PREPARATION OF BORON FROM BORAX* JACK T. THURSTON. UNIVERSITYOP NEVADA, RENO, NEVADA A very interesting and instructive experiment is the preparation of boron from borax. Boron is not one of the elements that are commonly prepared in the laboratory. Its preparation requires very careful technic and manipulation. Boron is not found in the free state, but in combination in boric acid, native borax, boricite, colemanite, and boronatrocalcite. Borax is obtained from minerals, such as colemanite, by treating them with sodium carbonate. Another important compound of boron is boric acid, H3BOa; obtained in the United States and South America by treating borax with a mineral acid. In the volcanic regions of Tuscany jets of steam contain nitrogen, ammonia, and boric acid.' The preparation of boron may be divided into six steps: first, treat borav with hydrochloric acid to obtain boric acid, second, dehydrate the acid to form the trioxide, third, fuse the trioxide with magnesium, fourth, dissolve the compounds of magnesium in hydrochloric acid and water, fifth, dissolve boron suboxide in nitric acid, and sixth, dissolve silicon and silica in hydrofluoric acid. Make a hot saturated solution by dissolving 80 grams of sodium tetraborate in 40 cc. of water. Filter the solution hot, with the aid of a hot water jacket, to remove the insoluble impurities. A t relatively low temperatures the solubility of borax and boric acid is low, but a t high temperatures these compounds are more soluble. With the aid of a hot water jacket borax and boric acid may be kept in solution. Add concentrated hydrochloric acid until the solution is strongly acid to litmus. On cooling scaly aystals of boric acid separate:
+
N ~ s B I O V ~ O H ~ 2HCI-+2NaCI O
+ 4HaBOa + 5H.O
Add water so as to make a volume of about 100 cc. Filter by means of suction and wash the crystals with cold water until they are free of salt. Evaporate filtrate and washings to about 15 cc. and filter with both hot water jacket and suction. Cool the solution and recover the boric acid as before. About 53 grams of the acid should have been obtained. The second step is to obtain boron trioxide from the boric acid. Place a small amount of the acid in a platinum crucible and heat to redness. If a porcelain or clay crucible is used the oxide will fuse with them. To prevent losses, add small portions of acid as the dehydration proceeds: 2H3BOs
+ heat-+B.OJ + 3H20
About 30 grams of the trioxide should be obtained from the 53 grams of acid. The oxide is very brittle. Like the oxide of aluminum i t is some-
* Winner of ten-dollar award in the student contest closing February 15, 1929. 1
J. R . Partington, "Textbook of Inorganic Chemistry," Macrnillan, p. 733, 1925.
what amphoteric; it is weakly acidic and yet it shows feebly basic properties toward very strong acids2 The next part of the preparation is the most important and also the most difficult to accomplish. Different proportions of horon trioxide and magnesium powder give different results. If boron trioxide is heated with the theoretical quantity or an excess of magnesium, magnesium horide is formed almost quantitatively. For the highest yield of boron one should fuse three parts by weight of boron trioxide with one part of magnesium p ~ w d e r . ~This ratio is necessary because several possible reactions may take place:
The silica comes from the clay crucible, in which the fusion takes place a t a red heat. When the vigorous reaction ceases, cool and place the crucible in dilute hydrochloric acid (12) and boil gently for about one hour. This treatment will loosen the mass so that it can be removed from the crucible. Grind up the fused mass to a fine powder, add about 75 cc. of concentrated hydrochloric acid and heat almost to boiling for one hour. Dilute with 100 cc. of hot water and filter with suction and hot water jacket. The filtrate will contain the soluble magnesium compounds:
+
MgO
+
+
MgO 2HC1+MgCln HnO 2HC1 2H20+MgCls 2HsBOs Mg(BO& GH20+2HaBOa 3MgClz 6H2 Mg,B1 6HC1 Mg(BO& Mg8B1 lOHCl 8HsO +5MgCh 4HaB01
+
+
+
+ + +
+
+ +
+
+
+ GHI
The mixture on the paper will contain boron, horon suboxide, silicon, and silica; about 7 grams should be obtained. The low yield shows that a large portion of the boron is lost, because after being reduced by the magnesium it reacts with more magnesium to form magnesium boride and magnesium borate. These two salts react with hydrochloric acid and form boric acid, which may he recovered. Before 1909 it was thought that boron at this stage contained only silicon and silica as impurities, hut in that year Weintraub stated that J. R. Partington, "Textbook of Inorganic Chemistry," Maemillan, p. 735, 1925.
".
W. Mellor. "A Comprehensive Treatise on Inorganic and Theoretical Chemistry." Longmans, Green and Co., Volume V, p. 8, 1922-27.
boron suboxide is present also. He also stated that pure boron is insoluble in 40% nitric acid:4 B40s
+ 2HNOa + 5H2O 4 4 H a B 0 3 + 2NO
In order to separate these add 6N nitric acid drop by drop. If the acid is added in large volumes the reaction is more violent and may cause a loss of some of the boron. When no further action occurs add about 25 cc. of water and heat to boiling. The remainder of the boron suboxide will be dissolved by the acid and leave the silicon, silica, and boron, which are insoluble in nitric acid. Filter with both suction and hot water jacket. The greater portion of the boron mixture is dissolved by the above treatment, showing that boron suboxide constitutes the greater part of the seven grams treated. Only 1.5 grams of boron containing silicon and silica are obtained. The chestnut brown powder, left on treating the mass with nitric acid, may be purified by treatment with hydrofluoric acid. Add about 10 cc. of hydrofluoric acid to the powder in a platinum dish and allow this to digest on a steam-bath for fifteen minutes: SiOz Si
+ GHF +H2SiFs + 2H?O
+ 6HF--t
H.SiFn
+ 2Hp
Filter the hot solution through a hard rubber funnel and take care to have sufficient water in the beaker receiving the filtrate, so that the acid will not attack the glass too readily. Wash the boron with hot water and finally dry in an oven. For the theoretical yield 9 grams of boron should have been obtained, but due to the formation of magnesium boride and magnesium borate only 1.2 grams are obtained. Boron so prepared is in the amorphous form and is a chestnut brown powder which is unaltered in air a t ordinary temperatures, but it smoulders when heated to a dull redness. Boron burns slightly when heated to redness with the formation of boron trioxide and boron nitride, which produce a superficial coating that prevents complete oxidation. Boron is one of the few elements that combines directly with nitrogen5 A crystalline form is said to have been prepared; this form is not really crystalline boron, but an aluminum, carbon, and boron compound, probably B18C2A4.6 Boron has practically no industrial uses, but its compounds are used for a number of commercial purposes. This method of production is too costly, and if boron finds a place in industrial chemistry, some other less expensive method of extraction will have to be devised. J. R. Partington, "Textbook of Inorganic Chemistry," Macmillan, p. 736, 1925. J. R. Partingon, "Textbookof Inorganic Chemistry," Maemillan, pp. 735-6, 1925. 6 J. W. Mellor, "A Comprehensive Treatise on Inorganic and Tl~eoreticalChem-
6
istry," Longmans, Green and Co., Volume V, p. 10, 1922-27.
