Determination of Boron in Metal Borides HERMAN BLLW ENTH.4 L imerican Electro WetaZ Corp., Yonkers, IV. Y . In the search for a method that could be used for the determination of boron in metal borides, it was found that the standard procedure of separating boron in the form of methyl borate from the metals by distillation was too time-consuming and not always accurate. The method described carries this separation out bj precipitating the metals with barium carbonate after using a sodium carbonate fusion to bring the sample into solution. It has
been successfully applied to the borides of iron, chromium, tungsten, titanium, zirconium, thorium, vanadium, niobium, tantalum, calcium, cerium and aluminum, boron carbide, boron nitride, and amorphous and electrolytic boron. The borides of transition metals have become recently of great interest because of their physical properties at high temperatures. This method should therefore be useful for fast and accurate routine determinations.
T
lib; borides of sonit: trarisitiori nietals have recently be(:ome of great terhnical interest. They are metallic refract,ories and good conductors of heat and electricity, have high mc:lting points, and some of them are corrosion-resistant a t high teinperat,ures. In the course of metallurgical invest,igations, ~ponsorrdby thr Office of Saval Research, the lack of a fast, axid :wcurate method for the det,ermination of boroii in these k)nrid(~,~ hecnmr obvious. I'KKVIOUS
unnecessary when barium carbonate is used. In addition, sodium hydroxide might cause the formation of a sodium borate of low solubility where the concentration of the boric acid is high. Calcium carbonate will behave in a similar manner, also forming a salt of lox solubility under these conditions, and there is a greater danger with high pH of retaining the boron by the metal precipitate. When barium carbonate+isused as precipitant, the solution never gets above pH 6.5, so that there is no danger of picking up excess carbon dioxide from the air, and barium borate is very soluble in water. Sodium hydroxide is used, but only partly to neutralize the hydrochloric acid. The precipitated metals are filtered off, and the filtrate is slightly acidified with hydrochloric acid and heated to boiling to expel carbon dioxide, which, if present, would titrate as boric acid. This boiling must be done gently and in a well covered beaker. Boiling for 1 to 3 minutes is sufficient, but experiments showed that boiling for 10 minutes under the above conditions caused no loss of boron. Thr 11sr of a reflux condenser is unnecessarv.
WOHh
l'x~I'(Jl1 has generally been determined in chemical cumpuuiiclh. alloys, and mixtures by transforming the element into boric acid, separating this acid from the rest of the constituenls, and titrating it, with sodium hydroxide. All elements that would interfere in the titration by react,ing with sodium hydroxide have been eliminated by three inherently different methods (2). The oldest and stJillmost common, originated by Chapin ( I ) , consists of distillat,ion of boron in the form of methyl borate. The second (7) ..ioparates the metals from t>het~oronby plating them on a mercury cathode, and the t,hird, recommrnded by the aut,hors only for the separation of boron from a very limited number of metals, precipifat,es the metals with calcium rarbonate (5), barium hydwxide (6), or a mixtuw of sodium hydroxide and sodium carlionate (4). The f r s t method, involving two distillations, is long and tedious. The second, which was not, tested by the author, re,quires special apparatus and is believed not to function satisfactorily (3). Special attention was given to the precipitation ,method, which was felt to require the least time, apparatus, and ;ttt,ention. The method developed in the course of this investigation is based on this principle and has been tested on many hundreds of samples of various borides, including so-called amorphoiis and electrolytic boron.
Table I. Reproducibility of Boron Determinations Saniple Iron boride, FeB Chromium boride Chromium boride, CrB Chromium boride, CrBn Tungsten boride, WB Titanium boride (TIB?) Titanium boride. TiBz Zirconium boride, ZrBn Thorium boride, ThBz Vanadium boride, VBn Siobium boride, NbBn Tantalum boride, TaBz Calcium boride, CrtB6 Cerium boride, CeBs Aluminum boride, AlBln Boron carbide. BIC Boron nitride, B N Amorphous boron Electrolytic boron
EXPERIME\TAL
Whereas available methods recommend solution of the sample by boiling nith acids under a reflux condenser to avoid loss of boron, the method discussed suggests fusion of the sample xith sodium or potassium carbonate in a platinum crucible, the ))asicnature of these salts effectively preventing loss of boron. The melt is decomposed either with hydrochloric acid 01, in some cases, with water. Esperiments were carried out in which the metals were then precipitated with barium hydroxide, sodium hydroxide, calcium carbonate, or barium carbonate. Result? were low when the f r s t three reagents were used. Barium carbonate is the only reagent which can be used successfully unless special precautions are taken. Barium hydroxide and sodium hydroxide make the solution too basic. thic can be avoided by careful adjustment of the pH, but it is
Boron Found, % I I1 16.59 16.66 9.20 9.20 14.52 14.70 28.46 28.30 5.59 5.76 16.03 16.18 25.84 25.91 18.80 18.94 8.98 8.92 21.69 21.85 17.37 17.20 9.96 9.85 42.45 42.56 30.29 30.09 65.48 65.40 75.66 75.76 43.29 43.23 81.90 82.10 100.20 100.00
The filtrate may be titrated with sodium hydroxide in the presence of mannitol visually or potentiometrically. If done visually, a mixed indicator solution of methyl red, bromocresol green, phenolphthalein, and thymolphthalein is used, which gives more distinct end points than the common pnitrophenol and phenolphthalein, especially at a temperature of about 50' C. This indicator mixture gives a distinct color change from pink to green m-hen the hydrochloric acid is neutralized, the starting point of the boric acid titration, and changes to purple a t the end point of the boric acid titration. Titration a t elevated temperature shortens the time for cooling the boiling solution and 992
V O L U M E 2 3 , NO. 7, J U L Y 1 9 5 1 there is, therefore, less danger of picking up carbon dioxide from the air. Potentiometric titrations were done a t about 40" C., as this was the highest temperature a t which the available pH meter could be operated. Figure 1 gives an actual boric acid titration curve for a chromium boride sample. Neutralization of hydrochloric acid occurs a t pH 6.3 and of the boric acid complex a t p H 9.1, while the indicator end points, indicated by arrows, are a t pH 5.3 and 8.9, respectively. Any pH between 5 and 7 may be chosen as a starting point and between 8.6 and 9.4 as an end point for the potentiometric titration, as the parts of the titration curve between these points are practically parallel. The sodium hydroxide solution used for the titrations must, of course, be standardized against a known amount of boric acid a t the same pH a t which the tit,ration is to be perfornird. 10
9
a
I
a
7
993 metal MXS precipitated and the boric acid determined by titration. Table TI1 shows the results of these experiments. These data prove the reliability and accuracy of the described method for the determination of boron in the metal borides mentioned :is well as in elemental boron. Only for tantaluni horide is the accuracy somewhat belox the usual analytical standard, For a number of other borides--e.g., those of nickel and inanyanese-the method is not applicable in its present form, as they are not completely precipitated with barium carbonate. These will be dealt with in a later puhlicatiorr. REAGENTS REQUIRED
The mixed iiidicator solution is made by dissolving 0.05 gram of methyl red, 0.1 gram of bromocresol green, 0.30 gram of phenolphthalein, and 0.30 gram of thymolphthalein in 100 ml. of methyl
alcohol. lfannitol, reagent grade. Sodium hydroxide stock solution is prepared by dissolving 50 grams of C.P. sodium hydroxide pellets in 50 ml. of distilled water and transferring to a large test tube. Enough barium hydroxide is added to precipitate carbonate present (see analysis on label), and the solution is shaken well, stoppered tightly, and let' s t a n d in vertical position until the supernatant liquid is clear. Sodium hydroxide, 0.05 N , is made by withdrawing 2.5 ml. of the clear stock solution with a pipet and running it into 1 liter ot' freshly boiled and cooled water. I t is mixed well and stored away from contact with air, pTotected a t all times by Caroxit>e tubes (indicating carbon dioxide absorbent). The solution is standardized against potassium acid phthalate or twice recrystallized C . P . horic acid. PROCEDURE
6
5
4
Solution of Sample. The well pulverized sample of about 100 mg. (all through a 100-mesh screen) is fused with ten times its weight of sodium carbonate in a platinum crucible. After about 5 minutes, when a good fusion has been accomplished, the burner is removed and the melt in the crucible is well shaken and spread around the walls with the help of crucible tongs. When the melt has solidified and somewhat cooled, about 100 t o to 200 mg. of sodium nitrate are introduced and the fusion is continued carefully, the temperature being raised gradually. After the fusion is complete, which takes about 20 minutes, the crucible is cooled and the melt is digested with 50 ml. d I to I hydrorhloric- :wid. Roiling muqt hr avoided.
3 -.
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Figure 1. Determination of Boron in Chromium Boride Samplr
The outcome of duplicate determinations given in Table I ahows the reproducibility of results obtained with the described method. Some of these borides were produced in the course of the metallurgical investigation; others are commercial grades. For this reason analysis results sometimes are far from the theoretical values. As no standard boride samples were available, two series of experiments were conducted to prove the reliability of the method.
Addition of Known Amount of Boron to Previously Analyzed Samples. After the fused sample of the respective boride was decomposed with hydrochloric acid, a known amount of boron was added in the form of 25 ml. of a boric acid solution containing about 1 gram of H3B03 per liter. Table I1 shows the amounts of boron added to the various borides and the amounts recovered. Addition of Known Amount of Boron to Solutions of Metals. A known amount of boron in the form of boric acid was added to solutions of the metals under study. The ratio of metal to boron in these mixtures was the same as in the respective borides. For these mixtures either a metal salt sqlution, chloride or nitrate, was used, or a metal oxide was fused according to the procedure described in this paper. The mixtures, metal salt plus boric acid, were then treated like a metal boride solution-namely, the
.-
Hemvery of Boron after Boric Acid Additions to Borides
Iron horide
Chroniium boride Tungsten boride Titanium boride Zirconium boride Thorium boride Vanadium boride Niobium boride Tantalum boride Calcium boride Cerium boride Aluminum boride Boron carbide Boron nitride
Boron Added
Boron Recovered
Deviatiorr
MQ.
.My.
MQ.
4.39 4.39 4.37 4.37 4.37 .4.37 4 37 1.37 4.33 4.39 4.37 4.39 4.37 4.88
4.40 4.40 4.43 4.52 4.25 4.41 4.26 4.25 4.14 4.51 4.42 4.40 4.29 4.82
+0.01
+O.OE $0.06 10.15 -0.12 +0.04 -0.11 -0.12 -0.19 +o. 12 +0.05
+0.01 -0.08 -0.06
Table 111. Recovery of Boron after Boric Acid Additions to Metal Salt Solutions Metal i n Solution Iron
Chromium Tungsten Titanium Zirconium Thorium Vanadium Niobium Tantalum Calcium Aluminum
Borpn Added
Boron Recovered
MQ.
MQ.
25.5 17.7 18.2 17.8 18.1 18.0 18.2 17.5 17.5 18.2 18.8
25.4 17.8 18.2 17.8 18.2 18.2 18.3 17.4 17.0 18.0 19.0
Deviatioii
MU. -0. 4.
+o
d
0 0
+o
+o +o -0
il
2 1 I
-0.5 -0.2 $0 2
994
ANALYTICAL CHEMISTRY
In the case of elemental boron and other high boron-containing materia$ which are aliquoted before precipitatiou of the nietds, it is advisable to double the amount of sodium carbonate. Aluminum boride and chromium boride have to be fused with a sodium carbonate-sodium nitrate mixture. Chromium boride does not dissolve in carbonate only, and aluminum boride forme metallic aluminum which alloys with platinum. The borides of tantalum and niobium are fused with potassium carbonate and potassium nitrate to avoid the formation of sodium salts of low solubility. Their melts as !vel1 as that of tungsten boride are later completely decomposed with cold Kater, then 50 ml. of 1 to 1 hydrochloric acid are added, and the analysis is completed as usual. To emure complete precipitation of vanadium, ferric chloride is added to the vanadium boride solut,ion before precipitation. ( A ferric chloride solution for this purpose was made by dissolving 50 mg. of Bureau of Standards open hearth iron, sample 558, in a lit,t,lehydrochloric acid and oxidizing it with hydrogen peroxide. ) Precipitation of Metals. The hydrochloric acid solution is diluted t o 200 ml., and 20 ml. of 1to 2 sodium hydroxide solution are added. Ten grams of finely powdered barium carbonate are added under stirring, and the solution is then slowly heated to hoiling, and kept boiling for about 5 minutes. Care should be taken that a small excess of barium carbonate is present a t the bottom of the beaker. The precipitate is allowed to settle on the hot plate for 0.5 hour, filtered off using suction, and wished Ivitli hot water. Harium carbonate will not precipitate chromate ions completely. 111 the case of chromium boride, it is therefore necessary to add 10 i d . of a 10% barium chloride solution and bring again to n rhort boil. Titration of Boric Acid. The filtrate, the volume of which a1 this point should be 400 ml., is made slightly acidic to lit,mus with 3 ' i I r o p ~of 1 to 3 hydrochloric acid and boiled for 3 minutes t o
espel all carbon dioxide. After rapid cooling to 50" to 60" C., 6 drops of mixed indicator solution are added and the solution is titrated with 0.05 il' sodium hydroxide solution until the first green tinge is observed, which indicates neutralization of the hydrochloric acid. Eight grains of mannitol are introduced and the boric acid is titrat,ed to a purple end point. In the pot'entiometric titration 0.05 .V sodium hydroside is added dropwise until pH 6.2 is reached, mannitol is added, and boric acid is titrated to pH 9.0. 9 blank titration with the same nmounts of ti11 reagents is subtracted from all determinations. ACKNOWLEDGMENT
The author wishes to acknowledge the assistance of William Fall, who carried out much of the experimental work. Appreciation is further expressed to the Office of Naval Research for support, of this investigation nnil permission to publish it. LITliR i T U R E CITED
(1) C h a p i n , \V. H.. J . Am. C h o n . Snc., 30,1691 (1908). ( 2 ) D e a n , H. Y., a n d Silkes, B., V. Y. Bur. Mines, Inform. Circ. 7363 (September 1946). Revien- of m e t h o d s for d e t e r m i n a t i o n of borides in irott a u d steel. (3) Ibid..p. 24. (4) Hazel, W . H., a n d Ogilvie. €1.. .Is.LL. ( ' H E M . , 22, ti97 (1950). (5) L i n d g r e n , J. hl., J . Am. Chem. Soc.. 37,1137 (1915). (6) Sabitiia. I,. T., a n d S t y u n k e l , T. V.,Zavodsknya Lab. 13, 752 (1947) ; translated by Henry Blutcher. (7) Tschischewski, N.. I t i d . Etig. Chcni., 18,ti07 (1926).
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RECEIVED July 10, 1948. Presented before the regional meeting, Long Island Section, - ~ X E R I C A XC H E J I I CSOCIETY, ~L AIaroh 16, 1951. Work done under e i i n t r a c t 36-onr-256 with the Offire of S a r a 1 Research.
lonophoretic Analyses in Agar Gels QUINTIN P. PENISTON, HILDA D. AGAK, AYD JOSEPH L. McC-IKTHY University of Washington, Seuttle, Wash. An ionophoretic procedure has been developed to provide a method for analyzing mixtures of low molecular weight ionic substances of similar ohemical behavior, and for characterization of these soliites. Of particular interest were mixtures of sulfonated aromatic substances comprising dialj zable lignin sulfonic acids, and of phenolic substances obtained by alkaline oxidative degradation of lignin. Ionic components of a sample mixture are made to migrate from an initial short sample section through an agar gel in a long straight tube under constant electrical potential gradient, so that distances migrated are proportional to net mobilities. Under favorable conditions approximatel? complete separation of
I
N WORKING toward separation and identification of lignins
and lignin degradation products of low molecular weight (6, 12, IS), an analytical procedure as needed which would permit determination of the concentration in which such ionically dispersed substances were present 'as constituents of complex mixtures in aqueous solution. Because the technique of electrophoresis or ionophoresis seemed well suited to this purpose, a relatively simple and inexpensive apparatus and procedure were devised which differ from the well-known Tiselius method (16) of dectrophoresis in that migration proceeds in an agar gel without substantial convection, and also over a considerable distance from the initial short sample region, so that more or less complete heparation of the ionic species ocmrs, depending on mobility differences and extent of dispersive effects. In this paper the apparatus and procedure are described, factors influencing sepa-
coniponents is achieved, so that individual solutes can be not only quantitativel?;determined, but also characterized by measurement of such properties as absorption spectra and diffusion constants. The use of agar gel to eliminate convection, and of direct photometric scanning of the ionophoresis tube for positional analysis, permits the ionophoretic procedure to be carried out relatively simply with apparatus of moderate cost and with limited expenditure of research time. In some cases, as little as 10 micrograms of a component snffices for analysis. The method should be useful in research on many types of substances such as antibiotics, alkaloids, hormonea, and radioactive compounds.
ration are discussed, and esaniples of application to known mixtures are given. APPARATUS AYD PROCEDURES
Migration Apparatus. The following parte make up the migration apparatus (see Figure 1). The migration tube, A , is a straight glass tube approximately 8 mm. in inside diameter and of variable length, de ending on the migration conditions. The authors have u s e l Vycor tubing because it has good transparency down to about 2500 A. and permits analysis after migration by direct scanning of the tube for absorption of ultravio1,et radiation by use of a spectrophotometer. The electrode vessels, B , are cylindrical borosilicate glrrss chambers with bend and constriction to join the migration tube,
