Spectrochemical Determination of Beryllium in Microquantities E. C. BAKNES, W. E. PIKOS, T. C. BRYSON, AND G. W. WIENEK
Westinghouse Electric Corporation, East Pittsburgh, Pa. A spectrochemical procedure has been developed, using a conventional direct current arc, for the quantitative determination of beryllium in amounts as l o w as 0.04 microgram of beryllium per liter of urine. The sample is prepared b y a phosphate precipitation to which aluminum is added to act as an internal standard. The amount of beryllium is determined by comparing the density of Be 2348.6 K . to .412321.6 In addition to the determination of beryllium in urine, the method has been successfully applied to tissue and air samples.
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HE dettdriniiiatioii of beryllium in biolugicd niaterial has he cwnr important t o industry. Cholak ( I j has described a method which gave a seiisitivity of 6.25 micrograms of berylliuni per liter of urine. This procedure was used in preliminary survey work arid found to be of insufficient sensitivity fat, many urine samples. Because it utilized only 20y0 of the s:rniplo o i i the electrode, experimental work was hegun in order to us(' t i t s tint ire sample. ITtilizing the phosphate precipitstion p r i i w i l n i ' t ~ ( I I , with sonit' niodjfication, for t,he conc.entration anrl ,.t~~i:jr:ition of the beryllium, a satisfactory concrntrnte has 1)eeri obtained for the spectrographic mcxthod. Quantitative dat,a have bwri obtained which show t,hat 0.04 microgram of beryllium per liter of urine can be determined with accuracy, and 0.02 microgram of beryllium per liter of urine can be qualitatively d e t e c t d . ('holak ( d ) has more recently published a quantitative procedure utilizing the cathode layer technique which, from the data given, yields a qunntitativr sensitivity of 0.10 microyr:tin of beryllium per liter of urine, and a qua1it:ttive detection of 0.025 microgram per liter of urine. The method described ninkes use of a conveiitional direct current arc tvith the powdered sample placed on the anode. Chemical preparat ion involves wet ashing, Precipitation, centrifuging, arid drying of all the beryllium (with phosphates) from a .5O-ml. sample of urine tvhich is then burned to completioii i n t l i t l :iw. SPECIAL REAGENTS
811 the reagent& specified u e analytical grailc. Aluminum Internal Standard (1.50 mg. of aluminuni per m1.j. Dissolve 0.750 gram of metallic aluminum in approximately 10 ml. of 1 t o 1 hydrochloric acid and make up t o 500 ml. with distilled watcr. Ammonium Sitrate Wash. Dissolve 10 grams of ammonium nitrate in vatt'r and make up to 1000 ml. -4djust to pH 7.5 with ammonium hvdrosidc. Calciuni Phosphate Solution. As described by Cholak ( I ) , dissolve 2.5 grams of calcium carbonate (calcite) in just sufficient conccntratcd hydrochloric acid (specific gravity 1.19) t o effect solution (titiout 7 nil.), add 2.5 grams of ammonium phosphate, and make up t,he solution to 100 ml. with distilled water. If a precipitat t> forms, redissolve by adding hydrochloric acid drop by 'drop. Standard Beryllium Solution. Dissolve a weighed amount of fused metallic beryllium in a small amount of 1 t o 1 hydrochloric acid and dilute t o volume a-ith 1% hydrochloric acid in distilled water. .\Iake further dilutions to required concentration in 1% hydrochloric acid, using a microburet and volumetric flasks. Take extreme care t o avoid contamination during these dilutions. A standard solution containing 0.02 microgram of beryllium per milliliter has been used. Synthetic Urine. Sodium chloride, 170.5 grams; potassium chloride, 63.5 grams; calcium chloride hexahydrate, 31.5 grams; magnesium chloride hexahydrate, 20 grams; sodium dihydrogen phosphate monoh drate, 37.5 grams; dissolved in and diluted t o 1000 nil. with 1 0 g h y volume nitric acid.
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:E1 EhlICA L PRO( :El I t ! K r:
To 50 nil. if ui,inv in a 300-nil. td1-t(JI'lll l'yrcs beaker add 20 nil. of nitric acid and 5 ml. of sulfuric ai-ill. Heat this mixture about 3 hour? on the hot plate, during M-hich time most of the liquid is waporated. Heating is not continued t o dryness. 1Tht.n tht. liquid h g i n s T O turn a brown 1-olor, add 1 ml. of nitrir acid. Continue heating at approsimatrly t,he same rate until the solution again Iieginr to turn bron.rr aiid then add 1 ml. of nitrir acid. Repeat thi. procedure until a vlear, colorless solution results. Greatthr amounts of sulfur trioxide funitis are evolved each sucressive time it beconies necessary to aiitl nitric acid. If spattering occurs during the addition of thr nitric*acid, reduce the heat slightly. After the solution a p p w r s c:olr>rle?s,continue additions of 1-ml, port ions of riit,ric acid at aliprosimatcly 15-minute intervals until t h c evolution of sulfur trioxide fumes becaomee noticeably less but has not entirely stopi)riI. Do not take to dryness but drive out most of t,he sulfuric acid. .-\tltf I ml. of perchloric acid in .such a way as to ririst, {IIJKII thc sides of the beaker and h a t the mixture to a frotliiirg v o i i i l i i i o n over a separate burner. As soon as the liquid h a woled (,nough to prevent spattering, add approsimate1J- 40 nil. of hot tlistilled water and transfer the clear solution to a 50-nil. gratluatt~ilPyrex centrifuge tube where it is allon-ed to cool. The eentrifiigt*tubes which seem best are those wit11 a short csniral Iiottoni. Add 2 drops oi' phenol red and theii coiic.t.iitrated ammonium i hydroxide until t h t . first evidence 1 ~ 1 ' colol, rhange. T l ~ e ~add dilute ammonium hydroride (1 t o 5 ) u n t i l t l i t , first dcfinite precipitation of phosphates occur3. The lxq-Ilium also precipitatee a t t,his time. Complete precipitation of the phospliat,cs IS not necessary. The aim should be t o precipitate a n amount which will yield a correct final precipitate weight,; with a little eupc,rience this can be done. I n some urine samples, the ainount of phosphat,e is so sinall that no precipitate occurs, even t,hough sufficient ammoniuni hydroxide has been added to turn t.ho indicator w r y red. In buch cases, add a feiy drops o f nitric. acid to make t,he solution slightly acid again. .kdd 0.5 nil. uf the calcium phosphate solution and again add dilute ammonium hydroxide until precipitation occurs. After precipitating the phosphates, add 1 ml. of the aluminurn internal standard solution from a microburet: it immediately forms a precipitate. Centrifuge for 10 minutes at 1800 r.p.m., the radius of rotation being 18.75 cm. (7.5 inches) at the bottom of the centrifuge cups. Pour off supernatant liquid. Add approximately 20 ml. of ammonium nitrate wash; swirl to break up and wash the precipitate. Centrifuge and pour off supernatant liquid. Repeat this wash once. Transfer recipitate to a weighed 10-nil. fused silica crucible with the ai! of a small volume of water, and place in oven at 130" C. until sample is completely dried. This may take from 2 t o 3 hours. Weigh and adjust weight, if necessary, to between 40 and 60 mg. of precipitate by addition of the appropriate amount of calcium phosphate solution, and dry again. Scrape the precipitate loose from the walls and bottom of the crucible with a small spatula and transfer this precipitate to a prepared spect,rographic carbon. SPECTROGRAPHIC E ~ ; I P . \ l ~ N T
.4 large Gaertner quartz spectrograph has been used in this method. Accessory equipment includes a Restinghouse Fkctox rectifier to provide a direct current sourre, R T,ctds & Northrup 1281
ANALYTICAL CHEMISTRY
1282 nonrecording microphotometer, and developing equipment of Applied Research Laboratories. A neutral filter and miscellaneous cutters for shaping electrodes are also used. The electrodes are regular grade spectroscopic carbons of the Xational Carbon Company. SPECTROGRAPHIC PROCEDURE
The lower electrodes used to contain the sample are cut into 5-cm. (2-inch) lengths and one end is cupped with a special cutter (4). I t has been found experimentally that the dimensions of the electrode must be held to close tolerances in order to secure the best results. The specifications are as follows: Outside diameter of electrode Inside diameter of electrode Drill angle Depth t o bottom of cup Length of electrode
0.250 0.002 inch 0.210 0.001 inch 48 degrees 0.180 * 0.020 inch 2.0 inches f f
The upper electrode is 2 inches long and 0.25 inch in diameter.
It is tapered and hemispherically tipped with a cutter to give an
end with 0.06-inch radius. This shaped eIectrode withstands the high amperage and mill not burn rapidly. The electrodes are mounted on the arc stand and a 2-mm. gap set between them. No adjustment of gap length is made during the arcing period. The optical arrangement consists only of a 40% neutral filter between the source and the slit. The slit is 20 mcrons wide and 3 mm. long. The sample is burned to completion in a conventional 220-volt direct current arc a t 16 amperes. I n order to prevent spattering, the arc is started a t 3 amperes until fusion of the sample has taken place, which requires approximately 30 seconds. The current is then rapidly increased to 16 amperes. The length of burning time is approximately 150 seconds; no pre-arc is used. The spectrum is recorded on Eastman Kodak Gompany S.h No. 1 plates in the wave-length region 2290 to 3100 A. The plates are developed in D-19 (Eastman Kodak) for 3 minutes, hardened in chrome alum, and fixed in x-ray fixer. The temperature is maintained a t 70" + 1' F. A two-line method, similar to that proposed by the Aluminum Company of America (S), is used for plate calibration, except that an iron spectrum is substituted for aluminum. DISCUSSION
Early in this investigation, it was recognized that beryllium is a refractory element that would not readily volatilize from the electrode until new the end of the arcing period. I t was desirable, therefore, to choose an internal standard that would have similar characteristics and remain in the arc until the sample burned to completion. Aluminum has been found satisfactory for this purpose. A moving plate study indicates that the density of the beryllium line increases with time and does not reach its maviniuni value until near the end of the arcing period. The aluminum line has a final density value little different from its value during the entire arcing period. The line pair used for the analysis is Be 2318.6 A. and A1 2321.6 I1 A. Background is measurable in this region but no corrections have becn necessary to obtain the required accuracy. Referring to Figure 1, the difference in slope of the two analytical curves may be partially attributed to the neglect of background corrections in the low concentration region. However, a straight-line relationship is obtained throughout the analytical range as shown. The technique as described is capable of quantitatively determining 0.04 microgram of beryllium per liter of urine and qualitatively detecting 0.02 microgram of beryllium per liter (corresponding to 0.001 microgram of beryllium in the arc). The standard deviation for a single determination a t 0.020 microgram is *0.004 or 20% of the amount present. This is based on a statistical analysis of over 50 standard samples, using all results obtained. The standard deviation suggests the need for duplicate deterniinations, which has been the practice. Certain precautions must be followed to secure optimum results. The standard so!ution containing 0.02 microgram of beryllium per milliliter must be acid to prevent adsorption on the glassware. Experimental data have been obtained on solutions with and without the acid addition. Consistently lorn results have been found for the nonacidified solutions. Statistical
Figure 1. Analytical Curve for D e t e r m i n a t i o n of Beryllium evidence on the acidified standard has shown that stability existe over a period of several months. The final precipitate as obtained from the chemical procedure is made up of refractory materials which are difficult to burn to completion. Experimental work has shown that the electrode shape and size, as mentioned above, must be met in all particulars. The weight of the dried precipitate n u s t also be held to a critical range, and should preferably lie between 40 and 50 nig. As much as 60 mg. can be burned successfully if the material is free from large amounts of sodium salts. Because the sodium salts are soluble, they can be removed by washing. Data have been obtained to show that losses of beryllium due to washing with ammonium nitrate wash are negligible. Excessively long burning times are to be avoided, as the background will then interfere with the lower end of the analytical range. When working with such small quantities of beryllium, the contamination problem becomes paramount. Glassware which hau contained strong solutions of beryllium should not be used for very dilute solutions. Pyrex does not contain detectable amounta of beryllium, but porcelain crucibles were found to contain small amounts and fused silica crucibles have been used instead. Contamination of samples during collection or preparation must be avoided by adopting extreme precautions. The spectrographic carbons have not shown the presence of beryllium. The technique, as described, has been successfully applied t o urine samples, lung tissue, skin, and dust samples. Lung tissue and skin are ashed by placing 10 grams of tissue in a 250-ml. Pyrex beaker and proceeding as previously described for urine samples. For lung tissue or other samples which contain little or no phosphate, it is necessary to add 0.8 ml. of the calcium phosphate solution before trying to precipitate the phosphates with dilute ammonium hydroxide. Analysis of dust samples collected by the electrostatic dust and fume sampler in-' dicates the method of analysis is satisfactory for these samples. The dust is washed from the stainless steel sampling tubes with water and a policeman, 5 ml. of hydrochloric acid are added, and the solution is heated to dissolve the beryllium compounds. Most, but not all, of the hydrochloric acid is driven off and the solutions made up to a definite volume with water. A measured portion of
V O L U M E 21, NO. 10, O C T O B E R 1 9 4 9 this is transferred to a centrifuge tube and water added to make approximately 40 ml. of solution. Two drops of bromothymol blue me used as an indicator instead of phenol red; 0.8 ml. of calcium phosphate solution is added and the analysis is continued previously described. Higher concentrations than the range specified are easily analyzed by using aliquot parts or by dilution. An aliquot can conveniently be taken while the sample is in liquid form in the centrifuge tube prior to the precipitation of phosphates. The above procedure does not depart from standard techniques used by the analytical chemist, and elaborate optical adjustments for the spectrographic procedure are avoided. This
1283 enables the spectrographer to perform the analyses on the prepared samples with speed and facility. The short time required for setup is particularly advantageous in an industrial laboratory where many different problems are encountered. LITERATURE CITED (1) Cholak, J., and Hubbard, D. M., ANAL.CHEM.,20, 73 (1948). (2) Ibid., 20, 970 (1948). (3) Churchill, J. R., IND.ENG.CHEM.,ANAL.ED., 16, 11 (1944) (4) Leichtele, P. A., J . Optical SOC.Am., 34, 8 (1944).
RECEIVEDjsnrlary 7 , 1'149.
All-Metal Needle Valve of the HershbergSouthworth Type Committee for the Standardization of 3Iicrochemical Apparatus Division of Analytical and Micro Chemistry, AMERICANCHEMICALSOCIETY A L STEYERMARK, Chairman HUFFMAN, J. A. I
