280 PRINCIPLES OF CHEMISTRY 



or carbonate (carbonic anhydride being disengaged in the latter case) it 

 forms the normal salt, K 2 Cr0 4 , known as yellow chromate of potassium. 

 Its specific gravity is 2*7, being almost the same as that of the dichro- 

 mate. It absorbs heat in dissolving ; one part of the salt dissolves in 

 1*75 part of water at the ordinary temperature, forming a yellow 

 solution. When mixed even with such feeble acids as acetic, and more 

 especially with the ordinary acids, it give's the dichromate, and Graham 

 obtained a trichromate, K 2 Cr 3 O l0 = K 2 Cr0 4 ,2Cr0 3 , by mixing a 

 solution of the latter salt with an excess of nitric acid. 



Chromic anhydride is obtained by preparing a saturated solution of 

 potassium dichromate at the ordinary temperature, and pouring it in a 

 thin stream into an equal volume of pure sulphuric acid. 4 On mixing, 

 the temperature naturally rises , when slowly cooled, the solution 

 deposits chromic anhydride in needle-shaped crystals of a red colour 

 sometimes several centimetres long. The crystals are freed from the 

 mother liquor by placing them on a porous tile. 4 bis It is very important 

 at this point to call attention to the fact that a hydrate of chromic 

 anhydride is never obtained in the decomposition of chromic compounds, 



* The Sulphuric acid should not contain any lower oxides of nitrogen, because they 

 reduce chromic anhydride into chromic oxide. If a solution of a chromate be heated 

 with an excess of acid for instance, sulphuric or hydrochloric acid oxygen or chlorine 

 is evolved, and a solution of a chromic salt is formed. Hence, under these circum- 

 stances, chromic acid cannot be obtained from its salts. One of the first methods 

 employed consisted in converting its salts into volatile chromium hexafluoride, CrF 6 . 

 This compound, obtained by Unverdorben, may be prepared by mixing lead chromate 

 with fluor spar in a dry state, and treating the mixture with fuming sulphuric acid in a 

 platinunt vessel : PbCrO 4 + 8CaF 2 + 4H 2 SO 4 = PbSO 4 + 3CaSO 4 + 4H 2 O + CrF 6 . Fuming 

 sulphuric acid is taken, and in considerable excess, because the chromium fluoride which 

 is formed is very easily decomposed by water. It is volatile, and forms a very caustic, 

 poisonous vapour, which condenses when cooled in a dry platinum vessel into a red, 

 exceedingly volatile liquid, which fumes powerfully in air. The vapours of this 

 substance when introduced into water are decomposed into hydrofluoric acid and 

 chromic anhydride : CrF 6 + 8H 2 O = CrO 3 -f 6HF. If very little water be taken the hydro- 

 fluoric acid volatilises, and chromic anhydride separates directly in crystals. The 

 chloranhydride of chromic acid, CrO 2 Cl 2 (Note 5), is also decomposed in the same 

 manner. A solution of chromic acid and a precipitate of barium sulphate are formed by 

 Creating the insoluble barium chromate with an equivalent quantity of sulphuric acid. 

 If carefully evaporated, the solution yields crystals of chromic anhydride. Fritzsche 

 gave a very convenient method of preparing chromic anhydride, based on the relation 

 of chromic to sulphuric acid. At the ordinary temperature the strong acid dissolves 

 both chromic anhydride "and potassium chromate, but if a certain amount of water is 

 added to the solution the chromic anhydride separates, and if the amount of water be 

 increased the precipitated chromic anhydride is again dissolved. The chromic anhy- 

 dride is almost all separated from the solution when it' contains two equivalents of 

 water to one equivalent of sulphuric acid. "Many methods for the preparation of chromic 

 anhydride are based on this fact. 



4 bis They cannot be filtered through paper or washed, because the chromic 

 anhydride is reduced by the filter-paper, and is _ dissolved during the process of 

 washing. 



