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PUBLIE TOUT CE QUI CONCERNE LES DES SUBSTANCES RADIOACTIVES. CHEMICAL NEWS,} Jan. 9, 1914 THE Dimethyl Phosphates of the Rare Earths. CHEMICAL NEWA VOL. CIX., No. 2824. THE study of methods for the separation of the rare earths and of new compounds of the rare earths that seem likely to be of use for such purposes has for a long time occupied the attention of those interested in this branch of chemistry. The rare earth salts of dimethyl phosphoric acid seemed, on preliminary investigation, to be possessed of properties which would make dimethyl phosphoric acid an extremely valuable reagent for the separation of some of the rare earths. Therefore a study of the properties of these compounds was commenced. This investigation involved the preparation of dimethyl phosphoric acid; the preparation of the rare earth compounds from this; the determination of the composition and solubility of these compounds; and, finally, several fractionations of rare earth mixtures. Preparation of Dimethyl Phosphoric Acid.-This acid was prepared, according to the method of Hugo Schiff (Chem. Central Blatt, 1857, 761-763, 864), by allowing methyl alcohol to drop slowly into phosphorus oxychloride. About 200 cc. of phosphorus oxychloride were placed in a litre flask, and 250 cc. of methyl alcohol were added, drop by drop, through a separatory funnel fitted to the flask. During the reaction the flask was shaken in a stream of cold water, and the temperature maintained between 25° and 30°. The large quantities of hydrochloric acid gas and methyl chloride which were evolved passed off through a side tube. At the completion of the reaction, which required from an hour to an hour and a half, there remained a colourless somewhat syrupy liquid, consisting of both dimethyl phosphoric and phosphoric acids with some hydrochloric acid and methyl chloride. This was removed to an evaporating dish, and heated upon the steam-bath until the hydrochloric acid and methyl chloride were expelled. The syrupy acids were then diluted and neutralised with barium carbonate. The phosphoric acid was removed as barium phosphate. The precipitate was filtered off, leaving a clear filtrate consisting of a solution of barium dimethyl phosphate, together with traces of barium monomethyl phosphate. This solution was treated with just sufficient sulphuric acid to precipitate all the barium as the sulphate, which after digestion at 95° was removed by filtration, leaving a clear solution of dimethyl phosphoric acid. Yttrium Dimethyl Phosphate.-This salt was obtained by dissolving yttrium hydroxide in dilute dimethyl phosphoric acid. After stirring for some time the liquid became clear. The solution was then filtered and placed upon a water-bath, the temperature of the latter being kept at about 90°. This treatment caused a quantity of crystalline yttrium dimethyl phosphate to form. The precipitate was filtered off and washed with boiling water. The compound was next dissolved in as small an amount of water as possible, and permitted to crystallise by spon taneous evaporation. Groups of white needle-like crystals, radiating from a common centre, were formed. These were removed, dried between filter-papers, and then placed in an oven at 100° for four hours. The results of the analysis seemed to show that a very small amount of water was retained. Further drying at the same tempera ture failed to make any difference. Yttrium dimethyl phosphate dissolves in water at 25° to • Contribution from the Chemical Laboratories of New Hampshire College. 13 the extent of 2.80 parts of the anhydrous salt to 100 parts of water. At 95 only about o'55 part dissolve in 100 parts of water. 4 YO; calculated, 24'34; found, 24'06. Lanthanum Dimethyl Phosphate.-This substance was prepared by dissolving the oxide in dilute dimethyl phos. phoric acid. The solution, after filtration, was concentrated upon a water-bath until a skin began to form upon the surface, after which it was permitted to crystallise by spontaneous evaporation. The resulting crystals were dissolved in water and re-crystallised once again by spon. taneous evaporation. These crystals were separated from excess of mother-liquor by pressing between filter-papers, and finally dried in air. Lanthanum dimethyl phosphate forms white hexagonal Crystals. One hundred parts of water dissolve 103.7 parts of the anhydrous salt at 25°. The solubility at 100° did appeared to be somewhat difficult to obtain owing to the not vary like the other rare earths. The solubility at 95° fact that the solution had a tendency to become colloidal. In order to determine the phosphorus a weighed sample of lanthanum dimethyl phosphate was first tused with sodium peroxide to oxidise the methyl groups and get the phosphorus present as phosphate. The usual method was then followed. Calculated: La2O3, 29·63; P2O, 38.75. Found: La203, 29 45; P2O, 38.63. From the above analysis it was considered that the compound contained 4 molecules of water of crystallisation, as shown by the formula Laz¦ (CH3)2PO4]6°4Н2O. Cerous Dimethyl Phosphate.-The cerous compound was obtained by treating cerous carbonate with a dilute solution of the acid. The clear solution was evaporated on the water-bath until a skin began to form upon the surface, as was observed in the case of the preceding substance. It was further concentrated by spontaneous evaporatior. The crystals obtained in this manner were again crystallised from water. Cerous dimethyl phosphate is a white crystalline solid belonging to the hexagonal system. It is very soluble in cold water, but less soluble in hot. One hundred parts of water dissolve 79 6 parts of the anhydrous salt at 25° and about 65 parts at 95°. Upon analysis 33 08 per cent CeO2 was found. This still remained after drying for some time at 100°, and corresponded to the formula Ce2 [(CH3)2PO4]6. H2O. tures. Praseodymium Dimethyl Phosphate.-A solution of this dimethyl phosphate was prepared by dissolving praseodymium oxide in the acid. The liquid was concentrated on the water-bath until small crystals began to form, after which it was set aside to evaporate at ordinary temperaThe compound purified by re-crystallisation formed 64 1 parts were dissolved by green hexagonal crystals. 100 parts of water at 25°. Neodymium Dimethyl Phosphate was prepared in a similar manner to the praseodymium compound. It forms pale lilac coloured hexagonal plates. At 25°, 561 parts dissolve in 100 parts of water, while at 95° only about 22.3 An analysis parts are dissolved by 100 parts of water. showed the presence of 32 27 per cent of Nd203, thus pointing to the formula Nd2 [(CH3)2PO4]6. Samarium Dimethyl Phosphate. A small quantity of the acid was neutralised with samarium oxide. T'he solution thus obtained was concentrated on the water-bath until nearly solid, owing to the formation of the dimethyl phosphate crystals. It was filtered while still hot. The salt was dissolved in water and crystallised by evaporation at ordinary room temperature. Samarium dimethyl phosphate forms cream coloured hexagonal prisms. One hundred parts of water dissolve 35 2 parts of the salt at 25° and about 10-8 parts at 95°. 33.11 per cent Sm203 was found to be present, indicating no water of crystallisation like most of the rare earth dimethyl phosphates. Gadolinium Dimethyl Phosphate was separated from its solution by carefully heating. It forms white needle-like crystals very similar to the yttrium compound. One hundred parts of water dissolve 230 parts of the salt at 25°, while only 6.7 parts are dissolved by the same quantity at about 95°. The compound contained 34.00 per cent Gd2O3. Erbium Dimethyl Phosphate was prepared in a similar way to the ytterbium salt described below. It forms very pale coloured needles, 1.78 parts dissolving in 100 parts of water at 25°. Ytterbium Dimethyl Phosphate.-The ytterbium oxide was dissolved in a slight excess of hydrochloric acid and the solution diluted considerably. A sufficient quantity of dimethyl phosphoric acid to react with the ytterbium present was neutralised with sodium carbonate, and then made slightly acid with a few drops of dimethyl phosphoric acid. This solution was diluted and slowly added to the ytterbium chloride with careful stirring. A precipitate of ytterbium dimethyl phosphate formed, which slightly increased in amount on heating the solution to 100°. The precipitate was filtered off, dissolved in cold water, and again precipitated by heating. The white needle-like crystals dissolve to the extent of 12 parts per 100 parts of water at 25°. Only 0.25 are dissolved by 100 parts of water at about 95°. An analysis showed 35.73 per cent Yb2O3, which corresponds to the amount contained in the formula of the anhydrous substance. Table of Solubilities of Dimethyl Phosphates. Crystal form. Hexagonal crystals 103.7 Element. Lanthanum Cerium plates 79.6 Fractionation of Material.-Since, as already stated, the study of new compounds of the rare earths is carried on mainly to obtain compounds adaptable to a more complete and more rapid separation of these elements, no study would be complete without determining the behaviour of mixtures of the dimethyl phosphates. Several fractionations were conducted, and the method employed was that of precipitation by heat, in the case of the less soluble compounds a dilute solution was prepared. The beaker containing this solution was then placed in a water-bath and the temperature of the bath gradually raised, the solution being constantly stirred. As soon as a fair amount of precipitate had formed, the liquid was filtered off and the precipitate retained as fraction I. The filtrate was again heated until another lot had separated and again filtered. The dimethyl phosphates collected from this second heating were put aside as fraction II. In this manner fractions were taken up to and inclusive of 95°. Additional fractions were obtained by fractionally evaporating the motherliquor. These fractions were further fractionated by dissolving the least soluble fraction in water, and heating in the water-bath to the temperature at which fraction I. was removed. The precipicate which had formed at this point was removed by filtration and retained as fractions I.-II. The filtrate was used to dissolve fraction II., and the solution heated as before to the temperature of removal of fraction II. The precipitate was taken as fraction II.-2, and the filtrate used to dissolve fraction III. By proceeding in this manner any degree of fractionation which was desired could be obtained. During and at the completion of the fractionation various fractions were examined with regard to colour of oxide, nitric acid solution, and spectrum. Gadolinium Material.—A material containing gadolinium with only sufficient traces of terbium to colour the oxide orange-brown was first submitted to fractionation with dimethyl phosphoric acid. The oxide of this crude gadolinium was dissolved by stirring with the acid until all had dissolved. The liquid was then treated as described above. The fractions were taken as follows:-Fractions I., I.—2, I.—3 at 65°; fractions II., II.—2, II.—3, at 90°; fractions III., III.-2, III.-3, after evaporating the mother-liquor from fractions II. to one-half volume; and fractions IV., IV.-2, IV.—3 upon complete evaporation. A fair idea can be formed with regard to the rapidity of the method by observing the change in colour of the oxides. The oxide from fraction II. was of a dark brown colour; that of fraction IV. light brown, and that of fraction IV.-3 pale cream. This showed a rapid concentration of the gadolinium in the most soluble fractions, while the impurity-terbium— collected in the least soluble. It is interesting to note that this fractionation required only about forty-eight hours to give a comparatively pure gadolinium. This shows a great increase in the rapidity of the separation over all previous methods. Fractionation of Yttrium Material.-The next oxides consisted of a mixture of those of yttrium, holmium, and dysprosium with traces of erbium, samarium, gadolinium, terbium, neodymium, and praseodymium. They were dissolved in hydrochloric acid, the solution diluted and boiled with an excess of sodium hydroxide. The precipitated hydroxides were filtered off, washed with hot water until free from chlorides, and dissolved by stirring with the dilute dimethyl phosphoric acid. The resulting solution was then submitted to the usual fractionation. Fraction I. was removed at 38°, fraction II. at 48°, fraction III. at 65°, fraction IV. at 96°, while fractions V., VI., VII., and VIII. were taken by fractionally evaporating the mother-liquor from fraction IV. Fraction I. The oxide was yellowish, and the absorption spectrum showed a rapid concentration of erbium. Holmium and dysprosium were also present. Fraction II. This portion gave a yellowish coloured oxide. The spectroscope showed holmium and dysprosium with very small quantities of erbium. Fraction VI. The oxide was coloured an orange-brown. An intense absorption proved the presence of very much dysprosium, less holmium, and the merest possible trace of neodymium. Fraction VII.-Reddish brown oxide. The absorption spectrum was very weak, showing the presence of a small amount of neodymium and traces of samarium and dysprosium. The green band of holmium could barely be detected. Fraction VIII.-This gave an orange-brown oxide. The spectrum showed intense absorption bands, indicating the presence of a large quantity of neodymium, a very little praseodymium, and only a trace of samarium. Fractionation of a mixture of earths, from monazite, giving more soluble double sulphates. The earths present consisted largely of gadolinium and dysprosium with small amounts of terbium, holmium, and neodymium. The oxides were warmed with dimethyl phosphoric acid until entirely converted into dimethyl phosphates, after which the thick mass was stirred with water until dissolved. The fractions were collected as follows:-Fractions I. to III. up to 95°; and fractions IV. and V. by evaporation of the mother-liquor from fraction III. Fraction I. gave a brownish yellow oxide, which, when dissolved in nitric acid, gave a yellowish green solution. With the aid of the spectroscope it was found that large amounts of dysprosium were present, accompanied by a little holmium. Its Fraction II.-The oxide was chocolate-brown. nitrate solution was faintly green and showed weak absorption bands of dysprosium and terbium. Fraction V.-Oxide red-brown. Absorption spectrum CHEMICAL NEWS,} Passivity of Metals. Jan. 9, 1914 indicated very small quantities of neodymium and dysprosium. It will be observed from the above fractionations that the rate of separation of the rare earths is vastly greater than practically all the methods given up to the present time. Lanthanum, cerium, praseodymium, neodymium are left at once in the mother-liquor. Samarium, europium, and gadolinium are much less soluble than those previously mentioned, while they are more soluble than terbium, dysprosium, and holmium. Erbium, thulium, yttrium, ytterbium, &c., collect in the least soluble portions. Since the solubilities of these compounds are the reverse of the usual type, they may be used for the rapid purifica tion of many of the rare earths. For instance, we can easily remove traces of neodymium from samarium by this means, as the samarium dimethyl phosphate separates before the neodymium compound. It is necessary to state that there is some inconvenience when working with the salts of dimethyl phosphoric acid, since they undergo gradual decomposition. A gelatinous precipitate is formed very slowly in the case of the rare earths, which filters with difficulty. Durham, N.H. REVIEW AND INTERPRETATION OF 15 than before. As a consequence some more zinc will be deposited on the electrode in accordance with the higher potential. The formation of the Pd-H alloy will proceed more slowly, and the polarisation will finally rise so high that zinc alone is deposited. Analogously a Cu electrode has a certain oxygen concentration in the electrode volume already before the current is turned on. The potentials of the systems Cu-Cu ions and OH' ions always remain equal to one another, with or without the current flowing. That may be supposed to become possible by the splitting of the electric current into two components: two primary processes are simultaneously taking place at the anode(1) Cu+2F(+) -> Cu.... (2) OH'+1F(+) → OH. Whilst the first process depresses the concentration of the Cu atoms in the electrode volume, the second process raises the O concentration there, and it is therefore the electric current which maintains the equilibrium in the electrode volume. Both the reactions increase the polarisation, and the purely chemical compensation processes oppose them. The process which is subject to chemical inertia is the oxidation of the massive copper by the aid of the oxygen in the electrode volume in the presence of the H' ions of the electrolyte (any reaction between the oxygen of the electrode volume and the Cu atoms present in it is excluded, because there is equili RECENT EXPERIMENTS WHICH EXTEND AND brium between the two systems, which, as we shall see, is a ELUCIDATE THE DOMAIN OF THE By Dr. D. REICHINSTEIN, Zurich. 5. First Theory. weak point of the first theory). When under special conditions the described oxidation takes place at a slow rate, the O concentration in the electrode volume and consequently the polarisation rise to a high degree, until finally another compensation process sets in, for example, O2 generation (passivity). The equations of this theory may be deduced as follows: the electrolyte, which are assumed to be very high (constant, practically not variable by small current densities). When the system tends to equilibrium, only P1 and P2 will change, and the conditions of equilibrium will be Let there be: Pr, the electrolytic solution tension of the THE first theory was proposed at a time (1910-11) when oxygen concentration in the alloy metal-oxygen, P2 the the experiments of Le Blanc concerning the chemical electrolytic solution tersion of its metal concentration, and polarisation of active metal electrodes were already, and p2 the corresponding concentrations of the ions in known, and when the palladium-zinc experiments were made in order to clear the matter up; the other experiments mentioned above were not yet known, however. The theory, therefore, aimed at devising a mechanism - which would admit of chemical polarisation without denying the possibility that a system, and a passive metalanode likewise, might primarily emit anodic ions. I should like to emphasise at this place already that this question is more satisfactorily answered by this first theory than by the second one. The first theory conceives a mechanism of the chemical polarisation after the type of the Pd-Zn electrode (Reichinstein, Zeit. Elektrochem., 1911, xvii., 699). The rate of formation of the Pd-H alloy in the cathodic polarisation of the Pd-H2SO4 electrode is given by the momentary concentration of the Pd and H atoms near the boundary of the solid electrode and of the liquid electrolyte. Let us now assume that the contact with the electrolyte at the boundary electrode/electrolyte is not formed by the solid metal, but by a solution (alloy) of all those substances whose ions are represented in the electrolyte. The volume which this hypothetical alloy occupies in the electrode is designated the electrode volume. When we add to the electrolyte of the Pd-H2-H2SO4 electrode some Zn salt, some zinc will be deposited in the electrode volume already at open circuit, because the thermodynamical equilibrium demands that the potentials H2H⚫ ion and Zn-Żn ion must finally be equal to one another. We now make the assumption that the zinc entering the electrode volume diminishes the concentration of the Pd atoms there. When we now treat the electrode cathodically, the rate of formation of the Pd-H alloy will be smaller, and the polarisation will be greater, * A Contribution to the General Discussion on "The Passivity of Metals" held before the Faraday Society, November 12, 1913. -10 In RT PI RT P2 Ρι P2 (1) (2) PIP2=pip2 = K (3) where K is a constant. When we bear in mind that according to our assumption there is always (current flowing or not) equilibrium between the systems oxygenoxygen ions and metal-metal ions, we may interpret equation (3) to mean that, when the solution tension of the metal decreases as the current flows because its concentration in the alloy is reduced, the solution tension of the oxygen must decrease at the same time. We therefore write¿P1.P2)=0. (4) By differentiating (4) we obtain— |