Separation and Determination of Aluminum
and Beryllium Using Tannin M. L. NICHOLS
AND
JOHN M. SCHEMPF, Cornel1 University, Ithaca, N. Y.
S
Chemicals and Apparatus INCE the chemical properties of aluminum and beryllium and their compounds are very similar, and because Baker and Adamson’s reagent grade aluminum sulfate octaaluminum occurs in many of the ores of beryllium, most decahydrate was recrystallized three times from distilled water containing a small amount of sulfuric acid. Pure beryllium methods for the analysis of beryllium take into account t,he carbonate, prepared in this laboratory, was acidified with sulfuric preliminary separation of aluminum. These’ include the biacid and the carbon dioxide expelled. The solid impurities were carbonate method (16), the method employing hydrochloric filtered off, and the beryllium sulfate tetrahydrate was recovered acid gas and a cold solution of the chlorides of aluminum and by evaporation and recrystallized twice from distilled water acidified with sulfuric acid. Solutions of these were prepared and beryllium in equal parts of hydrochloric acid and ether (e), found to contain no impurities when tested by spectroscopic the use of tannin (I.%?), and the most recent method proposed methods. The solutions were standardized by precipitating th; by Kolthoff and Sandell (9) and modified by Knowles (7) in hydroxides with ammonia (IO)and igniting in platinum at 1200 which 8-hydroxyquinoline is used to precipitate and remove to 1300” C. in a platinum-wound muffle furnace. The results of the aluminum. four determinations on each solution gave values of 2.464 * 0.005 mg. of aluminum oxide per ml. and 3.790 =t0.003 mg. of The use of tannin in analytical chemistry arises from the beryllium oxide per ml. The tannin and ammonium acetate left fact that a solution of common tannin or gallotannic acid no appreciable ash upon ignition. (ClaHl009.2HzO)is essentially a colloidal suspension of negaAll measurements of the hydro en ion concentration were tively charged particles capable of flocculating the positively made with a glass electrode (14), feeds 81. Northrup Type K potentiometer, and Type R No. E galvanometer, shielded with a charged particles of certain inorganic compounds such as the grounded copper case. An ultramicroscope was used t o deterhydrous oxide sols. I n 1925 Powell and Schoeller (16)recmine the presence or absence of colloidal material in the solutions. ommended the use of tannin for the separation of tantalum from columbium. Their eo-workers (17) extended the use of Experimental this reagent to the determination of such elements as titaI n Moser’s procedure (19) employing tannin to precipitate nium, zirconium, and hafnium. In 1927 Moser and Niessner aluminurn, the use of large amounts of ammonium acetate is proposed a method (19) for the separation and determination recommended. To determine whether the ammonium aceof aluminum and beryllium using tannin, in which the alumitate merely acted as a buffer or whether in some way it prenum is precipitated from a “weakly acid (acetic) or neutral vented the precipitation of the beryllium tannin complex, as solution” as the insoluble aluminum hydroxide tannin comstated by Moser, the effect of the acetate concentration on the plex while the beryllium forms a “soluble complex salt” in the precipitation of aluminum and beryllium hydroxides and the presence of tannin and ammonium acetate and remains in effect of the addition of tannin to such solutions a t different solution (13). I n a later article on the separation of beryl-, pH values were investigated. lium from other elements such as vanadium, tungsten, chroSolutions approximately 0.0014 N as regards the oxides mium, iron, etc., the acidity conditions are more definitely were adjusted to a pH of 1 with sulfuric acid and a saturated defined. solution of ammonium acetate was added until the ammonium The difficulty with the procedures recommended by Moser, acetate concentration reached 1 N-the concentration used as evidenced by the opinion of other analysts who have atby Moser. Then (1to 1) ammonium hydroxide was added to tempted to apply the methods, lies in the fact that the acidity increase the p H and the solutions were examined a t room conditions are not specified accurately nor definitely and that temperature for the appearance of precipitates. They were there is not a large enough difference in acidity between the then heated for several hours a t 85” C. and re-examined for precipitation point of the beryllium tannin complex and the precipitate formation. To these solutions tannin was added tannin complexes of some of the other metals to make cleanand, upon boiling for a few minutes, the presence of n tannin cut separations possible. Schoeller and Webb state ( I S ) complex was determined. that “even simple acetate solutions present difficulties in this The results (Tables I and 11) show that the addition of respect (adjustment of acidity) as in Moser and Kiessner’s ammonium acetate raises the precipitation pH of the solution nroposed method for the semration of aluminum from beryl- lium.” Mitchell and Ward found (11) that they ‘(did not obtain satisfactory separations by the method of Moser and Niessner, possibly owing TABLE I. PRECIPITATION OF ALuiurxwnx to the insufficiently definite specifications of the Total AI(?H)s acidity conditions in their description.” Dixon Precipitation Ai-Tannin SatuTotal after Heating Precipitation rated 1 to 1 criticizes (6) Moser and Singer’s method for the Visible 41(OH)3 2 Hours a t on Heating NHaOH % NHaOAc Sample Precipitation at 26’ C. 8 j 0 C. t o Boiling Added Added No. 25O C. separation of iron from beryllium, stating that M1. M1. it is likely to lead to coprecipitation of beryllium None None None None 0 1 .oo 1 with iron a t the reduced acidity required for None None None None 5 1.41 2 Heavy None None None 10 3.39 3 complete precipitation of the iron. Heavy None None 15 5+ 4.28 4 In the opinion of the authors there is no Heavy None None 20 3+ 4.57 5 Heavy None None 25 2 f 4.74 6 apparent reason why beryllium should form Heavy None None 35 I+ 4.92 7 Heavy Faint cloudipess None 1+ 37 4.96 a soluble salt, but satisfactory separations 8 Heavy Cloudiness None 39 I f 5.01 9 of beryllium from aluminum or other eleHeavy Very faint precipitate None I+ 45 5.08 10 Heavy Very faint precipitate None 50 1+ 6.15 11 ments using tannin should depend upon the acHeavy Faint precipitate 50 5 5.33 1+ 12 Heavy Large precipitate 20 50 curate specification and adjustment of p H con7.73 5+ 13 ditions. 278
MAY 15, 1939
ANALYTICAL EDITION
I
clear solution remains when i t is treated with a
TABLE11. PRECIPITATION OF BERYLLIUM
%
Sample No. 25'C.
Total SatuTotal Visible rated Be(0H)s 1 to 1 NHlOAa NHdOH Preclpitatlon Added Added a t 25" C. Ml. MZ. None 0 None 10 None None 20 None None None 30 None None 40 None None 50 None 50 5.0 None None 50 10.0 50 15.0 None
8 9
0.94 2.23 4.35 4.66 4.86 4.99 5.12 5.28 5.50
10 11 12 13
5.86 6.10 6.31 6.45
50 50 50 50
20 0 21.0 21.2 21 4
14
6.60
50
21.6
15 16 17 18 19
6.67 6.72 6.90 7.22 8.16
50 50 50 50 50
21.8 22.0 23.0 25.0 30.0
1 2 3 4 5
6
7
None None None Very light precipitate Lieht ureoipititte Precipitate Precipitate Precipitate Precipitate Precipitate
Visible Be(0Hh Precipitation on Heating 6 Hours a t 85O C.
None None None None None None None None None None None None Very llaht precipitate Lieht ureoipitite Precipitate Precipitate Precipitate Precipitate Precipitate
279
3 per cent tannin solution in saturated ammonium
Be-Tannin Precipitation on Heating t o Boiling None None None Very faint opalescence Very faint opalescence Faint oualescence Opalesience Faint precipitate Precipitate, settled rapidly Heavy precipitate Heavy precipitate Heavy precipitate Heavy curdy precipitate
acetate. Attempts to reproduce this latter experiment always yielded the usual voluminous light yellow beryllium tannin complex. Although the previous experiments indicated that the metal tannin complexes were formed through the mutual coagulation of the metal hydroxides by tannin, still there was a large difference in the p H a t which the hydroxides precipitated a t room temperature and when boiled with ammonium acetate and tannin-namely, with aluminum a t pH 4.96 and 3.39 and with beryllium a t pH 6.45 and 4.66.
To determine more accurately the pH at which incipient precipitation of the metal tannin complexes occurred, a mixture of 20 ml. of the solution of the precipitate pure metal sulfate, 25 ml. of saturated ammonium precipitate sulfate, and 50 ml. of 3 per cent tannin solution was precipitate precipitate diluted to 500 ml. and adjusted to a pH of 1 with precipitate sulfuric acid. The solution was heated to boiling and (1 to 1) ammonium hydroxide added dropwise until precipitation commenced. The solution was then cooled to room temperature and the pH determined.
Heavy curdy precipitate Heavy Heavy Heavy Heavy Heavy
curdy curdy curdy curdy curdy
above that a t which the hydroxides normally start to precipitate in the cold. Britton found that aluminum hydroxide normally precipitates in the cold a t a pH of 4.1 ( 2 ) but in the presence of acetate no precipitation occurred until a pH of 4.69 (4) was reached and with beryllium the precipitation in the presence of acetate occurred a t a pH of 5.90 (4). He says (3) that "even though pH values were established which were higher than those a t which aluminum hydroxide should precipitate, the solution remained clear. It is likely that some portion of the hydroxide was in the state of either a colloidal or pseudocolloidal solution which was stabiliaed by some kind of partial combination with acetic acid." I n this case the precipitation of the aluminum and beryllium hydroxides did not occur in the cold until pH values of 4.96 and 6.45 were reached, which is probably due to the fact that the authors' solutions contained a much larger acetate ion-metal ion ratio than the solutions used by Britton. Upon heating the solutions, aluminum hydroxide is precipitated, although in varying amounts, as soon as a pH greater than 4.1 is reached, while with beryllium hydroxide the value does not change. With aluminum the higher the acetate concentration the less the amount of precipitate although the pH is raised. This was shown with sample 7, Table I, where upon dilution with an equal volume of water and heating a voluminous precipitate of aluminum hydroxide formed although the change in pH in this well buffered solution was negligible. The aluminum solutions also showed "reversible hydrolysis" ( 1 ) as the precipitates formed in hot solution redissolved to some extent after the solutions were cooled. The results with beryllium hydroxide seem to indicate that beryllium forms a more stable complex with acetate than aluminum and is not so readily or completely hydrolyzed. The addition of tannin to these solutions followed by boiling gave heavy precipitates with aluminum in all cases where the pH was greater than 3.39 and a slight opalescence with beryllium a t a p H of 4.66 which increased as the pH was raised. I n the case of aluminum this would be expected, because of the reciprocal flocculation of tannin and a metal hydroxide. Even in those cases where no visible precipitate of aluminurn hydroxide was shown, ultramicroscopic investigation and dialysis experiments indicated that colloidal aluminum hydroxide was present. In the case of beryllium the formation of the tannin complex is not compatible with Moser's contention ( l a ) that beryllium hydroxide forms a soluble complex with ammonium acetate and tannin and that freshly precipitated beryllium hydroxide is immediately dissolved and a
The pH values found for the precipitation of the tannin complexes were 3.04 for aluminum and 4.90 for beryllium. It is well known (8) that although pK, changes with temperature, in acid solutions such as these the hydrogen-ion concentration remains practically constant. On this assumption a comparison of the pOH values a t which the precipitation occurs, as given in Table 111, shows that there is no great difference in these values a t different temperatures. TABLE111. pH PH POH
-Al(OH)a25OC. 4.96 9.04
AND
100°C. 3.04 9.16
pOH
Diff. 1.92 0.12
OF
PRECIPITATION -Be 25" C. 6.45 7.55
(OH) - 2 100°C. 4.90 7.30
Diff. 1.55 0.25
The effect of pH, tannin concentration, and digestion time after the addition of the tannin on the completeness of the recovery of aluminum from solution were investigated by the following procedure: A known amount of aluminum sulfate was added to 500 ml. of water, saturated ammonium acetate solution was added, and the pH was adjusted with 6 N sulfuric acid. The solution was heated to boiling, 3 per cent tannin solution was added, the heating was continued over a free flame for the short periods or on a steam bath for the longer time periods, and the solution was allowed to cool. The precipitate was filtered off in a Munroe crucible, mashed with ammonium acetate tannin solution of the same pH as used in the precipitation, dried to constant weight at 110"C., weighed, ignited to aluminum oxide at 1200" t o 1300" C., and weighed. The results are given in Table IV. These experiments show that: (1) complete recovery of aluminum from solution is effected when the pH is 4.6 =t0.1 and the digestion time after the addition of the tannin is a t least an hour. This length of time is necessary to complete the hydrolysis of the aluminum acetate and ensure complete precipitation of the aluminum tannin complex. (2) pH changes within the limits studied (samples, 3, 4, and 5) had no pronounced effect on the recovery of aluminum, although complete recovery was not secured in any of these cases since the digestion time was only 2 minutes. However, the pH should not rise above the point a t which the beryllium tannin complex starts to precipitate-i. e., 4.9 a t 100' C. (3) The amount of tannin used should be a t least 12 to 15 times the weight of aluminum oxide determined, but greatly increasing
INDUSTRIAL AND ENGINEERING CHEMISTRY
280
VOL. 11, NO: 5
TABLEIV. EFFECT OF pH, TANNIN CONCENTRATION, AND DIGESTION TIME Sample No. 1 2 3 4 5 6 7 8 9 10
AlzOs Taken Gram
Saturated NHaOAc Added MI.
pH
0,0246 0,0493 0.0740 0,0740 0,0740 0.0740 0.0740 0.0740 0.0740 0.0740
25 25 15 50 25 25 25 25 25 25
4.61 4.61 4.42 5.15 4.64 4.64 4.60 4.61 4.61 4.62
3%
Tannin Solution M1.
Digestion Time
Min.
A1-T Complex Gram
30 50 30 30 30 50 50 50 50 50
2 2 2 2 2 5 30 60 3.5 hours 5.5 hours
0.3862 0.6023 0.7545 0.7952 0.7087 0.9770 1.0874 1.0440 1.1735 1.2579
the amount of excess tannin has no effect. (4) The dried precipitates do not suffer a constant percentage loss on ignition. Moser and Niessner ( l a ) secured similar results which they interpreted as proof that a definite compound was not formed. Since complete recovery of aluminum from solution was effected as outlined above, solutions of known aluminumberyllium content were analyzed according to the following recommended procedure : The solution containing the aluminum and beryllium is introduced into a large (800-ml.) beaker. If more than 0.08 gram of aluminum oxide or beryllium oxide is present, the tannin precipitates become too bulky and large to be handled conveniently. In this event, an aliquot of the sample is takenfor analysis. Twentyfive milliliters of saturated ammonium acetate solution are added, the solution is diluted to 500 ml., and the pH is adjusted to about 4.6 with 6 N sulfuric acid and (1 to 1) ammonium hydroxide. After heating t o boiling, 50 ml. of 3 per cent tannin solution, or at least 12 to 15 times the combined weight of beryllium oxide and aluminum oxide to be determined, are added slowly and the whole is digested on a steam bath for 1hour. The solution is allowed to cool to room temperature and the aluminum-tannin complex is filtered off on a coarse-textured quantitative filter paper. After thorough washing with a wash solution containing 5 per cent ammonium acetate and a little tannin and adjusted to pH 4.6, the precipitate is placed in a covered platinum crucible, carefully dried and ignited, and finally heated a t 1200” to 1300’ C. to constant weight. At these elevated temperatures, the loss in weight of the platinum crucibles must be taken into account. The pH of the wash solution used on the aluminum tannin complex must be carefully adjusted, because if the pH is too high, beryllium-tannin complex might be precipitated on the surface of the aluminum tannin precipitate and thus make the results for aluminum too high and for beryllium too low. If the pH of the wash solution is below 4.1 it is possible that some of the aluminum tannin DreciDitate might dissolve and pass into the filtrate to be determined as berylliG. The beryllium is determined in the filtrate by Moser’s alternate procedure ( I S ) . Tannin equivalent to 10 to 12 times the weight of beryllium oxide to be determined is added in the form of a 3 per cent solution to the weakly acid filtrate and washings from the aluminum separation. After heating this solution to boiling, (1 to 1) ammonium hydroxide is added dropwise. A pale yellow precipitate of beryllium tannin complex forms as soon as the pOH reaches 7.3. In order to ensure complete precipitation which occurs a t the isoelectric point of beryllium hydroxide, pH approximately 7.5 at room temperature (Z), the addition of ammonium hydroxide is continued until the solution is just basic to litmus. The flame is then removed and the precipitate allowed t o settle. It is not necessary to digest after the beryllium precipitation has taken place, since beryllium hydroxide is completely and practically instantaneously precipitated and removed by the tannin at the isoelectric point. Care must be taken to have an excess of tannin present and to avoid a large excess of ammonia. In a hot, strongly basic solution tannin itself forms a gummy mass which adheres to the side of the beaker and coats the beryllium tannin precipitate. This makes it impossible t o transfer the precipitate quantitatively from the beaker and to wash it free from impurities (18). The precipitated beryllium tannin complex is filtered off on a coarse filter paper and washed with a 5 per cent ammonium acetate solution containing a little tannin and made just basic to litmus, The precipitate is dried and ignited to constant weight in a platinum crucible at 1200” to 1300”C.
Also8 Tannin in A1 Recovered A1-T Complex Recovered Gram Gram Mole 0.0245 0.0483 0.0715 0.0718 0.0721 0.0729 0,0738 0.0742 0.0739 0.0743
0.3617 0.5540 0.6830 0.7234 0.6366 0.9041 1.0136 0.9698 1.0996 1.1836
~~~~i~ in Moles A1 Complex Moles Tannin Mole
0.00048 0.00095 0.00140 0.00141 0.00141 0.00143 0.00145 0.00145 0.00145 0,00146
0.00112 0.00172 0.00212 0.00224 0.00198 0.00281 0.00315 0.00301 0.00342 0.00368
0.43 0.55 0.66 0.63 0.71 0.51 0.46 0.48 0.42 0.40
The ignited precipitates of aluminum oxide and beryllium oxide should be white, indicating complete removal of organic matter. If the precipitates are not white, they are cautiously fumed once or twice with a few drops of nitric acid before the final ignition. The results of determinations using the above procedures, as given in Table V, show that satisfactory separations of aluminum from beryllium are obtained.
TABLE V. ANALYSIS OF KNOWN SOLUTIONS Determination
Me.
Be0 Taken Gram
Be0 Reaovered Gram
+0.4 +0.3 +0.3 +0.2 f0.2 f0.2 fO.0 +0.3
0.0266 0.0266 0.0265 0.0532 0.0797 0.0797 0.0114 0.0266
0.0264 0.0268 0.0268 0.0531 0.0799 0.0797 0.0112 0.0268
Taken Gram
AlzOa Recovered Gram
Error
0,0124 0,0247 0,0493 0,0740 0.0740 0.0124 0.0739 0.0247
0.0128 0.0250 0.0496 0.0742 0.0742 0.0126 0.0739 0.0250
A1z03
Error
Me. -0.2 +0.2 f0.3 -0.1 +0.2
+o.o
-0.2 +0.2
Determination 8 was made in the presence of large amounts of chloride ion, although Moser and Niessner state that the presence of chloride ion causes premature precipitation of the beryllium tannin complex and makes the separation of aluminum fiom beryllium impossible.
Summary Moser and Niessner’s method for the separation and determination of aluminum and beryllium using tannin will give satisfactory results with careful regulation of the pH of the solution and time of digestion.
Literature Cited Bancroft, W. D., J. Phys. Chem., 26, 501 (1922). Britton, H. T. S., “Hydrogen Ions”, pp. 259-62, London, Chapman and Hall, Ltd., 1929. (3) Britton, H. T. S., and Meek, F. H., J . Chem. Soc., 1931, 2835.
(1) (2)
(4) Ibid., 1932, 187. (5) Dixon, B. E., AnaEyst, 54, 268 (1929). (6) Gooch, F. A., and Havens, F. S., Am. J . Sci., (4) 2,416 (1896). (7) Knowles, H. B., Bur. Standards J . Research, 15, 87 (1935). (8) Kolthoff, I. M., and Rosenblum, C., “Acid Base Indicators”, pp. 189-91, New York, Macmillan Co., 1937. (9) Kolthoff, I. M., and Sandell, E. B., J . Am. Chem. Soc., 50, 1900 (1928). (10) Kolthoff, I. M., and Sandell, E. B., “Textbook of Inorganic Quantitative Analysis”, p. 304, New York, Macmillan Co., 1936. (11) Mitchell, A. D., and Ward, A. M., “Modern Methods in Quantitative Analysis”, p. 43, New York, Longmans, Green & Co., 1932. (12) Moser, L., and Niessner, M., Monatsh., 48, 113 (1927). (13) Moser, L., and Singer, J., Ibid., 48, 673-87 (1927). Ewa. CHEM.,Anal. (14) Nichols, M. L., and Schempf, J. M., IND. Ed., 10,286 (1938). (15) Parsons, C. L., and Barnes, S. K., J . Am. Chem. Soc., 28, 1589 (1906). (16) Powell, A. R., and Schoeller,W. R., Analyst, 50, 485 (1925). (17) Schoeller, W. R. et al., Ibid., 52, 504 (1927); 54, 709 (1929); 57, 550 (1932); 58, 143 (1933). (18) Schoeller, W. R., and Webb, H. W., Ibid., 61, 237 (1936).
