Chromium Tetroxide Triammine, CrO₄.3NH₃

Chromium Tetroxide Triammine, CrO₄.3NH₃, may be obtained by several methods, one consisting of heating the red ammonium perchromate with 10 per cent, ammonia at 40° C., when crystals are obtained of which the form appears to depend upon the concentration of ammonia.5 The mother-liquor, from which red ammonium perchromate separates in the preparation of that salt, also yields chromium tetroxide triammine upon heating and cooling. It separates in long brown needles or rhombic plates, the density of which at 15.8° C. is 1.964.

The compound is soluble in ammonia and in water (though with partial decomposition), and explodes on moderately heating. With alkalies it yields chromates; with acids, chromic salts and hydrogen peroxide are formed. In both cases gaseous oxygen is evolved. Its properties being those of an ammine and not of an ammonium salt, it may be formulated thus:



The compounds CrO₄.3KCN,

and , have been prepared, together with analogous ethylenediamine and hexamethylenetetramine compounds.

Compounds of perchromic acid with certain organic bases, the formulae of which are of the type HCrO₅.X, are also blue or violet in colour. The triethy 1 amine, aniline, pyridine, piperidine, and quinoline salts have been prepared. The pyridine salt, for example, is prepared as a blue crystalline compound by the addition of hydrogen peroxide to an aqueous solution of chromic acid containing excess of pyridine, or by the addition of excess of pyridine to either the red or the blue ammonium perchromate. It is only slightly soluble in water, and is stable only when in a dry condition. It is decomposed with detonation by moderate heating, or by addition to concentrated sulphuric acid.

A tetramethyl-ammonium derivative, N(CH₃)₄CrO₅, is also known. Constitution of Perchromic Acid and the Perchromates. - The properties of the blue perchromic acid, H₃CrO₈.2H₂O, indicate that the water is present as water of constitution, and the chromium appears to function as a heptavalent element, a condition which would not be anticipated by consideration of its position in the Periodic classification of the elements. The following formula has therefore been suggested:



and the red perchromates may be regarded as anhydro-salts of the blue perchromic acid, thus:



whereas the blue perchromates and compounds of the type HCrO₅.X may be regarded as derivatives of an unknown acid of composition

.

These formulae are based on the proportions of peroxidic oxygen which appear to be present in the compounds. The accurate determination of such oxygen, however, is attended with difficulty, and it must not be assumed that the question of the constitution of the perchromates has been finally settled.