The Br-Hg (Bromine-Mercury) System
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The Br-Hg (Bromine-Mercury) System C. Guminski University of Warsaw
Hg, and (5) (Br) with also unknown but certainly small solubility of Hg.
Equilibrium Diagram Only fragments of the Br-Hg phase diagram are known. Due to significant volatility of both component elements, the solubility in liquid Br and Hg cannot be investigated without pressure elevation. The assessed phase diagram, presented in Fig. 1, is based on work of [1970Dwo], who performed some experiments allowing presentation of the general shape of the phase relationships. Special points of the Br-Hg phase diagram are collected in Table 1. One should remember that the system of the reactions is not complete because the composition range 3 to 33 at. % Hg was not investigated. Although the compound HgBr2 melts congruently, it is likely that another immiscibility gap will be observed between pure Br and HgBr2 because immeasurably small solubility of HgBr2 in liquid Br was observed even at 80 ⬚C in thermal analysis experiments of [1970Dwo]. The Br-Hg system consists of the following phases: (1) L, the liquid, (2) L1 and L2, the partly miscible liquids separating molten Hg2Br2 from Hg liquid, (3) two (probably) line compounds Hg2Br2 and HgBr2, the first one may be present in two polymorphic forms below and above ⫺129 ⬚C, (4) (Hg) with unknown but certainly small solid solubility of Br in
Liquidus Parts of the Br-Hg liquidus were determined in several studies. [1970Dwo] used thermal analysis, [1907Bec] and [1951Jan] performed cryoscopic measurements and [1959Jan], [1974Wen], [1975Bar1], [1982Pod], [1989Pan1], and [1989Pan2] determined the melting point of HgBr2. The data obtained in these works are collected in Table 2. There is nearly perfect agreement of the liquidus data from [1907Bec], [1951Jan], and [1970Dwo] in the vicinity of HgBr2 composition. A correction of 3.5 ⬚C was made to all results of [1907Bec] because of the melting point of HgBr2 accepted in this assessment. Some experiments of [1970Dwo] at the highest temperatures were certainly obtained at elevated pressure because Hg boils at 356.58 ⬚C and HgBr2 sublimes at 345 ⬚C [1989Gre], whereas a few thermal arrests of [1970Dwo] were recorded at temperatures higher than these. It is quite well known to electrochemists that Hg2Br2 electrodeposited on a Hg electrode is not measurably soluble in liquid Hg at room temperature [1954Bou]. The solubility value ⬃10⫺6 at.% Br, obtained by extrapolation of
Fig. 1 Assessed Br-Hg phase diagram
Journal of Phase Equilibria Vol. 21 No. 6 2000
539
Section II: Phase Diagram Evaluations Table 1 Special points of the Br-Hg assessed phase diagram at pressures sufficient to keep all phases as solid or liquid Composition of respective phases, at.% Hg
Reaction L } Br L } Br(G) L } (Br) ⫹ HgBr2 ? L } HgBr2 L } HgBr2 ⫹ Hg2Br2 ␣Hg2Br2 } Hg2Br2 L1 ⫹ L2 } Hg2Br2 L } (Hg) ⫹ Hg2Br2 L } Hg(G) L } Hg
⫺0
34.2 ⫺46 ⫺100
0 0 ⫺0 33.33 33.33 50 ⫺94 ⫺100 100 100
Temperature, ⴗC
33.33
50 50 50
Type of reaction
⫺7.25 58.78 ⫺7.8
Melting point Boiling Point(a) Eutectic
238.
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