either iron or uranium in air dust or ore samples can be eliminated by addition of sodium dithionite and shaking again before drawing off the extract. Add sufficient ammonium hydroxide to neutralize the acid of the dithionite as described under Uranium. Alternatively, shake the estract with 1 ml. of 1to 1 sulfuric acid for 30 seconds, add 50 ml. of water, EDTA, and ammonium hydroxide, and continue as described.
LITERATURE CITED
(1) Anderegg, G., Nligeli, P., Miiller, F.,
Schwarzenbach, G., Helv. Chim. Acta 42,827 (1959). (2) Bogucki, R. F., Martell, A. E., J. Am. Chem. SOC.80,4170 (1958).
( 3 ) Durham, E. J., Ryskiewich, D. P., Ibid., 80,4812 (1958). (4) Sill, C. W., ANAL. CHEM.33, 1579 (1961). (5) Ibid., p. 1584. (6) Zbid., p. 1684.
(.~ 7 ) Sill. C. W.. Willis. C. P.. Ibid.. 31. ,
r
598 (‘1959). ‘ (8) Turner, G. K., G. K. Turner Associates, Palo Alto, Calif., 1959,. Drivate -
communication.
’
(9) White, C. E., Ho, M., Weimer, E. Q., Spectrochim. Acta 16, 236 (1960). (10) White, C. E., Hoffman, D. E., Magee, J. S., Jr., Ibid., 9, 105 (1957). (11) Vosburgh, W. C., Cooper, G. R., J . Am. C h a . SOC.63,437 (1941).
R E C E I ~for D review February 21, 1961. Accepted July 17, 1961.
Decomposition of Refractory Silicates in UItramicro Analysis CLAUDE W. SILL Health and Safety Division, U. S. Atomic Energy Commission, ldaho Falls, Idaho
b A procedure is described for the complete decomposition of mixtures of refractory silicates and oxides. A potassium fluoride fusion in a platinum dish is followed by addition of sulfuric acid and transposition to a pyrosulfate fusion in the same vessel. Fluorides and silica are volatilized simultaneously, and sample decomposition is so complete that all time-consuming evaporations and filtrations are eliminated. Conventional glassware can be used in all subsequent steps and contamination from impure reagents or from corrosion of the container walls is reduced to a minimum. The decomposition is so rapid and effective and the contamination from environmental sources so small that the procedure can be used for the determination of submicrogram quantities of nonvolatile elements in siliceous materials. The procedure should be particularly valuable in geochemical investigations.
F
with anhydrous potassium fluoride is very effective for the dissolution of many kinds of refractory materials and deserves much wider use and popularity than it currently has. Many silicates that are very refractory and generally intractable such as talc, asbestos, etc., will dissolve completely in less than 1 minute of contact with the molten flux. Yet the procedure is not even mentioned in many of the standard reference texts on applied analytical methods. Probably, the main reasons for its limited use are the difficulty of eliminating hydrofluoric acid and fluorides that are objectionable later in the analysis and the inability to use glass containers. until such elimination has been accomplished. Both objections are overcome in the present method by adding USION
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ANALYTICAL CHEMISTRY
concentrated sulfuric acid and transposing the solid potassium fluoride cake to a pyrosulfate fusion in the same platinum vessel in which the fluoride fusion was made. Both fluorides and silica are volatilized rapidly and completely in one operation so that glassware can be used in all subsequent steps, tedious and time-consuming evaporations or filtrations are eliminated and loss of nonvolatile components in insoluble materials is avoided even from large samples. Present methods for the decomposition of refractory silicates (1, 2, 6) are not entirely satisfactory for application to the determination of submicrogram quantities of metals. While hydrofluoric acid will dissolve silica and some silicates, many samples contain refractory silicates and/or oxides that are not dissolved by either hydrofluoric acid or by subsequent fuming with sulfuric acid. Pyrosulfate fusion is a very powerful and convenient method for the dissolution of refractory oxides but is very ineffective with refractory silicates. The insoluble material must be filtered off and dissolved by some other method. Alkaline fluxes also leave much to be desired. Sodium peroxide is a rather impure substance to use in trace metal analysis, particularly when large samples are used. Its attack on the container in which the fusion is made is so great that iron, nickel, silica, or other impurities depending on the container are added to the sample in prohibitively large quantities. Fusion with sodium carbonate is relatively mild and the dissolution of refractories is time consuming or incomplete unless very small samples are used. Even if the decomposition is complete, the problem of eliminating the dissolved silica still remains with either alkaline
flux. The solution must generally be evaporated with concentrated hydrochloric or perchloric acid to dehydrate the silica before filtering it off which is difficult and time consuming. After ignition, the silica must be volatilized with hydrofluoric acid and the residue dissolved and added to the main fraction‘to ensure quantitative recovery. Potassium fluoride is much more effective than either sodium or lithium fluoride in dissolving refractory silicates. Also, the melting points of the sodium and lithium salts are significantly higher and more difficult to reach. Ammonium fluoride or bifluoride dissociates readily, and the high temperatures necessary for the dissolution of most refractories cannot be attained. Potassium acid fluoride has been recommended as a general flux (3, 5) and for dissolving monazite sands prior to the determination of thorium (d), but its use is neither necessary nor desirable. The acid fluoride melts a t a relatively low temperature and gives little if any more solvent action than hydrofluoric acid alone. More important, the acid fluoride is converted to the normal fluoride during heating, and the melt will solidify due to its much higher melting point. Considerable spattering and loss of sample will accompany the change in state unless care is taken. The temperature must then be increased sufficiently to melt the normal fluoride before sample decomposition becomes really effective. Rodden (4) notes that fusion with potassium acid fluoride ‘[tends to foam, creep, and sputter.” I n contrast, if the sample is perfectly dry, the maximum temperature obtainable from the blast burner can be applied immediately without loss of sample when anhydrous potassium fluoride is used. Oxides are dissolved less rapidly and
completely in general than silicates during the potassium fluoride fusion. This is particularly true with iron, zirconium, and, to a lesser extent, aluminum vhose oxides or potassium fluosalts are relatively insoluble in the alkaline flux. If the quantity of insoluble material is small and the fluoride fusion is prolonged for a few minutes, the entire sample will dissolve completely in the subsequent pyrosulfate fusion. If relatively large quantities of such insoluble oxides are present as with zircon, dunite, etc., some of the original siliceous material will be protected from attack while the fluoride is still present and will not dissolve subsequently. Addition of a small quantity of sodium pyrosulfate will dissolve many of the oxides, particularly iron, and expose the remainder of the siliceous material to the action of the fluoride so that sample decomposition is complete. In fact, the mixed sodium pyrosulfate-potassium fluoride flux is more effective for many materials than the potassium fluoride alone. Levine and Grimaldi (3) used a mixed flux of 2 parts of sodium fluoride and 3 parts of potassium pyrosulfate and found that it “will decompose essentially all of the refractory minerals associated with thorium.” However, mixed fluxes containing pyrosulfate are not as clean as straight potassium fluoride and require much more care because of their low melting point and a pronounced tendency to creep over the sides of the dish. Also, the two compounds react with each other, presumably in the presence of moisture either from the air or from the combustion products of the blast burner. The original effect of the mixed flus lasts for only a few minutes and pyrosulfate is most effective when added after most of the sample has been dissolved in a potassium fluoride fusion. If potassium fluoride is in excess, the flux again becomes alkaline as it is heated because of loss of hydrogen fluoride, and the insoluble oxides will reprecipitate if the fusion is prolonged. However, all siliceous material will have been dissolved, and the osides will dissolve subsequently in the pyrosulfate fusion. If pyrosulfate is in excess, decomposition of siliceous material may be incomplete. I n samples containing large quantities of aluminum such as clays, decomposition of the potassium fluoride cake with sulfuric acid causes precipitation of aluminum sulfate in a form that retains sulfuric acid tenaciously. The resultant slurry is virtually impossible e0 evaporate to a pyrosulfate fusion without serious spattering and loss of sample. Addition of sodium sulfate causes the evaporation to proceed smoothly without personal attention. Sodium sulfate is also very
beneficial in the presence of many other elements such as calcium, iron, and rare earths although they are not as serious as aluminum and do not require as much sodium sulfate. Unless the additional salt is objectionable because of solubility or other considerations or unless small samples are used, sodium sulfate should be added routinely. At least 0.5 gram of calcium oxide will dissolve in both the potassium fluoride fusion and in the mixed sodiumpotassium pyrosulfate fusion although it will not dissolve in either of the alkali pyrosulfates separately. The insoluble double sulfates formed with yttrium, lanthanum, and rare earths are also much more readily soluble in the mixed pyrosulfates than in either one alone. Combination of two of the most powerful methods of attack available thus results in a method of sample decomposition of wide applicability. Some of the materials that have been decomposed completely and easily include zircon, monazite sands, dunite, beryl, corrundum, talc, asbestos, kaolin, fire-clay, soils, ores, and many highfired oxides and phosphates of beryllium, thorium, zirconium, and alum inum , PROCEDURE
Place 0.5 gram of the silicate into a 50-ml. platinum dish and add 1 ml. of water dropwise around the edge of the sample so that the sample is confined and moistened completely with as little dusting as possible. Add 1 or 2 drops of 48% hydrofluoric acid from a polyethylene dropper a t the edge of the slurry to determine the reactibility of the material. As fast as the vigor of the reaction &-ill permit, add enough more hydrofluoric acid to make a total of about 1 ml. Rlix the solutions by tilting the dish gently, but do not swirl the solution around the sides. Evaporate the solution to dryness slowly to prevent spattering and loss of sample. Use either a steam bath or a hot plate covered with a piece of asbestos cloth folded back on itself two or three times to lessen heat transfer. R h e n the sample is completely dry, sprinkle 3 grams of anhydrous potassium fluoride uniformly over the dry residue. Heat the dish over a blast burner gently until the last trace of moisture is expelled from the cake, then increase the heat until the flux melts. If the sample has not dissolved after two or three minutes in the molten flux, cool for a few seconds to let the melt solidify, add 0.5 to 1 gram of sodium pyrosulfate and continue heating very carefully until the flus has remelted. If the sample dissolves completely, roll the melt gently around the sides of the dish while continuing the application of heat to dissolve any particles of sample that may have escaped the fusion and set the dish on a clean stainless steel plate to cool. If the sample does not dissolve completely, a second addition of pyrosulfate
may be helpful. If the insoluble material remaining appears to be different than the original sample, it will likely dissolve completely in the subsequent pyrosulfate fusion. Add 3 ml. of concentrated sulfuric acid around the sides of the dish to moisten all potassium fluoride. Heat the dish very gently on the asbestoscovered hot plate or steam bath until the first appearance of gas bubbles in the solution and set the dish immediately on a cold piece of stainless steel to moderate the reaction. Continue heating intermittently as required to maintain the transposition of the potassium fluoride a t controllable rate. This is the most critical step in the entire procedure and if the dish is not cooled soon enough, evolution of hydrogen fluoride may become so vigorous that some loss of sample will result, particularly if very little sample or pyrosulfate have been used. When the cake has been completely disintegrated and the frothing has subsided, place the dish on the hottest part of the uncovered hot plate until all water and most of the excess sulfuric acid has been expelled. Use a thin piece of stainless steel between dish and hot plate to keep the dish clean and undamaged. If the sample contains considerable aluminum, iron, rare earths, etc., add up t>o4 grams of anhydrous sodium sulfate and 2 ml. of additional sulfuric acid as required to give a smooth clean transposition. After removing most of the excess sulfuric acid a t the temperature of the hot plate to minimize spattering and personal attention, place the dish on a ring stand and heat over a blast burner to a pyrosulfate fusion. Begin heating the slurry a t the lowest temperature and with the smallest velocity of gases possible with the burner. Gradually, increase the temperature as fast as frothing or spattering will allow until a quiet fusion is obtained. Heat just hot enough to melt the flux and maintain a t this minimum temperature until the solid has dissolved. Roll the melt gently around the sides of the dish with continued application of heat to ensure complete transposition of all fluorides and set off to cool immediately. Do not overheat or some of the metal sulfates in the thin layer of melt on the sides of the dish above the liquid level will be decomposed to refractory oxides and will not redissolve. The melt should not be swirled appreciably before the last step or part of the sample might be carried too high in the dish and will be difficult to get down in the fused flus. If a large mantle of high-velocity gases from the burner are allowed to envelop the dish, the liquid flux will also be blown too high in the dish and will form a crust on the outside of the rim from which it cannot be recovered. In either case, turbid solutions and loss of sample will result. Cool the pyrosulfate melt and clean the outside of the platinum dish if necessary. Place the dish in a n upright position in a 400-ml. beaker. Add 10 ml. of concentrated hydrochloric acid and 10 ml. of water to the platinum dish and cover the beaker with a watch glass. VOL 33, NO. 12, NOVEMBER 1961
1685
Let the solution stand for a few minutes and then swirl the beaker gently until the cake has become detached completely from the sides of the dish. Heating may be required with certain types of samples or if the platinum dish is not perfectly smooth. Invert the dish and allow the contents to fall into the beaker. Rinse the dish with about 20 ml. of water and remove. Boil the solution in the covered beaker until the cake has dissolved completely and the volume has been reduced to about 15 ml. The evaporation will require 20 to 30 minutes and is necessary to hydrolyze any condensed phosphates that will have been formed during fusion from orthophosphates present in the sample and which will interfere subsequently in many cases (6). Sulfuric acid is ineffective in the hydrolysis, and perchloric acid cannot be used because of formation of insoluble potassium perchlorate. The solution is then diluted to any desired volume and all or Dart taken for analysis. The fluoride treatment given above is of general auidicabilitv for the dissolution of silic&us matehais, but can be shortened somewhat with specific types of samples by eliminating the pretreatment with hydrofluoric acid. Talc asbestos, silica, and many other types of silicates can be dissolved quickly and complctely by direct fusion with anhydrous potassium fluoride. The aluminum silicates such as kaolin, beryl, etc., are dissolved more cleanly by including the acid treatment. Material containing large proportions of free silica is also preferably evaporated to dryness with hydrofluoric acid to volatilize the silica and then fused with potassium fluoride to dissolre the more refractory components. If the silica is finely divided, water must be added before the hydrofluoric acid to moderate the reaction or loss of sample may result. The procedure described will handle samples as large as 1 to 2 grams depending on the sample composition. However, when the ratio of sample to flux becomes too large, particularly with ores, the sample might dissolve in the potassium fluoride melt but the cake will not transpose completely with sulfuric acid to give either a clear pyrosulfate fusion or aqueous solution. The difficulty becomes more pronounced the longer the fusion is heated and is thought to be due to insufficient fluoride to volatilize silica completely. The difficulty disappears when sufficient flus is used. Ten grams of soil or 5 grams of ore in a 100-ml. platinum dish will require about 20 grams of potassium fluoride for the first fusion and 20 nil. of concentrated sulfuric acid and 20 grams of sodium sulfate for transposition of the cake to a pyrosulfate fusion. As much as 50 grams of soil has been decomposed
1686
ANALYTICAL CHEMISTRY
successfully using a 250-ml. platinum dish and a proportionate increase in all reagents except sodium sulfate where a twofold increase is adequate. The fluoride fusion may need to be maintained a t the maximum temperature obtainable from the blast burner for 15 minutes before the last of the sample will dissolve. Addition of pyrosulfate is also effective with the large samples. DISCUSSION
Although the complete procedure may appear to be mechanically difficult, it is actually very smooth and easily accomplished without significant loss of sample if it is carried out slowly, particularly during the transposition to the pyrosulfate fusion. The time required through the pyrosulfate fusion can be as little as 30 minutes with 0.5-gram samples without serious loss although somewhat slower operation is recommended. The decomposition is so complete that the pyrosulfate cake can generally be dissolved in water to give a perfectly clear solution that does not require filtration. If elements are present that form insoluble sulfates or hydrolytic precipitates in the acidic solution of the pyrosulfate cake, such as alkaline earths, rare earths, niobium, and tantalum, filtration nil1 be necessary and additional treatment of the precipitate may be required. Since the primary objective of the present procedure is only to obtain complete decomposition of the sample, the remaining treatment of the resulting solution necessarily depends on the analysis desired. Although it is highly unlikely that more than traces of silica or fluoride could have survived the pyrosulfate fusion, some of the potassium fluoride melt might have been carried too high in the platinum dish to have been included in the pyrosulfate fusion and might interfere in some particularly sensitive determination. To determine if sufficient fluorides were present to interfere with the extraction of beryllium by acetylacetone ( 6 ) , 1 ml. of a solution containing 2 X lo5 counts per minute of beryllium-7 tracer was placed in an empty platinum dish, the sample was added and the complete procedure was carried out as described including the evaporation of hydrofluoric acid. Zircon, monazite, beryl, talc, asbestos. and kaolin have all been used as the sample in similar tracer experiments. More than 99.5% of the total activity was present in the aqueous solution of the pyrosulfate cake showing that mechanical losses
through the complete decomposition were less than 0 5 % in every case. The beryllium was then estracted with acetylacetone and chloroform and 99.4% of the remaining beryllium was recovered in a single extraction. The present procedure has been used successfully in the determination of strontium-90 in soil, of radiurn-226 and thorium-230 in ores and sludges, and of beryllium and thorium-232 in many kinds of refractories. The complete procedures and data will be published in subsequent publications. ACKNOWLEDGMENT
The author acknowledges the assistance of his associates in applying the procedure to practical use. Special thanks are extended to K. UT.Puphal for the suggestion to use pyrosulfate in the potassium fluoride fusion. LITERATURE CITED
(1) Furman, N. H., “Scott’s Standard
Methods of Chemical Analysis,” 5th ed., Van Sostrand, New York, 1938. ( 2 ) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., Kiley, S e w York, 1953. (3) Levine, H., Grimaldi, F. S., U. Y. Atomic Energy Comm. Doc. AECD3186, 1950. (4)Rodden, C. J., U. S. Atomic Energy Comm. Doc. MDDC-1220, 1947. (5) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, New York, 1959. (6) Sill, C. W., Willie, C. P., A x . 4 ~ . CHEM. 31,598 (1959). RECEIVEDfor review January 3, 1961. Accepted July 12, 1961.
Correction Analysis of Tungsten Tantalum -Rhenium AI Ioys
-
In this article by J. F. Reed [ASAL. CHEW 33, 1337 (1961)], on page 1338, column 3, Table 11, the caption should read Determination of Rhenium in Tungsten-Rhenium.
