Acids, Alkalis and Salts

by George Henry Joseph Adlam

Fig. 9. BORIC ACID

Boric acid is a natural product; the method by which it is obtained is of some interest, because it is so simple, and because it shows how mere traces can be gradually accumulated until a very fair total is ultimately obtained. Moreover, the method is copied directly from Nature.

In the early years of the nineteenth century, certain jets of natural steam, called suffioni, which issue from the earth in Tuscany, were found to contain the vapour of boric acid. These jets of steam are of volcanic origin. The quantity of boric acid in the vapour is very small indeed; nevertheless, by the method which is adopted, it can be profitably recovered, and more than a ton of the solid is daily produced.

In the same country there are many lagoons, the water of which contains boric acid. It was rightly conjectured that this boric acid came from jets of steam which issued from the earth in the bed of the lagoon. This suggested the idea of building up an artificial lagoon around a group of jets.

Series of about five of these collecting basins ([Fig. 9]) are formed, each one at a slightly lower level than the one which precedes it. The first basin is filled with water from an adjacent spring, and this is allowed to remain for twenty-four hours. A sluice is then opened and the liquid contained in the first basin flows down to the second, where it remains for another day, and so on until it reaches the last basin of the series. The liquid by this time is almost fully charged with boric acid, but it contains only about 2 per cent., because the acid is so sparingly soluble in water.

From the last basin (A), the liquid runs into large vats (B, D), where the suspended impurities settle down. The solution of boric acid is then concentrated by causing it to flow over a broad inclined plane made of corrugated lead or through a series of shallow vessels heated by jets of natural steam. The hot liquid flows into another vat (C), and, as it cools, boric acid crystallizes out and is removed by perforated ladles.

The mother liquor from which the crystals have been withdrawn is, of course, a cold saturated solution of the acid, and this is returned to the top of the incline to flow down again and lose more water. The boric acid is finally transferred to drying chambers, which are also heated by the natural steam.

Native borax or “tinkal” comes from Thibet and also from Ceylon. In California, a large quantity of borax is obtained from a borax lake, and also from the mud of marshes in its neighbourhood.

Silica. The element silicon does not occur in the free state in Nature, neither has any particular use been found for it, and therefore it is not often isolated except to provide a lecture specimen. The compounds of silicon, however, are both plentiful and important, especially silica, the oxide, and the silicates or salts of silicic acid.