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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 22, No. 8
Fluoaluminates of the Alkali Metals' R. H. Carter INSECTICIDE DIVISION,BUREAUOF CHEMISTRY A N D SOJLS, W A S H I N G T O N , U. C.
N E of the fluoaluminates of an alkali metal, cryolite (Na&F6), has recently received considerable attention as an insecticide that has given good control experimentally of several kinds of insect pests against which arsenicals are not entirely satisfactory. There are several similar compounds whose physical and chemical properties have not been determined and whose toxicity to insects has not been investigated. For these reasons the preparation and the determination of the physical and chemical properties and possible insecticidal value of several others of this series were undertaken. Several of the double fluorides of aluminum with the alkali metals occur in nature as minerals, some of which are rare. Among these may be mentioned: cryolite (AlF3.3NaF), cryolithionite (2A1F3.3NaF.3LiF), chiloite (3AIF3.5NaF), ralstonite [3A1(F.0H)3.(Na2Mg)Fz.2H20 1, and pachnolite (AlF3.CaFz.NaF.H20).
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Previous Work
Cryolite, or sodium fluoaluminate, besides occurring in nature, may be made by several wet processes, and synthetic cryolite of a high degree of purity is sold commercially. Synthetic cryolite is a white amorphous powder, but natural cryolite occurs in monoclinic crystals. Investigation of the physical and chemical properties of this compound has been rather exhaustive and the results are summarized by Mellor (10)*
Besides these compounds of natural occurrence, there are several that have been prepared in the laboratory and their properties investigated more or less completely. The following are listed by Gmelin-Kraut (7): AlF3.2NaF ( 6 ) , A1F3.2KF (Z), A1F3.3KF (Z), A1F3.3LiF ( d ) , AIF3.3RbF ( I I ) , AlF3.3CsF (11), AlF3.3NHdF (9), AlF3.2NHdF.1 '/zHzO ( 1 ) . Berzelius ( 2 ) precipitated potassium pentafluoaluminate ( K A F , ) by stirring an aqueous solution of aluminum fluoride with insufficient potassium fluoride for complete saturation, but if a slight excess of potassium fluoride was used the hexafluoaluminate (KAF6) was formed. He stated that the pentafluoaluminate was also formed by digestion of aluminum hydroxide with aqueous potassium hydrogen fluoride and this gelatinous precipitate was converted by boiling into the hexafluoaluminate. Continued boiling hydrolyzed this compound. Schuch (12) formed gelatinous hexafluoaluminate by boiling aluminum hydroxide with potassium fluoride. Baud ( 1 ) used dilute aqueous solutions of the component salts and obtained a gelatinous precipitate, the composition of which corresponded with K~AlF6.3~/2&0. Deville (4) also reported the formation of this compound by fusing aluminum chloride with an excess of potassium fluoride; Duboin (6) by fusing alumina with potassium fluosilicate and an excess of potassium fluoride; Puschin and Baskoff (11) by fusing molecular proportions of the component salts. According to these investigators, the precipitated potassium fluoaluminates are gelatinous colloids which dry to white powders. They are reported as being little soluble in water and decomposed by sulfuric acid with the evolution of hydroa Received M a y 21, 1930. Presented as a part of the Insecticide Symposium before the Division of Agricultural and Food Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, Ga., April 7 to 11, 1930.
fluoric acid, after which treatment aluminum hydroxide may be precipitated from the solution by ammonia. The solubility of AlF3.3KF.3l/2HzO is given by Baud ( I ) as 0.385 gram per 100 cc. at 16" C. Duboin (5) examined the optical properties of KAlFe prepared by a fusion method, and stated that it was probably isomorphous with NaaAlFe. According to Berzelius (Z), lithium with hydrofluoric acid and aluminum oxide forms an insoluble double salt which is also found in amblygonit mixed with a similar basic salt of lithium and aluminum phosphate. Puschin and Baskoff (11) obtained fusion curves for binary mixtures of aluminum fluoride with rubidium and caesium fluorides. They found eutectics corresponding to the melting points of the two hexafluoaluminates. Ammonium fluoaluminate was prepared by Berzelius (5) by the digestion of aluminum hydroxide with ammonium fluoride. This compound was dried to a white powder which when heated in platinum first gave off ammonia, then ammonium acid fluoride, and there remained in the retort basic aluminum fluoride. Von Helmholt (8) added freshly precipitated aluminum hydroxide to a concentrated ammonium fluoride solution. The precipitate was filtered, washed with alcohol, and dried. His analysis indicated the composition ("4)AlFa. Under the microscope this compound was a finely divided crystalline powder, and single crystals were sharply defined octahedra of the regular system. The compound was white and rather easily soluble. The aqueous solution did not attack glass. Ammonium pentafluoaluminate was reported by Baud ( 1 ) who mixed aqueous aluminum fluoride with ammonium fluoride in excess. This was a gelatinous precipitate with a solubility of 1 part in 100 parts of water a t 16" C. Preparation
As previously stated, methods for the preparation of artificial cryolite, or sodium fluoaluminate, have been developed, and the use of this compound as a substitute for arsenical insecticides has been under investigation for several years. From an economic standpoint, other available compounds of this type are the potassium, ammonium, and lithium salts of the hypothetical acid H3AIF6. The physical and chemical properties of these compounds have not been definitely determined and no mention has been made in the literature of their possible insecticidal value. Accordingly, these compounds were made in the laboratory by the following methods. POTASSIUM HESAFLUOALUMINATE-The double fluoride of potassium and aluminum was made by treating freshly precipitated aluminum hydroxide with an aqueous solution of potassium acid fluoride in the ratio Al(OH)a 3KHF2 10 per cent in excess and heated to boiling, a t which temperature it was maintained for 15 minutes. A gelatinous precipitate was formed which was washed by decantation several times with a considerable volume of water, and then filtered and dried a t 110' C. Analyses of this compound indicate its composition to be K3AlF6. LITHIUMHEX AFLUOALUMINATE-AlUmiIlUm hydroxide and lithium carbonate were mixed together and then treated with aqueous hydrofluoric acid in the ratio 2A1(OH)3 3Li2COs 12HF. This reaction mixture was digested for several hours on the steam bath and finally evaporated to
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I.VDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1930
dryness. The product, a white powder, was washed several times with water, filtered, and dried. Analyses of this compound indicated that it was composed of LisAlFs, containing also some hydrated alumina. AMMOKIUXHESAFLUOALUMINATE-AlUmhUm hydroxide was treated with ammonium acid fluoride in the ratio ,kl(OH)3 3?;H4F.HF (in slight excess) and heated to boiling, a t which temperature it was maintained for 15 minutes. A fine white precipitate was formed, which was filtered off, washed, and dried a t 110” C. Analyses of this compound indicated its composition to be (”4)3AlFe containing some hydrated alumina, probably a result of decomposition of the double salt during drying.
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Analyses
Analyses of these compounds are shown in ‘Table I. Tahle I-Analytical Data COMPOUND THEORETICAL FOU\D Per cent Per cent K3AIFs K 45.41 4 6 . l!) (by diff.) AI 10.48 10.41 F 44.11 43.40 12.8 9 . 7 (by diff.) LijAlFa Li 16.7 25.4 AI 70.5 64.9 F 24, 53 27.6 NH4 16.33 13.8 AI 58.6 F 55.04
The analyses were made by standard methods given in “Methods of Analysis of the Association of Official Agricultural Chemists,” second edition, and “Standard Methods of Chemical Analysis,” second edition, by Scott. Fluorine was determined as CaFz after fusion of the samples with a mixture of sodium and potassium carbonates. Aluminum was determined as Al2O3after the sample had been heated to white fumes in platinum, with sulfuric acid to liberate hydrofluoric acid. This procedure was carried out twice on each sample. Sitrogen was determined by distillation of the ammonia from an alkaline soliltion. Very good duplicate checks were obtained. The physical and chemical properties of these compounds were also determined and are shown in Table 11. Table 11-Physical
and C h e m i c a l Properties
PEI OF
SATD SOLN. DETD. SOLU- COLORI- PHYSICAL COMPOUNDBILITY METRICALLY CONDITION G r a m s per 100 cc. 0 061 3 7 White powder NaaAlFs 0 158 6 Y White powder KsAIFs 0 107 White powder LiaAlFa 5 8 White powder (XH4)rAIFa 1 031
APPEARAUCE OF CRYSTALS FROM AQUEOUSSOLN Cubes singly refracting Rods doubly refracting Octahedra singly refracting
Large-Scale Production
Preliminary reports on the determination of the toxicity of these compounds, especially the potassium fluoaluminate, and demands for larger amounts for experimental tests necessitated the development of methods of preparation applicable to larger production. Considerable amoiints of the potassium hexafluoaluminate were made by the following procedure: One thousand grams of aluminum sulfate were dissolved in about 8 liters of water in a galvanized iron bucket, 1240 grams of potassium carbonate were added and then commercial hydrofluoric acid in the amount calculated from the equation A12(S04)a.18H20
+ 6K2COs + 12HF+2KsA1F6 + 3K~S04+ 24HzO
6C02
Considerable heat was generated from the reactions and more heat was applied to bring the reaction mixture to boiling at which temperature it was maintained for 20 to 30 minutes. A gelatinous precipitate was formed which was filtered off, washed, and dried to a fine white powder. Yields as high as 90 per cent of the theoretical amount of KaAIFe were ob-
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tained and analyses agreed closely with the theoretical percentages of the different constituents. Other water-soluble salts of aluminum-such as sodium or potassium alum, etc.-may be substituted in this reaction with other alkali metal compounds (as the hydroxides, oxides, bicarbonates, etc.). Also hydrofluosilicic acid may be substituted in place of hydrofluoric acid in the formation of these compounds according to the equation A12(S04)3,18H20
+ 6K2C03 2S102 +.2H2SiFe+2K~AlFs + 3KzS04 + 26H20+ + 6COi
I n this reaction equimolecular amounts of hydrated silica are precipitated simultaneously with the potassium hexafluoaluminate, thereby forming a lighter, fluffier compound which has advantages for dusting purposes in insecticidal use. As hydrofluosilicic acid is made by the solution of silicon tetrafluoride in water, the following reaction should also take place when a mixture of the aluminum and alkali metal compounds in water is treated with gaseous silicon tetrafluoride ‘according to the equation In this case silica and potassium hexafluoaluminate are obtained in the ratio of 3 to 2 , making a still lighter powder. Hydrated alumina may be also formed in these reactions by using an insufficient amount of the acid to convert all the aluminum hydroxide to aluminum fluoride. The preceding reactions have all been carried out with water-soluble compounds of aluminum. However, there are commercially available several water-insoluble compounds of aluminum as the oxide, etc. As aluminum is soluble in either concentrated alkali solutions or strong acid, these compounds may also be used for the formation of the double fluorides. Using the oxide and potassium hydroxide as examples, these reactions may be written
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A1203 6KOH 12HF-2KaAlFe 6KOH f 2H2SiFe+2K3A1F6 A1203 6KOH 3SiF4+2K3AlFe
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25102 + 5H20 ++3Si02 + 3H20
I n these reactions hydrated silica is precipitated simultaneously with the double fluorides when hydrofluosilicic acid or gaseous silicon tetrafluoride is used. These coinpounds as precipitated are gelatinous precipitates which can be filtered easily through cloth, dried on the steam bath or a t 110” C. without decomposition, and easily ground to a fine powder. When properly washed during the process of manufacture, the suspension or solution of these compounds in water gives a practically neutral reaction to phenolphthalein and litmus indicators, and as their solubility is low they may be considered much safer to use on plant foliage without danger of burning than the fluosilicates for example. Considering their fluorine content alone as a measure of toxicity they compare favorably with the fluosilicates and metallic fluorides which have definite insecticidal value supposedly due to this element. Further practical experiments are now in progress to determine the insecticidal value and the plant tolerance of these compounds. Literature Cited Baud, Comfit. rend., 135, 1337 (1902). Berzelius, Pogg. A n n . , 1, 41 (1824). Berzelius, Lehrbuch, Vol. 111, p. 462. Deville, Ann. c h i m . fihys., [31 46, 450 (1856). Duboin, Comfit. r e n d . , 114, 1361 (1892); 115, 56 (1892); Bull. SOC. min., 15, 191 (1892). Gmelin-Kraut, Handbuch der anorganischen Chemie, Vol. 11, p. 665. Gmelin-Kraut, I b i d . . Vol. V, p. 59. Helmholt. v., Z . anorg. Chem., 2, 115 (1893). Helmholt, v . , I b i d . , 3, 127 (1893). Mellor, “Inorganic and Theoretical Chemistry,” Vol. V, p. 303. Puschin and Baskoff, Z. anorg. Chem., 81, 347 (1913). Schuch, Ann., 126, 108 (1863).
