We may remark that the latent heat of vaporisation of the molecular weight Br2 is about 7·2, and of iodine 6·0 thousand heat units, whilst the latent heat of fusion of Br2 is about 0·3, and of I2 about 3·0 thousand heat units. From this it is evident that the difference between the amounts of heat evolved does not depend on the difference in physical state. For instance, the vapour of iodine in combining with Zn to form ZnI2 would give 48 + 8 + 3, or about sixty thousand heat units, or 1½ times less than Zn + Cl2.
[56] One litre of sea-water contains about 20 grams of chlorine, and about 0·07 gram of bromine. The Dead Sea contains about ten times as much bromine.
[57] But there is no iodine in Stassfurt carnallite.
[58] The chlorine must not, however, be in large excess, as otherwise the bromine would contain chlorine. Commercial bromine not unfrequently contains chlorine, as bromine chloride; this is more soluble in water than bromine, from which it may thus be freed. To obtain pure bromine the commercial bromine is washed with water, dried by sulphuric acid, and distilled, the portion coming over at 58° being collected; the greater part is then converted into potassium bromide and dissolved, and the remainder is added to the solution in order to separate iodine, which is removed by shaking with carbon bisulphide. By heating the potassium bromide thus obtained with manganese peroxide and sulphuric acid, bromine is obtained quite free from iodine, which, however, is not present in certain kinds of commercial bromine (the Stassfurt, for instance). By treatment with potash, the bromine is then converted into a mixture of potassium bromide and bromate, and the mixture (which is in the proportion given in the equation) is distilled with sulphuric acid, bromine being then evolved: 5KBr + KBrO3 + 6H2SO4 = 6KHSO4 + 3H2O + 3Br2. After dissolving the bromine in a strong solution of calcium bromide and precipitating with an excess of water, it loses all the chlorine it contained, because chlorine forms calcium chloride with CaBr2.
[59] There has long existed a difference of opinion as to the melting point of pure bromine. By some investigators (Regnault, Pierre) it was given as between -7° and -8°, and by others (Balard, Liebig, Quincke, Baumhauer) as between -20° and -25°. There is now no doubt, thanks more especially to the researches of Ramsay and Young (1885), that pure bromine melts at about -7°. This figure is not only established by direct experiment (Van der Plaats confirmed it), but also by means of the determination of the vapour tensions. For solid bromine the vapour tension p in mm. at t was found to be—
| p = | 20 | 25 | 30 | 35 | 40 | 45 mm. |
| t = | -16°·6 | -14° | -12° | -10° | -8·5° | -7° |
For liquid bromine—
| p = | 50 | 100 | 200 | 400 | 600 | 760 mm. |
| t = | -5°·0 | +8°·2 | 23°·4 | 40°4 | 51°·9 | 58°·7 |
These curves intersect at -7°·05. Besides which, in comparing the vapour tension of many liquids (for example, those given in Chapter II., Note [27]), Ramsay and Young observed that the ratio of the absolute temperatures (t + 273) corresponding with equal tension varies for every pair of substances in rectilinear proportion in dependence upon t, and, therefore, for the above pressure p, Ramsay and Young determined the ratio of t + 273 for water and bromine, and found that the straight lines expressing these ratios for liquid and solid bromine intersect also at 7°·05; thus, for example, for solid bromine—
| p = | 20 | 25 | 30 | 35 | 40 | 45 |
| 273 + t = | 256·4 | 259 | 261 | 263 | 264·6 | 266 |
| 273 + t′ = | 295·3 | 299 | 302·1 | 304·8 | 307·2 | 309·3 |
| c = | 1·152 | 1·154 | 1·157 | 1·159 | 1·161 | 1·163 |