Precipitation of Beryllium from Homogeneous Solution by Decomposition of an Acetylacetone Species in Basic Solution Walter G . Boyle and Charles H. Otto, Jr. Lawrence Radiation Laboratory, University of California, Livermore, Calif.
ACETYLACETONE NORMALLY forms a chelate with beryllium (two moles of ligand to one mole of beryllium) which is only slightly soluble in water. However, while investigating different masking agents for beryllium, it was found that a relatively soluble species was formed when a ratio of between one and two moles of acetylacetone to one of beryllium was maintained. This species held beryllium in solution even in quite basic media, but upon heating the species was destroyed and beryllium hydroxide precipitated from homogeneous solution. The precipitation of beryllium hydroxide is usually done in hot solution with dilute ammonium hydroxide (1, 2). The precipitate so obtained is voluminous and difficult to filter, and the proper temperature for ignition to the oxide is determined somewhat empirically. As precipitation from homogeneous solution often results in a purer, more easily filterable precipitate (3), it was felt that the technique might improve the analytical characteristics of the beryllium hydroxide precipitate. EXPERIMENTAL Reagents and Solutions. Acetylacetone and ethylenediamine were “research grade” and were distilled before use. The water used for this experiment was distilled from a Barnstead still and contained about 10 ppb silicon. Beryllium standard solutions were made by dissolving high-purity beryllium metal (General Astrometals Corp., New York) in dilute sulfuric or hydrochloric acid so that the final solution was 1M in beryllium ion and contained 5 % sulfuric or hydrochloric acid. The standard solution was checked by evaporation and ignition of an aliquot to the oxide. Fast Sulfon Black F was obtained from K and K Laboratories, Plainview, N. Y. All other chemicals were ACS reagent grade. Procedure. About 10 ml of 1M beryllium ion in 5 % H2SOd of HC1, an additional 10 ml of 5 % H2S04or HC1, and 15 ml of 1M acetylacetone in water are added to a 400-ml Teflon beaker. The volume is brought up to about 250 ml and the pH adjusted with cooling to 9.7 with 20% ethylenediamine in water. The final clear solution should have a volume of about 300 ml and should be at room temperature or below. The basic solution is placed on a cold, magneticstirring hot plate, and the hot plate is turned to a setting which will eventually boil the solution. When the temperature of the solution reaches 60”-70” C, a slight turbidity is noticeable. The heating is continued and the solution is brought to a boil and held there for 1 hour, at which time precipitation is complete. Total time for the precipitation is about 1 hour and 45 minutes. The final pH of the solution after precipitation and at room temperature is about 9.2. After cooling to room temperature the condensed, almost granular precipitate is filtered through a Whatman No. 40 (1) W. F. Hillebrand et al., “Applied Inorganic Analysis,” Wiley, New York, 1953, p. 522. (2) 6. J, Rodden, “Analysis of Essential Nuclear Reactor Materials,’’Chap. 4, U. s. At. Energy Comm., 1964. (3) L. Gordon, M. L. Salutsky, and H. H. Willard, “Precipitation
paper. About 150 ml of 2 % NH4N03solution made basic with ethylenediamine is used to wash the precipitate. The paper and precipitate are dried at 150” C for at least 1 hour, placed in a furnace at 350” C until well charred, and then the paper is burned off at 575” C. The precipitate is ignited once at 1150” C for 2 hours. If the precipitate is to be weighed as beryllium hydroxide, it is filtered through a medium-porosity glass frit, washed first with the ethylenediamine-NHdN03 wash solution, then with water made basic with ammonia, and finally with absolute alcohol. It is dried at 150” C for 2 hours. Teflon beakers were used to avoid excessive silica contamination which would occur if these basic solutions were boiled in glass. The ethylenediamine was chosen so as to keep the pH as constant as possible during the boiling and cooling of the solution. DISCUSSIOhTAND RESULTS
Figure 1 is a picture of two forms of precipitated beryllium hydroxide after overnight settling. Each beaker contains 180 mg of beryllium and would yield 0.5 gram of Be0 on ignition. The beaker on the left contains beryllium hydroxide precipitated from hot solution with NH40H and the beaker on the right contains the precipitate obtained from homogeneous solution. The improvement in the analytical characteristics of the homogeneous precipitate is readily discernible. This precipitate filters and washes very easily. It was, however, still necessary to ignite the precipitate at 1150” C, and in this respect it was similar to the usual N H 4 0 H precipitate. The homogeneous precipitate exhibited the unusual property of a large expansion in its apparent volume when ignited. The beryllium hydroxide produced by NHIOH of course shrinks enormously when ignited. Table I shows the results of precipitations from homogeneous solution at various concentrations of beryllium. The relative standard deviation was about 1 0 . 1%. There seems to be very little bias in these results. The beryllium hydroxide precipitated from homogeneous solution condenses to a very small volume when filtered through a medium-porosity sintered glass frit. Qualitative thermograms taken on an Aminco Thermograv for purposes of comparing the two forms of the hydroxide revealed that there might be a level position corresponding to Be(OH)2 between 160” C and 200 O C for the homogeneous precipitate. This was not present in the NH40H precipitate. An x-ray powder pattern showed that the homogeneous precipitate
Table I. Determination of Beryllium as Oxide Be taken Rel. std.a as BeO, mg Be0 obtained, mg dev., % 124.8 124.9 125.0 124.6 0.17 250.4 250.1 250.1 250.4 250.3 0.06 376.0 375.8 376.5 376.0 376.2 0.08 501.4 502.7 503.0 502.3 502.4 0.06 a Average 0,09.
from Homogeneous Solution,” Wiley, New York, 1959. VOL. 39, NO. 13, NOVEMBER 1967
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Figure 1. Be(0H).
Two forms of
Pptd. by direct addition B. Pptd. from homogeneoussolution
A.
was mostly amorphous and not the beta-hydroxide; however, there was some evidence of crystallinity probably indicating the presence of the metastable alpha form of the hydroxide. These facts led us to try an analysis based on weighing the hydroxide. Table I1 shows the results of weighing the hydroxide after drying at 150" C for 2 hours. The time of drying was not critical; two different tests on a number of precipitates, each for 60 hours at 150" C, gave no change in weight over the weight obtained for the 2-hour drying time. Because of the positive bias and also because of the importance of anions in contributing to the character of the homogeneous precipitate, several of the precipitates listed in Table I1 were analyzed for total sulfur by an induction-
Table 11. Determination of Beryllium as Hydroxide Sulfuric Acid Solution Be taken as Be(OH)*, Be(OH12 Rel. Rel. mg obtained, mg std. dev., %a error, +o. 51 214.8 216.2 216.1 215.3 215.9 0.19 +O. 53 430.8 433.4 432.8 432.9 433.4 0.07 644.7 649.4 649.5 0.01 $0.73 a
Average 0.09.
Table 111. Determination of Beryllium as the Hydroxide Hydrochloric Acid Solution Be taken Rel. Be(OH)2 Rel. std. as Be(OH)*, dev.,a error, obtained, mg mg +0.56 215.7 215.9 0.06 214.6 0.23 +0.65 431.4 431.4 429.2 431.4 433.1 433.0 643.8 647.4 647.8 0.05 $0.60 a
Average 0.11. ~~~~
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0
~
ANALYTICAL CHEMISTRY
furnace technique ( 4 ) and were found to contain from 0.080.11 sulfur by weight. Converted to this is 0.240.33x; as S04-2-2H20,0.33-0.45x; and as SO4-2.4Hz0, 0.42-0.58 . The tetrahydrate decomposes to the dihydrate at 89" C and the dihydrate does not decompose to the anhydrous form until around 270" C. Thus, it seems likely that a large part of the positive bias which is found when beryllium is determined as the hydroxide in sulfate solution is due to the dihydrate of beryllium sulfate. These precipitates contained only about 100 ppm of chloride. While this work was in progress, Prasad and Sastri ( 5 ) reported on a study of the homogeneous precipitation of beryllium by hydrolysis of urea. They found that sulfate ion was necessary for the production of a dense, easily-filterable precipitate. In order to test whether sulfate was necessary in this procedure, several precipitations were carried out substituting hydrochloric acid for sulfuric acid and using beryllium standards prepared with hydrochloric acid. The beryllium chloride solution behaved very much like the beryllium sulfate solutions during the precipitation from homogeneous solution, and the precipitates produced were practically identical in appearance to those produced in sulfuric acid. Table I11 shows the results obtained when these precipitates were weighed as the hydroxide. Approximately 740 ppm C1 were found in these precipitates, which is not enough chloride to account for much of the large positive error even assuming the chloride is present as the tetrahydrate. These precipitates were found to contain negligible amounts of sulfates. Although these analyses for sulfate and chloride do not account for the total bias present when the hydroxide was weighed, they do show that the anion is present, even if only in small amounts, and that it might have an important effect on the analytical and thermal characteristics of the precipitate. EDTA is often used as a masking agent for impurities in procedures in which beryllium is precipitated as the hydroxide. EDTA complexes many of the common impurities present in beryllium, but forms only a weak complex with beryllium
x
x
(4) W. G. Boyle and L. J. Gregory, U.S . At. Energy Comm. Rept., UCRL-7204 (1963). ( 5 ) T. P. Prasad and M. N. Sastri, Talunta, 13, 1517 (1966).
itself, During the investigation, difficulties were encountered with some of these procedures. With this method, considerable beryllium was found in the filtrates when equimolar quantities of EDTA and beryllium were used. In any event, EDTA cannot be used indiscriminately. Table IV gives the amount of beryllium found in the filtrate for various mole ratios of EDTA to beryllium. Even the standard ammonium hydroxide procedure showed a significant amount of beryllium in solution when a mole ratio of 1 : 1 EDTA to beryllium was used. With the homogeneous precipitate and at a mole ratio of 1 :1, the equivalent of 8 mg of Be0 was found in the filtrate, As the ratio of EDTA to beryllium was gradually Iowered, the amount of beryllium found in the filtrate disappeared. The relatively high pH (9.7) of the homogeneous precipitation method might possibly explain the large amount of beryllium apparently complexed by EDTA; however, homogeneous precipitations which were done at a lower starting pH of 9.2 showed even more beryllium in solution with 1 : 1 EDTA to beryllium. When no EDTA was present, these precipitations were always quantitative. It has been brought to the attention of the authors that Menis et al. (6) have also reported finding beryllium in solution with certain procedures calling for the use of EDTA. These results were obtained by using a colorimetric method employing Fast Sulphon Black F (7). The filtrate was diluted to 500 ml, and beryllium was determined directly on an aliquot of this filtrate without concentration or separation of the beryllium. I n order to minimize salt effects and develop the color, the aliquot was diluted still further. The reproducibility of this method was therefore found to be h0.1 mg of beryllium as BeO, which was adequate to show the analytical significance of beryllium remaining in the filtrate, It is possible when using a beryllium hydroxide precipitation that in many cases the beryllium present in the filtrate due to the presence of EDTA is compensated for by other impurities in the precipitate such as silica. However, EDTA can be used in the homogeneous precipitation procedure if caution is exercised. Table V shows the results of some separations accomplished in a single precipitation. The foreign ions were present in a 1 : 1 mole ratio to beryllium. Copper required no EDTA; the ethylenediamine itself was a sufficient masking agent. Iron, calcium, and magnesium required only a slight excess of EDTA. Aluminum, however, did not give satisfactory separation even with a 25 excess of EDTA. All of the precipitates listed in Table V, except the one with added aluminum, were analyzed for impurities by emission spectroscopy. The amounts of added impurity found in the precipitates were: 50 ppm in the case of calcium, 20 ppm for magnesium, 80 ppm for iron, and 2 ppm for copper. These results show that the amount of impurity remaining with the oxide was negligible. Excepting for the precipitate with added aluminum, conditions were chosen (6) 0. Menis, R. K. Bell, and E. R. Deardorff, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 1967. (7) A. M. Cabera and T. S. West, ANAL.CHEM., 35, 311 (1963).
Table IV.
Beryllium Found in Filtrate Moles EDTA Be in filtrate to moles Be= as mg Be0 Procedure NHiOH hot 1:l 1.2 8.0 Homogeneous 1:l Homogeneous 1 :4 0.3 0.1 Homogeneous 1 :10 a 250 mg Be as Be0 total added. ~~
Table V. Beryllium Separations Be taken as BeO, mg Ion added, mg B e 0 found, mg 250.4 Cu 6300 251.1 250.4 Fe 550b 251.5 249.5 Mg 240b 250.7 250.7 249.5 Ca 400b 250.4 A1 270c 255.4 a No EDTA. b Slight excess of EDTA. c 2.5 mmoles of EDTA in excess.
(using the results of Table IV) so that no beryllium would reasonably be expected to be in the filtrate. Attempts to mask aluminum were not successful. Even with 21/2 times the amount of EDTA necessary to complex aluminum, results were significantly high. With this amount EDTA in excess over beryllium, appreciable beryllium would be in the filtrate. Thus, compensating errors have probably made this situation appear more favorable than it really is. The suggestion of a slight bias in the results presented in Table V might possibly be explained by the presence of trace silicates. Emission spectroscopy showed some silica to be present. It was probably added along with the controlled impurities. The general procedure has applications other than the precipitation of beryllium hydroxide. For instance, a beryllium complex containing @-hydroxy-a-naphthoic aldehyde can be precipitated from homogeneous solution using 2aminoethanol for a base in a modification of the procedure by Gusev et a(. (8). It has also been possible to produce condensed precipitates of magnesium and uranium by decomposing the acetylacetone complexes in basic solution. ACKNOWLEDGMENT
The authors thank Carl Schoenfelder of Sandia Corp., Livermore, for providing thermogravimetric data, and Vernon Silveira of this laboratory for providing x-ray powder patterns. RECEIVED for review July 10, 1967. Accepted August 219 1967. Division of Analytical Chemistry, 153rd Meeting, ACS, Miami Beach, Fla., April 1967. Work performed under the auspices of the U. S. Atomic Energy Commission. (8) S. I. Gusev, V. I. Kumov, and E. V. Sokolova, J . Anal. Chem. USSR,12, 51 (1957).
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