Vapour tension

If V is the volume, t the temperature, B the barometric pressure, and f the vapour tension of water at then the corrected volume is given by the formula... 

H. Erdmann considers that the volatility of a perfume does not depend on its vapour tension alone but also on its specific solubility in the air. This he deduced from the fact that certain bodies lose, more or less completely, their odours in liquid air, but that on shaking the mixtures the odours become strongly apparent. He argues therefore that... 

This is a doubtful conclusion since if temperature-vapour tension curves for volatile substances be examined it will be seen that at low temperatures the rate of diminution of the vapour tension falls off rapidly and hence the-vapour tension at — 190° C. is often not vastly different from what it is at normal temperature, and hence is not by any means negligible when we take into account the very small quantity of substance that needs to be inspired in order for its odour to be perceptible. 

Since the vapour tension over the sodium hydroxide-asbestos is less than that over the specially dried calcium chloride, some ordinary not specially dried calcium chloride must be interposed between those two layers. 

Berthoud (loc. dt.) calculated from vapour tension values Berthollet (Corrupt. rend., 1880, 90, 1510) gives 11-8 Calories. 

The density of the hydride is 0-92,8 and the vapour-tension for each interval of 10° between 300° C. and 410° C. is 15, 17, 21, 27, 38, 55, 87, 186, 201, 285, 396, and 540 mm. respectively.7 Sodium hydride is the most stable of the alkali-metal hydrides, and caesium hydride the least. The sodium derivative is unaffected by dry air, but decomposes in presence of traces of moisture. Although insoluble in organic solvents such as carbon disulphide, carbon tetrachloride, benzene, and turpentine, it dissolves in the alkali-metals and their amalgams. 

Potassium hydride, KH.—Moissan5 prepared the hydride by a method analogous to that employed by him for the corresponding sodium derivative, the excess of potassium being dissolved by liquid ammonia. Ephraim and Michel6 passed hydrogen into potassium at 350° C., and found the reaction to be promoted by the presence of calcium. The hydride forms white crystals of density 0-80. The vapour-tension for each temperature-interval of 10° between 350° and 410° C. corresponds with the values 56, 83, 120, 168, 228, 308, and 430 mm. respectively.7 In chemical properties potassium hydride resembles the sodium compound, but is less stable. Its stability is greater than that of rubidium hydride or caesium hydride. Carbon dioxide converts it into potassium formate. 

A small contact angle implies that the drop spreads over the surface if the contact angle is zero, the surface will be wetted completely (Figure C2-8). This happens when the solid/vapour tension is much larger than the solid/liquid tension the system then avoids any solid/vapour interface. If the contact angle is equal to Jt, there is no wetting. This happens when the solid/liquid tension is much higher than the solid/vapour tension. The system then minimizes the liquid/vapour interface, as with a drop of mercury on paper. 

Crystals of the heptahydrate possess the same vapour tension at 44 01° C. as magnesium sulphate, MgS04.7H20. Below this temperature their dissociation pressure is greater, and above it is less, than that of the magnesium salt.5 In the case of zinc sulphate, ZnS04.7H20, the equilibrium temperature between the two salts is 16 4° C.6... 

Chemical interference is practically non existent as a result of the high temperature of the plasma. On the other hand, physical interference may be observed. This stems from variations in the sample atomisation speed which is usually due to changes in nebulisation efficiency caused by differences in the physical properties of the solutions. Such effects may be caused by differences in viscosity or vapour tension between the sample solutions and the standards due, for example, to differences in acidity or total salt content. The technique most commonly used to correct this physical interference is the use of internal standards. In this technique a reference element is added at an identical concentration level to all the solutions under analysis, standards, blank and samples. For each element, the ratio of simultaneous measurements of the lines of the element and the internal standard is then determined in order to compensate for any deviation in the response of the plasma. If the internal standard behaves in the same way as the element to be determined, this method can be used to improve the reliability of the result by a factor of 2 to 5. It can also, however, introduce significant errors because not all elements behave in the same way. It is thus necessary to take care when using it. Alternatives to the internal standard method include incorporating the matrix into the standards and the blank, sample dilution, and the standard addition method. 

Ammonia was found by L. Troost i to unite with ammonium bromide, forming a series of ammino-compounds which are analogous with the corresponding ammino-compounds of ammonium chloride and they are formed in the same manner. Ammonium ammino-bromide, HBr.2NH3, is formed when pulverulent ammonium bromide is exposed to ammonia gas below 5°. The vapour tension is 90 mm. at —27° j 350 mm. at 0° 775 mm. at 14 8° and 1660 mm. at 31°. The absorption of ammonia may continue until a liquid ammonium triammino-bromide,... 

There are various methods (static, dynamic, etc.) of determining the vapour tension of a substance, but it is not proposed to describe these here, as they may be found in special treatises. Moreover, various formulae may be employed to calculate the vapour tension of a substance at different temperatures. That in most general use is the empirical formula of Regnault ... 

Baxter, Mumford and others, applying this formula to the determination of the vapour tension of war gases, have determined the values of A and B for various substances as follows ... 

By substituting the values of Table IV in the formula already given, the vapour tensions of the various substances may be found at different temperatures. 

In Table V the values of the vapour tensions of several substances are given in mm. of mercury at 20° C. 

It is seen from this table that the variation in the vapour tensions of the war gases is very great. For example, some of these substances have a vapour tension greater than one atmosphere (phosgene, cyanogen chloride), while others (dichloroethyl sulphide, bromobenzyl cyanide) have an extremely low vapour tension, and for this reason special methods are necessary in order to obtain efficient results in using them in warfare. 

The boiling point of a substance is that temperature at which its vapour tension attains the value of the atmospheric pressure. The lower the boiling point of a substance, the higher is its vapour tension and its volatility. 

The boiling point is an important characteristic of a war gas, not only because of its connection with vapour tension and therefore with the tactical aims attainable in warfare, but also because of its influence on the ease of storage and transport of the substance. A war gas whose boiling point is lower than ordinary temperatures, as, for example, phosgene, is difficult to pack and necessitates the use of refrigerating apparatus during transport in order to keep it below its boiling point. 

This classification also lacks neatness and precision, for the place of some substances in it is doubtful. It is proposed nowadays to add a third intermediate group, to include substances whose vapour tension lies between the gases of the first group and those of the second. This third group has been termed that of the Semi-persistent Gases."... 

Green Cross Gases. This includes substances with a high vapour tension and great toxic power on the respiratory tract phosgene, trichloromethyl chloroformate (diphosgene), chloro-picrin, etc. 

Yellow Cross Gases. This includes substances with a low vapour tension and high toxic and vesicatory power dichloroethyl sulphide (mustard gas), chlorovinyl dichloroarsine (lewisite), etc. 

Liquid chlorine is green with a tinge of yellow and is very mobile. It boils at ordinary pressure at — 33-6° C. The value of the vapour tension of liquid chlorine at different temperatures is given in the following table ... 

The vapour tension of bromine varies with temperature as follows (Landolt) ... 

In the following table, the vapour tension and the volatility at various temperatures are given ... 

The vapour tension of liquid phosgene at temperatures between — 15° C. and + 23° C. can be calculated from the formula ... 

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