Vapour pressure, 4.18

The vapour pressure of a liquid provides an essential safety parameter and it is mandatory that safety sheets contain these values (when they are known). This parameter is taken into account in some classification methods of inflammability risk. It enables one to determine the equilibrium vapour concentration of a liquid in air. This concentration can then be used to ascertain whether a working environment presents an inflammability risk (by reference to the inflammability limits) or a toxicity hazard (by comparison with the exposure values). 

But it will also be seen that vapour pressure estimation methods provide critical analysis of all parameters involved in a fire hazard and thus allow refinement of the methods leading to a quantification of this risk. 

1 Published data problems posed by experimental unrartainty 

There are two disadvantages to the existing vapour pressure tables. Rrst of all, like any experimental data, there is no agreement between sources. This is worsened by the decision to take only one value into account for each chemical substance. This fact may encourage the user to take on trust the figure proposed, which is sometimes unjustified. Secondly, these values are given for a temperature that does not always correspond to the thermal conditions in which the chemical substance will be handled. Some references, to overcome this difficulty, offer several values. For instance, Weka most often gives three values, 20, 30 and 50°C, and the coefficients A, B, C in Antoine s equation can thus be calculated  

There are to be found lists of chemical substances in handbooks for each of which log P = f (T), and whose coefficients are to be inserted, are given. These lists are limited but nevertheless provide solutions for the most common chemical substances. When there are several experimental estimates of vapour pressures it is possible to estimate the importance of the experimental uncertainty from the standard deviation of the measurements. The relevance of the values can be verified from a series of different sources (to be rigorous, checking that it is a Gaussian sequence would be required). 

At 200°C, the vapour pressure must first be estimated, from the Antoine equation  

If the normal boiling point (vapour pressure = 1 atm) and the critical temperature and pressure are known, then a straight line drawn through these two points on a plot of log-pressure versus reciprocal absolute temperature can be used to make a rough estimation of the vapour pressure at intermediate temperatures. 

Several equations have been developed to express vapour pressure as a function of temperature. One of the most commonly used is the three-term Antoine equation, Antoine (1888)  

Several studies have measured vapour pressures of the FTOHs [59,64, 71-73] (Table 3.2) however, reported values are variable, differing by several orders of magnitude. Vapour pressures of the FTOHs are influenced by the length of the perfluoroalkyl chain, and increase as length of the perfluoroalkyl chain decreases. Reported vapour pressures of the FTOHs, at 25 °C, range from minimum values of 0.2 Pa (10 2 FTOH) to 216 Pa (4 2 FTOFl) reported by Krusic et al. [72] to maximum values of 53 Pa (10 2 FTOH) to 1670 Pa (4 2 FTOH) reported by Lei et al. [71]. 

Two studies [71, 74] in the peer-reviewed literature have investigated the vapour pressures of the FSAs (Table 3.2). Similar to the FTOHs, reported values are variable, differing by several orders of magnitude. The vapour pressure of NEtFOSE has also been determined by the 3M Company [75]. It is possible that some of the difficulties encountered in measuring vapour pressures of the FTOHs are also applicable to the FSAs. 

Vapour pressures of the free acid forms of PFCAs have also been determined and values vary by several orders of magnitude (Table 3.2). Kaiser et al [76] measured the vapour pressures of PFCAs and extrapolated values at 25 °C, which range from 4.19 Pa (PFOA) to 8.03 X 10 Pa (PFDoA). The vapour pressure of the free acid form of PFOS, at 20 °C, has been reported as 3.31 x 10 Pa [70]. Vapour pressures of the dissociated forms of the PFSAs and PFCAs have not been determined and are expected to be much less. 

The residence time of chemicals in soil and water is, among other factors, determined by the volatility of the substances - the tendency to evaporate into the air compartment. Partitioning and transport of substances between environmental media are thus affected by vapour pressure ip ). Highly volatile chemicals have the potential for rapid, long-distance dispersion in the atmosphere, and can be taken up by terrestrial animals by inhalation or skin absorption, by terrestrial plants through stomatal pores or the cuticle, as well as into water bodies. 

Vapour pressure p represents the partial pressure of a compound above the pure solid or liquid phase at thermal equilibrium it corresponds to a steady state with a continuous exchange, but no net transfer, of molecules between the two phases. From thermodynamic considerations, the vapour pressure of a chemical is determined by its enthalpy of vaporization (A/f ) and the temperature (7) as described by the Clausius-Clapeyron equation  

Because is not directly accessible, the total enthalpy change may be aggregated from the individual processes involved (Yalkowsky and Mishra, 1991), which include the heat capacities and enthalpies of melting and boiling as well as the melting point (TJ and the boiling point (7 ), with the melt- 

For illustrative purposes, vapour pressure may be portrayed as solubility in air. This parameter is strongly dependent on the ambient temperature if measured at different temperatures, the logarithm of can be linearly related to the reciprocal temperatures (K). For most liquids, vapour pressure ranges between 10 and 4 x 10 Pa at room temperature. It is experimentally accessible using a (mercury) manometer to measure the pressure established in the gas phase above the pure compound at defined temperatures. For volatile chemicals p 100 Pa), measured data are generally accurate, whereas for low-volatility compounds (p 100 Pa), the experimental results may scatter by one order of magnitude (Schwarzenbach, Geschwend and Imboden, 1993). 

No validated QSARs are available to predict directly from chemical structure, but there are several methods for calculating p based on derivations of the Clausius-Clapeyron equation (Table 4.4). 

Gas masks contain activated charcoai which absorbs toxic gases in the surrounding air. This photograph of a family out shopping in London was taken during a tear gas test in 1941 (World War II). 

The forces that bind the adsorber to adsorbent may be physical (i.e. intermolecu-lar forces) or chemical (when chemical bonds are formed). The adsorption of gases by charcoal is physical, whereas the adsorption of gases on some catalysts is chemical. Toxic gases adsorb to charcoal better than the oxygen and nitrogen of the air because they are often relatively large molecules and are frequently polar. This increases the intermolecular forces between adsorbate and adsorbent. 

The charcoal used in gas masks is called activated charcoaT, and is made by heating wood or coal in carbon dioxide, water vapour or in a limited supply of air. Activated charcoal has an enormous surface area - crucial to adsorption - as high as 1000 m of gas per gram of charcoal At a given temperature and pressure, a sample of activated charcoal will adsorb only a fixed mass of adsorbate, but the mass increases as the pressure of adsorbate gas is increased or if the temperature is lowered. The charcoal maybe regenerated by heating, which drives off the adsorbate. 

Adsorption will also occur in solution. For example, charcoal beds are used to adsorb trace organic compounds (such as pesticides and dyes) from water. 

A vapour is a gas in contact with a liquid of the same substance. For example, the water gas above the surface of liquid water is described as water vapour. The gas pressure of the water vapour is known as its vapour pressure. 

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At z in the curve, however (the minimum of vapour pressure), the solution and vapour are in equilibrium and the liquid at this point will distil without any change in composition. The mixture at z is said to be azeotropic or a constant boiling mixture. The composition of the azeotropic mixture does vary with pressure. 

Raoult s law When a solute is dissolved in a solvent, the vapour pressure of the latter is lowered proportionally to the mole fraction of solute present. Since the lowering of vapour pressure causes an elevation of the boiling point and a depression of the freezing point, Raoult s law also applies and leads to the conclusion that the elevation of boiling point or depression of freezing point is proportional to the weight of the solute and inversely proportional to its molecular weight. Raoult s law is strictly only applicable to ideal solutions since it assumes that there is no chemical interaction between the solute and solvent molecules. 

Oil True vapour pressure (TVP), base sediment and water (BS W) content,... 

Vapour Pressure Base Sed. and Water Teperature Salinity... 

Experiments on sufficiently dilute solutions of non-electrolytes yield Henry s laM>, that the vapour pressure of a volatile solute, i.e. its partial pressure in a gas mixture in equilibrium with the solution, is directly proportional to its concentration, expressed in any units (molar concentrations, molality, mole fraction, weight fraction, etc.) because in sufficiently dilute solution these are all proportional to each other. 

Figure A2.3.6 illustrates the corresponding states principle for the reduced vapour pressure P and the second virial coefficient as fiinctions of the reduced temperature showmg that the law of corresponding states is obeyed approximately by the substances indicated in the figures. The useflilness of the law also lies in its predictive value.

Figure A2.5.1. Schematic phase diagram (pressure p versus temperature 7) for a typical one-component substance. The full lines mark the transitions from one phase to another (g, gas liquid s, solid). The liquid-gas line (the vapour pressure curve) ends at a critical point (c). The dotted line is a constant pressure line. The dashed lines represent metastable extensions of the stable phases.

Figure A2.5.11. Typical pressure-temperature phase diagrams for a two-component fluid system. The fiill curves are vapour pressure lines for the pure fluids, ending at critical points. The dotted curves are critical lines, while the dashed curves are tliree-phase lines. The dashed horizontal lines are not part of the phase diagram, but indicate constant-pressure paths for the T, x) diagrams in figure A2.5.12. All but the type VI diagrams are predicted by the van der Waals equation for binary mixtures. Adapted from figures in [3].

As a special development in recent years, SEMs have been designed which no longer necessitate high vacuum (enviromnental SEM, ESEM variable pressure SEM, VPSEM). This development is important for the imaging of samples with a residual vapour pressure, such as aqueous biological or medical samples, but also samples in materials science (wet rock) or organic chemistry (polymers). 

Flere is the volume of gas required to saturate the monolayer, V the total volume of gas adsorbed, P the sample pressure, P the saturation vapour pressure and C a constant related to the enthalpy of adsorption. The resulting shape of the isothemi is shown plotted in figure Bl.26.6 for C = 500. A plot of P/V(P - Pq) against P/Pq should give a straight line having a slope (C - )/y C and an intercept The BET surface area is... 

Accurate enthalpies of solid-solid transitions and solid-liquid transitions (fiision) are usually detennined in an adiabatic heat capacity calorimeter. Measurements of lower precision can be made with a differential scaiming calorimeter (see later). Enthalpies of vaporization are usually detennined by the measurement of the amount of energy required to vaporize a known mass of sample. The various measurement methods have been critically reviewed by Majer and Svoboda [9]. The actual teclmique used depends on the vapour pressure of the material. Methods based on... 

Apart from the techniques described in this chapter other methods of organic film fonnation are vacuum deposition or film fonnation by allowing a melt or a solution of the material to spread on the substrate and subsequently to solidify. Vacuum deposition is limited to molecules with a sufficiently high vapour pressure while a prerequisite for the latter is an even spreading of the solution or melt over the substrate, which depends on the nature of the intennolecular forces. This subject is of general relevance to the fonnation of organic films. 

The equilibrium vapour pressure, P, over a curved surface is defined by tlie Kelvin equation... 

White phosphorus is very reactive. It has an appreciable vapour pressure at room temperature and inflames in dry air at about 320 K or at even lower temperatures if finely divided. In air at room temperature it emits a faint green light called phosphorescence the reaction occurring is a complex oxidation process, but this happens only at certain partial pressures of oxygen. It is necessary, therefore, to store white phosphorus under water, unlike the less reactive red and black allotropes which do not react with air at room temperature. Both red and black phosphorus burn to form oxides when heated in air, the red form igniting at temperatures exceeding 600 K,... 

Phosphorus(V) oxide is an extremely effective desiccating agent, reducing the vapour pressure of water over it to a negligibly small... 

Iodine is a dark-coloured solid which has a glittering crystalline appearance. It is easily sublimed to form a bluish vapour in vacuo. but in air, the vapour is brownish-violet. Since it has a small vapour pressure at ordinary temperatures, iodine slowly sublimes if left in an open vessel for the same reason, iodine is best weighed in a stoppered bottle containing some potassium iodide solution, in which the iodine dissolves to form potassium tri-iodide. The vapour of iodine is composed of I2 molecules up to about 1000 K above this temperature, dissociation into iodine atoms becomes appreciable. 

These can be prepared by electrolytic oxidation of chlorates(V) or by neutralisation of the acid with metals. Many chlorates(VII) are very soluble in water and indeed barium and magnesium chlorates-(VII) form hydrates of such low vapour pressure that they can be used as desiccants. The chlorate(VII) ion shows the least tendency of any negative ion to behave as a ligand, i.e. to form complexes with cations, and hence solutions of chlorates (VII) are used when it is desired to avoid complex formation in solution. 

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