A Highly Sensitive Spectrophotometric Method for the Determination of Rare Earths and Nickel W. J. de Wet and G . B. Behrens Chemistry Dicision, National Nuclear Research Institute, Pelindaba, South Africa
XYLENOL ORANGE in the presence of Cetylpyridinium bromide has been found exceptionally sensitive for the spectrophotometric determination of submicrogram quantities of any of the rare earths. The complexes formed have the general formula RE[(XO) (CPB),],, with log stability constant values of about 14. A procedure is described whereby the individual rare earths can be determined by this method subsequent to separation of a mixture on a cation exchange column. Nickel also forms an intensely colored blue complex under similar conditions which from a sensitivity point of view could be of analytical value. A large number of papers ( I ) describe the determination of rare earths in small quantities either by complexometric titration or colorimetric procedures. Unless very large samples are taken, however, these methods d o not readily afford means of assessing rare earths at the 1-ppm level as are frequently encountered (2-6) in the analysis of certain materials. Although neutron activation analysis offers satisfactory sensitivities (7,8) for most of the rare earths, it is not very attractive for several of these elements because of difficulties attached t o the subsequent counting of the low y-energies of their induced isotopes. A spectrophotometric method of determining minute quantities of lanthanum which is based on a complex formation with a metallochromic indicator of the complexan-type, in particular Xylenol Orange (XO), and a long-chain quaternary salt like cetylpyridinium bromide (CPB), has been reported (9). It is based on the following observations. The absorption spectra of alkaline XO solutions (intense purple-red) show a characteristic absorption maximum at 580 mp and addition of CPB as a cation-active detergent results in a considerable decrease of this maximum while a second maximum appears at 440 m p (color changes drastically to pale grayish pink) (10, 11). This absorption spectrum is almost identical t o the absorption spectrum of XO in acid solution (10) except for a small bathochromic shift of the 580 m p absorption maximum t o approximately 590 mp, The addition of even a very small amount of a lanthanum salt to such a discolored solution results in the formation of an intensely blue colored solution which shows a pronounced absorption maximum at 618 mp. The bathochromic shift of about 30 m p and the relatively marked increase in absorbance enabled a very sensitive spectrophotometric method to be developed for this element (9). (1) P. C. Stevenson and W. E. Nervik, U . S . At. Energy Comm. Booklet NAS-Ns-3020 (1961). ( 2 ) L. H. Ahrens, “Progress in the Science and Technology of the Rare Earths,” I, p. 1, Pergamon Press, Oxford, 1964. (3) ~, W. J. de Wet and E. A. C. Crouch, J . Inorn. Nucl. Chem., 27, 1735 (1965). 141 A. A. Schilt and W. C. Hovle. ANAL.CHEM..39, 127 (1967). {5j L. Kosta an! G. B. Cook, faluntu, 12,977 (1965). (6) M. Vobecky, A. Mastalka, and J. Marecek, Collection Czech. Cliem. Commun., 31, 3309 (1966). (7) H. P. Yule, ANAL.CHEM.,37,129 (1965). (8) E. M. Lobanov, E. S. Cureev, A. G. Duntov, and A. A. Kist, J. Anal. Cliem. U.S.S. R., 21,,258 (1966). (9) V. Svoboda and V. Chormy,,Tuluntu, 13,237 (1966). (10) V. Svoboda and V. Chromy, Tuluntu, 12,431 (1965). (1 1) [bid.,p. 437.
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Z Figure 1. Variation of absorption maxima of the RE-XO-CPB complexes with rare earth atom number A value of 9.2 X IO4 at 625 mp (the wavelength at which the
difference between complex and reagent blank absorption was a maximum) has been given for the molar extinction coefficient (E). With a double beam instrument of 2-cm actual optical path length, this value of E should easily enable determinations of lanthanum in concentrations between -0.04 and 0.4 ppm in working solutions. This is claimed (9) t o be the most sensitive spectrophotometric method known for lanthanum. The principle of this method has been followed up for the other rare earths. Investigations revealed that it is equally satisfactory for all the rare earths (yttrium was not investigated) as well as for nickel. These were the only elements for which the method appeared more sensitive than the most frequently used spectrophotometric methods. Work done on the rare earths and nickel is discussed below. EXPERIMENTAL
Reagents. RAREEARTHSOLUTIONS.Solutions containing approximately 1 mg/ml of the individual rare earths were prepared by dissolving appropriate amounts of the oxides (British Drug Houses) in minimum quantities of concentrated nitric acid which were subsequently evaporated almost t o dryness before dissolving each in 100 ml of deionized water. The solutions were standardized by chelatometric titration with EDTA with X O as indicator (12). Working solutions were prepared by appropriate dilutions of portions of these standardized solutions. NICKELSOLUTION.2.5.10-’ mole/ml Ni+2, prepared by appropriate dilution of a standardized solution of about 10-2M NiSOa. XO SOLUTION. lO-3M aqueous solution in 2 0 2 aqueous methanol. BUFFER SOLUTION. Tris(hydroxymethy1)aminomethane and borate solution. Apparatus. Spectrophotometer. Recording spectrophotometer RPQ2OAV (Zeiss) with 10 mm silica cells was used. The pH meter was a Metrohm E300. (12) E. A. C. Crouch and I. G. Swainbank, Report A.E.R.E. C/R 2843 (1959).
Recommended Procedure. Mix 0.37 ml of the XO solution, 2.5 ml of CPB solution, and 2.5 ml of tris (hydroxymethyl) aminomethane buffer of pH 7.5. A neutral solution with an absolute minimum of mineral salts containing not more than 8 Mg of a rare earth or nickel is added to this mixture and diluted with deionized water to a volume of 25 ml. The solution is allowed to stand for 1 hour and the absorbance measured at the appropriate wavelength (given below for the various rare earths and nickel) against a reagent blank. Compare the absorbance with that on a calibration curve of the corresponding rare earth or nickel (prepared from standard solutions treated in the same way). For small amounts of a rare earth or nickel, use smaller volumes of the reagents but in the same proportions and dilute to a correspondingly smaller final volume (which may be as small as the volume of the silica cell of the spectrophotometer).
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RESULTS
Absorption Spectra, Complex Formation and Stability Constants of the Rare Earth (RE)-XO-CPB Complexes and the Ni-XO-CPB Complex. The absorption spectra of all the REXO-CPB complexes and Ni-XO-CPB complex have been recorded between 400-700 mp against reagent blank solutions. The shapes of the spectra are virtually the same as that found for lanthanum (9), but definite differences have been observed in the position of the absorption maxima for the various rare earths as shown in Figure 1. Nickel forms an almost pure blue colored complex with an absorption maximum at 610 mp. The composition of the complexes has been studied and it was ascertained to be the same for all the rare earths and nickel, viz. RE[(XO)(CPB),], and Ni[(XO)(CPB),],. The method of continuous variations and spectrophotometric titrations of RE(XO), and Ni(X0)2solutions with CPB under various conditions was applied. Stability constants for these complexes have been determined from the curves of the method of continuous variations. The logarithm of this constant obtained for cerium is 14.8, while the published value for lanthanum is 13.6 (9). The remaining rare earths gave values in between these two figures. A value of 12.2 has been derived for the nickel complex at 20". Calibration Curves. Calibration curves were obtained for the various rare earths and nickel by using the standardized solutions described above. Straight lines through the origin were found for all the rare earths and nickel up to concentrations of 0.4 ppm which was the maximum concentration used in all the cases. The variation of the molar absorptivities for the various rare earths is depicted in Figure 2. A molar absorption coefficient of 27,800 was calculated for nickel. Application of Method to Known Amounts of Individual Rare Earths after Being Passed through a Cation Exchange Column. The method has been tested on separate submicrogram amounts of several of the rare earths subsequent to elution through a cation exchange column for which 1M a-hydroxyisobutyric acid solution adjusted to pH 3-5 has been used as eluant. Fractions of about 5 ml were collected which were facilitated by the use of radio isotopes. The organic reagent in these fractions was destroyed by evaporation to dryness after addition of 1 ml of concentrated sulfuric acid to each and, toward the end, hydrogen peroxide. The dry residues were dissolved in water, and XO, CPB, and buffer solution added before they were diluted to the final volumes, Measurements on these solutions confirmed quantitative recovery and determination within by this method of analysis,
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Z Figure 2. Variation of molar absorptivities of the RE-XO-CPB complexes at maximum absorption with rare earth atom number
provided the concentrations of ammonium sulfate in the final working solutions are negligible. Ammonium salts or even the organic reagent may be effectively separated by using a short cation exchange column with 6 N HCI acid solution as eluant ( I ) . DISCUSSION
Compared with published spectrophotometric methods, the described method is at least three times and in several instances in excess of six times more sensitive for the determination of small amounts of rare earths. The method is furthermore several-fold more sensitive for any of the rare earths in cornparison with the normal application of XO in slightly acid solutions as chromogenic agent (I.?, 14). From experience it may be stated that for the first nine rare earths (La to Dy) one should be capable of determining quantities as small as 0.15 pg in a sample and for the remainder about 0.2 pg with a reasonable degree of precision. The useful practical range falls between about 0.1 to 0.5 ppm (or more) concentration levels in working solutions (for 1-cm cells on a double beam instrument). The value found for the molar absorptivities for lanthanum agrees very closely with the published value (9) especially in view of the fact that a different wavelength has been used in the present work. This resulted in a slightly lower value, as expected. Cerium gave the highest molar absorptivity immaterial of being present in the three or four valency state. An acceptable explanation for the variation in molar absorptivity and associated bathochromic shifts with increasing atom number of the rare earths has not as yet been found for these com(13) A. A. Tikhonova and N. I. Tirnofeeva, . I Anal. . Chem. U.S.S.R., 21, 255 (1966). (14) K. Tonosaki and M. Otorno, Bull. Chem. Soc., Japan, 35,
1682 (1962). VOL 40, NO. 1, JANUARY 1968
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plexes. Further studies on these as well as on analogous complexes of other elements are at present under way with the purpose of elucidating the nature and behavior of these complexes. The stabilities of the complexes are excellent in the presence of the relatively large excess of CPB used, and no changes have been noticed even at high pH values after standing times of one day or more. The color development for all the rare earths is not immediate and for this reason measurements were done one hour after mixing of the solutions during which time solutions were intermittently shaken. In conjunction with a cation exchange column which is the most frequently used method of separating mixtures of rare earths (1,15), the method offers a considerable improvement to existing methods for analysis of materials containing traces of rare earths. In such applications it is essential, however, to guard against interferences and especially an excess of mineral salts in the final working solutions (9, 10). For rare earth determinations, most interfering elements can fortunately be removed or marked (10) quite readily. Excess mineral salts can also be eliminated without difficulties. At the time of determination of rare earths, the rare earth fractions are usually clean and the method can be applied directly. Total rare earths can also be determined in minute quantities using an average molar absorption coefficient at a certain wavelength if it assumed that the relative abundances of the individual rare earths are approximately constant in the mixtures to be analyzed. (15) L. Wish and S. C. Foti, J . Chromatog., 20,585 (1965).
The most widely colorimetric reagent for nickel is dimethylglyoxime which exhibits a very high molar absorption coefficient of 13,600 at 465 mp (16). Compared with the method described above, the dimethylglyoxime method is only about half as sensitive. No evidence has further been found that more than one complex forms which must be guarded against when using dimethylglyoxime as coloring reagent. Unfortunately, to a more serious degree than for the rare earths, colored complexes formed by various other cations may interfere with the determination of nickel, if present. Interfering cations forming red, grey, or pink complexes like Lif, Zrf4, Pbf2, Fef3, Vf5, and UOZf2,do not seriously interfere with the method. Cations forming violet to blue complexes, however, greatly interfere and even relatively small amounts of cations like Caf2, Mg+2, Mn+2, Znf2, Coc2, Cuf2, Cdf2 and Ag+ cannot be tolerated. Cations of all the rare earths, Be+2 and In+3 should be completely absent. The method should thus be applied with great caution for the spectrophotometric determination of nickel, preferably by removing interfering cations which cannot be masked. The advantage of the high sensitivity of this method may, nevertheless, be of great value in many analytical applications.
RECEIVED for review June 12, 1967. Accepted August 28, 1967. (16) “Oak Ridge National Laboratory Master Analytical Manual,” Subsection 12, Method Nos. 1 215410(900715410).
Conductometric Determilnation of Sulfur in Beryllium Oxide Ceramics Using Induction Heating and Peroxide Absorption James F. Chapman1 and Malcolm Thackray
Australian Atomic Energy Commission, Research Establishment, Lucas Heights, New South Wales, Australia A VARIETY OF METHODS have been used to determine the sulfur content of ceramic oxides. Frequently the material is dissolved in a nonsulfur acid, the sulfur reduced to H2S,and this is then reacted with zinc acetate to give Lauth’s Violet which can be determined colorimetrically ( I ) .Phosphoric acid is one of the few liquids which are suitable for the dissolution. However, high density beryllium oxide dissolves very slowly in this medium and is almost unaffected by all other nonsulfur acids. Even fusion mixtures have only a slight effect on this resistant material. Boyle, Gregory, and Sunderland ( 2 ) mixed the beryllium oxide with a flux of iron and tin which was then combusted in a stream of oxygen with radiofrequency heating. Very high temperatures are attained using this method and there is a high probability that all the sulfur is released into the oxygen stream. The combustion furnace is available commercially (Leco) and is probably used more frequently for the determination of 1 Present address, University of New South Wales, Sydney, Australia.
(1) R. C.Richards,U.K.A.E.R.E. Rept.AM86(1961). (2) W. G. Boyle, L. J. Gregory, and W. Sunderland, Rep. UCRL 7204 (1963).
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carbon than sulfur. The manufacturers supply an apparatus in which carbon is determined conductometrically by absorption of C 0 2in barium hydroxide solution. For the determination of sulfur a separate and more complex attachment is supplied in which the sulfur dioxide resulting from combustion is absorbed in a solution of iodine in acidified potassium iodide. The iodine which reacts is continuously replaced by automatic titration with potassium iodate (2). Similar apparatus using the electrogeneration of bromine from potassium bromide to titrate the sulfur has been described (3, 4 ) and is also available commercially (Consolidated Electrodynamics Corp.). Since Ericson, Elgh, and Wiman (5) have stated that sulfur dioxide can be conductometrically determined by absorption in hydrogen peroxide, the commercial apparatus for sulfur determination, although more specific, seems unnecessarily complex and expensive for many applications. In view of the limited number of sulfur determinations required in our work, it was decided to explore the use of the conductometric cell, normally used for carbon analysis, to determine sulfur. (3) L. E. Hobbs and D. H. Wilkins, Anal. Chim. Acta, 20, 344 (1959). (4) E. Barendrecht and W. Marten, ANAL.CHEM. 34(1), 138 (1962). (5) G. Ericson, B. Elgh, and A. Wiman, ASEA Res., 2,83 (1959).
