Sodium perborate, NaBO₃

Sodium perborate, NaBO₃.4H₂O, may be prepared by adding to a saturated solution of borax an equivalent quantity of sodium hydroxide and twice the amount of hydrogen peroxide theoretically necessary; after some time the perborate commences to crystallise out: -

Na₂B₄O₇ + 2NaOH + 4H₂O₂ = 4NaBO₃ + 5H₂O.

In preparing the salt on a large scale the necessary ingredients are mixed in small batches, which are then united and slowly cooled with stirring. Wooden vats and stirrers are used, and metal pipes, etc., well tinned to minimise decomposition of the hydrogen peroxide. The temperature after mixing is 25°, and the concentrations are chosen so that the perborate separates out slowly. Large and fairly stable crystals are deposited, which are centri- fuged free from mother liquor and dried in a stream of pure air at a temperature not exceeding 50° C.

Sodium perborate is only formed in minute quantities at the anode when aqueous sodium orthoborate, with or without excess of alkali, is electrolysed. Sodium perborate, NaBO₃.4H₂O, crystallises in large, transparent, monoclinic prisms, and is stable in air free from carbon dioxide. The monohydrate may be obtained by careful drying, and this may be completely dehydrated in vacuo over phosphoric anhydride. The solubility of the salt in water, in grams per litre of solution, is as follows: -

Temp. °C	15°	21°	26°	32°
Grams of NaBO.4H20	25.5	26.9	28.5	37.8

The solubility is augmented by the addition of boric, tartaric, or citric acid, glycerol, or ammonium and magnesium sulphates in small amount. The heat of solution in water is -11.56 Cals. at 16°; in seminormal sulphuric acid it is -8.95 Cals. at 17° C.

In aqueous solution at 0° C. the hydrolysis of sodium perborate is very slight, and from conductivity measurements it follows that the molecular formula of the salt is NaBO₃. With rise of temperature, hydrolysis proceeds further, and at 25° it is very considerable. The aqueous solution has an alkaline reaction. According to Pissarjewsky, the hydrolysis proceeds in two stages, (i.) and (ii.), the third reaction (iii.) then taking place between products formed in (i.) and (ii.): -

(i.) NaBO₃ + 2H₂O ⇔ NaO.OH + H₃BO₃
(ii.) NaO.OH + H₂O ⇔ NaOH + H₂O₂
(iii.) 3NaOH + H₃BO₃ ⇔ Na₃BO₃ + 3H₂O

When heated above 40°, sodium perborate solution commences to evolve oxygen. The solution is an energetic oxidiser, converting ferrous salts to ferric, chromic salts to chromic acid, manganous salts to manganese dioxide, liberating chlorine from hydrochloric acid, iodine from potassium iodide, etc. Sodium perborate is used commercially under the name of " perborin " for bleaching purposes, since it may be incorporated into soaps and washing-powders. " Perborin M" contains soap, alkali, and sodium perborate; " persil " contains soap, alkali carbonate and silicate, and sodium perborate; "ozonite" is similar to " persiland "clarax" contains borax, sodium phosphate, and sodium perborate.

A sodium perborate of the composition NaB₂O₄.5H₂O is said by Jaubert to be obtained when an intimate mixture of 248 grams of boric acid and 78 grams of sodium peroxide is slowly added to 2 litres of cold water. It cannot be recrystallised from water.