Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/611

Rh POTASSIUM 589 Water 4 5 Insoluble matter. ..27 composition of this substance is very variable, the per centage of real K 2 CO 3 varying from 40 to 80 per cent. The following analysis of an American &quot;potashes&quot; is quoted as an example. Carbonate of potash ... 7 1 4 ,, soda 2-3 I Sulphate of potash 14 4 Chloride of potassium ... 3 6 Crude potashes is used for the manufacture of glass, and after being causticized for the making of soft soap. For many other purposes it is too impure and must be refined, which is done by treating the crude product with the mini mum of cold water required to dissolve the carbonate, removing the undissolved part (which consists chiefly of sulphate), and evaporating the clear liquor to dryness in an iron pan. The purified carbonate (which still contains most of the chloride of the raw material and other im purities) is known as &quot; pearl ashes.&quot; Large quantities of carbonate used to be manufactured from the aqueous residue left in the distillation of beetroot spirit, i.e., indirectly from beetroot molasses. The liquors are evaporated to dryness and the residue is ignited to obtain a very impure carbonate, which is purified by methods founded on the different solubilities of the several components. Such potashes, however, is exceptionally rich in soda : Grandeau found in crude ashes from 16 to 21 per cent, of potash and from 23 to 50 of soda carbonate. This industry would have expired by this time were it not that the beetroot spirit residues are worked for tri- methylamine (see METHYL, vol. xvi. p. 196), and the carbonate thus obtained incidentally. Most of the car bonate of potash which now occurs in commerce is made from Stassfurt chloride by means of an adaptation of the &quot; Leblanc process &quot; for the conversion of common salt into soda ash (see SODIUM). Chemically pure carbonate of potash is best prepared by the ignition of pure bicarbonate (see below) in iron or (better) in silver or platinum vessels, or else by the calcina tion of pure bitartrate (see TARTARIC ACID). The latter operation furnishes an intimate mixture of the carbonate with charcoal, from which the carbonate is extracted by lixiviation with water and filtration. The filtrate is evaporated to dryness (in iron or platinum) and the residue fully dehydrated by gentle ignition. The salt is thus obtained as a white porous mass, fusible at a red heat (838 C., Carnelley) into a colourless liquid, which freezes into a white opaque mass. The dry salt is very hygro scopic ; it deliquesces into an oily solution (&quot;oleum tar- tari &quot;) in ordinary air. 100 parts of water dissolve at o c. 83 20 C. 94 135&quot; C. (boiling point of saturated solution) 205 parts. Carbonate of potash, being insoluble in strong alco hol (and many other liquid organic compounds), is much used for the dehydration of the corresponding aqueous pre parations. From its very concentrated solution in hot water the salt crystallizes on cooling with a certain pro portion of water ; but these crystals are little known even to chemists. Pure carbonate of potash is being constantly used in the laboratory, as a basic substance generally, for the disintegration of silicates, and as a precipitant. The industrial preparation serves for the making of flint-glass, of potash soap (soft soap), and of caustic potash. It is also used in medicine, where its old name of &quot; sal tartari &quot; is not yet quite obsolete. Bicarbonate of Potash (K 2 OCO 2 + H 2 OC0 2 = 2KHCO 3 ) is obtained when carbonic acid is passed through a cold solution of the ordinary carbonate as long as it is absorbed. If silicate is present, it likewise is converted into bicar bonate with elimination of silica, which must be filtered off. The filtrate is evaporated at a temperature not exceeding 60 or at most 70 C. ; after sufficient concen tration it deposits on cooling anhydrous crystals of the salt, while the chloride of potassium, which may be present as -an impurity, remains mostly in the mother-liquor ; the rest is easily removed by repeated recrystallization. If an absolutely pure preparation is wanted, it is best to follow Wohler and start with the &quot; black flux &quot; produced by the ignition of pure bitartrate. The flux is moistened with water and exposed to a current of carbonic acid, which, on account of the condensing action of the charcoal, is absorbed with great avidity. The rest explains itself. Bicarbonate of potash forms large monoclinic prisms, permanent in the air. 100 parts of water dissolve at 10 20 CO 70 19-61 23-23 26-91 41 35 45 24 parts of salt. At higher temperatures than 70 the solu tion loses carbonic acid quickly. The solution is far less violently alkaline to the taste and test-papers than that of the normal carbonate. Hence it is preferred in medicine as an anti-acid. When the dry salt is treated it breaks up below redness into normal carbonate, carbonic acid, and water. Caustic Potash (Hydrate of Potassium}, KHO. It has been known for a long time that a solution of carbonate of potash becomes more intensely alkaline, acts more strongly on the epidermis, and dissolves fats more promptly after it has been treated with slaked lime. It used to be supposed that the latent fire in the quick-lime went into the &quot; mild &quot; alkali and made it &quot; caustic,&quot; until Black, about the middle of last century, showed that the chemical difference between the two preparations is that the mild is a com pound of carbonic acid and the caustic one of water with the same base (potash), the causticizing action of the lime consisting in this, that it withdraws the carbonic acid from the alkali and substitutes its own water. Add to this that the exchange takes place only in the presence of a sufficient proportion of water, and that it is undone if the mixture is allowed to get concentrated by evaporation beyond a certain (uncertain) point, and you have a full theory of the process. A good concentration is twelve parts of water for one of carbonate of potash ; the lime is best employed in the shape of a semi-fluid paste, made by slaking quick -lime with three parts of water poured on at a time. The alkali solution is heated to boiling in a cast-iron vessel (industrially by means of steam-pipes) and the lime paste added in instalments until a sample of the filtered mixture no longer effervesces on addition of an excess of acid. The mixture is then allowed to settle in the iron vessel, access of air being prevented as much as practicable, and the clear liquor is draAvn off by means of a syphon. The remaining mud of carbonate and hydrate of lime is washed, by decantation, with small instalments of hot water to recover at least part of the alkali diffused throughout it, but this process must not be continued too long or else some of the lime passes into solution. The united liquors are boiled down in an iron vessel until the desired degree of concentration is reached. In obedience to an old tradition, the concentration is habitually continued until the specific gravity of the cold ley is 1 333, which is a rather incon veniently high degree of strength for most purposes, but in the case of the ordinary commercial article offers this advantage, that any sulphate of potash which may be present as an impurity crystallizes out completely on standing (Liebig). If solid caustic potash is wanted, the ley (after removal of the deposit of sulphate, &c.) is trans ferred to a silver dish, and the evaporation continued until, instead of steam, the heavy vapour of KHO itself is seen to go off. The residual oily liquid is then poured out into