1902 Encyclopedia > Potassium Metals

Potassium Metals




POTASSIUM METALS. Under this heading we treat of potassium, rubidium, and caesium ; SODIUM and LITHIUM, being less closely allied to potassium, have special articles devoted to them.

Potassium.—The three metals under consideration are all very widely diffused throughout nature ; but only potassium is at all abundant, and therefore we begin with it. The richest natural store is in the ocean, which, according to Boguslawski's calculation (in his Océanographie) of its total volume and the present writer's analysis of sea water, contains potassium equal to 1111 times 10 tons of sulphate, K2S04. This inexhaustible store, however, is not much drawn upon at present ; the " salt-gardens " on the coast of France have lost their industrial importance as potash-pro-ducers, if not otherwise, since the rich deposits at Stassfurt in Germany have come to be so largely worked. These deposits, in addition to common salt, include the following minerals:—sylvine, KC1; carnallite, KCl.MgCl2 +6H20 (transparent deliquescent crystals, often red with diffused oxide of iron); kainite, K2S04.MgS04.MgCl.2 + 6H20 (hard crystalline masses, permanent in the air) ; kieserite (a hydrated sulphate of magnesia which is only very slowly dissolved by water) ; besides boracite, anhydrite (CaS04), and other minor components lying outside the subject of this article. The potassium minerals named are not confined to Stassfurt ; far larger quantities of sylvine and kainite are met with in the salt-mines of Kalusz in the eastern Carpathian Mountains, but they have not yet come to be worked so extensively. The Stassfurt potassiferous minerals owe their industrial importance to their solu-bility in water and consequent ready amenability to chemical operations. In point of absolute mass they are insignificant compared with the abundance and variety of potassiferous silicates, which occur everywhere in the earth's crust ; orthoclase (potash felspar) and potash mica may be quoted as prominent examples. Such potassiferous silicates are found in almost all rocks, if not as normal at least as subsidiary components; and their disintegration furnishes, directly or indirectly, the soluble potassium salts which are found in all fertile soils. These salts are sucked up by the roots of plants, and by taking part in the process of nutrition are partly converted into oxalate, tartrate, and other organic salts, which, when the plants are burned, assume the form of carbonate, K2C03. It is a remarkable fact that, although in a given soil the soda may pre-dominate largely over the potash salts, the plants growing in the soil take up the latter by preference : in the ashes of most land plants the potash (calculated as K,0) forms upwards of 90 per cent, of the total alkali (K20 or N"a20).1:! The proposition holds, in its general sense, for sea plants likewise. In ocean water the ratio of soda (Na00) to potash (K20) is 100 : 3'23 (Dittmar); in kelp it is, on the average, 100 : 5'26 (Richardson). Ashes particularly rich in potash are those of burning nettles, wormwood (Artemisia Absin-thium), tansy (Tanacetum vulgare), fumitory (Fumaria officinalis), tobacco. In fact the ashes of herbs generally are richer in potash than those of the trunks and branches of trees; yet, for obvious reasons, the latter are of greater industrial importance as sources of carbonate of potash.

Carbonate of Potash (K2C03) in former times used to be made exclusively from wood-ashes, and even now the industry survives in Canada, Russia, Hungary, and other countries, where wood is used as the general fuel. In some places—for instance, in certain districts of Hungary— wood is burned expressly for the purpose; as a rule, how-ever, the ashes produced in households form the raw material. The ashes are lixiviated with water, which dissolves all the carbonate of potash along with more or less of chloride, sulphate, and a little silicate, while the earthy phosphates and carbonates and other insoluble matters remain as a residue. The clarified solution is evaporated to dryness in iron basins and the residue cal-cined to burn away particles of charcoal and half-burned organic matter. In former times this calcination used to be effected in iron pots, whence the name " potashes" was given to the product; at present it is generally conducted in reverberatory furnaces on soles of cast-iron. The cal-cined product goes into commerce as crude potashes. The composition of this substance is very variable,—the per-centage of real K2C03 varying from 40 to 80 per cent. The following analysis of an American " potashes" is quoted as an example.

Carbonate of potash ...71'4 Water 4>5
,, soda 2'3 Insoluble matter 2'7
Sulphate of potash 14-4
Chloride of potassium... 3 -6 98-9

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 " pearl ashes."

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 " Leblanc process " for the conversion of common salt into soda ash (see SODIUM).

Chemically pure carbonate of potash is best prejiared 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 ("oleum tar-tari") in ordinary air. 100 parts of water dissolve—
at 0° C. 20° C. 135° C. (boiling point of saturated solution)
83 94 205
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 " sal tartari" is not yet quite obsolete.

Bicarbonate of Potash (K2OC02 + H2OC02 = 2KHC03) 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 " black flux " 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 0' 10* 20° 60° 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 wyent into the "mild " alkali and made it " caustic," 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 drawn off by means of a syphon. The remaining mud of carbonate and hydrate of lime is washed, by décantation, 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 a polished iron tray, or into an iron mould to produce the customary form of " sticks," and allowed to cool. The solidified preparation must be at once bottled up, because it attracts the moisture and carbonic acid of the air with great avidity and deliquesces. According to the present writer's experience (Journ. Soc. Chem. hid., May 1884), nickel basins are far better adapted than iron basins for the concentration of potash ley. The latter begin to oxidize before the ley has come up to the traditional strength, while nickel is not attacked so long as the percentage of real KHO is short of 60. For the fusion of the dry hydrate nickel vessels cannot be used; in fact, even silver is perceptibly attacked as soon as all the excess of water is away; absolutely pure KHO can be produced only in gold vessels. Regarding the action of potash on platinum, see PLATINUM (supra, p. 191). Glass and (to a less extent) porcelain are attacked by caustic potash ley, slowly in the cold, more readily on boiling.

Frozen caustic potash forms an opaque, white, stone-like mass of dense granular fracture; specific gravity = 2*1. It fuses considerably below and is perceptibly volatile at a red heat. It is extremely soluble in even cold water, and in any proportion of water on boiling. The solution is intensely " alkaline " to test-papers. It readily dissolves the epidermis of the skin and many other kinds of animal tissue,—hence the well-known application of the " sticks " in surgery. A dilute potash ley readily emulsionizes fats, and on boiling " saponifies " them with formation of a soap and of glycerin. Caustic potash is the very type of an energetic (mono-acid) basic hydrate (see CHEMISTRY, vol. v. pp. 486, 488).

According to Tiinnermann's and Schiff's determinations, as calculated by Gerlach, the relation in pure potash ley between specific gravity at 15" C. and percentage strength is as follows :—

Percentages of KHO or K30. Specific Gravity, if percentage refers to Percentages of KHO or K20. Specific Gravity, if percentage refers to

K.,0. KHO.
K„0. KHO.
0 1
10 15 20 1-000 1-010 1-099 1-154 1-215 1-000 1-009 1-083 1-128 1-177 25 30 40 50 00 1-285 1-355 1-504 1-660 1-810 1 -230 1 "288 1-411 1 -539 1-667

All commercial caustic potash is contaminated with ex-cess of water (over and above that in the KHO) and with carbonate and chloride of rjotassium; sulphate, as a rule, is absent. Absolutely pure potash has perhaps never been seen; a preparation sufficing for most purposes of the analyst is obtained by digesting the commercial article in strong (85 per cent, by weight) pure alcohol. The hydrate KHO dissolves in the alcohol of the solvent; the chloride and the carbonate unite with the water and form a lower layer or magma, from which the alcoholic solution of the KHO is decanted off, to be evaporated to dryness and fused in silver vessels ("potasse a l'alcool").

The metal (potassium) has been known to exist since Lavoisier, but was first obtained as a substance by Hum-phry Davy in 1807. He prepared it from the hydrate by electrolysis. Gay-Lussac and Thenard subsequently found that this substance can be reduced to the metallic state more easily by passing its vapour over white hot metallic iron; but even their method as a mode of preparation was soon superseded by Brunner's, who, to the surprise of his contemporaries, produced the metal by simply distilling its carbonate with charcoal—applying an old-established principle of ordinary metallurgy. Brunner's process is used to the present day for the production of the metal.

One of those cylindrical, neckless, wrought-iron bottles which serve for the storing of quicksilver is made into a retort by taking out the screw-plug at the centre of one of the round ends and substituting for it a short, ground-in, iron outlet pipe. This retort is charged with a black flux made from a mixture of pure and crude bitartrate so adjusted that the flux contains as nearly as possible the proportion of free carbon demanded by the equation K„C03 + 2C = 2K + 3CO. It is then suspended horizontally within a powerful wind-furnace, constructed for coke as fuel. At first a mixture of coke and charcoal is applied, to produce the right tem-perature for chasing away the moisture and enabling one to, so to say, varnish over the retort with borax and thus protect it against the subsequent intense heat. After these preliminaries coke alone is used and the fire urged on to, and maintained at, its maximum pitch, when potassium vapour soon begins to make its appearance. The condensation of this vapour, however, demands special methods, because even the cold metal would quickly oxidize in the air and act most violently on liquid water. Brunner used to condense the vapour by passing it into a small copper vessel charged with rock-oil (see PARAFFIN, vol. xviii. p. 237), in which liquid the condensed metal sinks to the bottom and thus escapes the air. Donne and Maresca dispense with rock-oil altogether ; they receive the vapour in a dry condenser made of tw7o Hat rectangular trays of wrought iron which fit closely upon each other, enclosing a space such as might be used as a mould for casting a thin cake of any ordinary metal. This condenser has a short neck into wdiich the outlet pipe of the retort fits ; and the pipe must be as short as possible, be-cause it is essential (Donne and Maresca) that the hot vapour pass-abruptly from its original high to a low temperature, to evade a certain range of medium temperatures at which the metal com-bines with carbonic oxide into a black solid, which may obstruct the outlet pipe. The formation of this bye-product cannot be altogether avoided ; hence a long borer is inserted into the con-denser from the first to enable one to clear the throat of the retort, at a moment's notice. The condenser is kept as far as possible cold by the constant application to it of damp cloths. As soon as the distillation is finished the (still hot) condenser is plunged into a bucketful of rock-oil, to cool it down, the mould opened (under the oil), and the now solid metal taken out. The crude metal is always contaminated with some of the black solid and other mechanical impurities. To remove these the best method is to redistil it from out of a small iron retort and condense the vapour in rock-oil according to Brunner's original plan. The purified metal i^ soft enough to be moulded (under rock-oil) into globular pieces, wdiich are preserved in bottles filled to the top with the protecting liquid. But even this does not prevent gradual oxidation ; bright, metallic potassium can be maintained in this condition only by preserving it in a sealed-up glass tube within a vacuum or in an atmosphere of hydrogen or some other inert gas. The black solio) above referred to is a most dangerous substance. When exposed! to the air it turns red and then explodes either spontaneously or on the slightest provocation by friction or pressure. Even if kept under rock-oil it gradually becomes explosive. The distillation of potassium, in fact, is a dangerous operation, which had better be-left in the hands of specialists.

Pure potassium is a bluish-white metal; but on exposure to ordinary air it at once draws a film of oxide, and on prolonged exposure deliquesces into a solution of hydrate and carbonate. At temperatures below 0° C. it is pretty hard and brittle; at the ordinary temperature it is so soft that it can be kneaded between the fingers and cut with a blunt knife; specific gravity = 0'865. It fuses at 62°-5 C. (Bunsen), and at 720° to 730° C. (Carnelley and Williams), i.e., considerably below its boiling point, begins to distil with formation of an intensely green vapour. When heated in air it fuses and then takes fire and burns into a mixture of oxides. Most remarkable, and charac-teristic for the group it represents, is its action on water. A pellet of potassium when thrown on water at once bursts, out into a violet flame and the burning metal fizzes about on the surface, its extremely high temperature precluding absolute contact with the liquid, except at the very end,, when the last remnant, through loss of temperature, is w-etted by the water and bursts with explosive violence. What really goes on chemically is that the metal decom- -poses the water thus, K + H,0 = KHO + H, and that the hydrogen catches fire, the violet colour of the flame being due to the potassium vapour diffused throughout it. Similar to that on water is its action on alcohol: the alcohol is converted into ethylate, while hydrogen escapes, K + C2H5. OH = C2H5. OK + H, this time without inflam-mation. So strong is the basilous character of the element that, in opposition to it, even ammonia behaves like an acid. When the oxide-free metal is heated gently within the dry gas it is gradually transformed into a blue liquid, which on cooling freezes into a yellowish-brown or flesh-coloured solid. This body is known as " potassamide," KNH2. When heated by itself to redness the amide is decomposed into ammonia and nitride of potassium, 3NH2K = NK3 + 2NH3. The nitride is an almost black solid. Both it and the amide decompose water readily with for-mation of ammonia and caustic potash. Potassium at temperatures from 200° to 400° C. " occludes " hydrogen gas, as palladium does (see " Palladium," under PLATINTBI, supra, p. 193). The highest degree of saturation corre-sponds approximately to the formula K2H for the " alloy," or to about 126 volumes of gas. (measured cold) for one volume of metal. In a vacuum or in sufficiently dilute hydrogen the compound from 200° upwards loses hydro-gen, until the tension of the free gas has arrived at the maximum value characteristic of that temperature (Troost and Hautefeuille).





Potassium Oxides, singularly, can be produced only from the metal, and another remarkable fact is that the one with which all chemical students imagine they are so familiar—namely, "anhydrous potash," K20—is little more than a fiction. According to Vernon Harcourt, when the metal is heated cautiously, first in dry air and then in dry oxygen, it is transformed into a white mass (K202 ?), which, however, at once takes up more oxygen with formations ultimately of a yellow powdery tetroxide (K204), fusible at a red heat without decomposition. At a white heat it loses oxygen and leaves a residue of lower oxides (K.20 ?). "When heated in hydrogen it is reduced to ordinary potash, KHO. When dissolved in excess of dilute acid it yields a mixed solution of the respective potash salt and peroxide of hydrogen, with abundant evolution of oxygen gas.

Potassium Salts.—There is only one series of these known,— namely, the salts produced by the union of potash (KHO) with acids.

Chloride, KC1.—This salt (commercial name, "muriate of potash") is at present being produced in immense quantities at Stassfurt from the so-called '' Abraumsalze." For the purpose of the manu-facturer of muriate these are assorted into a raw material contain-ing approximately in 100 parts—55-65 of carnallite (representing 16 parts of chloride of potassium); 20-25 of common salt; 15-20 of kieserite, a peculiar, very slowly soluble sulphate of magnesia, MgSo4.H20; 2-4 of tachhydrite (CaCl2. 2MgCl2 + 12H20); and minor components. This mixture is now wrought mainly in two ways. (1) The salt is dissolved in water with the help of steam, and the solution is cooled down to from 60° to 70°, when a quantity of impure common salt crystallizes out, which is re-moved. The decanted ley deposits on cooling and standing a 70 per cent, muriate of potash, which is purified, if desired, by washing it by displacement with cold water. Common salt prin-cipally goes into solution, and the percentage may thus be brought up to from 80 to 95. The mother - liquor from the 70 per cent, muriate is evaporated down further, the common salt which separates out in the heat removed as it appears, and the suffi-ciently concentrated liquor allowed to crystallize, when almost .pure carnallite separates out, which is easily decomposed into its components (see infra). (2) Ziervogel and Tuchen's method. The crude salt is ground up and then heated in concentrated solution of chloride of magnesium with mechanical agitation. The carnallite principally dissolves and crystallizes out relatively pure on cooling. The mother-liquor is used for a subsequent extraction of fresh raw salt. The carnallite produced is dissolved in hot water and the solution allowed to cool, when it deposits a coarse granular muriate of potash containing up to 99 per cent, of the pure substance. The undissolved residue produced in either process consists chiefly of kieserite and common salt. It is worked up either for Epsom salt and common salt, or for sulphate of soda and chloride of magnesium. The potassiferous bye-products are utilized for the manufacture of manures.

Chemically pure chloride of potassium is most conveniently pre-pared from pure perchlorate (see infra) by dioxygenating it in a platinum basin at the lowest temperature and then fusing the residue in a well-covered platinum crucible. The fused product solidifies on cooling into a colourless glass. Chloride of potassium dissolves in water and crystallizes from the solution in anhydrous cubes. 100 parts of water dissolve—
at 0° 10° 20° S0° 100° C.
29-2 32-0 34-7 42-8 56'6
parts of the salt. When a sufficiency of hydrochloric-acid gas is passed into the solution the salt is completely precipitated as a fine powder. If the original solution contained chloride of mag-nesium or calcium or sulphate of potash, all impurities remain in the mother-liquor (the S03 as KHS04), and can be removed by washing the precipitate with strong hydrochloric acid. Chloride of potassium fuses at 738° C. (Carnelley), and at a red heat vola-tilizes rather abundantly.

Chlorate, KC103. — This industrially important salt was dis-covered in 1786 by Berthollet, who correctly designated it as "peroxidized muriate." Chlorine gas is largely absorbed by cold caustic - potash ley with formation of chloride and hypochlorite, 2KHO + Cl2 = KCi + KC10 + H20. When the mixed solution is boiled it suffers, strictly speaking, a complicated decomposition, which, however, in the main comes to the same as if the hypo-chlorite broke up into chloride and chlorate, 3KC10 = 2KC1 + KC103. Hence chlorate of potash is easily produced by passing chlorine into hot caustic - potash ley so as at once to realize the change, 6KH0 + 3C12 = 3H20 + 5KC1 + KC103; and this method used to be followed industrially until Liebig pointed out that five-sixths of the potash can be saved by first substituting milk of lime, Ca(0H)2 = 2ca0H, for the potash ley and from the mixed solution of lime-salts precipitating, so to say, the chloric acid as potash salt by adding 1KC1 for every lcaC103 present, concentrating by evaporation, and allowing the KC103 to crystallize out. This in the present industrial process. For the technical details we must refer to the handbooks of chemistry. Suffice it to say that in practice about 1'03 times KC1 are used for every lcaC103, ami that the salt produced is almost chemically pure after one recrys-tallization. By repeated recrystallization every trace of impurities, is easily removed. The crystals are colourless transparent mono-clinic plates, which, unless formed very slowdy, are very thin, so as often to exhibit the Newton's colours. 100 parts of water dissolve—_
at 0° 15° 50' 104°-S (on boiling)
3-3 6 19 60
parts of the salt (Gay-Lussac). The salt is almost insoluble in strong alcohol. It is permanent in the air. It fuses at 359° C. (Carnelley), and at about 18° above the temperature of its formation the liquid gives off oxygen with evolution of heat, and formation ultimately of chloride (and oxygen). The salt accordingly, in opposition to any combustible matter with which it may bo mixed, behaves at the same time as a store of highly-condensed loosely-combined oxygen and of potential heat. Hence its manifold applications in artillery and pyrotechnics are easily understood. To give one example of the readiness with which it acts as a burning agent: a mixture of it and sulphur when struck with a hammer explodes loudly, the mechanical blow sufficing to produce locally the temperature neces-sary for starting the reaction. AVhen the salt was still a novelty it was tried, as a substitute for the nitre in gunpowder. Such powder, however, proved too good to be safe. More recently a mixture of 49 parts of the chlorate, 23 of sugar, and 28 of prussiate of potash was recommended by Pohl as a preferable substitute for gunpowder, but this powder has never come into actual use any-where. We must not forget to point out that mixtures of chlorate of potash and combustible substances must on no account be made in a mortar ; this would be sure to lead to dangerous explosions. The several ingredients must be powdered separately and only then be mixed together on a sheet of paper or on a table, all unnecessary pressure or friction being carefully avoided.

The decomposition of chlorate of potash by heat is greatly facili-tated by admixture of even small proportions of certain solid oxides, e.g., oxide of copper, of iron, or of manganese. The oxygen, in the case of binoxide of manganese, for instance, comes off below the fusing point of the salt. Hence a salt contaminated with even a small proportion of heavy metallic chlorate cannot (in general) be fused without decomposition. The writer observed this anomaly with a commercial chlorate which happened to contain about oris half per cent, of chlorate of zinc. The aqueous solution of the sa'.t is neutral and bears prolonged boiling without decomposition. Oil acidification with dilute sulphuric acid it assumes the reactions <n a solution of chloric acid, i.e., of a powerful but readily controllabie oxydant. In this capacity it is used in calico-printing as a "discharge." In the same industry it serves for making tho chlorate of soda needed for the production of aniline black. In the chemical laboratory it is in constant requisition as a source of oxygen and as an oxidizing agent. In the hands of Marignac it served for the determination of tho important ratio KCl: 30.
Perchlorate, KC104.—The decomposition of chlorate of potash by heat, if catalytic agents like Mn0.2, &c., are absent, proceeds by two stages. In the first the salt breaks up thus, 2KCl03=ECl + 0.2 + KC104 ; in the second the perchlorate at a higher temperature is decomposed into chloride and oxygen. The termination of the first stage is marked by a slackening in the evolution of the oxygen and by tho residual salt (which, at the beginning, is a thin fluid) becoming pasty. From the mixture KCl + KC104 the chloride is extracted by lixiviation with successive instalments of cold water. The residual perchlorate is very easily purified by recrystallization (compare pure chloride of potassium, supra). Perchlorate of potash dissolves in 88 parts of water of 10°C, and in far less of boiling water. It is absolutely insoluble in absolute alcohol. It begins to give off its oxygen at about 400° C., which is below its fusing point.

The salt has been recommended as a substitute for chlorate in pyrotechnic mixtures, because it contains more oxygen, and yet, on account of its greater stability, is a less dangerous ingredient.

Bromide, KBr.-—This salt is formed when bromine is dissolved in caustic-potash ley. The reaction is quite analogous to that go-ing on in the case of chlorine ; only the hypobromite (KBrO) first produced is far less stable than hypochlorite, and vanishes after short heating. The addition of bromine is continued until the liquid is permanently yellow and retains its colour after short heat-ing. The solution is then evaporated to dryness and the bromate decomposed by cautious heating. A small portion of the bromate breaks up into K„0 + Br2 + 50 ; hence the residual bromide is con-taminated with a little free alkali; but this is easily set right by neutralizing its solution with hydrobromic acid. The salt crystal-lizes in colourless transparent cubes, easily soluble in water. It is used in medicine for quieting the nerves,—to cure sleeplessness, for instance ; also (internally) as a local anaesthetic preparatory to operations on the larynx or the eye. The dose of the pure (KI free) salt for adults can safely be raised to 2 grammes (about 30 grains). It is also used in photography.

Iodide, KI.—Of the very numerous methods which have been recommended for the preparation of this important salt the simplest (and probably the best) is to dissolve in a caustic-potash ley (which is dilute enough to hold the rather difficultly soluble iodate KI03 in solution) enough iodine to produce a permanent yellow colour (the iodine passes at once into 5KI + KIO.,; the hypo body KIO has no existence practically) and to deoxidize the iodate, which is done most conveniently by adding a sufficiency of powdered char-coal to the solution, evaporating to dryness in an iron vessel, and heating the residue. The oxygen goes off as C02 at a lower tem-perature than that which would be needed for its expulsion as oxygen gas. The residue is dissolved, and the solution filtered and evaporated to crystallization. The salt comes out in colourless transparent cubes, very easily soluble in even cold water. The commercial salt forms opaque milk-white crystals, which, as a matter of habit, are preferred to the clear salt, although they are produced by causing the salt to crystallize from a strongly alkaline solution and by drying the crystals (finally) in a stream of hot air, and although through the former operation they are at least liable to contain carbonate. Iodide of potassium acts far more powerfully on the human system than bromide, and therefore is administered in smaller doses. It is used against skin-diseases, and also for eliminating the mercury which settles in the system after long-continued administration of mercurial medicines. It is also used, far more largely than the bromide, in photography. See PHOTO-GRAPHY, passim.

Sulphate (K2S04) used to be extracted from kainite, but the process is now given up because the salt can be produced cheaply inough from the muriate by decomposing it with its exact equi-valent of oil of vitriol and calcining the residue. To purify the crude product it is dissolved in hot water and the solution filtered and allowed to cool, when the bulk of the dissolved salt crystallizes _out with characteristic promptitude. The very beautiful (anhydrous) crystals have as a rule the habitus of a double six-sided pyramid, but really belong to the rhombic system. They are transparent, very hard, and absolutely permanent in the air. They have a bitter salty taste. 100 parts of water dissolve—
at 0° 12° 10(T C.
8-36 10 26
parts of the salt. Sulphate of potash fuses at a strong red heat, and at this temperature volatilizes, for an alkaline salt, rather slowly. The chloride, weight for weight, volatilizes at ten times the rate (Bunsen). Sulphate of potash used to be employed in medicine, but is now obsolete. The crude salt is used occasionally in the manufacture of glass.
Bisulphate (KHS04) is readily produced by fusing thirteen parts of the powdered normal salt with eight parts of oil of vitriol. It dissolves in three parts of water of 0° C. The solution behaves pretty much as if its two congeners, K2S04 and H2S04, were present side by side of each other uncombined. An excess of alcohol, in fact, precipitates normal sulphate (with little bisulphate) and free acid remains in solution. Similar is the behaviour of the fused dry salt at a dull red heat; it acts on silicates, titanates, &e., as if it were sulphuric acid raised beyond its natural boiling point. Hence its frequent application in analysis as a disintegrating agent.

For the following potash salts we refer to the articles named :— Chromates, see CHROMIUM; Cyanide and Ferrocyanide, PKUSSIC ACID; Chloroplatinate, PLATINUM (supra, p. 192); Nitrate, NITROGEN (vol. xvii. p. 518) ; Phosphates, PHOSPHORUS (vol. xviii. pp. 818-19); Oxalates, OXALIC ACID; Sulphides and Sulphites, SULPHUR; Silicates, GLASS (vol. x. p. 655 sg.) and SILICA ; Tartrates, TARTARIC ACID. For potash salts not named, see the handbooks of chemistry.





Rubidium and Caesium.—When Bunsen and Kirchhoff in 1860 applied their method of spectrum analysis to the alkali salts which they had extracted analytically from Purkheim mineral water, they obtained a spectrum which, in addition to the lines characteristic for sodium, potassium, and lithium, exhibited two blue lines which were foreign to any other spectrum they had ever seen. They accord-ingly concluded that these lines must be owing to the presence of a new alkali metal, which they called "cresium." Bunsen at once resumed the preparation of the mixed alka-line salt with 44,000 litres of Diirkheim water, with the view of isolating the caesium in the form of a pure salt; and he was more than successful—for the new alkali salt, after elimination of all the ordinary alkali metals, proved to be a mixture of the salts of two new alkali metals, which he succeeded in separating from each other. For one he retained the name already chosen; the other he called "rubidium," on account of the presence in his spectrum of certain characteristic red lines. Since Bunsen's time these two metals have been discovered in a great many native potassiferous materials—minerals, mineral waters, plant ashes, &c.—but in all cases they form only a small fraction of the alkali, the caesium in general amounting to only a fraction of even the rubidium. One solitary exception to both rules is afforded by a rare mineral called "pollux," which is found only on the island of Elba. Plattner analysed this mineral in 1846 and recognized it as a compound silicate of alumina, oxide of iron, soda, potash, and water; but his quantitative analysis came up to only 92-75 per cent., and he could not account for the 7'25 per cent, of loss. After Bunsen's discovery Pisani analysed the mineral again, and he found that it contained no potash at all, but, instead of it, a large percentage (34-l) of csesia. Recalculating Plattner's analy-sis on the assumption that the presumed chloroplatinate of potassium was really chloroplatinate of caesium, he found that the corrected numbers did add up to near 100 and agreed with his own. Rubidium, singularly, is absent from this mineral.

That both rubidium and caesium are contained in sea water might well be taken for granted; but it is worth while to state that Schmidt of Dorpat actually proved the presence of rubidium, and even determined it quantita-tively.

For the preparation of rubidium compounds one of the best materials is a mixture of alkaline salts, which falls as a bye-product in the industrial preparation of carbonate of lithia from lepidolite. A supply of this salt mixture which Bunsen worked up contained 20 per cent, of chloride of rubidium, 33 of chloride of potassium, and 36 of common salt, but very little caesium ; his supply came from the Saxon or Bohemian mineral. The lepidolite of Hebron, Maine, United States, on the other hand, is rich in caesium. Another practically available source for caesium is the mother-liquor salt of Nauheim in Germany. It yielded to Bottcher 1 per cent, of its weight of the chloroplatinate PtCl6Cs2.

Bunsen's method for the extraction of the two rare potassium metals from a given mixture of alkaline salts is founded upon the different solubility of the several alkaline chloroplatinates. Accord-ing to him 100 parts of water dissolve—
Potassium Rubidium Ceesium
at 0°C 0-74 0-13 0-024
„ 20° C 1-12 0-14 0-079
,, 100° C 5-13 0-63 0-3/7
parts of the several salts. The chloroplatinates of sodium and lithium are easily soluble even in cold water, so that chloride of platinum does not precipitate these two metals at all. Hence, supposing we boil a given mixture of chloroplatinates of potassium and (say) rubidium with a quantity of water insufficient to dissolve the whole, part of both salts will dissolve ; but the residual chloro-platinate will be richer in rubidium than the dissolved part. And supposing, on the other hand, we add to a mixed solution of the two chlorides a quantity of chloroplatinic-acid solution insufficient to bring down the wdiole of both metals, the rubidium will accumu-late in the precipitate and the potassium in the solution. It is also easily understood that, if the amount of reagent added falls short even of that which would be needed by the rubidium if present alone, a very nearly pure PtClr,Bb2 may be expected to come down. Any dry chloroplatinate is easily reduced to a mixture of metallic platinum and alkaline chloride by the simple operation of heat-ing in hydrogen to about 300° C. The chloride can be dissolved out, and thus again made amenable to fractional precipitation by platinum solution, and the platinum be reconverted into reagent by means of aqua regia. Hence the process is not so expensive as it might at first sight appear.

Redtenbacher has worked out an analogous process to Bunsen's, Sounded upon the different solubility of the three alums—Al. K(S04)a t-12HaO. At 17° G. 100 parts of water dissolve of the alum of
Potassium Rubidium Ceesium
13-5 2-27 0-62
parts. Sodium and lithium alum are very easily soluble in water, and remain dissolved in the first mother-liquor when the mixed alum of K, Rb, and Cs crystallizes out. These three alums are parted by repeated crystallization, and the rare alkalis recovered irom their respective alums by precipitation with chloride of platinum.

The separation of rubidium and caesium offers great difficulties. According to Godeffroy an approximate separation may be effected by dissolving the mixed chlorides in strong hydrochloric acid, and adding a solution of terchloride of antimony in the same menstruum; the caesium (chiefly) comes down as SbCl3 + 6CsCl; the bulk of the rubidium remains dissolved. The two rare alkali metals are so closely similar to potassium that it will suffice to give a tabular statement of the principal points of difference. By way of intro-duction, however, we may state that rubidium metal was prepared by Bunsen from the black flux obtained by igniting the bitartrate, by Brunner's method for potassium. Metallic caesium, it seems, cannot be thus obtained ; but in 1883 Setterberg made it by the electrolysis of a fused mixture of the cyanides of caesium and barium.
Potassium. Rubidium. Csesium.
Atomic weights 0 = 16 K=39-136 Rb=85-4 Cs = 133-0
Free Metals—
Specific gravity 0-865 1-52 1-88
Fusing point 62°-5 38°-5 26° to 27° C.
Volatility increases >—>
Hydrates, RHO—Very similar to one another; the basility increases =.—>
{
Vide supra. Permanent in air. Deliquescent.
Almost insoluble More soluble than KC1.
in alcohol. Soluble in alcohol.
Sulphates, R2S04—
100 parts 0: water dis- (At- 2° 0. 8 ? 159
solve \ „ 70° C. 19-8 42 ?
Carbonates, R2CO3—All very soluble in water.
100 pp.Hs of alcohol dis- |At lg. c 0 0.-4 n.j
Alums ) Solubility decreases ?—>'
Chloroplatinates ) (vide supra).

Analysis.—In this section we treat of the detection and determin-ation of alkali metals generally. If the given substance is a solid, a good preliminary test is to heat about one centigramme of it at one end of a fine platinum wire in the flame-mantle of a Bunsen lamp, or in a blow-pipe flame just at the end of the inner cone. Most alkali salts are sufficiently volatile to impart to the flame the colour characteristic of the respective metallic vapour. Certain native silicates and certain other compounds do not volatilize, but these can be rendered amenable to the test by mixing them with sulphate of lime and then applying the flame, whereupon alkaline sulphate is formed which volatilizes. The flame-colours are—
Potassium, Rubidium, Caesium. Sodium. Lithium.
Violet. Yellow. Red.

These flame-reactions are very delicate but not conclusive, because in the case of mixtures several colours may be radiated out at the same time, and one may eclipse all the rest—this holds, for instance, for things containing sodium, whose flame-colour is more intense than that of any other metal—or a mixed colour may be produced which the eye is incompetent to analyse. The spectrum apparatus here comes in usefully ; and by means of it it is in general possible to see the lines characteristic of the several metals in presence of, or at least after, one another, because as a rule the several metals are present as compounds of different volatility.

For a thorough analysis it is necessary to begin by bringing the substance into aqueous or acid solution, and next to eliminate all that is not alkali metal by suitable methods. A certain set of heavy metals can be precipitated as sulphides by means of sulphur-etted hydrogen in the presence of acid, all the rest of these by means of sulphide of ammonium from an alkaline solution. From the filtrate, barium, strontium, and calcium are easily precipitated by means of carbonate of ammonia on boiling, so that, if the filtrate from these carbonates is evaporated to dryness and the residue kept at a dull red heat long enough to drive away the ammonia salts, nothing can be left but salts of alkali metals and magnesium. This residue is dissolved in a small quantity of water, and any residual basic salt of magnesium filtered off. The filtrate is then ready to be tested for alkali metals as follows : if magnesia be oisent, potassium or rubidium (not caesium) can be detected by addition (to a neutral or feebly acetic solution) of a saturated solution of bitartrate of soda. Potassium and rubidium come down as crys-talline bitartrates. The reaction may take some time to become manifest, but can be accelerated by vigorous stirring. In a separate quantity of the solution lithium may be searched for by means of -carbonate of soda or trisodic phosphate as explained under LITHIUM (vol. xiv. p. 697). For soda we have no characteristic precipitant. In any case the spectrum apparatus should be used for controlling and, if necessary, supplementing the wet-way tests. The case of magnesia being present need not be specially considered, because the qualitative method will easily be deduced from wdiat is said in the following paragraph.

Quantitative Determinations.—An exhaustive treatment of this subject would be out of place here. "We confine ourselves to two cases. (1) A mixture of alkaline chlorides only. In this case the potassium (including Rb and Cs) is best separated out by adding a quantity of chloroplatinic-acid solution sufficient to convert all the metals into chloroplatinates, to evaporate to dryness over a water-bath, and from the residue to extract the lithium and sodium salts by lixiviation with alcohol of 70 per cent, (by weight). The residual chloroplatinate is collected on a filter, dried at 110° C., and, if Rb and Cs are absent, weighed as chloroplatinate of potassium, PtCleK^P tCl6K2 x 0 -3071 = 2KC1). The chloride of sodium is determined by difference—if lithium be absent. The case of its presence cannot be here considered. (2) A mixture of alkalis combined with sulphuric acid, or such volatile acids as can be expelled by sulphuric. In this case it is best to begin by converting the whole into neutral sulphates, and then to apply the method of Finkener, which, amongst other advantages, offers the one that it does not demand the absence of magnesia. The mixed sulphate is dissolved in water and the solution mixed with a little more than the volume of chloroplatinic acid (" platinum solution") demanded by the potassium (Rb and Cs). The mixture is placed in a water bath and, if necessary, diluted with sufficient water to bring the whole of the precipitated chloroplatinate into hot solution. The solution is then evaporated very nearly to dryness (on the water bath, with continuous stirring towards the end. to avoid formation of crusts), allowed to cool, and the residue mixed, first with twenty times its volume of absolute alcohol, then with ten volumes of absolute ether. The mixture is allowed to stand in a well-covered vessel for some hours, to enable the precipitate to settle completely. The precipitate contains all the potassium as chloroplatinate, and most of the sodium and magnesium, and also part of the lithium in the sulphate form. It is washed with ether-alcohol (to complete filtrate A), and then lixiviated as quickly as possible with cold concentrated solution of sal-ammoniac, which dissolves away the sulphates (filtrate B). The residual chloroplatinate is dried within the filter in a porcelain crucible, which is next heated so as to char the paper at the lowest temperature. The residue is then ignited gently in hydrogen, and from the resulting residue the chloride of potassium is extracted by water, to be determined as chloroplatinate, as shown in (1), or otherwise. From the undissolved residue the charcoal is burned away and the residual platinum weighed to check the potassium determination. After removal of the ether and alcohol from filtrate A by distillation, the two filtrates A and B are mixed, evaporated to dryness, the ammonia salts chased away by heating, and the residue is reduced (at about 300° C.) in hydrogen to bring the platinum into the form of metal, from which the magnesia and alkali salts are easily dissolved away by means of water or dilute acid. The whole of the salts are then made into neutral sulphate, which is weighed and then dissolved in a known weight of water. The lithium and the magnesium are determined in aliquot parts of the solution and calculated as sulphates. The soda is found by difference. A case intermediate between (1) and (2) often presents itself in practice. We refer to the commercial muriate from Stassfurt. In such an impure muriate the potassium can be determined promptly and accurately by adding to the very concentrated solution of the substance a large excess of a very concentrated solution of chloro-platinic acid,—"excess" meaning more platinum than necessary to make all the metals into chloroplatinates. The precipitate is allowed to settle, collected on a small filter, and washed, first with successive instalments of a platinum solution (containing 5 per cent, of metal), then with ordinary alcohol; it is next dried, and weighed as above (Tatlock's method slightly modified). In exact analyses the small quantity of potassium which passes into the filtrate is recovered—ultimately by Finkener's method—and allowed for. (W. D.)


Footnotes

Compare the interesting paper by C. Bischoff in the Journ. f. Pracl. Chem., vol. xlvii. p. 193 (1849).




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