(2) Benfey, 0. T., Stanmeyer, J. R.,
Jr., Milligan, B., Westhead, E. W., Jr. J . Org. Chem. 20, 1777 (1955). (3) dhang, Ming-Che, Kao, Chen-Heng, J . Chinese Chem. Soc. 3, 256 (1935). ( 4 ) Dajac Laboratories, Leominster, Mass., data sheet on 2,4,7-trinitrofluorenone, 1954. (5) Gilman, ,H Blatt, 9. H., “Organic Syntheses, 6011. 1701. I, p. 543, Wiley, Sew York, 1947.
(6) Kofler, L., Kofler, A., ‘(Mikromethoden Bur Kennzeichnung organischer Stoffe und Stoffegemische,” Univ. Wagner, Innsbruck, 1948. (7) Laskoffski, D. E,, ~ ~ D. G,, ~ ‘IcCrone, T.TT. c., ANAL. CHEM. 25, 1400 (1953). (8) LaSkowSki, D. E., McCrone, W *c.7 Ibzd., 26, 1497 (1954). (9) Ibzd., 30, 542 (1958).
(10) Sutter, P., Helv. Chim. Acta 21, 1266 (1938). bRECEIVEI) ~ for ~ review , Ma!: 14, 1958. Acce ted August 18, 1958. Based in part to be submitted by Otis It’. on
Bdams to the graduate school of Illinois Institute of Technology in partial fulfillment of requirements for the degree of doctor of philosophy.
Spectrophotometric Determination of Beryllium and Fluoride Using Chrome AzuroI S LOUIS SILVERMAN and MARY E. SHIDELER Atomics International, A Division o f North American Aviation, Inc., P.O. Box
b Chrome Azurol S has been applied to the spectrophotometric determination of beryllium and fluoride. The colored beryllium-Chrome Azurol S complex is formed a t pH 6.0 in the presence of a pyridine-hydrochloric acid buffer, which enhances the sensitivity of the dye to beryllium and increases the sensitivity of the metal-dye complex to fluoride. The procedure described can be used to determine from 1 to 30 y of fluoride per 50-ml. volume with a precision of =k 1 y, and from 0.2 to 10 y of beryllium per 50ml. volume with a precision of k0.2 y. Studies were made on the variables of the system, interferences, and an ion exchange method to separate uranium from beryllium.
A
!vas required in this laboratory to determine microgram amounts of fluoride in enriched uranium sulfate, which is used as the fuel in the water-boiler type of nuclear reactors. A commercial preparation of uranium sulfate for reactor fuel necessitates its conversion from the uranium hexafluoride, and some fluoride would be expected in the sulfate salt produced. As fluoride in acid solution speeds the corrosion of stainless steel vessels, the quantity of fluoride present, however small, must be known. * The number of methods reported for the determination of microgram amounts of fluoride shows that the determination is difficult and not completely satisfactory. Fluoride is best determined spectrophotometrically by its bleaching action on metal-dye colored complexes. A great variety of metals and dyes have been used for this purpose (1, 9, 4-7)J including the Chrome Azurol S-aluminum complex ( 1 ) . Willard and Horton (13) report that Chrome Azurol S is one of the more sensitive in152
METHOD
ANALYTICAL CHEMISTRY
309, Canoga Park, Calif.
a broivn bottle. The solution is stable dicators for the titration of fluoride using for several weeks. thorium. Pyridine-Hydrochloric Acid Buffer, Theis (8)and Wood (14) used Chrome p H 6.0. Slowly add 35 ml. of conAzurol S as a sensitive indicator for the centrated hydrochloric acid t o 215 ml. presence of small amounts of beryllium. of pyridine. This solution is stable for As beryllium also forms a strong complex at least 3 weeks. with fluoride, it seemed that beryllium might have advantages as a Chrome PROCEDURES Azurol S complex for the determination of microgram quantities of fluoride. Standard Curve for Fluoride. PreChrome Azurol S, the sodium salt of pare t h e standard curve by pipetting 3”-sulfo -2”,6”-dichloro-3,3’-dimethyl-4- aliquots containing from 0 to 30 y of fluoride into a series of 50-ml. voluhydroxyfuchson-5,5’-dicarboxylic acid, metric flasks. Add water to bring the is also known as Solochrome Brilliant volume to about 40 ml., and add an Blue B, Polytrop Blue R, and has the aliquot containing 10 y of beryllium British Colour Index No. 723. I n to each flask. Then add 3 ml. of 2% weakly acidic beryllium solution the dye hydroxylamine hydrochloride solution, forms a pink to purple-blue color which 2 ml. of pyridine-hydrochloride buffer, is bleached by the presence of fluorides. and 1 ml. of Chrome Azurol S reagent, Although the preliminary aim of this mixing the contents of the flask after study was to investigate the berylliumeach addition. (The p H should be 6.0 at this point.) Dilute the solutions Chrome Azurol S complex for use in t o volume with distilled water and allow determining fluoride, it was immediately to stand for 15 minutes. Measure the apparent that this work could also be absorbance with a Becknian Model applied to a method for determining D U spectrophotometer in 5-cm. cells microgram quantities of beryllium. Bea t 575 mp, using as the reference cause the majority of the experimental solution a reagent blank containing all work was essentially the same for the reagents except beryllium and fluoride. determination of either fluoride or Prepare a standard curve from these beryllium, a procedure for beryllium is values. Fluoride Determination. If interincluded here. fering substances are present in t h e sample, they must be separated a s REAGENTS discussed below. If necessary, t h e sample may be adjusted to p H 6 f Standard Beryllium Solutions. Dis0.1 with a minimum amount of solve 1 gram of beryllium metal in dilute (10%) hydrochloric acid and hydrochloric acid or ammonium hydroxide. Then follow the procedure for the dilute t o 1 liter with distilled water. standard curve. Determine the microMake suitable dilutions t o obtain solugrams of fluoride present from the tions containing 1 and 10 y of beryllium standard curve. per ml. It is recommended t h a t all Standard Curve for Beryllium. solid or powdered beryllium compounds Prepare t h e standard curve b y pipetbe handled in a hood, and that an aspirating aliquots containing from 0 t o 10 tor mask be worn. y of beryllium into 50-ml. volumetric Chrome Azurol S, 0.05%. Disflasks. Dilute with distilled water to solve 0.50 gram of Chrome Azurol S 40 ml. Add the amounts of hydroxyl(Geigy Chemical Co., Los Angeles, amine hydrochloride, pyridine-hydroCalif.) in 1 liter of distilled water chloride buffer, and Chrome Azurol containing 2 grams of gum arabic powder. Allow the solution to stand S specified for the fluoride procedure, then follow the fluoride procedure. several days, then filter and store in
W
W
0
z a
m
E 0
cn m
400
300
a
500
575
700
600
WAVE LENGTH, m p
Figure 2. Spectrophotometric curves of beryllium-Chrome Azurol S-fluoride system
_ _ _ - N-o beryllium .._.
500
10
20
30
40
50
WAVE
80 90 600 LENGTH, rnb 60 70
- 10 y of
beryllium
10 y of beryllium
Figure 1 . Effect of p H on beryllium-Chrome Azurol S complex with no buffer present
+ 30
Y
of fluoride
Table 1. Effect of Pyridine Buffer on Beryllium-Chrome Azurol S Complex
Three upper curves, p H 5, 6, 7. 30 y of beryllium Two lower curves, p H 5, 6, 7. 0 y of beryllium
(10 y of beryllium present)
Fluoride, Beryllium Determination. After interferences have been removed, use t h e above procedure t o determine t h e a m o u n t of beryllium present. EXPERIMENTAL RESULTS
pH and Buffer. According t o Theis (8) and Kood (14). a neutral or weakly acidic solution is optimum for the formation of the beryllium-Chrome Azurol S complex. The present studies showed that a t p H 5 to 6, the color due to the complex is a t a maximum, while the absorbance of the dye is small. The peak of maximum absorbance varies \\ ith the change in pH, as does the color intensity (Figure 1 ) . With no buffer present, the complex, a t pH 6 to 7 , has its peak in the 54O-mp region, while a t p H 5, the peak is in the 570-mp region. Figure 1 indicates that the optimum p H is 5 to 6 . The preliminary investigation carried out in this p H range showed t h a t a p H of 6 is optimum when the pyridine-hydrochloric acid buffer is usrd. The p H is critical and should be maintained a t 6.0 i 0.1. Others (8. 14) who have worked with the beryllium-Chrome Azurol S system maintained the desired p H x alues (pH 6 to 7 ) nith acetate buffer; contrary to this. Reiinson and Harley ( 7 ) reported that the thorium-Chrome Azurol S lake is bleached by carbonyl and carboxyl groups. They finally chose an amine (0-toluidine) as the buffer for a p H of 4.5. The preliminary investigations in this laboratory s h o w d that the beryllium-Chrome ..izurol S lake is also appreciably bleached by acetate and tartrate buffers. I n additicn to these buffers, others were tried, Khile mannitol (10) has little effect on the color, i t does not effectively buffer the solution. Hydrazine and hyclrazine hydrochloride were
used, but full color does not develop in the presence of this amine. When the pyridine-hydrochloric acid buffer was used, an enhancement of color intensity for the metal-dye complex was observed and good buffering was obtained. Table I shows the effect of the pyridine buffer on 10 y of beryllium in a 50-ml. volume with varying amounts of fluoride present. The p H was maintained at 6 in all tests. These solutions mere measured in 1-cm. cells. Pyridine enhances the intensity of the metal-dye complex and, alternatively, the bleaching effect of the fluoride. The amount of pyridine buffer present must be kept reasonably constant to ensure precise results, as a variance of 1 ml. of buffer per 50 ml. causes a change of 0.04 absorbance unit. Spectra. Figure 2 shows t h e spectrophotometric curves of t h e beryllium-Chrome Azurol S-fluoride s) st e m obtained by t h e procedure given above, except t h a t 1-cm. cells were used for t h e absorbance measurements. T h e dye alone has a low absorbance a t 575 mp, vc hile the beryllium-Chrome L4zurol S complex has a maximum absorbance pcak a t this wave length. The addition of fluoride to the metal-dye lakc bleachrs the color, giving a decrease in absorbance proportional to the fluoride concentration. The standard curves for both beryllium and fluoride are linear and reproducible. By the method of continuous \.ariations (If) Chrome Azurol S was found to combine with beryllium in a mole ratio of 1 to 1. Color Stability. T h e color of t h e metal-dye complex, even mith fluoride present, is relatively stable for a t least 5 hours when developed according t o t h e standard curve procedure. During the first 15 minutes the absorb-
05
30= 50a
y
Change in dbsorbance Absorbance 0 M I . of Buffer 0.312 0.245 0.220
0.06i 0.092
2 111. of Buffer
nb
0.345
30b 50
a t.
0.238 0.180 Measured at 535 mp. Measured at 5 i 5 nifi.
Table II.
NaCl
0.107 0.165
Interferences with Bleaching Effect
F-
Gram Per
Equiv-
50 111. 0.1
Y
TL"4CI 0.1 (N&)zSO4 0.1 IYazCaH4Oe.2H20 0.002 Na2COs 0 02
alent, 5 7 12 30 7
ance readings decrease very slightly when fluoride is present, and then remain constant. The optimum time for reading the samples is not critical, but if the samples are alloned to stand a t least 15 minutes before reading, constant results are aswrcd. INTERFERENCES
The beryllium-Chrome Azurol S complex is subject to several interferences. Some of the substances n-hich bleach the color (causing loiv results in the beryllium anal? sis and high results in the fluoride analysis) are shonn in Table 11, which includes the extent of interferencr in terms of fluoride rquivalent. I n addition to thc substances listed in Table 11, acetatr, T'erscne, and fluoride also bleach the color of the VOL. 31, NO. 1, JANUARY 1 9 5 9
153
Distillation of Fluoride in Presence of 1 M g . of Selected Fluoride Complexing Agents
Table 111.
Metal Present
UOnSO,, Grams
Present
A1
2 0
36 30
18 19 18
- 18
36
37,30 30
+I -6 ,
27 26
34
-9 -4 -2
22 31
-8 -5
0
Be
20
2 0
Fe
30 36 30
2 0 2
Th
Zr
0
36 30
2 0
36 30
Sulfate added (0.5 gram).
Table IV. Recovery of Known Quantities of Beryllium
Be Added,
Be Recovered,
Y
Y
1.0 5.0
0.9,0.8,0.8,0.8,0.8
10.0 ~
0
ANALYTICAL CHEMISTRY
0
-27, -29 -2
Found, Y
per
Sample
Gram
A
0.3, 0 2.0, 1 . 4 1.7, 1 . 4 1.3, 1 . 0 0.5, 0 1 .O, 0 ,8,O. 6
€3
c D E F
~~
154
-2
Table V. Determination of Beryllium in Graphite Ash Samples
5.2,5.0,5.0,5.0,4.9, 4 . 8 , 4.8 10.2, 10.1, 10.1, 10.0, 9.9, 9 . 9
complex. The concentration of these substances must be kept low. Theis (8) reported that, under the conditions he used, ferric iron, copper, aluminum, zirconium, and lead cause interference b y forming colored complexes with the dye, and t h a t the alkaline earth metals do not form violet compounds with the dye. He alleviated the iron effect with tartrate, the zirconium effect by the addition of sodium fluoride, and the copper interference by Complexon; however, the studies reported here clearly shorn that each of these additions (tartrate, fluoride, and Complexon) bleaches the color of the berylliumChrome pzurol S lake when it is developed according to the given procedure. Wood (14) carried out semiquantitative tests on possible interfering ions and found that nickel(II), copper(I1) , bismu th(II), tin(II), iron(I1) , titanium(111), and antimony(II1) interfere. Under the conditions described herein, uranium(V1) and iron(III), even when present in microgram quantities, form a blue color with the dye, thus causing interference. Investigation showed that the addition of 3 ml. of 2% hydroxylamine hydrochloride solution does not affect the beryllium-Chrome Azurol S color, but does alleviate the interference caused b y 1 mg. of iron or uranium. I n general, however, to attain maximum sensitivity and to ensure accurate results using the Chrome Azurol S method, all interfering ions must be removed. Distillation of Fluoride from Various Metals. I n analyzing for fluoride, interferences are usually removed
Dev. -11
Y
30
Qa
a
Fluoride, Micrograms Recovered
Table VI. Determination of Fluoride in Uranyl Sulfate Samples
(Distillation method) Sample Fluoride, Wt., Y per Sample Grams Gram A B
C D
1.0 1.0
2.0 2.0 1.0
1.5
E
F G
1.0 1.5 1.0 1.5 1.0 1.5
0, 0 12 12
3, 3 1 3 J 0 4 9, 8
2J
5 6 8 9 6
by t h e distillation of t h e fluoride as fluosilicic acid from a perchloric acid solution (7) or by the pyrohydrolysis method (12). Studies were made to determine if microgram amounts of fluoride could be quantitatively distilled from 1 mg. each of aluminum, beryllium, zirconium, thorium, and iron. The distillations were made from perchloric acid solutions and the temperature was maintained between 135' and 140' C. (7). Two sets of distillations were madeone containing only 1 mg. of the metal and the other containing, in addition to the metal, 2 grams of a uranyl sulfate which had been previously analyzed for
fluoride. The fluoride was quantitatively recovered in the presence of beryllium, thorium, and iron. The addition of 0.5 gram of sulfate ion aided in obtaining complete recovery of the fluoride when 1 mg. of zirconium was present. Recovery was low when the distillation was made from aluminum. Table 111 shows these data. Separation of Beryllium by Ion Exchange. Ion exchange was investigated as a means of removing large amounts of metal ions, uranium in particular, from microgram quantities of beryllium. T h e anionic resin IRA400 in t h e sulfate form removes t h e negatively charged uranyl sulfate complex (S), and the beryllium passes through the resin (at p H 1.5). One to 2 grams of uranyl sulfate can be separated from microgram amounts of beryllium by this technique; however, some interference picked up from the resin itself results in an increase in absorbance. (This could be a small amount of a n amine from resin breakdown, but no work was done to determine the exact cause.) For this reason, the eluate containing the beryllium xas first fumed to near dryness with 0.5 ml. of perchloric acid in order to destroy any organic matter, and the beryllium color was then developed in the usual manner. However, the beryllium recovery was consistently low when the ion exchangeperchloric acid fuming method was used. Low recovery of the beryllium was also obtained rrith evaporation alone, and a still slightly lower result was obtained when the ion exchange step was added. Therefore, when this procedure is to be followed, a standard curve must be prepared using these same conditions. I n order to investigate the lon beryllium recovery obtained, evaporations were niade in glass, quartz, and platinum, The losses were similar for each type of vessel. Other workers have encountered this type of loss, and it is believed that the losses are due, not to volatilization of the beryllium, but to the partial conversion of the beryllium t o a relatively insoluble form (9). Because the results obtained by ion exchange -evaporation technique are low but reproducible, the method can be applied if the standard curve is prepared in the same way. DISCUSSION
Data tabulated in Table IV show the deviation of a group of synthetic samples which were analyzed for berylliuni by the Chrome Azurol S method. The method is accurate to *0.2 y of beryllium per 50-ml. volume. Beryllium determinations on the residues from ashed graphite samples are given in Table V. These samples were being examined for boron, strontium, calcium, magesium, iron, and other elements, as well as for beryllium.
The limits of the method as outlined in the bcryllium procedure are from 0.2 to 10 y per 50-ml. volume; however, the procedure can be changed by reducing or increasing the volume and reagents to include lower or higher ranges. Table VI shows the data obtained from the fluoride analysis of several uranyl sulfate samples which had been produced commercially from the hexafluoride salt. The samples were first distilled from perchloric acid and the fluoride in the distillate was then determined by the beryllium-Chrome Azurol S method. T h e duplicates are in satisfactory agreement, and the fluoride content of the commercial uranyl sulfate was shown to be sufficiently low. Consideration should also be given to the impurities listed in Table 111. The beryllium-Chrome Azurol S
method for fluoride has the advantage of being more sensitive than other methods tried in this laboratory. It is easily reproducible to =kl y of fluoride per 50ml. volume. The sensitivity would be increased if smaller volumes were used. The interfering ions mentioned for beryllium and fluoride must be removed or accounted for in the standard curve preparation. LITERATURE CITED
(1) Babko, A. K., Khodulina, P. V., J . Anal. Chem. U.S.S.R. 7, 317 (1952). (2) Bumsted, H. E., Wells, J. C., AXAL. CHEM.24, 1595 (1952). (3) Fisher, S., Kunin, R., Zbid., 29, 400 (1957). (4) Horton, A. D., Thomason, P. F., Miller, F. J., Zbid., 24, 548 (1952). (5) hlacxulty, B. J., Roollard, L. D., Anal. Chim. Acta 14, 452 (1956).
(6) Price, M. J., Walker, 0. J., AXAL. CHEM.24, 1593 (1952). (7) Revinson, D., Harley, J. H., Zbid., 25,
794 (1953). (8) Theis, M., 2. anal. Chem. 144, 192 (195.5). \ - - - - I
(9) Toribara, T. Y., Sherman, R. E., ANAL.CHEM.25, 1594 (1953). (10) Underwood, A. L., Keuman, W. F., Ibid., 21, 1348 (1949). (11) Vosburgh, W.C., Cooper, G. R., J. Am. Chem. Soc. 63, 437 (1941). (12) Warf, J. C., AKAL. CHEM.26, 342 (1954). (13) Willard, H. H., Horton, C. A., Zbid., 22, 1190 (1950). (14) U'ood, J. H., Mikrochim. Acta 1955, * < 11.
RECEIVEDfor review September 30, 1957. Accepted iZugust 26, 1958. Division of ilnalytical Chemistry, 132nd meeting, ACS, New York, N.Y., September 1957. Based on studies conducted for the U.S. Atomic Energy Commission under Contract AT-11-1-GEN-8.
Ce rimetric Tit rati o n of Iron Using a Mixed indicator SIR: Under the working conditions proposed b y Heumann and Belovic ( I ) for the cerimetric titration of iron, the calculated total indicator error amounts to approximately O.lS%, 0.15% being due to the warning indicator diphenylamine sulfonate. To reduce the error this indicator was replaced by the sodium salt of diphenylbenzidine decasulfonic acid, prepared according to Sarver and Fischer (3) and used in 0.001M solution (0.136 gram per 100 mi. of water). From 0.30 to 0.35 ml. of this solution per 100 ml. of titrated solution produced the same color change as 0.1 ml. of 0.01M solution of diphenylaminesulfonic acid sodium salt. Figure 1 shows the zones of color change of both warning indicators. The warning signal of the diphenylbenzidine decasulfonate extends over roughly 1% of titrant solution prior to the end point, which corresponds to 8 to 10 drops in a titration requiring about 40 ml. of standard cerate solution. T o compare the color and its intensity for both indicators, solutions in 1N sulfuric acid were prepared with the same concentrations of indicator as in the titrations. The indicators were oxidized b y adding O.Ol11i cerate solution in slight excess. By using a Beclrman DK-1 recording spectrophotometer, the absorption spectra of the colored solutions were taken within 1 minute after oxidation. Diphenylamine sulfonate showed its maximum absorption at 550mp; that of diphenylbenzidine decasulfonate appeared at 540 mp. T h e corresponding slight difference of color is difficult t o detect b y the eye. The
absorption produced by 0.1 nil. of 0.01M diphenylamine sulfonate in 100 ml. of 1 N sulfuric acid was equal to that obtained with 0.36 ml. of O.O001;1f diphenylbenzidine decasulfonate. This
I
98
I
I
99 100 Percentage of reoction
I 1
101
Figure 1. Zones of color change of warning indicators shown on cerium(lV)iron(II) titration curve
(Potential readings with reference to calomel electrode) % of Reaction at Start of PermaIndicator nent Color Change a. Diphenylaminesulfonic acid 99.1 b. Diphenylbenzidinesulfonic acid 99.5
compares f a i r k ne11 n-ith the 0.1 to 0.30 to 0.35 found visually in the titration of iron. Theoretically 0.5 ml. of 0.001M diphenylbenzidine decasulfonate should be equivalent to 0.1 nil. of 0.01M diphenylamine sulfonate, but the latter is probably oxidized to a lesser degree to diphenylbenzidine violet or the further oxidation of the violet to colorless products proceeds faster in the case of the diphenylamine sulfonate than v-ith t!ie other indicator. The titration errnrs for both indicators were determined hy adding to 300 ml. of I N sulfuric acid the same amount of indicator as in the titration of iron and titrating the solution with 0.01N standard cerate solution until the violet color was fully developed. Table I shows the indicator errors for the titration of iron, calculated on the basis of these determinations. The error due to the diphenplbenzidine dccasulfonate is only one fourth of that cnnscd by the diphenylamine sulfonate instead of one half, because a relatively larger amount of diphenylamine sulfonate is required for obtaining the same color intensity and a larger than the theoretical amount of oxidizing agent is required for its oxidation. The total indicator error, including that of the ferroin, is thus with diphenylbenzidine decasulfons te roughly one third of that with the other indicator. On the other hand, the latter provides a more extended warning zone. ACKNOWLEDGMENT
The authors wish t o thank G. Frederick Smith, TTniversity of Illinois, who suggested the use of the diphenylVOL. 31, NO. 1, JANUARY 1959
* 155
