethanol and water for example, the mass density will range from about 790 to 1000 kg/m3 whereas the molar density will range from about 17 to 56 kmol/m3. For this reason the diffusion equations are frequently written in the form of a mass flux JA (mass/area x time) and the concentration gradients in terms of mass concentrations, such as cA. 

Tabic 4. Specific heats at constant pressure of gases and vapours at 101.3 kN/mJ ... 

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