540 CHEMIST II Y [ARSENIC GROUP.. elements have a great tendency to unite with nitrogen. Vanadium dichloride crystallizes in apple-green coloured plates, and vanadium trichloride forms peach-blossom coloured tables ; vanadium tetrachloride is a reddish-brown liquid, which boils at 154 C. An oxychloride of vanadium, VOC1 3 , corresponding to phosphorus oxychloride, POC1 3 , is also known ; it is a yellow liquid, which boils at 126 C. The vapour density of the tetrachloride, according to lloscoe, corresponds with the formula VC1 4 , but as it is resolved into the trichloride and free chlorine on boiling, and even undergoes the same decomposition at ordinary temperatures, it may be doubted whether the density observed was really that of the tetrachloride, and not that of a mixture of a lower chloride and chlorine, especially as from the analogy which vanadium presents to chromium the formula V 2 C1 8 appears to be more probable than the simpler formula. The vanadium chlorides are dissolved and decomposed by water, the tetrachloride forming a blue, the trichloride a green, the dichloride a lavender-coloured, and the oxychloride a yellow liquid ; the solution of the dichloride is a most powerful reducing agent, and the solutions of the tri- and tetra-chloride also exhibit reducing properties, owing to the tendency of the dissolved compounds to combine with oxgyen to form vanadic acid. Vanadium pentoxide or vanadic anhydride, V 2 5 , corresponds in composition and properties to phosphoric anhydride ; it has a brownish-red colour, and fuses at a red heat ; it dissolves sparingly in water, forming a yellow, strongly acid solution of vanadic acid. By the combination of vanadic anhydride with basic oxides in various proportions salts corresponding to the three series of phosphates, as well as salts of a more complex character, are obtained ; thus : Sodium metavanadate, NaY0 3 ,, orthovanadate, Na 3 T 6 4 ,, pyrovanadate, Na 4 V 2 7 Sodium metaphosphate, NaP0 3 orthophosphate, Na 3 ?6 4 ,, pyrophosphate, Na 4 P 2 7 Isomorphism has been observed between several corre sponding phosphates and vanadates. The tetroxide and trioxide of vanadium also possess feeble acid properties and unite with basic oxides. The vanadium oxides dissolve in acids, mostly forming salts such as the sulphate V 2 2 (SO 4 ) 2 , in which the hydrogen of the acid is displaced by the radicle VO ; the formation of salts of this kind is characteristic of all the metalloids included in this group, and apparently of the metalloids generally. A solution of vanadium pentoxide in sulphuric acid, which is red, becomes blue on treatment with sulphurous acid or hydrogen sulphide, the salt of the pentoxide being reduced to a salt of the tetroxide ; by the aid of magnesium the reduction may be carried a stage further, and a salt of the trioxide is formed, the solution becoming green; and if zinc, cadmium, or sodium amalgam is employed, the solution finally becomes lavender-coloured, and contains a salt of the dioxide. Arsenic trichloride, AsCl 3 , the only known chloride of arsenic, is a heavy, colourless, oily liquid, which boils at 132 C. ; it is dissolved by water, probably as a chlorhydrin, as its solution in the smallest possible quantity of water deposits after some days crystals of the composition AsCl(OH) 2 . Antimony trichloride is a crystalline compound which boils at 223 C.; its vapour density corresponds with the formula SbCl 3 . Antimony pentachloride, SbCl 5 , is a heavy, yellowish liquid, which cannot be distilled without decomposition. These chlorides are dissolved by hydro chloric acid or a small quantity of water, but a large quantity of water converts the trichloride into an insoluble oxychloride, SbOCl, and the pentaciiloride into a compound which corresponds in composition to pyrophosphoric acid, viz., II 4 Sb.,O 7 . No oxychloride of arsenic or antimony corresponding to phosphorus oxychloride is known, but a. crystalline antimony sulphochloride, SbSCl 3 , exists. Bismuth trichloride is a very fusible, volatile, delique scent, colourless compound ; its vapour density corresponds with the formula BiCl 3 ; water converts it into an insoluble oxychloride, BiOCl ; when heated in hydrogen it furnishes an unstable dichloride, Bi 2 Cl 4 . Niobium pentachloride, NbCl 5 , is a yellow, easily fusible, crystalline substance, which boils at 240 C. ; niobium oxychloride, NbOCl 3 , is a white, infusible, but volatile, crystalline compound. The formulae of these bodies have been established by the determination of their vapour den sities ; both are decomposed by water. The solution of niobium chloride in hydrochloric acid becomes blue when a plate of zinc is immersed in it. Tantalum pentachloride is a yellow, fusible, crystalline substance, which boils at about 242 C. ; its vapour density corresponds with the formula TaCl 5 ; it is decomposed by water. Both oxides of arsenic are white ; the trioxide sublimes at about 200 C. without fusing ; the pentoxide, when suddenly heated to redness, fuses and is decomposed into the trioxide and oxygen. The vapour density of the tri oxide corresponds with the formula As 4 O 6 . Arsenic trioxids is sparingly soluble in cold water, more readily in hot water, and freely in hot hydrochloric acid ; the aque ous solution reddens litmus. The pentoxide is delique scent and easily soluble in water, forming a solution of arsenic acid, H 3 AsO 4 ; on evaporating the solution, however, even at ordinary temperatures, crystals of an acid corre sponding in composition to pyrophosphoric acid, H 4 As 2 O 7 , are obtained, which is converted into the pentoxide when heated at about 2 GO 3 C. Arsenic acid furnishes a series of salts analogous in composition to and isomorphous with the corresponding phosphates. Arsenic trioxide is almost desti tute of basic properties, and the salts formed by its union with basic oxides are not very stable, so that it differs greatly from the corresponding oxide of phosphorus ; thermochemical investigation confirms this conclusion. Thus, on inspection of the following table, which shows the amounts of heat which are developed on adding so dium hydroxide solution to solutions of the acids of phos phorus and arsenic, it will be evident that, whereas the behaviour of arsenic acid is similar to that of phosphoric acid, the behaviour of a solution of arsenic trioxide is altogether different from that of a solution of phosphorus trioxide (phosphorous acid), the molecule of arsenic trioxide regarded as As 2 O 3 , like boron oxide, evidently existing in solution as a dibasic acid ; it is not known whether the molecular composition of phosphorus trioxide is. correctly represented by the formula P 2 O 3 , and whether therefore the difference between the phosphorus and arsenic compounds is to be referred to the existence of a difference similar to that exhibited by the formulae 1O 3 and As 4 6 . NaOH. PO,H. P0 2 H 3 . rO.II,. POJI 3 . As0 4 H 3 . P 2 7 H 4 . As 2 3 . ^ mol. 1 2 3 4 6 7,100 14,380 7,700 15,160 7,430 14,830 28,370 7,330 14,830 27,080 34,030 7,360 14,990 27,580 35,920 14J880 28,640 52,740 7,300 13,780] 15,270 28,940 15,070 15,580 35,280 37,400 54,480 It will be evident also on comparing the following table
with that on p. 518 that much less heat is developed ID