Determination of Boron in Beryllium, Zirconium, Thorium, and Uranium Dissolution in Bromine-Metha no1 A. R. EBERLE and M. W. LERNER New Brunswick laboratory, U. S. Atomic Energy Commission, New Brunswick,
b Boron in metals important in nuclear technology can b e determined by dissolution of the metal in brominemethanol, followed by distillation of the boron ester and color development with diaminochrysazin. With the anhydrous reaction mixture, traces of boron can b e completely recovered by a partial distillation. The procedure, adaptable to many metals, yields excellent precision and accuracy.
E
spectrophotometric procedures for determining boron in beryllium (7), zirconium (3), thorium (6), and uranium (6) involve dissolution of the metal in the appropriate acid, distillation of methyl borate, and development of a colored boron complex with a chromogenic reagent such as curcumin. These procedures generally suffer from a lack of good precision in the lower microgram range and significant losses in the higher ranges. These errors arise partially from the difficulty of recovering the boron as the ester from an aqueous methanol solution even when repeated distillations are used. I n the case of zirconium and thorium, suppression of the fluoride ion, necessary for dissolution, by calcium fluoride precipitation prior to the distillation may also reduce the recovery. The results are generally corrected for incomplete recovery by standardizing the procedure with known samples. In other work in this laboratory on beryllium metal, it was found that bromine in methanol, used by Werner (9) for the determination of oxide in aluminum by selective dissolution of the metal, also smoothly dissolved beryllium and other metals including those listed above. The dissolution of uranium, zirconium, and other metals in bromine-ethyl acetate has been reported recently (1). With the anhydrous solutions resuIting from the dissolution of the metals in bromine-methanol, the distillation of the boron is facilitated. Complete recovery of the boron is obtained with only a partial distillation. Fluoride present in the sample is not found in the distillate. XISTING
146
ANALYTICAL CHEMISTRY
N. 1.
and to speed up hydrogen bromide evolution in the sample flasks, Measure the absorbance of the solutions in 2-cm. cells a t 525 mu with the 0-7 standard as a reference. Wait 2 to 3 minutes after filling the cells before reading the samples t o allow any bubbles of hydrogen bromide to disappear. Draw the REAGENTS A N D APPARATUS calibration curve through the origin and the points. BORON-FREE WATERAND METHANOL. For samples containing more than 2 Distill both water and absolute methp.p.m., prepare standards with 0, 2.00, anol over redistilled calcium. Store in 5.00, and 10.0 y. Use a diaminopolyethylene, chrysazin solution of 0.12 mg. per ml. CALCIUhl HYDROXIDESUSPENSION, Zirconium and Zircaloy. Place a 0.37'%, Dissolve 1 gram of redistilled 1-gram sample of thin shavings in the calcium in 500 ml. of boron-free water reaction flask of the distillation unit. in a quartz storage flask. Add 15 ml. of methanol and a Teflon STANDARD BORONSOLUTIONS,100 magnetic stirring bar, and connect AND 1 y per ml. Dissolve 0.5716 gram the flask. Cool the flask in an ice of boric acid in water and dilute to 1 bath. Add 45 ml. of boron-free water liter, Dilute 10 ml. of this solution to to the receiving flask previously Cali1 liter. brated for 100 ml. and position the DIAMINOCHRYSAZIN SOLUTIONS.Disflask so t h a t the condenser-adapter solve the appropriate quantities of red i m into the water. Cool this flask agent, Boran (LaMotte Chemical Prodalso in a n ice bath. ucts Co.), in 95 to 98% boron-free waterAdd 2.0 ml. of bromine to the distillawhite acid (Merck & Co., Inc.). tion flask and quickly replace the BERYLLIUM METAL, high-purity stopper. Control the reaction with the powder, boron less than 0.2 p.p.m. ice bath, allowing the reaction to proceed DISSOLUTION AND DISTILLATION fairly rapidly, but not to become SO hot APPARATUS, quartz, consisting of 250that more than one drop of condensate ml. reaction flask, 75" three-way conforms a t the end of the condenser. necting tube with %inch water-cooled With samples of zirconium, which may condenser-adapter (borosilicate) and not react as vigorously as Zircaloy, stir 250-ml. receiving flask, all T joints. the solution occasionally to assist the SPECTROPHOTORIETER, Becknian DU dissolution. with 2-cm. cells. When dissolution is complete, cool the solution and auicklv add 85 ml. of PROCEDURES methanol and 1:0 gram of the lorn-boron beryllium scavenger. Again control Color Development and Calibration the reaction with the ice bath until the Curve. For samples containing less beryllium is dissolved. than 2 p.p.m. of boron, prepare standReplace the reaction flask ice bath ards by adding 0, 0.20, 1.00, and with a hot water bath a t 85" to 95O C. 2.00 y of boron to 15 ml. of the lime Immerse the flask about 2 to 3 cm. in suspension in 250-ml. quartz Erlenmeyer the hot water and heat until 55 ml. of flasks. Add 50 ml. of boron-free water. distillate is collected. Wash the adapter At this stage, treat standards and tip with boron-free water and remove samples concurrently. Evaporate the the receiving flask. solutions on a hot plate a t medium heat If the sample contains less than 10 to dryness and continue heating an p.p.m. of boron, add 15.0 ml. of the additional 3 to 5 minutes until no lime suspension. If the sample conmoisture is present on the flasks. tains more than 10 p.p.m., dilute the Transfer the flasks to an oven a t 160' distillate to an appropriate volume SO to 170" C. for 30 minutes. Insert that an aliquot no greater than 50 ml. stoppers and cool. contains less than 10 y . To the aliquot Add 10.0 ml. of diaminochrysazin in a 250-ml. quartz Erlenmeyer flask solution, 1.25 mg. per ml., and again add 15.0 ml. of the lime suspension and stopper to reduce moisture pickup. sufficient water to bring the total volume Swirl the flasks frequently over a 30- to of water t o about 65 ml. After evapo60-minute period to dissolve the residue
Traces of bromine interfere with the formation of the boron-curcumin complex. However, it was found that diaminochrysazin, recently described by Cogbill and Yoe (P),is a satisfactory chromogenic reagent.
rating the solution and drying the residue, develop the color and measure the absorbance as described under the calibration curve preparation. Determine the boron content of the sample plus added beryllium by referring to the calibration curve. Subtract the boron content of the added beryllium. Thorium. Pickle the sample, chips or chunks, in concentrated nitric acid containing a trace of fluoride ion until the sample appears bright, wash with water and ethyl alcohol, and dry. Procerd as for zirconium samples, using a &gram sample with 2.0 ml. of bromine in 25 nil. of methanol. Cool the solution well before adding the brominc, hecause the reaction is vigorous. Add 75 ml. of methanol before adding 0.5 gram of beryllium scavenger. Uranium. Prior to weighing, pickle the samnle, chins or chunks, in 8 M nitric acid; &sh with water and ethyl alcohol, and dry. Proceed as with tliorium sampll?s, 11-ith the exception t h a t the receiving flask contains 40 nil. of methanol. When 60 ml. of distillate are collected, remove the distillation flask and replace i t with the flask containing the methanol and distillate. Ji'ith a receiving flask containing 45 ml. of water in place, distill off 65 ml. of methanol and proceed as with zirconium samples. Beryllium. Use 100 ml. of methanol in the distilling flask and 2.0 ml. of bromine for 1-gram samples. Do not add more methanol or beryllium after the sample is dissolved. Cool the methanol well before adding the bromine, because the reaction is vigorous, especially with powders. EXPERIMENTAL
Reagent Concentration. Preliminary tests revealed t h a t the sensitivity of the particular batch of diaminochrysazin used initially was much less than that reported by Cogbill and Yoe ( 2 ) . b t the recommended concentration of 0.03 mg. per ml., a sensitivity of 0.0044 y per sq. em. was obtained as compared to the expected value of 0.0022 y per sq. em. Although the reagent was used without prior drying, only part of the lonered sensitivity could be attributed to the water content. Later work indicated that the purity of the reagent varies greatly I n an attempt to increase the sensitivity, a study was made of the effect of reagent concentration on the equilibrium concentration of the boron complex. -4solution containing 250 y of boron was treated with 5.0 ml. of the lime suspension and evaporated t o dryness. After the residue was ovendried and dissolved in 100.0 nil. of contrated sulfuric acid to give a solution containing 2.5 y per ml., 1.0 ml. was diluted to 10 nil. n i t h various concentrations of diaminoclirysazin in sulfuric wid. The absorbance of each solution was measured in 2-em. cells a t 520 mp
PI
I
0 5 ;
I
Y
7,
0
4 z
E
/
03-
*
I
2
Ca, Ti, Al, Bi, Fe, In, T1, Ag,b Hg,b Cr, Ta, Ph, hu. Pt. Rh. Sb, Co, stainless steel S o . 310 Sb, S i Re, 3In, B a Essentially complete dissolution \Tithin 30 minutes. b Dissolution apparently stops after an initial reaction.
Table II. Recovery of Boron Added to Zirconium Sample Boron Added, Boron Recovered, P.P.M. P.P.R.I. 0.0 0.20 0.40
1.0
5.0
12.30 24.0
100 500
0. l o a
0.20b 0.40b 1 .l b
5.0b 12.30 24.26 97 495, 500
average of 3 results. Results corrected for 0.10 p.p.m. found in zirconium and 0.15 p.p.m. present in beryllium scavenger; single determinations. cBoron added as neighed amount of Zircaloy A.
the results may be slightly low. With a large excess of beryllium the final solution is somewhat alkaline. This alkalinity reduces the distillation recovery. The solution, after the dissolution of the beryllium sample or the scavenger beryllium, can be slightly yellow with bromine a t the start of the distillation. In the case of uranium samples, any slight excess of bromine cannot be seen. With uranium samples, interfering traces of uranium are found in the distillate. These traces do not survive a second distillation. Fluoride interferes with the color reaction, 36 y causing a -44'3* error in the determination of 5 y of boron (a). Although no fluoride is used in the dissolution step, the fluoride impurity in a metal is a potential interference. However, from considerations of the complexing action of beryllium and the conditions of the distillation, it was expected that little fluoride present in the sample would appear in the distillate. This supposition was confirmed by tests in which 12 mg. of fluoride as sodium fluoride were added to brominemethanol in which 1 gram of beryllium was dissolved and containing 1 and 100 y of boron. Boron in the distillate was completely recovered in each case. Because beryllium is used as a scavenger in the analysis of the other metals, and the amount of fluoride tested was much larger than that normally found, it is evident that fluoride present in the sample will not interfere. Evaporation of Distillate. To prevent loss of boron upon evaporation of the distillate, water and base must 148
ANALYTICAL CHEMISTRY
Table Ill. Analysis of Zircaloy Samples
Average,
Boron Found, % ' Zircaloy A Zircaloy B 0.0243 0 0293
KO.of determina-
tions, N 15 15 Standard deviation, /z(z - R ) * 0.00058 0 0010;
1 . _. Y-1,
be added in sufficient quantity bath t o hydrolyze the methyl borate and also t o prevent hydrolysis of the resulting salt t o the volatile free acid. Spicer and Strickland (8) suggest the addition of a volume of water equal to one half the volume of alcohol and a small amount of glycerol in addition to the alkali. Cogbill and Yoe ( 2 ) > on the other hand, reconmend that ammonium hydroxide be added t o the normally used calcium hydroxide, 2 eporting that the latter alone is iiieffectual in 0.5-meq. amounts in preventing some loss with their 80315 mdhanolwater distillates. In this study it was found that 0.75 meq. of calcium hydroxide in 15 nil. of water makes the 55 ml. of distillate distinctly alkaline, and that this quantity of lime and water alone is sufficient to retain the boron completely. The recovery was shown to be essentially 100% in the following indirect manner. The absorbance obtained with 1.0 ml. of a solution prepared by evaporating 250 y of boron with 5.0 ml. of the lime suspension and making up to 100 nil. with sulfuric acid was identical with that obta'ned with 2.5 y of boron added t o sulfuric acid. Thus no boron is lost upon evaporating an aqueous lime suspension containing boron. The absorbance obtained after evaporating 2.5 y of boron in 15 ml. of the lime suspension xas identical with that found after evaporating 2.5 y of boron with 15 nil. of the lime suspension plus 60 ml. of methanol. The addition of 15 meq. of animoniuni hydroxide prior to the evaporation in the latter case did not increase the resulting absorbance. When the distillate n as diluted and a suitable aliquot evaporated, 2.0 nil. of the lime suspension was found to be sufficient to retain the boron. I n practice it was desirable to have about 50 ml. of water, in addition to that in the lime suspension, present
during the evaporation step. K i t h this amount' of n-aler, the residue is deposited in a fine. evenly distributed film that can be dried uniformly. For this same reason, it is reconmiended that flasks of the same size be used for both the saniples and standards. The adverse effect of water i n the sulfuric acid on the m i i t i v i t y of hydroxyanthraquinone derivat,ives has been studied ( 2 ) . Calcium hydroxide has a similar effect due t o the ivater of neutralization formed during color development. The importance of using the same quantit'y of calcium hydroxide for the standards and samples \vas shown by a study made by evaporating various volumes of the lime suspension on the hot plate, drying a t high heat for 15 minutes, and further drying in the oven a t 170" C. for 60 minutes. To the residues were added 5.0 ml. of sulfuric acid containing 2.5 y of boron plus 5.Oml. of acid containing 12.0 mg. of dianiinochrysazin. The absorbance of each solution was measured in 2-em. cells against a reference of sulfuric acid containing the same amount of diaminochrysazin. The data show that the absorbance, initially 0.555 with no added calcium hydroxide suspension, decreases linearly with increasing volumes of suspension taken. With 20.0 ml. of suspension the absorbance was 0.387. Standard Curves. Although the curves made with varying quantities of diaminochrysazin and 2.0 nil. of the lime suspension xere linear from 0 to at least 5 y, with 15.0 ml. of the lime suspension as used in the procedure, the calibration curves are linear only down to 0.2 y and then curve over to the origin. The absorbances obtained wit,h standards containing less than 0.2 y are also somewhat erratic. If the time of color development is estended to 20 hours, the absorbances of the standards are a fe\v per cent higher, but the curvature and the unreliability of the 0- to 0.2-y region are still present. The analysis of samples containing less than 0.2 y can be improved by adding 0.2 y of boron to the sample to ensure being above the unreliable portion of the curve. In the anaIysis of samples other than beryllium, the addition of the scavenger beryllium automatically brings the boron present to the more reliahle portion in most cases. Blank. K i t h scrupulously clean apparatus the blank boron must arise from the methanol and bromine only. A true blank determination cannot be made directly because the bromine must be consunied before the ester can be distilled. An indirect estimation can be made by analyzing samples of known low boron content. K i t h this method, the blank has been found to be consistent'ly less than 0.03 y. Bromine-Methanol-Metal Reaction. I n the initial n-ork. the dissolution
of t h r samples n a s rarried out under reflux n i t h a n ater-cooled 18-inch quartz condenser filled n ith quartz chips connected to t h e reaction flask. After the sample and scavenger were dissolved, t h e condenser was TI ashed down n i t h nicthanol and the flask was connected t o the distillation unit. This step mas eliminated in subsequent work, when if, was found that n i t h some care the dissolution could be carried out as described without too much bromine being carried over. However, in the analysis of finely divided samples, it may be aclvisable to use the reflux apparatus. The reaction of the metals with hromine-methanol is apparently not simple. Kerner (9) suggested that the products with aluminum were aluminum methylate, aluminum bromide, and hydrogen. Dangyan (4)reported the production of large amounts of mclthyl bromide. Only a brief study of the reaction v a s made in the investigation reported here. The apparent stoichiometry of metal to hydrogen evolved is near 4 : 3 with both beryllium and thorium, although the value n ith thorium is somewhat variable. K i t h zirconium the metal-hydrogen ratio is very variable, ranging from 4:1 to 5 : 3 . The metal-bromine stoichiometry with thorium appears to be 4 : s . K i t h beryllium, if a slight excess of bromine is used the ratio is 4 : 1. If a n excess of metal is used, the reaction still proceeds after the yellow color of bromine disappears. The residues, after evaporation of most of the methanol, are intractable gums. Because the basic procedure possibly could be applied to any metal which dissolves in bromine-methanol with the formation of a relatively nonvolatile metal compound, the dissolution of all metals available was studied briefly. About 0.5 grain of bulk or powdered metal was added to 2 ml. of bromine in 15 ml. of methanol and the suspension N-as warmed if no immediate reaction occurred. The results are shown in Table I. The fact that the boron gave no visible evidence of dissolving was not surprising. KO apparent reaction can be seen with a mixture of hydrochloric and nitric acids a4 used in the conventional acid dissolution-curcumin procedure. A test made by carrying 100 mg. of crystalline boron through the proposed procedure gave a boron value of about 50 y in the distillate, indicating that some attack dow occur. If the boron is present as boride, this anomaly may not appcar. Zirconium boride dissolves easily. nearly violently, in methanol-bromine.
Table IV.
Comparative Analyses of Beryllium Samples
(Boron, p.p.m.)
0.61, 0 . 5 4
SBS-2868
XBS-2869
0 51, 0 51 0 14 6.5 1.2
0 , 1 4 >0.15
NBS-2700 XBL-D-8950 SBL-D-8952 SBL-D-8953 SBL-E-4195 SBL-E-4496 KBL-E-4497 SBL-E-4498
6.7
1.3, 1.2. 1.2 1.6, 1.5
11
14 0 32 0 26 0 38 0 54, 0 30 0 30 0 39
1.4
XBL-E-4499 SBL-E-4500 Range of multiple determinations by different analj.sts.
b
Average of duplicate analyses. Table V.
Analysis
of Uranium and Thorium Samples
Sample Uranium SBL-16 analj,zcd sample Thorium XBL-D-9065 Thorium SBL-B-6885 “as is’’ Thorium KBL-B-6885 ‘bright“ a Accepted value. * Range of multiple determinations
Boron, P.P.N. Spectrographic Bromine-methanol 0,234 0.20, 0.20, 0.20 0.35-0,;’ 0.34, 0.38 0 . 7 -0.8‘ 0 . 3 8 , 0.38 0 . 3 7 , 0.37
RESULTS
Recovery tests were carried out by analyzing the same zirconium metal both with and without the addition of boron (Table 11). I n one test, a weighed amount of Zircaloy A was added; in the others, boric acid in methanol was added before the sample dissolution. A precision study was carried out by analyzing two samples of Zircaloy 15 times (Table 111). The larger deviations obtained for sample B may be explained by sample inhomogeneity. A number of beryllium samples, previously analyzed by the acid dissolution-distillation-curcumin procedure (7) were analyzed with the results shown in Table IV. The fact that the borosilicate adapter of the apparatus did not interfere with the procedure prompted the testing of an all-borosilicate apparatus. The only quartz ware used in these tests was the receiving and drying flasks. The results, also given in Table IV, show that successful determinations can be carried out in glass. However, other tests made by adding 200 mg. of finely powdered glass to the bromine-methanol before dissolution gave slightly high results. Accordingly, the use of the all-glass apparatus should be viewed with some concern if extremely low boron samples are to be analyzed. Finally, a sample of uranium and
two samples of thorium previously analyzed by spectrographic procedures were analyzed (Table V). One of the thorium samples in the form of small chips with a black surface was analyzed both “as isJ’ and after a pickling treatment. LITERATURE CITED
(1) Beederman, hI., Munnecke, V. H., Vogel, R. C., Vogler, S., U. S.Atomic Energy Comm. ANL-4720, 18 (19511. (2) Cogbill, E. C., Yoe, J. H., ASAL.
CHEJI.29,1251 (1957). (3) Craig, Jessie, Richmond, hl. S., U. S. Atomic Enerav Comm. NY00-2001 I”
(1949). (4) Dangyan, ?*I. T., Bull. Armenian Branch Acad. Sci. U.S.tS.R. 314, 83 (1942). (5) Rodden, C. J., Proceedings of International Conference on Peaceful Uses of Atomic Energy, Geneva, 1955, Vol. 8, p. 197, United Xations, N. Y., 1956. (6) Rodden, C. J., Lerner, h l . VI.,“The Metal Thorium,” 11. W. White, J E. Burke, eds., p. 352, Am. Soc. Metals, Cleveland, Ohio, 1958. ( 7 ) Rodden, C: J .,,,Vinci, F. A,, “The Metal Beryllium, D. W. White, J. E. Burke, eds., p. 641, ,4m. Soc. Lletals, Cleveland, Ohio, 1955. (8) Spicer, G. S., StrickIand, J. D. H., Anal. Chim. Acta 18,523 (1958). (9) Werner, Otto, Z . anal. Chem. 121, 385 (1941). RECEIVED for review July 6, 1959. Accepted Octobrr 29, 1959.
VOL. 32, NO. 2, FEBRUARY 1960
149
