Voltammetrilc Method for Determination of Zirconium H. E. Zittel and T. A I . Florence’ Analytical Chemistry Dicision, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
ALIZARIN Red S (sodi um alizarin-3-sulfonate) and similar hydroxyanthraquinone dyes have been used for the spectrophotometric determination of zirconium and several other metals ( I ) . Zirconium is one of the few metals that forms a colored complex with Alizarin Red S in highly acid solutions (2). Alizarin Red S shows both anodic and cathodic reactions at the pyrolytic graphite electrode (3). A solution containing a mixture of free Alizarin Red S and its zirconium complex exhibits two discrete oxidation waves, and the zirconium concentration can be determined by measuring either the height of the zirconium-Alizarin Red S wave, or the decrease in height of the free dye wave. Zirconium may be determined at concentrations as low as 0.1 ppm with simple polarographic apparatus, and the precision and accuracy of the method compare favorably with other methods available for the determination of traces of zirconium. EXPERIMENTAL
Apparatus and Reagents. The controlled potential d.c. polarograph-voltammeter, the rotating graphite electrode, the purification of Alizarin Red S, and the preparation of the zirconium stock solution have all been described previously (3). Potentials quoted refer to the saturated calomel electrode. Procedure. Fume an aliquot of the sample solution containing 3-20 kg of zirconium almost to dryness with HCIOa. Add sufficient HCIOa to make the final solution 0.3M in acid, and transfer to a 25-ml volumetric flask. Add 2.00 ml of a 1.5 x lO-4M aqueous solution of Alizarin Red S (ARS). If the amounl: of zirconium in the sample aliquot is outside the range given, use a proportionately adjusted concentration of ARS. Clilute to volume and heat in a water bath at 50-60” C for 15 min. Cool, transfer to the voltammetric cell and record the current-voltage curve between +0.4 and +1.3 volts. Both the decrease in the height of the = 4-0.7volt) and the height of the Zr-ARS free dye wave complex wave (Eli2 = +1.1 volts) are proportional to the zirconium concentration. The results should be compared to a calibration curve otitained by carrying standard zirconium solutions through the s3me procedure. After each potential scan, clean the electrode of any absorbed material by allowing it to rotate briefly in warm 1M NaOH followed by 5M HC104.
Attached to Oak Ridge National Laboratory from the Australian Atomic Energy Commission, N. S. W., Australia. 1
(1) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 2nd ed., Interscience, New York, 1950. (2) D. E. Green, ANAL.C’HEM.,20, 370 (1948). (3) H. E. Zittel and T. M Florence, Zbid.,39, 320 (1967).
Table I. Effect of Acid Concentration Height of Zr-dye HC10, Height of free complex Total wave concn., Ma dye wave, pa wave, pa height, pab 0.1 0.3 0.5 0.7 0.9
0.83 0.87 0.90 0.92 0.94
1.20 2.03 3.17 2.04 1.15 2.05 1.12 2.04 1.09 2.03 a Ionic strength adjusted to 1.OM with NaC104. Zr = 4.75 X lO-jM, Alizarin Red S = 9.68 X lO-jM, scan speed = 0.1 volt min-I.
RESULTS AND DISCUSSION
The behavior of a number of anthraquinones at the dropping mercury electrode has been investigated by Furman and Stone ( 4 ) . Studies carried out in this laboratory have shown that at the RPGE, Alizarin Red S displays a 2-electron reversible reduction wave, and also an irreversible 2-electron oxidation wave (3). The Zr-ARS complex formed in strongly acid medium exhibits an irreversible but well-formed oxidation wave, and a rather erratic, irreversible reduction wave. The determination of small amounts of zirconium is made possible by the wide (0.35 volt) separation of the free dye and dye-complex oxidation waves (3). The reaction between zirconium and Alizarin Red S proceeds via a hydrolyzed zirconium species (3) probably Zr (OH)z+2, so it is important to observe the correct order of addition of reagents to avoid uncontrolled hydrolysis of the Zr(1V) ion. Zirconium must not be transferred from a less acid solution to one more acid, since the reversal of hydrolysis is slow. The sample aliquot should be taken to fumes of perchloric acid before analysis to ensure destruction of zirconium hydrolysis products. The relationships between zirconium concentration and the heights of the free dye and dye-complex waves are essentially linear except at the lower concentration ends of each curve, and either wave may be used as a measure of the zirconium present. Although the Zr-ARS complex is very stable (3), the acid concentration does affect the degree of dissociation, and therefore also affects the limiting currents of both the free dye and dye-complex waves. Table I shows the effect of acidity on the limiting currents, and it is evident thet a small change in acid concentration would cause very little error in the zir-
(4) H. H. Furman and K. G. Stone, J. Am. Chem. Soc., 70, 3055 (1949). VOL. 39, NO. 3, MARCH 1967
0
355
.
Alizarin Red S concn., M
x 105~ 0.41 1.03
4.12 8.24 20,60
a
Table 11. Precision Study Zirconium concn., Relative standard deviationb M X Free dye wave Complex wave 0.24
0.60 2.38
5.8 4.5
3.6
5.95 3.2 11.90 2.9 0.3M HCI04,scan speed 0.1 volt rnin-l.
10.7 5.8 3.3
3.0 2.5
Seven determinations.
conium determination. It should be noted that the acid effect becomes more important at low concentrations of free dye. The precision of the method for various levels of zirconium and Alizarin Red S concentrations is shown in Table 11. The large relative standard deviation noted in the results at the lowest zirconium concentration is mainly due to the fact that, at the level of current sensitivity required, the complex wave begins to merge with the following water decomposition wave. This makes exact measurements difficult. Therefore, at very low zirconium concentrations it is advantageous to use derivative voltammetric techniques.
The following metals were tested at the 200-ppm level and found not to interfere in the determination of 5 ppm of zirconium : Ag(I), Al(III), Be(II), Bi(III), Ca(II), Cd(II), Co(II), Cu(II), Hg(II), Li(I), Mg(II), Mn(II), Mo(VI), Ni(II), Pb(II), Pr(III), Sb(V), Sn(IV), Th(IV), Tl(I), U(VI), W(VI), and Zn(I1). Ce(1V) and Cr(V1) interfere, but reduction with hydroxylamine perchlorate prior to addition of the Alizarin Red S reagent eliminates their interference. Iron (111) reduces the height of the free dye wave, but does not interfere with the Zr-ARS complex wave. Vanadium(V) at the 10-ppm level shows no interference, but at the 50-ppm level causes the zirconium values to be low by about 10%. Hafnium reacts in an identical manner to zirconium. At the 500-ppm level the following anions had no effect on the zirconium determination: C1-, CH3COO-, NO3-, and Sodp2. Both fluoride and phosphate cause serious negative errors, but since fluoride would normally be removed by the perchloric acid fuming step in the procedure, phosphate is the only serious interference. Oxygen does not affect the anodic electrode reactions, so deaeration is unnecessary. RECEIVED for review September 14, 1966. Accepted December 23, 1966. Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp.
X-Ray Spectrographic Determination of Rare Earths in SiIica-AIumina CataIysts Irving C. Stone, Jr., and Kenneth A. Rayburn W. R . Grace and Co., Washington Research Center, Clarksuille, Md. 21029 THE DETERMINATION of rare earths by classical chemical analysis requires from six to 24 hours to complete. The most common method ( I ) involves an oxalate precipitation and calcination to the oxide to determine the total rare earth content. The accuracy of the method depends on the aging time of the oxalate precipitate and is sensitive to small pH changes. X-ray spectrometry has been used to determine yttrium in rare earth solutions (2), yttrium, thorium, and the rare earths in ore fractions (3),and trace rare earths in high purity rare earths (4). We have developed an x-ray spectrographic method which requires less than 2 hours for a single determination of the five most abundant rare earths present in silica-alumina catalysts. The data yield individual rare earth contents which may be summed for a total rare earth value.
(1) R. C. Vickery, "Analytical Chemistry of the Rare Earths," p. 60, Pergamon Press, New York, 1961. (2) R. H. Heidel and V. A. Fassel, ANAL.CHEM., 30, 176 (1958). ( 3 ) F. W. Lytle, J. I. Botsford, and H. Heller, Bur. Mines Report Incest., 5378 (1957). (4) F. W. Lytle and H. H. Heady, ANAL. CHEM., 31,809 (1959).
356
ANALYTICAL CHEMISTRY
EXPERIMENTAL
Standards. Rare earth stock solutions are prepared from the calcined oxides at concentrations of 0.03 gram/ml for CeOz, Laz03, and Nd203, and 0.01 gram/ml for PrGOliand Sm203. One hundred milliliters of 10% HCl are added to 24.00 grams of appropriate silica-alumina base material in a 400-ml beaker. Varying amounts of the stock solutions are then added followed by 100 ml of hot, saturated oxalic acid solution. The solution is evaporated, without boiling, to dryness. Constant stirring is necessary to obtain thorough mixing. The dry powder is then heated for 1 hr at 200" C, slowly raised to 1000" C, and held for 1 hr. Recovery should be 98 % or better. Samples. Samples are ground to pass 325 mesh and dried for 10 min at 200" C. Pellet Preparation. Pellets are prepared of standards and samples by mixing Elvanol (Du Pont) 71-17, a polyvinyl alcohol (PVA), at a samplelbinder ratio of 2/1. The PVA was obtained from E. I. Du Pont, Niagara Falls, N. Y . Samples are mixed in a Spex mixer mill before being pelletized at 40,000 psi in a 1'/,-inch diameter mold. About 0.5 gram of PVA is added to the bottom of the die before addition of the sample for increased pellet strength. Four 6-gram pellets of each standard and two pellets of each sample were prepared. Instrument Conditions. A General Electric XRD-6 vacuum spectrometer equipped with gas flow proportional counter, W-Cr dual target x-ray tube, LiF analyzing crystal, 0.030inch beam slit, and 0.005-inch receiving slit was used. The
