P T P O T 593 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, founded upon the different solubility of the three alums A1.R(S0 4 ) 2 + 12H 2 0. At 17 C. 100 parts of water dissolve of the alum of Potassium 13-5 Rubidium 2-27 Ceesium 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 from 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 SbCl 3 + 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 [K>ints 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 Brimner 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. K = 39 13G Rubidium. Rb=85 4 Cxsium. Cs = 133 1-52 3S 5 More soluble than KC1. Soluble in alcohol. 0-74 Atomic weights O = 16 free Metals Specific gravity .......... 0-865 Fusing point .............. 62 5 Volatility increases 3 > Hydrates, EHO Very similar to one another ; the basility increases > ( Vide supra. Permanent in air. Deliquescent. Chlorides, RC1 .......... ] Almost insoluble ( in alcohol. Sulphates, R.>SO 4 100 parts of water dis- (At - 2 C. 8 solve .............. t 70 C. 19-8 Carbonates, R2CC>3 All very soluble in water. 100 parts of alcohol dis- ) <.inn n solve .............. f At19 C Alums ) Solubility decreases > > Chloroplatinates f (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, Ciesium. Violet. Sodium. Yellow. Lithium. 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 absent, 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 what 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, PtCl 6 K a: (PtCl 6 K 2 x -3071 = 2KC1). The chloride of sodium is deter mined 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 pot assium (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 continu ous 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 con tains 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 cru cible, which is next heated so as to char the paper at the lowest tem perature. 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 dis tillation, the two filtrates A and B are mixed, evaporated to dry- ness, 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 Aveight 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.) POTATO. The potato (Solanum tuberosum) is too well known to need detailed description. It owes its value to the peculiar habit of developing underground slender leafless shoots or branches which differ in character and office from the true roots, and which gradually swell at the free end and thus produce the tubers with which we are so familiar. The nature of these tubers is further rendered evident by the presence of " eyes " or leaf-buds, which in due time lengthen into shoots and form the haulm or stems of the plant. Such buds are not, under ordinary circumstances, formed on roots. What the determining cause of the formation of the tubers may be
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