BERYLLIUM IV. Micro Qualitative Analysis of Beryllium * BY HAROLD SIMMONS BOOTH AND SPENCER GUILD FRARY
Introduction
,
Beryllium, although discovered in I 793 by Vauquelin and first prepared by Wohler in 1828, has not shown much possibility of commercial importance until quite recently. However the demands of industry for light, strong alloys for aircraft and highly specialized alloys for other purposes will probably increase manufacturing of the metal and bring the price down to the point where it can be profitably employed.z2 Beryllium is not a rare element and in certain localities is rather abundant. Washington26 states that “beryllia has never, so far as I know, been determined in the analysis of an igneous rock. Yet the fact that the aluminosilicate, beryl, is not a rare mineral indicates that beryllium is fairly widespread. Beryl occurs in granitic and syenitic pegmatites, especially sodic ones. It is also found in metamorphic rocks. It has been suggested that in some cases beryllia has been weighed with, and supposed to be alumina, unidentified beryl being present, thus accounting for an apparent excess of AlzOa, which it is difficult to reconcile with the mineral composition of the rock. It has been shown recently that alumina has been mistaken for beryllia in the mineral vesuvianite:o in which the presence of beryllium had not been suspected. The need of a closer study of some minerals and rocks as regards this point is obvious. It might be well to determine B e 0 in granites associated with berylbearing pegmatites and in some nephelite syenites.’’ In a more recent article Washingtonz4 again stresses the importance of analyzing silicate minerals and igneous rocks for beryllium. Barylite was found to contain beryllium in place of the aluminum hitherto supposed to be present. Because of its low atomic weight the percentage of beryllium inmost minerals is not very great; beryl, the most common, contains only 4 to 5 per cent beryllium as the metal. Since beryl is the present commercial source of the element despite a hardness of 7.5 to 8 and the natural difficulty of decomposing silicates which make it far from satisfactory, it is of interest to know if there are other sources of the element that might prove of value. To prevent the element from being overlooked in the future it is important that the methods of its detection be improved, if possible. To this end a study of the microqualitative detection of beryllium has been made.
* Contribution from the Morley Chemical Laboratory, Weatern Reserve University. For Beryllium I, 11, and I11 see J. Phys. Chem., 35, 2465, 2492, 3111 (1931).
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HAROLD SIMMOSS BOOTH AXD SPESCER GUILD FRARY
Historical Beryllium at the head of group I1 of the periodic table has many of the chemical characteristics of aluminum in group 111. The two hydroxides closely resemble each other and it is their separation that offers the chief difficulty in quantitative analysis. In the ordinary course of analysis they are precipitated together and weighed as alumina. There have been many separations suggested for beryllium and aluminum. Of these methods, Britton2 found that the best were: I. Decomposition of sodium beryllate solutions by boiling to precipitate beryllium hydroxide.I1 2. Parsons and Barnes’ method of momentarily boiling a saturated solution of sodium bicarbonate.21 3 . Wunder and Wenger’s method of fusing the oxides of beryllium and aluminum in sodium carbonate. Sodium aluminate is formed while the beryllia is u n a f f e ~ t e d . ~ ~ 4. Haven’s method in which a concentrated solution of the chlorides is treated with an equal volume of ether and dry hydrogen chloride gas passed in, to precipitate the aluminum chloride.l* Since that time a newer separation in which 8-hydroxyquinoline is used to precipitate the aluminum has been developed which is quite satisfactory for quantitative analysis.16 For the determination of small amounts of beryllium in large amounts of aluminum, a combination of the Havens, and 8-hydroxyquinoline methods has been suggested.’ By this means most of the aluminum is removed first, avoiding the use of an excessive amount of the rather expensive 8-hydroxyquinoline. In the analysis of minerals, Hills1*recommends the use of Parsons’ method of boiling a concentrated solution of sodium bicarbonate, followed by the 8-hydroxyquinoline separation to remove the last of the aluminum. There are very few really distinctive qualitative tests for the detection of either beryllium or aluminum. H. Fischerg suggests the use of quinalizarin (1,2,3,8 tetrahydroxy anthraquinone alizarin bordeaux) for the detection of small amounts of beryllium and I. M. KolthofP suggests curcumin for the detection of the element. Both of these are adsorbed by beryllium hydroxide, giving characteristic colors.13 Microqualitative Methods of Analysis Chemical microscopy, on the other hand, offers simple, easy methods of qualitative analysis in many cases where the ordinary methods are laborious and often inexact. In the case of beryllium there are several tests that distinguish it quite definitely from aluminum and other elements with which it might be associated. A satisfactory microqualitative test should have the following characteristics: ( I ) the resulting product should have a large molecular weight in proportion t o the amount of the element actually present; ( 2 ) a small amount of
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the substance should give a positive test; (3) the products formed should have characteristics that distinguish them from the reagent and from any other elements that might be present; (4) the test should give good results under not too rigid conditions; ( 5 ) the test should be comparatively simple and not involve too many steps; (6) the resulting products should have an index of refraction so different from the solution that they may be easi$ distinguished. Double salts of beryllium should give good tests because of the low atomic weight of the element. For example, in the potassium oxalate test the double salt formed has a molecular weight of 263, of which only g parts are due to the beryllium present. Likewise in the chloroplatinic acid test the crystals obtained have the composition BePtCle.8H20, with a molecular weight of 560, only g parts of which are due to the element being detected. The third requirement, that of characteristic properties of the products, is not so well met in the case of beryllium, due to the fact that so many of the insoluble compounds of beryllium separate as amorphous, basic salts that have no distinguishing characteristics. Since solutions of beryllium salts have the property of dissolving beryllium hydroxide and the salts also hydrolyze quite readily, it is, in many cases, difficult to get crystalline products of definite composition. This property probably accounts for many of the strange, unsubstantiated compounds found in the literature of beryllium. Many reagents cause the precipitation of the hydroxide which in itself is useless as a microqualitative test for beryllium; though failure to obtain any hydroxide precipitate can be taken as a negative test showing the absence of the element, Or the precipitate can be filtered and dissolved in a small amount of acid thus concentrating it and increasing the accuracy of the test as well as effecting a separation from other material that might interfere. Precipitation with ammonia and filtration through a microfilter, or careful evaporation of the test drop, will show amounts of the hydroxide down to 0.0007 mg. With smaller amounts a spectrographic analysis is the best means for the detection of beryllium.
Expekmental To determine the limit of accuracy of the various methods a procedure similar to that given by Chamot and Cole was used? A series of four beryllium nitrate solutions were made up such that each was one-tenth the concentration of the previous one. I n order to be sure of getting the same amount of solution each time, micropipettes made by drawing out glass tubing to a capillary tip and attaching rubber bulbs to facilitate handling the solutions, were used. Measurements into a graduated cylinder showed the drops to average fifty to a cubic centimeter hence each contained 0.02 cc. Gravimetric analysis of the first solution showed it to contain 3.3 mg. of beryllium per cubic centimeter, or 0.066 mg. per drop. The beryllium content per drop of the different solutions was:
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No. I . No. 2 . No. 3 . No. 4 .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . .
0.066 mg. beryllium per drop 0.0066 mg. per drop 0.00066 mg. per drop o.000066 mg. per drop
Most limits of accuracy are given in micro milligrams (y mg.), equivalent to 0.001 mg. Expressed in this manner the four solutions would have the following beryllium content per drop: No. I , 6 6 . 0 ~mg.; No. 2 , 6 . 6 ~mg.; No. 3, 0 . 6 6 ~mg. and No. 4, 0 . 0 6 6 ~mg. The textbooks on chemical microscopy6*17p1 give the following methods for the detection of beryllium: I . Reaction with potassium oxalate to form the double salt, beryllium potassium oxalate (KzCz04.BeG04). 2. Reaction with chloroplatinic acid to form beryllium chloroplatinate, (BePtCh.8Hz0). 3. Reaction with sodium and uranyl acetates to form the triple salt, ) ~ .(GH302)~.9 H ~ 0 . sodium uranyl acetate, Na (C2H302).Be (C Z H ~ O 3ZUO All three of these books recommend the potassium oxalate method as the best for beryllium.
Beryllium Potassium Oxalate. K2G04.BeC204. Chamotc recommends a neutral, moderately concentrated test drop to which a little acetic acid is added; then the reagent is added in a fragment that, in this case, must be more than twice the size ordinarily used. According to Chamot, “K2GO4.BeG04separates in large, stout, clear, colorless, monoclinic prisms; single, in twins, or in groups of radiating, irregularly formed prisms. Thin plates in the form of rhombs arealsoobtained. The salt is strongly birefringent and exhibits an extinction angle of about 39’. “Unless the reagent is present in sufficient excess the test is apt to prove unsatisfactory. Acid potassium oxalate or the oxalates of sodium or ammonium are not suitable reagents. “During the disintegration and solution of the reagent fragment wellformed crystals of KzCz04usually appear momentarily. These crystals often bear a striking resemblance to the double salt, and it is therefore imperative that the analyst shall be on his guard lest he fall into error through too hasty a decision.” Chamot adds that the double salt can be recrystallized by gently warming the slide and cooling, whereupon well-formed crystals are obtained. Behrens’ suggests the addition of a little mercuric chloride to cause the formation of elongated prisms, thus making it easier to distinguish the double salt from the reagent. Lindsley” states that ammonium salts should be absent when testing for beryllium with potassium oxalate. When this test was made according to Chamot6 crystals were obtained with all the solutions, It was subsequently found that acetic acid in a drop of water gave similar crystals, Solutions of beryllium salts hydrolyze suf-
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ficiently to give the slight acidity necessary for the formation of the double salt so that the addition of acetic acid is unnecessary and should be avoided as it interferes with the test. Behrensl gives 0.08 mg. as the limit of accuracy of the potassium oxalate method. This is the minimum amount of beryllium in a test drop that will give a positive test. This would correspond to slightly more than one drop of the No. 4 solution. A positive test was obtained with the No. 3 solution equivalent to alimit of accuracy of 0 . 7 ~mg. Before arriving at these conclusions, several methods of obtaining the right acidity and at the same time avoiding ammonium salts were attempted. It was found, however, that ammonium salts did not interfere and that beryllium hydroxide, dissolved in a little hydrochloric or sulfuric acid, or solutions of the salts, could be used for the test without any preliminary treatment. Very dilute solutions can be concentrated by gentle evaporation and highly acidic solutions had best be neutralized with ammonia and the precipitated hydroxide dissolved in acid. Aluminum, ferric iron and chromium gave no crystals with the reagent. They are the elements most likely to be present in the ordinary course of analysis. Adding mercuric chloride to the solution made no appreciable change in the shape of the crystals obtained and appeared to interfere with the test to some extent.
Beryllium Chloroplatinate, BePtCh.8HtO. This test is performed by evaporating the test drop to a nearly dry film and then drawing the concentrated reagent solution across the film in a narrow channel. Beryllium chloroplatinate separates out in hydroscopic, faintly yellow, square and rectangular plates, usually singly, but sometimes penetration twins are formed. Chamot6 says that the formation of the crystals can be induced by the addition of alcohol. The precipitate should be examined with crossed nicols. The beryllium salt exhibits parallel extinction and is only weakly birefringent. As the alkali metals form characteristic crystals with chloroplatinic agid they should not be present in too great amounts. Chamot states further that in his opinion this test is valueless unless the material is so high in beryllium as to constitute a fairly pure salt. When this test was first tried no crystals were obtained due to the reagent solution being too dilute; after evaporating it down to a thick sirup good results were obtained with the No. 3 solution. Therefore, the limit of accuracy was found to be 0.71.1 mg. However, the addition of alcohol recommended by Chamot caused everything to dissolve and spoiled all the cryst@ rather than aiding in their formation. Aluminum did not interfere with this test and it is probable that, if the beryllium and aluminum were precipitated as the hydroxides and washed free from everything else, except possibly a slight trace of ammonium salts, this test would work reasonably well.
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HAROLD SIMMONS BOOTH AND SPENCER GUILD FRARY
Sodium Beryllium Uranyl Acetate. Beryllium is supposed to form a triple salt with sodium and uranyl acetates similar to those formed by magnesium and zinc. This method is mentioned by Caglioti3 as a test for beryllium but neither Miholic'g nor Bedient4 were able to obtain the triple acetate unless the solution went to dryness. When this test was tried no crystals were obtained that could be used as a test for beryllium. It seemed impossible to get the salts to crystallize unless the solution was evaporated almost to dryness. Caglioti3 also suggests the acetyl acetone derivative prepared by treating acetyl acetone with beryllium acetate solution, slightly acidified with acetic acid. Monoclinic tablets, or thin prisms, are supposed to be obtained by slow crystallization. Caglioti states that this is preferable to the triple acetate as a microqualitative test for beryllium. This test recommended by Caglioti was also attempted but no crystals were obtained. On one occasion some crystals were observed but, on repeating the test under apparently the same conditions the crystals failed to reappear.
Search for New Reagents W7ith the hope of finding a reagent better than potassium oxalate for the microqualitative analysis of beryllium, many substances which give distinctive tests, with other elements, were tried with beryllium salts. Since the great majority of these failed to give any definite reactions with beryllium the results are only briefly given in tabular form.
TABLE I Reactions of several organic compounds with beryllium nitrate solutions, first alone, then with KCNS, and finally with KCNS in acid solution were investigated, and also those with aluminum chloride were tried to determine what effect it would have as an impurity. Reagent
Acridine Resorcine Benzidine Diphenylamine Phthalic anhydride Picric acid Pyrogallic acid Uric acid Eosine Fluorescene
Beryllium nitrate
No action alone. Reacted with KCNS to give crystals No action No action; crystals of reagent on heating No action No action No action; crystals of reagent on heating No action; crystals of reagent on heating Reagent insoluble; no action No action No action
Aluminum Chloride
No action No action No action No action No action No action No action No action No action No action
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TABLE I1 Results of reactions of inorganic compounds on solutions of beryllium nitrate. No reaction ' Potassium dichromate No reaction Potassium permanganate No reaction Potassium tartrate Precipitated beryllium hydroxide Potassium antimonate Precipitated beryllium hydroxide Sodium arsenate Precipitated beryllium hydroxide Sodium bismuthate No reaction Sodium nitrate White curdy precipitate Sodium phosphotungstate No reaction Sodium nitroprusside No reaction Sodium thiosulfate White curdy precipitate Disodium phosphate White curdy precipitate Ammonium persulfate Ammonium phosphomolybdate Only soluble in alkaline solutions; beryllium hydroxide precipitates No reaction Dimethylglyoxime Orange color and precipitate. (Aluminum Ammonium vanadate gave a pale yellow color) No reaction (aluminum gave a precipitate) Ammonium fluostannate No reaction (aluminum gave a precipitate) Ammonium fluotitanate No reaction (aluminum gave a precipitate) Potassium fluoride No reaction (etched the slide rapidly). Potassium bifluoride
TABLE I11 Beryllium hydroxide was dissolved in solutions of the following acids and evaporated. No reaction Succinic acid No reaction Citric acid No reaction Tartaric acid No reaction Malonic acid No reaction Salicylic acid Rectangular crystals Cinnamic acid Needle crystals Benzoic acid The majority of the substances studied gave no particular reaction but there were a few that appeared to have interesting possibilities, that deserve a more detailed discussion. The reaction of sodium fluoride and some of the fluo-salts with solutions of beryllium and aluminum salts is a means of distinguishing between them and has been proposed as a method of separation.1° Britton2 states that this method is not quite quantitative due to the slight solubility of sodium aluminum fluoride. However, a sample of beryllium oxide made by the Copauxs process in which the separation is based on
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the insolubility of the sodium aluminum fluoride, showed little if any aluminum when examined spectrographically. However, the use of fluorides under the microscope is rather objectionable as special precautions have to be observed to protect the lenses from corrosive fumes. Cover glasses can be fastened to the objective with cedarwood or Canada balsam, but even so there is danger of corrosion if used for any length of time.
Test with Potassium Malonate Potassium malonate was studied because malonic acid is the second mcmber of the series of organic acids of which oxalic is the first. As the potassium oxalate test is the best so far known for beryllium, it was of interest to determine if potassium malonate would give a similar reaction with solutions of beryllium salts. A fragment of the salt was added to the test drop and the solution carefully evaporated. With more concentrated solutions, well shaped rhombs were formed a t the edge of the drop. When these were pushed into the center and the evaporation continued, long monoclinic prisms were formed. The rhombs, observed under polariaed light, with crossed nicols, gave oblique extinction a t an angle of 38'. The prisms showed parallel extinction. The rhombs were obtained in a drop of the No. I solution; the No. 2 solution gave only the prisms. The limit of accuracy of this method would be 6 . 6 mg. ~ Test forming Beryllium Basic Acetate Beryllium is unique in forming a volatile basic acetate. This compound is preparedza by dissolving the hydroxide in acetic acid, evaporating the solution to a gummy mass, dissolving in glacial acetic acid, and boiling off the excess acid; on cooling the solution, the basic acetate separates out in the form of octahedral crystals. While this method is not easily applicable to tiny amounts of unknown, if about a cubic centimeter of unknown solution is taken and the beryllium converted to the basic acetate, only a small amount of the resulting solution need be taken. Although the absolute accuracy of this test is in the order of one milligram, this is more than compensated by the fact that the crystals obtained are highly refractive and quite characteristic. The glacial acetic acid is very mobile and it is best to place the drop to be observed in a hollowed-out slide to prevent its spreading over the slide and evaporating too rapidly. TABLE IV The limit of accuracy of these methods: Beryllium potassium oxalate Beryllium chloroplatinate Beryllium malonate Beryllium basic acetate
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Conclusions The most satisfactory microqualitative test for beryllium so far found is the potassium oxalate method. It does not require any elaborate procedure and the sensitivity is more than sufficient for all practical purposes. The limit of accuracy is about 0 . 7 ~mg. Contrary to the literature, acetic acid should be avoided and ammonium salts do not interfere. The chloroplatinic acid test is suitable where the beryllium is not too contaminated with other material. In some cases the beryllium and aluminum can be separated from other material as the hydroxides, before applying the test. With pure beryllium solutions the limit of accuracy of this test is 0.7l-4 mg. Potassium malonate can be used as a reagent for the detection of beryllium. The limit of accuracy is 7 . 0 ~mg. Beryllium basic acetate recrystallized from acetic acid can be used as a confirmatory test for the element though its limit of accuracy is only about one milligram. The reactions of the fluorides can be used as a means of distinguishing beryllium from aluminum provided precautions are taken to protect the apparatus from fluoride fumes. The above methods of microqualitative analysis offer the best means for t,he detection of beryllium. They are superior to the'ordinary methods of qualitative analysis both in simplicity and in accuracy. If these methods are applied to the precipitated hydroxides ordinarily obtained in macro qualitative analysis, they should show conclusively the presence of any hitherto unsuspected beryllium. summary The application of the methods of chemical microscopy to the qualitative analysis of beryllium was studied. 2. A study was first made of the present method to determine their limitations and advantages. The potassium oxalate method was found to be the most satisfactory. The chloroplatinic acid method was found satisfactory under certain conditions. 3. An attempt was made to find new methods that would supplement or replace the present ones. 4. As a result of this investigation two methods are proposed: The use of potassium malonate in a manner similar to potassium oxalate. The use of the basic acetate recrystallized from glacial acetic acid as a confirmatory test. 5 . The above micro methods are believed to be superior to the macro qualitative methods, as a means of identification of beryllium, especially if applied to the precipitate of aluminum and beryllium hydroxides, as ordinarily obtained in macro qualitative separations. I.
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Bibliography Behrens and Kley: “Mikrochemische Analyse,” Vol. I, 4th ed. (1920). 2 Britton: Analyst, 46, 359 (1921); 47, 5 0 (1922). 3 Caglioti: Rend. Accad. Sci., 33, IIIa, 177 (1927). 4 Chamot and Bedient: Mikrochemie, 6, 13 (1928). 5 Chamot and Cole: J. Ind. Eng. Chem., 9, 969 (1917). 6 Chamot and Mason: “Handbook of Chemical Microscopy,!’ Vol. I1 (1931). 7 Churchill, Bridges and Lee: Ind. Eng. Chem. Anal. Ed., 2, 405 (1930). 8 Copaux: Cornpt. rend., 168, 610 (1919). 0 Fischer: Wiss. Veroff. aus dem Siemens-Konzern, 2, 99 (1926). 10 Gibbs: Am. J. Sci., ( 2 ) 37, 356 (1864). 11 Gmelin: Ann. Physik, (Pogg)., 50, 175 (1840. 12Havens: Am. 3. Sei., (4) 4, 1 1 1 (1897). 13 Hillebrand and Lundell: “Applied Inorganic Analysis,” (1929). 14 Hills: Ind. Eng. Chem. Anal. Ed., 4, 31 (1932). 15 Kolthoff: J. Am. Chem. Soc., 50, 393 (1926). 16 Kolthoff and Sandell: J. Am. Chem. Soc., 50, 1900 (1928). 1 7 Lindsley: “Industrial Microscopy” (1928). 1 8 Lundell and Knowles: Bur. Stds. J. Research, 3, 91 (1929). 1 Q Miholic: Bull. Acad. Sei. Zagreb Univ., 1920, 16. 20 Paladau and Bauer: Am. Min., 15, 30 (1930). 21 Parsons and Barnes: J. Am. Chem. SOC.,28, 1569 (1906). 22 Stock: Trans. Am. Eleotrochem. Soc. Preprint 61-35,451 (1932). 23 Urbain and Lacombe: Compt. rend., 133, 874 (1901). 34 Washington: Am. Min., 16, 37 (1931). 25 Washington: “The Chemical Analysis of Rocks,” 4th ed., z z (1930). 26 Tl’under and Wenger: 21. anal. Chem., 5 1 , 470 (1912). 1
