Measurement of the vapour pressure

As the medium is still further diluted, until nitronium ion is not detectable, the second-order rate coefficient decreases by a factor of about 10 for each decrease of 10% in the concentration of the sulphuric acid (figs. 2.1, 2.3, 2.4). The active electrophile under these conditions is not molecular nitric acid because the variation in the rate is not similar to the correspondii chaise in the concentration of this species, determined by ultraviolet spectroscopy or measurements of the vapour pressure. " ... 

Evidence from the viscosities, densities, refractive indices and measurements of the vapour pressure of these mixtures also supports the above conclusions. Acetyl nitrate has been prepared from a mixture of acetic anhydride and dinitrogen pentoxide, and characterised, showing that the equilibria discussed do lead to the formation of that compound. The initial reaction between nitric acid and acetic anhydride is rapid at room temperature nitric acid (0-05 mol 1 ) is reported to be converted into acetyl nitrate with a half-life of about i minute. This observation is consistent with the results of some preparative experiments, in which it was found that nitric acid could be precipitated quantitatively with urea from solutions of it in acetic anhydride at —10 °C, whereas similar solutions prepared at room temperature and cooled rapidly to — 10 °C yielded only a part of their nitric acid ( 5.3.2). The following equilibrium has been investigated in detail ... 

The relation between the octahedral and prismatic modifications has not yet been satisfactorily elucidated. The former is the stable form at ordinary temperatures and the latter at higher temperatures the transition point according to Rushton and Daniels is 250° C. and according to Smits and Beljaars 200° C., but the prismatic form is persistent at much lower temperatures and the change from octahedral to prismatic may be monotropic. Interesting information has been obtained from measurements of the vapour pressure of the oxide. The following values have been obtained ... 

Dobbs, A.J., Grant, C. (1980) Pesticide volatilisation rates—A new measure of the vapour pressure of pentachlorophenol at room temperature. Pest. Sci. 11, 29-32. 

Figure 26.1 (a) Plot of In (P/P°) (P° = standard pressure) versus 1/7. (b) The measurement of the vapour pressure over a liquid, (c) Representation of vapour pressure, P, over a liquid as used in this text. 

Measurements of the vapour pressures of solutions of nitric acid in ether carried out by Dalmon and his co-workers confirmed the existence of the compound. It was shown that the vapour pressure of ether, which at 0°C was 185 mm Hg, decreased on addition of nitric acid and attained 1 mm Hg when equimolar proportions of the two components were present in the solution. 

Beryllium— The most recent measurement of the vapour pressure of beryllium is due to Holden, Speiser, and Johnston 228 who used both the Langmuir rate of evaporation method and the Knudsen method. A third law calculation of AHq for vaporization showed no drift over a temperature range from 1,172° to 1,552° K, with a mean value of 76-57 iO-37. Previous measurements in this temperature range 2 gave a bad trend in third law... 

Nickel— The most recent measurements of the vapour pressure of nickel are those of Jones, Langmuir, and Mackay254 Bryce 6L. nd Johnston and Marshall 251. All these used the rate of evaporation technique, and the results for the vapour pressure get lower, and those for AHq higher, in the order given. The earliest lead to A//q =98 13 kcal, the latest to 10T14 kcal. A value for A///(Ni g)2s c of 101 kcal seems indicated. 

A differential manometer of the Rayleigh type ( 14.VII A) was used in accurate measurements of the vapour pressure of water, and a simple apparatus for measuring the difference in vapour pressures of two liquids was described by Lord Kelvin. Stock s vapour pressure thermometer ( 19.V1 Q can be used to measure the vapour pressures of liquids, including those at low temperatures. Special techniques are necessary for very small vapour pressures,... 

Other studies of niobium or tantalum halides have included measurements of the vapour pressures of solid and liquid niobium and tantalum pentachloride, investi-... 

This composition was also confirmed by measurements of the vapour pressure... 

Although complete measurements of the vapour pressure of the diflerent systems of pure iodine to pure chlorine have not been made, the experimental data are nevertheless sufficient to allow of the general form of the curves being indicated with certainty. 

II. Measurement of the Vapour Pressure.—In the preceding pages it has been pointed out repeatedly that the vapour pressures of the two systems undergoing reciprocal transformation become identical at the transition point (more strictly, at the triple or multiple point), and the latter can therefore be determined by ascertaining the temperature at which this identity of vapour pressure is established. The apparatus usually employed for this purpose is the Bremer-Frowein tensimeter. ... 

In the past, the basic experiment used to study the general properties of solutions was the measurement of the vapour pressure lowering of the solvent (Raoult 1890). But this method which is, in general, efficient is not really useful when the vapour pressure reduction is small. In such situations, it has been replaced by another method which is quite similar it consists in measuring the osmotic pressure of the solution with respect to the solvent. 

Bell [40BEL] reported on measurements of the vapour pressure of D2O over mixtures of the tetradeuterate and hexadeuterate, and it was found that the ratio of equilibrium values of was greater than 1.0 near 25°C. 

Sano [37SAN] and Auftfedic, Carel and Weigel [72AUF/CAR], reported measurements of the vapour pressure of water over Ni(N03)2 6H20(cr) as a function of temperature (Figure V-39). The measured vapour pressures differ systematically by a factor of 1.5 at similar temperatures. Sano interpreted his results in terms of the reaction ... 

This is a short progress report describing measurements of the vapour pressure of Thl4(cr) and Thl4(l). The work was presented in a Coirference paper [1980PRA/NAG] on the vaporisation of all the thorium halides, but no details are given it has not been published elsewhere. 

The second essay, On the Force of Steam , describes measurements of the vapour pressure of water from 32° to 212° F. (see p. 308), and calculated values for other temperatures, given in a long table, and describes a dew-point hygrometer (see p. 763). Dalton says the water vapour in the atmosphere behaves like any other gas in a mixture of gases, and he showed that the maximum pressure of water vapour is the same in air as in a vacuum. He thought he had established the result that ... 

An aneroid static-bomb combustion calorimeter has been employed to determine the standard enthalpy of formation of a small sample (ca. 20 mg) of bicyclo[3,3,3]nonane (— 36-4g + 0-42 kcal mol ), and a temperature-scanning technique makes use of a commercial gas chromatograph in the measurement of the vapour pressures of several adamantane and diamantane derivatives. Results were converted into heats of sublimation by employing the Clausius-Clapeyron equation doubt is cast on the accuracy of a previously recorded value for diamantane. 

The osmotic coefficient of the solvent substance A in a solution containing molality ms of an involatile solute substance B can be determined from measurements of the vapour pressures p of the solution and p of the pure solvent according to the relation ... 

The activity of the solvent in a solution ean thus be determined from measurements of the vapour pressure of the solution, p, and of the pure solvent, p° at a given temperature. It is obvious that for an ideal solution obeying Raoult s law p/p° will be equal to v. the mole fraction of solvent. The activity coefficient as given by Equation 1.56 will then be unity. It is with the object of obtaining this result that the particular standard state of pure solvent was chosen. For a non-ideal solution the activity coefficient of the solvent will, of course, differ from unity, and its value can be determined by dividing the activity as given by Equation 1.57 by the mole fraction of the solvent. 

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