Acids, Alkalis and Salts

by George Henry Joseph Adlam

Calcium Phosphate and Bone Black. Instead of being ground up, bone may be heated in a retort in much the same way as coal is treated for the manufacture of coal gas; bone oil is distilled off, and a non-volatile residue, called bone black or animal charcoal, remains. This contains about 90 per cent. of calcium phosphate and 10 per cent. of finely divided carbon disseminated throughout the mass. It has the peculiar property of absorbing colouring matter, and is used for this purpose in the sugar industry and in the preparation of fine chemicals.

Phosphoric Acid. After being some time in use, bone black loses the property of absorbing colouring matter; and though it can be “revived” several times by heating it strongly in a closed retort, it ultimately becomes spent and of no further use to the sugar refiner. It is then heated again, this time in an open vessel, until all the carbon is burnt away. The residue is now a greyish solid consisting mainly of calcium phosphate. This, supplemented with native phosphate, which is probably fossilized bone, is used for the preparation of phosphoric acid.

The salt is decomposed by sulphuric acid in wooden vats; calcium sulphate is formed, and ultimately settles on the bottom of the vat, leaving a clear supernatant liquid, which is a dilute solution of phosphoric acid. This liquid is drawn off and evaporated to a syrup. This is “syrupy” phosphoric acid. On being still more strongly heated, the syrup loses still more water, and a semi-transparent glassy-looking substance, called metaphosphoric acid, remains.

Superphosphate. All fertile soils, especially those on which wheat is to be grown, must contain a certain amount of phosphate. With this, as with all other plant foods, the actual percentage weight required in the soil is very small indeed, but it is necessary that it should be disseminated throughout the soil. Even distribution is very difficult to secure in the case of a substance like calcium phosphate, which is practically insoluble in water.

To get over this difficulty, calcium phosphate is converted into a mixture known as “superphosphate” by the following process. Bone ash or the mineral phosphate is finely ground and thoroughly mixed by machinery with two-thirds its weight of sulphuric acid from the lead chambers. After a time, this mixture sets to a hard mass, containing principally gypsum and calcium tetrahydrogen phosphate. It is then ground up finely and is ready for use.

The special modification of calcium phosphate contained in superphosphate is soluble in water. It is, therefore, carried into the soil in solution, and in this way very evenly distributed. In the soil it reacts with the lime or chalk which is present, and is gradually reconverted into insoluble calcium phosphate.

The manufacture of superphosphate is a very important industry. The weight of the substance produced annually in Great Britain alone is not far below a million tons.

Basic Slag. In the Bessemer process for converting iron into steel, cast iron is melted up in a vessel called a converter and, by the aid of a powerful blast blown through the molten iron, most of the impurities are burnt off. If, however, phosphorus and sulphur are present, they are not removed if the converter has a silica (acid) lining. The original Bessemer process was, therefore, modified by Thomas and Gilchrist, and the converter for this kind of iron is lined with dolomite and lime (basic lining). Phosphorus is then converted into phosphate and retained by the lining, which is subsequently removed, ground up finely, and sold as “basic slag.”

Boric Acid, or boracic acid, is familiar because it is used in medicine as a mild antiseptic; it is also employed as a preservative for food. It is a white crystalline compound, sparingly soluble in water. It has no well-marked taste, and causes only a partial change in the colour of litmus solution; it is, therefore, one of the weak acids. It does not dissolve metals, but it displaces carbon dioxide from carbonates, forming salts.

Borax, the best known salt of boric acid, is used in laundry work and also for making some enamels, for when it is strongly heated it loses water, and ultimately melts down to a clear “glass” in which the oxides of metals will dissolve, yielding transparent substances which are beautifully coloured according to the nature of the oxide used. This property is often made use of in chemical analysis in what is known as the “borax-bead” test.