The X-ray structure of a sodium peroxide hydrate, Na2O28H2O, and its reactions with carbon dioxide: relevance to the brightening of mechanical pulps

The main component of the solid originally believed to be a peroxosilicate with pulp-brightening properties has been shown to be Na 2 O 2 *8H 2 O. The solid crystallizes in the monoclinic space group C2/c, with an empirical formula H 8 O 5 Na, and with a = 14.335(3), b = 6.461(1), c = 11.432(2) Å, β = 118.28(3)°, and Z = 8. The centrosymmetric structure consists of a peroxide anion with an OO distance of 1.499(2) Å. Each of these oxygen atoms is at the apex of an approximate square-based pyramid, the base of which consists of four oxygen atoms of water molecules. The bases of the two pyramids are staggered when viewed down the peroxide bond. Each sodium is at the centre of an approximate octahedron of water molecules, four of which bridge other sodium atoms and two bridge to the peroxide anions. One hydrogen atom of each of these two water molecules is terminal and the other two are hydrogen bonded to peroxide oxygen atoms. The compound reacts very rapidly with CO 2 in moist air to form Na 2 CO 3 , but in drier conditions, formation of the carbonate can take many days and proceeds via a percarbonate, believed to be Na 2 CO 4 . This has been identified by infrared spectroscopy and X-ray powder diffraction and can persist for long periods in dry air. Key words: sodium peroxide hydrate, sodium percarbonate, pulp brightening, X-ray diffraction, infrared.