Perceric carbonate

Cerous carbonate dissolves to a considerable extent in concentrated potassium carbonate solution. When hydrogen peroxide is added to the solution, the latter turns blood-red in colour owing to oxidation, and the colour darkens with the addition of hydrogen peroxide up to a certain point; beyond this, the addition of more peroxide lightens the colour and throws down an orange-yellow precipitate, and by the addition of sufficient reagent all the cerium is precipitated.

When the colour reaches its maximum intensity, all the cerium is present in solution as perceric carbonate. The solution of perceric carbonate in potassium carbonate may be prepared from ceric carbonate as well as from cerous carbonate, by oxidation with hydrogen peroxide. By a suitable process of crystallisation, Job obtained perceric potassium carbonate, Ce₂(CO₃)₃O₃.4K₂CO₃.12H₂O, from this solution in dark-red, triclinic crystals.

This curious double salt is remarkably stable. It may be dehydrated at 110°, but withstands a temperature of 200° for several hours without decomposition being perceptible. At 240°, however, it loses oxygen, leaving a residue of a basic ceric carbonate.

By modifying Job's method of preparation in certain particulars, Meloche has prepared a crystalline perceric potassium carbonate of the composition Ce₂O₄(CO₃)₂.4K₂CO₃.12H₂O. This compound readily loses part of its water when exposed to dry air, and may be almost completely dehydrated at 110°- 120° without loss of available oxygen.

As already mentioned, the addition of hydrogen peroxide to perceric potassium carbonate solution causes all the cerium to be thrown out of solution as an orange-yellow precipitate. This precipitate appears to be a derivative of CeO₄, and is very unstable. When covered with a concentrated solution of potassium carbonate it slowly evolves oxygen, and beautiful red crystals of a perceric potassium carbonate are formed. According to Job, this is the best method for preparing the double salt; according to Baur, however, the crystals formed are rather different in composition from those described by Job, and have the formula Ce₂O₄(CO₃)₂.4K₂CO₃.10H₂O.

In alkaline solution the perceric potassium carbonates have three atoms of available oxygen per two atoms of cerium. In acid solution, however, only one-third of this oxygen is available, the remainder being set free in the gaseous state, e.g.: -

Ce₂O₄(CO₃)₂.4K₂CO₃.12 H₂O + 7H₂SO₄ = Ce₂(SO₄)₃ + O₂ + H₂O₂ + 4K₂SO₄ + 6CO₂ + 18H₂O.

A solution of ceric carbonate in potassium carbonate is quite stable towards air or oxygen. On the other hand, the corresponding cerous solution readily absorbs oxygen, passing into perceric and not into ceric carbonate. Simultaneously with this reaction another change occurs, namely, the interaction of the perceric carbonate produced with the unchanged cerous carbonate to produce ceric carbonate; and by varying the conditions of experiment the relative speeds of these two reactions may, within certain limits, be altered at will. This auto-oxidation of cerous carbonate may be used to affect the oxidation of various substances by the air, a small quantity of cerous salt acting as the oxygen-carrier. It is only necessary for this purpose that the 4'acceptor" shall be able to reduce both ceric and perceric carbonates to the cerous state; glucose is one such substance. When, however, the "acceptor" can only reduce perceric carbonate to the ceric state, the cerous salt soon becomes quantitatively transformed into ceric salt, when the auto-oxidation ceases. According to Engler, the auto-oxidation of cerous carbonate takes place in two stages, as follows: -

1. Ce₂(CO₃)₃ + 2H₂O + O₂ = Ce₂(CO₃)₃(OH)₂ + H₂O₂
2. Ce₂(CO₃)₃(OH)₂ + 2H₂O₂ = Ce₂(CO₃)₃O₃ + 3H₂O

the initial products of oxidation being basic ceric carbonate and hydrogen peroxide, which then interact to produce the perceric compound.