REFERENCES
Table V.
Recovery of Individual Rare Earths in Mixtures, Using
Mixture No. 1
Recovered, Element Gd Dv
Taken, 0.200 0.400
0,500 1.00 0.0200 2.00
Ce Iid
Y
Zn 2
0.0200
Not measured
(as suppressant) 0.080
DY Sm Ce Sd Y La
0.240 1 50 2 00 0 I0200
0 0 0 1 2 0
0.080
0.80 (as suppressant)
The experiments whose results are described in Table V were performed primarily to check the effectiveness of zinc in eliminating the above-described interelement effect. These solutions were
100
(1) -4m. SOC.Testing Materials, Phil-
adelphia, Pa., Photographic Photometry (SM 2-2) in “hfethods for Emission Spectrochemical Analysis,” 1953; Tentative Method E 116-56T, 1957. (2) Blackmore, R. H., Bearse, A. E., Calkins, G. D., Battelle Memorial Institute, Columbus, Ohio, Rept. BMI-261. (3) Bronaugh, H. J., Suttle, J. F., U. S.
075
083
94 103
245 50
08
102 100
104
0190
95
S o t measured
30 0
RECOVERYOF IXDI~IDUAL RARE EARTHSIN RARE EBRTH RIIXTURES.
105
Xot measured
(as buffer)
deviation was calculated for each intensity ratio. These relative standard deviations ranged from 0.0 t o 2.8%; their mean was 1.3%.
Recovery, % 96 95 100 99
Kot measured
30.0 (as buffer)
Gd
Zn
0.192 0.190 0.400 0.495 1.05
0.200
Sr;l
La
Y
30-7Zinc Buffer
not put through the chemical procedure, but were mounted directly on electrodes. Table V s h o w that the inclusion of 30 y of zinc in standard and sample residues overcomes the serious matrix effects exhibited by cerium and lanthanum in Table 11. Two years of application of this procedure have shown that when the latter elements are absent, spectrographic recoveries range from 98 to 102%.
Atomic Energy Commission Rept. LA-1561 (1953). (4) Feldman, C., Ellenburg, J. Y.. ANAL.CHEY.27, 1714 (1955). (5) Gordon, L., Firsching, F. H., Shaver, K. H., I b i d . , 28, 1476 (1956). (6) Hettel, H. J., master’s thesis, Ioiva State College, Ames, Iowa, 1956. (7) Hughes, D. J., Harvey, J. A., Brookhaven National Laboratory Rept. BNL-325 (1955). (8) Hyde, E. K., Tolmach, J., U. S. -%tomicEnergy Commission Rept. ANL-4248.
(9) Kember, N. F., United Kingdom Department of Atomic Energy (Ministry of Supply) Rept. CRLAE-23; reissued by Technical Information Division, U. S. Atomic Energy Commission, Oak Ridge, Tenn. (10) Lerner, M. TI’., Petretic, G. J., ANAL.CHEM.28, 227 (1956). RECEIVEDfor review April 7.0, 1957. A4cceptedNovember 21, 1957.
Microdetermination of Zirconium in Sulfuric Acid Solutions with Pyrocatechol Violet J. P. YOUNG, J. R. FRENCH, and J. C. WHITE Oak Ridge National laboratory, Oak Ridge, Tenn. ,The blue color of the zirconiumpyrocatechol violet complex has been utilized in the development of a very sensitive method for the colorimetric determination of zirconium in sulfuric acid solutions. The molar absorbance index of this complex is 32,600 a t a wave length of 650 mp. Its absorbance conforms to Beer’s law up to a concentration of zirconium of 2 y per ml. Trace amounts of zirconium can b e determined in the presence of thorium, uranium, lanthanum, cerium, iron, nickel, and chromium. Fluoride, citrate, oxalate, and tartrate ions interfere by decolorizing the complex. The coefficient of variation for the determination of zirconium by this method is 2%.
A
s
THE importance of zirconium grows in the fields of chemistry and metallurgy, a method is needed for
422
ANALYTICAL CHEMISTRY
the colorimetric determination of trace amounts of zirconium in solutions that contain sulfuric acid. One of the more common reagents for spectrophotometric determination of zirconium is alizarinsulfonic acid. The molar absorbance index of the zirconiumalizarin red lake is approximately 7000 (6); sulfate ion decreases its absorbance (13). Another reagent for the determination of zirconium, p-dimethylaminoazophenylarsonic acid, is also adversely affected by sulfate (20). The spectrophotometric determination of zirconium with quercetin (7) is very sensitive, but in the presence of sulfuric acid, the color reaction is grossly hindered; furthermore, thorium and iron cations interfere seriously. Horton (9) developed a method for the determination of zirconium with 2-(2-hydroxy-3,6disulfo-1-naphthylazo) benzenearsonic acid (Thoron). The molar absorbance
index in this system is approximately 3200. Sulfate ion causes moderate interference. A fairly sensitive determination of zirconium (1) makes use of 2-(p-sulfophenylazo-)-l,8-dihydroxy-3,5-naphthalenedisulfonic acid (SPADNS); thorium likewise forms a complex with SPADNS. No mention is made of the effect of sulfate ion; however, it is recommended that the sample be present in a hydrochloric acid solution. Chloranilic acid has been studied rather extensively for the determination of zirconium by Thamer and Voigt (19), Frost-Jones and Yardley (5), Menis ( 1 4 , and Bricker and Waterbury (2). Pyrocatechol violet, 3,3’4‘-trihydroxyfuchsone-2”-sulfonic acid, forms colored complexes with many cations. Suk and Malat (16) have reviewed the properties of this dye and described some of its applications in
colorimetry and chelatometric titrations with ethylenedianiinetetraacetic acid [(ethylenedinitrilo)tetraacetic acid, EDTA]. Pyrocatechol violet has been applied to the colorimetric determination of b i m u t h ( 1 6 ) , thorium ( l 7 ) , ant1 coppci (18). hlacek and hIoravek f l 2 ) ha\(, described the use of pyrocatechol inkt as a very sensitive reagent in the ehroiiiatographic detection of v:trious metals. Flsschka and Ssdek (4) have reported a highly specific spot test for the detection of zirconium, based on the formation of the blue color of zirconium and pyrocatechol violet in a weakly acidic solution. EDTA is added to remove the interference of many cations that would normally react with the dye. Subsequent to the completion of the experimental work described herein, Flaschka and Farah (3) described a method for the spectrophotometric determination of zirconium with pyrocatechol violet, but it is less sensitive than that described. APPARATUS AND REAGENTS
811 absorbance measurements were made with a Beckman Model DU spectrophotometer with 1-em. Corex cells. A Beckman Model H-2 pH meter was used for all determinations of PH. Pyrocatechol Violet, 0.05% weight per volume. Transfer 250 mg. of pyrocatechol violet (available from J. T. Baker Chemical Co.) into a 500-ml. volumetric flask. Fill the flask to the mark with water and mix well. This solution is stable for a t least 1 month. Standard Zirconium Stock Solution in 4M Sulfuric Acid (1.OO mg. of zirconium per ml.). Weigh 0.100 gram of crystal bar zirconium and transfer it to a platinum dish. Add 10 ml. of concentrated sulfuric acid and then 0.5 ml. of concentrated hydrofluoric acid. Heat this mixture gradually until fumes of sulfur trioxide are copiously evolved. Wash down the sides of the dish with water, add 10 ml. of concentrated sulfuric acid, and continue heating until white fumes have been evolved for approximately 10 minutes. Transfer the contents of the dish to a 250-ml. beaker; then cautiously add approximately 30 ml. of water. Rinse the platinum dish several times with water and add washings to the beaker. Warm the solution in the beaker until the salts are completely dissolved. Transfer the resultant solution to a 100-ml. volumetric flask and dilute to volume with water. Hahn and Joseph (8) have found that standard solutions of zirconium in the microgram range can be prepared satisfactorily from zirconium oxychloride octahydrate. Zirconium, Standard Solution in 0.5M Sulfuric Acid (10 y of zirconium per ml.). Transfer, by pipet, 5.0 ml. of the standard zirconium stock solution (1.00 mg. of zirconium per ml.), into a 500-ml. volumetric flask. Cautiously
add 35 ml. of water and dilute the solution to the mark with 0.5M sulfuric acid. METHOD
Transfer a suitable test portion (up to 15 ml.) of the sample into a 50-ml. beaker. Adjust the concentration of sulfuric acid in the test portion, so that about 5 mmoles of sulfate ion are present. Add to the beaker 1 ml. of thioglycolic acid, 4% volume per volume; 2 ml. of gelatin, 0.67, weight per volume; and 1 ml. of pyrocatechol violet, 0.057, weight per volume. Add 5 ml. of sodium acetate, 30% weight per volume and adjust the pH of the solution to 5.1 with 7M ammonium hydroxide. Transfer the solution to a 25-ml. volumetric flask and dilute t o the mark with water. I n the same manner prepare a reference solution which contains all reagents. Measure the absorbance of the test solution against the reference solution a t a wave length of 650 mp. EXPERIMENTAL
The partial absorbance spectrum of the zirconium-pyrocatechol violet complex is shown in Figure 1. Maximum absorbance occurs a t 650 mp. The absorbance of the reference solution is essentially zero abox-e 600 mp. As shown in Figure 2, the absorbance of the zirconium-pyrocatechol violet complex conforms to Beer’s law up to a concentration of 2 y of zirconium per ml. The molar absorbance index of this complex is 32,600 a t a wave length of 650 mp. Hafnium forms a complex with pyrocatechol violet of equivalent molar absorbancy index. Order of Addition of Reagents. T o obtain maximum color development, pyrocatechol violet must be added to an acidic solution of zirconium which also contains gelatin. The acidity must be adjusted t o less than p H 3 prior to adding the reagent; otherwise the final absorbance is decreased nearly 5001,. Addition of gelatin to a solution which already contains the zirconium-pyrocatechol violet complex a t pH 5.1 will result in an absorbance approximately 207, lower than expected. For these reasons, the order of addition of the reagents as described in the procedure should not be changed. Effect of Sulfate. The effect of sulfate on the absorbance of the zirconium-pyrocatechol violet complex was determined by measuring the absorbance of a given concentration of the complex in solutions that contained from 2.5 to 10 mmoles of sulfate (Table I). The reference solution contained 5 mmoles of sulfate. The absorbance of the zirconiumpyrocatechol violet complex is constant in the presence of 5 mmoles of sulfate ion or less. I n contrast to the customary bleaching effect for most zirconium
a m
0.300 550
600 650 WAVE LENGTH, rnp
700
Figure 1. Absorption spectrum of zirconium-pyrocatechol violet complex Zr concentration, 2 y/mI. Beckman spectrophotometer, Model DU 1 -cm. cells
Table I. Effect of Sulfate Ion on Determination of Zirconium with Pyrocatechol Violet
Sulfate, Zirconium, y/Ml. Mmoles -4bsorbance Present Found 1.16 1.15 2.5 0.414 1.16 0.417 1.16 5.0 1.16 1.25 7.5 0.451 10.0 0.453 1.16 1.26
reagents that is exhibited by high sulfate ion concentrations, the absorbance of the pyrocatechol-violet complex is somewhat enhanced in the presence of high concentrations of sulfate. A reference solution that contained 10 mmoles of sulfate ion had a slight absorbance relative to a reference solution that contained only 5 mmoles of sulfate ion. For these reasons it is recommended that 2.5 t o 5 mmoles of sulfate ion be present in the final test solution. If samples of relatively high concentrations of sulfate ion are encountered. a calibration curve should be established in the desired range of sulfate ion concentration to obtain the most accurate results. Effect of Sodium Acetate. Variations in the concentration af sodium acetate, in the final test solution, from 1.2 to 2.0 grams per 25 ml. do not affect the absorbance of the zirconium -pyrocatechol violet complex. Over the concentration range of 0.8 t o 1.2 grams of sodium acetate per 25 ml., the absorbance of the coniplex increases n-ith increasing concentrations of sodium acetate. Below a concentration of 0.8 gram of sodium acetate per 25 ml., the absorbance decreases with increasing concentration of sodium acetate. If no sodium acetate is present in the final test solution, the adjustment of the pH is very difficult and many metals may be hydrolyzed a t pH 5.1. Effect of pH. Variations in the hydrogen ion concentration of the VOL. 30, NO. 3, MARCH 1958
e
423
w’i /---
0.800 w
io,, u
0.600
/-
0 200-
/
M ~ O Absorbancy I Index 32, 6 0 0 ? 2 % 4.0
45
5.5
50
60
PH
Figure 3. Effect of p H on absorbance of zirconiumpyrocatechol violet complex A. 6.
Figure 2. Typical calibration curve for determination of zirconium with pyrocatechol violet
Zirconium-pyrocatechol violet complex Pyrocatechol violet
Beckmon spectrophotometer, Model DU 1 -cm. cell X = 650 m p
final test solution from 4.9 to 5.7 do not affect the absorbance of the zirconium-pyrocatechol violet complex. If the final p H is less than 4.9, the absorbance of the complex decreases with decreasing pH. The absorbance of pyrocatechol violet increases markedly as the p H is increased beyond 5.7 (Figure 3). Stability of Zirconium-Pyrocatechol Violet Complex. The color of the zirconium-pyrocatechol violet
Table II. Effect of Cations on Determination of Zirconium with Pyrocatechol Violet Cation Zirconium, -y/?vII.
-y/ml. Present Thf4
10 20 30 40 64 8 20 40 10 20 30 40 4
Ce +3 La +3 UOn +z
u
+4
t
Cr +e Cr +a Mo+B
20 10 20
1.05
1.05
Fe +3
Fe +2 +4
Ti +4 A1 +a
1.05
8 20 8 20
1.05
10
1.05
20
424
0.81 1.05
8
p\’i+2
v
1.05
2 4 20 2 4 8 16 2 8
1.05
1.05 1.22 1.05 1.05 1.05 1.00
ANALYTICAL CHEMISTRY
Found 1.09 1.14 1.19 1.22 1.09 1.04 1.06 1.08 1.07 1.04 1.08 1.06 1.11 1.18 1.31 1.05 1.07 1.05 1.04 1.03 1.02 1.13 1.16 1.07 1.19 1.15 1.05 1.21 1.30 1.56 >2 >2
complex prepared according to the procedure is stable for 2 hours. If the complex is formed in the absence of gelatin, blue crystals of zirconiumpyrocatechol violet appear within an hour. I n the presence of a small amount of gelatin, however, no separation of blue crystals was observed and no change in absorbance of the test solutions was noted in 24 hours. In the presence of gelatin and thioglycolic acid the stability of the absorbance of the zirconium-pyrocatechol violet is reduced, no separation of blue crystals occurs, but the color of the test solution changes in hue over a period of time. Stoichiometry of Zirconium-Pyrocatechol Violet Complex. The method
of continuous variation ( I O ) was applied t o the zirconium-pyrocatechol violet complex in order to determine its composition (Figure 4). The complex contains 2 moles of pyrocatechol violet to 1mole of zirconium. Effect of Diverse Ions. To study the effect of various cations on the determination of zirconium in sulfate solutions with pyrocatechol violet, a solution that contained both zirconium and the cation t o be studied was treated according to the pro-
Table Ill.
Summary of Effect of Diverse Ions
(Concn. of Zr, 1.0 per ml.) Wt. Ratio. Cation Error, Cation Zr % Th +4
Ce + 3
La + 3
uo2 +z u +4
Ni Cr +e Cr +3 Mo Fe+3 Fe +2
v
+4
Ti +4, A l + 3
10 15 30 30 3 15 15 15 6 6
6 1 1
5 5 5 0 5 0 0 0 5 5 5 5 10
MOLE
PVromtechol V Zirmntum
M
Figure 4. Molar combining ratio of zirconium and pyrocatechol violet
cedure. The results are given in Table I1 and summarized in Table 111. Based on a zirconium concentration of 1 y per ml., Table I11 presents the error that can be expected for given weight ratios of various cations to zirconium, This method was not affected by the presence of chloride, perchlorate, nitrate, or borate ions, up to concentration of 5 meq. of each ion in the final test solution. Higher concentrations were not studied. Anions such as citrate, oxalate, tartrate, and fluoride interfere seriously by destroying the color of zirconiumpyrocatechol violet complex. A determination of fluoride has been described ( I I ) , in which the decrease in absorbance of the zirconium-pyrocatechol violet complex in hydrochloric acid at pH 1 is a measure of the fluoride ion concentration. In the presence of EDTA the spectrum of the complex is altered and the resultant maximum absorbance is decreased by a factor of approximately 2. Aluminum, titanium, and quadrivalent vanadium interfere seriously. -41~minum forms a colored complex with pyrocatechol violet. The interference of titanium and quadrivalent vanadium is not unexpected, as quadrivalent ions usually react similarly with chromogenic reagents. Up to a ratio of 4, however,
moderate amounts of quadrivalent uranium can be tolerated. The tolerance of the method for thorium is large. A positive error of only 5% was observed when the thoriumzirconium ratio was as large as 10. Even higher ratios of thorium-zirconium are tolerable if thorium is added to the reference solution, so that the concentration of thorium in the reference and test solutions is approximately the same. The method is thus useful for determining zirconium in the presence of uranium and thorium. Divalent nickel, trivalent chromium, lanthanum, and hexavalent uranium apparently form no complex with p y o catechol violet. Hexavalent molybdenum, di- and trivalent iron, and trivalent aluminum interfere by forming very sensitive colors with the chromogenic reagent. The interference of either ferric or ferrous ions is eliminated, up to an iron-zirconium ratio of 6, by adding thioglycolic acid, which eliminates entirely the blue iron complex; however, the absorbance of the zirconium-pyrocatechol violet complex in the presence of thioglycolic acid decreases with increasing iron concentration. Relatively high concentrations of ferric ion, approximately 30 y per ml. and up, form a blue color with thioglycolic acid. Therefore, the absorbance of a solution which contains a given concentration of zirconium, pyrocatechol violet, and thioglycolic acid will decrease, go through a minimum, and then increase as the concentration of ferric ion is increased. I t is probable that zirconium could be determined in the presence of large amounts of iron if most of the iron were removed a t a mercury cathode prior to application of the pyrocatechol violet method.
Table IV. Determination of Zirconium in Sulfate Solutions with Pyrocatechol Violet
20 19 41 42 26 25 19 50
y
Dl
%
63.0
d 0
0.0
63.9
0.6
0.9
72.9
1.8
2.5
4 4
199
1 0
5 0
3
428
0 9
2 1
1 8
260
0 3
1 2
9 6
502
0 5
1 0
ff
63.0 63.0 64.2 63.6 73.8 72.0
4
Relative std. dev., 2%
sulfate and the melt was dissolved in dilute sulfuric acid. The zirconium in the resultant solution was determined by the procedure described, except that the pH of the test solution was adjusted to 4.5. This modification was necessary because the weight ratio of cerium to zirconium was much greater than 15 (Table 111). At p H 4.5, the interference of the cerium ion is practically eliminated, while the zirconium can be determined with only a slight loss of sensitivity. Some typical results for the determination of zirconium in residues of cerium and lanthanum oxides are shown in Table I-. The relative stand-
Table V. Typical Results for Determination of Zirconium with Pyrocatechol Violet at pH 4.5 in Presence of Cerium and Lanthanum
RESULTS
Precision. Typical results for the determination of zirconium in pure solutions or in the presence of thorium by the pyrocatechol violet method are presented in Table IV. The relative standard deviation is 2%. Determination of Zirconium in Residues of Cerium and Lanthanum Oxides. The method for the determination of zirconium with pyrocatechol violet was applied to cerium and lanthanum oxides. Cerium and lanthanum were precipitated as their oxalates from sample solutions which also contained zirconium. The oxalate precipitate was ignited to the oxides, and the rare-earth elements were determined gravimetrically. It was desired to determine the amount of zirconium contamination in these ignited residues. The cerium and lanthanum oxides were fused with potassium pyro-
Zirconium,
X
Zirconium, P.P.M.
D, 70
842
d 40
4.8
654
12
1.8
702
4
0.6
X
2
862 822 648 660 700 704 362 357 739 744 648 644 424 433 888 912 459 452 717 746
360
5
1.4
ard deviation for these determinations is 2%. COMPARISON WITH METHOD OF FLASCHKA AND FARAH
There are two essential differences between the procedure described in this article and the procedure of Flaschka and Farah (3). In the latter, EDTA is present in the test solution and pyrocatechol violet is added after the pH of the test solution is adjusted to 5.2. The absorption spectrum of the complex formed by either method is unique; thus, it is strongly indicated that a different complex is formed in each case. Nevertheless, eifher procedure could be modified to give essentially the results of the other, depending on the presence or absence of EDTA and the pH of the test solution during the addition of pyrocatechol violet. The proposed method is approximately twice as sensitive as the method of Flaschka and Farah.
LITERATURE CITED
Banerjee, G., Anal. Chim. Acta 16, 62 (1957).
Bricker, C . E., Vaterburv, G. R., ANAL.CHEY.29, 558 (1957). Flaschka. H.. Farah. M. Y . . Z. anal. Chem. i52,’401(1956).
’
Flaschka, H., Sadek, F. Z., Zbid., 150,339 (1956). Frost-Jones, R. E. V., Yardiey, J. T., Ana2yst 77, 468 (1952). Green, D. E., ANAL.CHERT. 20, 370 (1948). Grimaldi, F. S., White, C. E., Zbid., 25,1886 (1953). Hahn, R. B., Joseph, P. T., Zbid., 28,2019 (1956). Horton. A. D.. Ibid.. 25. 1331 (1953). Job, P., Ann. chim. 9, 113 (1928). Krahulec, L., Ceskoslov. hyg., epidemiol., mikrobiol., immunal. 4, 376 (1955). hlacek, K., Moravek, L., Nature 178, 102 (1956). Manning, D. L., White, J. C., AKAL. CHEM.27, 1389 (1955). I
,
Menis, O., Oak Ridge National Laboratory, “Determination of Zirconium by the Chloranilic Acid Method, ORNL-1626 (May 19, 1954).
Suk, V., Malat, M., Chemist-Analyst 45,30 (1956).
Svach, M., 2. anal. Chem. 149, 325
742
5
0.7
646
4
0.6
429
9
2.1
900
24
2.4
456
7
1.5
732
29
3.9
(1956). Ibid., p. 414. Zbid., p. 417.
Thamer, B. J., Voigt, A. F., J . Am. Chem. SOC.73,3197 (1951).
Relative std. dev., 2 %
Welcher, F.,;J., “Organic Analytical Reagents, Vol. 111, p. 64, .Van Nostrand, New York, 1948. RECEIVEDfor review June 8, 1957. Accepted November 13, 1957. Work carried out under contract No. W-7405eng-26 at Oak Ridge National Laboratory, operated b Union Carbide Nuclear Co., division o? Union Carbide and Carbon Corp., for the Atomic Energy Commission. VOL. 30, NO. 3, MARCH 1958
425
