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The vaporization of H3BO3(s) was studied by using a commercial thermogravimetric apparatus and a Knudsen effusion mass spectrometer. The thermogravimetric measurements involved use of argon as the carrier gas for vapor transport and derivation of vapor pressures of H3BO3(g) in the temperature range 315-352 K through many flow dependence and temperature dependence runs. The vapor pressures as well as the enthalpy of sublimation obtained in this study represent the first results from measurements at low temperatures that are in accord with the previously reported near-classical transpiration measurements (by Stackelberg et al. 70 years ago) at higher temperatures (382-413 K with steam as the carrier gas). The KEMS measurements performed for the first time on boric acid showed H3BO3(g) as the principal vapor species with no meaningful information discernible on H2O(g) though. The thermodynamic parameters, both p(H3BO3) and Delta sub H degrees m(H3BO3,g), deduced from KEMS results in the temperature range 295-342 K are in excellent agreement with the transpiration results lending further credibility to the latter. All this information points toward congruent vaporization at the H3BO3 composition in the H2O-B2O3 binary system. The vapor pressures obtained from transpiration (this study and that of Stackelberg et al.) as well as from KEMS measurements are combined to recommend the following: log [p(H3BO3)/Pa]=-(5199+/-74)/(T/K)+(15.65+/-0.23), valid for T=295-413 K; and Delta sub H degrees m=98.3+/-9.5 kJ mol (-1) at T=298 K for H3BO3(s)=H3BO3(g).

Thermobalances are used for equilibrium vapor pressure measurements based on both effusion and transpiration methods. In the case of the transpiration method, however, despite the numerous advantages a thermogravimetric apparatus can offer, it is not as widely used as is the conventional apparatus. In this paper, the difference that can exist in the vapor phase compositions in an effusion cell and in a transpiration cell is shown first with two examples. Subsequently, how a commercial thermobalance was utilized to perform transpiration experiments that conform to the basic principle of the transpiration method and yield vapor pressures consistent with the Knudsen effusion mass spectrometric method is described. The three systems investigated are CsI(s), TeO(2)(s), and Te(s), each known to vaporize congruently, but in different manner. A critical analysis was performed on the information available in the literature on transpiration measurements using thermogravimetric apparatuses, and the salient findings are discussed. Smaller plateau regions than with conventional transpiration apparatuses and the lack of evidence for perfect transpiration conditions in some transpiration thermogravimetric investigations are shown with a few examples. A recommendation is made for the use of the rate of mass loss versus flow rate plot to ascertain that the usual apparent vapor pressure versus flow rate plot corresponds to a meaningful transpiration experiment.

Vaporization studies on Mn-Te samples of initial compositions 49.6 and 54.9 at. % Te were conducted by Knudsen effusion mass spectrometry. Both these samples on continuous vaporization reached the congruently effusing compositions (CECs) ≈ 50 at. % Te. Vaporization chemistry around this composition was studied from 1194 to 1343 K to examine the effects associated with the reported phase transitions in MnTe(s) at 1228 (R T β), 1293 (β T γ), and 1323 K (γ T δ). Among these, only the R T β phase transition showed significant effects. During the phase transition, the vapor phase was relatively richer in manganese in the increasing temperature direction (i.e., as the sample was heated from T e 1203 K to T g 1238 K) and was relatively richer in tellurium in the decreasing temperature direction (i.e., as the sample was cooled from T g 1238 K to T e 1218 K). Evidence for a slight variation in the CEC with temperature was also obtained but was distinguishable from the effects associated with the R T β phase transition. From the results obtained in four series of experiments, the partial pressures of Mn(g), Te(g), and Te 2 (g) were deduced, and by neglecting the variations in the CECs with temperature, the enthalpy changes for the following vaporization reactions during congruent effusion were evaluated: MnTe(s) ) Mn(g) + 0.5Te 2 (g), ∆ r H°m(298.15K) ) 463.6 ( 2.1 kJ mol -1 ; MnTe(s) ) Mn(g) + Te(g), ∆ r H°m(298.15K) ) 592.4 ( 2.3 kJ mol -1 ; MnTe(s) ) Mn (g) + yTe(g) + (1y)/2Te 2 (g), ∆ r H°m (298.15 K) ) 554.0 ( 4.6 kJ mol -1 , y mean ≈ 0.7. These results yielded ∆ f H°m(MnTe,s,298.15K) ) -99.2 ( 6.8 kJ mol -1 . An equation for the total vapor pressure during congruent effusion of MnTe(s), valid for the temperature range from 1194 to 1343 K was obtained: log(P/Pa) ) -(14539 ( 273)/(T/K) + (11.159 ( 0.222).