The boiling point method

The boiling point method is also often called the effusion method in the literature. There are two variations of the boiling point method. In the first, the temperature is increased slowly at constant pressure, and in the second, the external pressure is decreased slowly at constant temperature. In the isothermal method, difficulties associated with superheating 

Boiling usually implies vapor bubble formation in a liquid. Since the method is also applicable to solid samples, it may seem inappropriate to call it the boiling point method. However, in this context, the term boiling may be taken to mean the evolution of vapor from a condensed sample when its saturated vapor pressure is equal to the external pressure. The term boiling point then refers to the temperature at which the vapor pressure equals an arbitrarily chosen external pressure, and not only 101.325 kPa. 

Regarding assumption (c), Wagner (1943), on the other hand, assumed that the pressure remains uniform along the capillary. However, this assumption limits the applicability of his results to certain experimental conditions therefore a new treatment seemed to be necessary. 

The total fluxes J and J2 for the case of gradients both of composition and total pressure can be expressed by general equations 

A pressure gradient causes viscous flow, which can be described in terms of its mean linear velocity Vyisc- For a straight, cylindrical tube with radius r, this velocity is given by the equation 

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Molecular Weight Determinatioos. Details of the determination of molecular weights on a semi-micro scale by the Freezing-point method are given on p. 436, and by the Boiling-point method on p. 440. 

Although the boiling-point method is able to dispose of a greater numbei of convenient solvents than are suitable for freezing-point determinations, it is never so accurate, mainly on account of the difficulty of avoiding fluctuations in the boiling-point, due to radiation, to the dripping of cold liquid from the condenser, to impure solvent, and to barometric fluctuations. 

The first five measure the activity of the solvent and the last five measure the activity of the solute. The boiling point method is generally not included in evaluations for two reasons there are little data from these measurements or the thermal data are not adequate to apply an accurate correction to obtain an activity at 298 K. 

Tabus II, 9, B.—Reference Substances for the Calibration of Thermometers by the Boiling Point Method... 

The molecular weight of cobalt chloride as determined by the freezing-point method, with urethane as solvent, corresponds to the double formula, Co2Cl4 (compare ferrous chloride),1 but the results obtained by the boiling-point method 2 indicate that under those conditions the molecule is single, namely, CoCl2. 

Ballast bulb (5 L) pressure gauge three long pieces of heavy-wall rubber pressure tubing. If the boiling-point method is used distillation flask appropriate thermometer to cover the desired range, with one-hole rubber stopper to allow adjustment of thermometer... 

The vapour pressures of metals at high temperatures have been determined by the boiling-point method under various pressures of indifierent gas, by the efflision method ( 12.VIII J), and by other methods. ... 

The boiling point method connected with the evaluation of experimental results according to the method of Motzfeldt et al. (1977) was applied to the determination of vapor pressure of various molten salt systems at a temperature range of 600-1200°C. The most accurate results were, however, obtained by computer fitting of the complete Eq. (7.19) using a sophisticated computer program developed by Hertzberg (1983). 

The boiling point method using the apparatus described was applied to a number of cryolite-based melts, in the following studies... 

The boiling point method originated from Ruff and Bergdahl (1919) and was further developed by Ruff (1929). The principle of the isothermal method is as follows. The system to be investigated is taken in a cell (crucible) with a narrow capillary opening in the lid. The cell is suspended from a balance into a furnace with an inert gas atmosphere at constant temperature. The initial pressure of the inert gas is higher than the equilibrium... 

The transpiration method is a simple and versatile method for vapor pressure measurement at high temperatures. An inert carrier gas is passed over the condensed substance in a constant temperature furnace zone. The flow rate of the carrier gas is constant and sufficiently small so that the carrier gas is saturated with vapor, which condenses at some point downstream. The mass of vapor transported by a known volume of carrier gas is determined. If the total vapor pressure is known, from the boiling point method, the results from the transpiration method may be used to calculate the average molar mass of the vapor. 

Similarly as in the theoretical treatment of the boiling point method discussed in Section 7.2.1.1., the correct transport equations for a binary gaseous mixture with gradients in both composition and total pressure are also applied to the case of the transpiration... 

For the explanation of parameters A, C, and y, look into the theory of the boiling point method. The above equations consider diffusion and flow of vapor only through the exit capillary of the apparatus. The equations become very complex when the diffusion upstream through the entrance capillary is taken into account. Their application is not practical, as they contain too many unknown parameters. 

The determination of molecular weights by means of the depression of the freezing point is based upon the same principle as that underlying the determination by means of the rise in boiling point. The same laws hold. The constants, however, for the various solvents are not the same as those used in calculating molecular weights by the boiling point method. 

A variant of the boiling point method for samples which attack sihca was developed, and used to measure the vapom pressures of ThC lcr) and ThCLt(l), ThBr4(cr) and ThBr4(l) and Thl4(l). Samples were contained in a molybdenum bell, immersed in liquid tin, and the pressure within the bell was balanced by argon on the outside. 

The vapour pressures of ThF4(cr) and ThF4(l) were determined, by effusion for the solid (1055 to 1297 K) and the boiling point method for the liquid (1437 to 1595 K). Mass-spectrometric studies showed that the monomer is the only important species in the vapour. 

The boiling point method was nsed for the measnrements on the liquid, from 1437 to 1595 K, again from a nickel container, the npper temperatnre limit being determined by the loss of strength of the nickel cell. 

This paper deals mainly with the pressure and composition of the vapour in the KCl-ThCk system, but does include six measmements of the vapour pressure of pme ThCklcr) from 933 to 1043 K, by the boiling point method. 

Vapour pressures of sohd and hquid thorinm tetrachloride were measured at 880-1024, 1055 -1126 K by the transpiration techniqne and from 923 -1043, 1045 -1161 K by the boiling point method. The ThCLt nsed was prepared from thorium hydride and chlorine gas at 500 K, and pnrified by vacnnm distillation at 1000 K. The analysed Th/Cl ratio was reported to be 1/4. The container for both techniques was sihca, which could perhaps have reacted with ThCLt to form ThOCp. However, the results from the two techniques are self-consistent and agree well with those by [1939FIS/GEW2] and [1989LAU/HIL]. The melting temperatnre and enthalpy of fusion of ThCp were evaln-ated from the vapom pressure curves to be 1043 K, and (62.1 + 3.7) kJ-moF, respectively. 

The vaponr pressnre of solid ThF4(cr) was measnred by the transpiration technique and that of ThF4(l) by both transpiration and the boiling point method, with self-consistent resnlts. 

A similar analysis with L/ replaced by L , the molar heat of vaporization of the pure solvent, relates the boiling-point elevation for nonvolatile solutes to the activity coefficients and the solution molecular weight of the solute. Although more accurate results are obtained by use of freezing-point determination, the boiling-point method is often used when only a small quantity of the solution is available. 

Extensive research on the oxalates, tartrates, molybdates and sulphites of beryllium to show their differences from the corresponding salts of Al, Fe and Cr, ending with the preparation of BeClj and the determination of its molecular weight in pyridine by the boiling point method. 

Saturated vapor pressures of [BMIm]BF4 + 2,2,2-trifluoroethanol (TFE) and [BMImJBr + TFE mixtures were measured by KS. Kim et al. [4] using the boiling point method in the concentration range of 40.0 90.0 mass% of ionic liquids and in the temperature range of 298.2 K 323.2 K. The data were correlated with an Antoine-type equation. The average absolute deviations between experimental and calculated values were 0.6% and 0.4% for the [BMIm]BF4 + TFE and the [BMImJBr + TFE system, respectively. As shown in Figure 3, the [BMIm]Br + TFE system was found to be more favorable as working pairs in absorption heat pumps or chillers than the [BMIm]BF4 + TFE from the results of VLE. 

Apparatus and Method.—The apparatus which is generally employed for the purpose of molar weight determinations by the boiling-point method is that designed by Beckmann, one form of which, together with the manner of using it, will be described here. 

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