An Improved Flux-Fusion Technique for X-Ray Emission Analysis D. A. Stephenson Research and Development Laboratories, Corning Glass Works, Corning, N . Y. 14830
THE FLUX-FUSIONtechnique used in the preparation of X-ray emission analysis samples was introduced by Claisse (7) more than a decade ago and has proved to be extremely useful in eliminating particle size effects on X-ray intensity measurements and in reducing interelement effects. Variations upon the technique have been reported by Townsend (2)) Rose, Adler, and Flanagan (3)) Rinaldi and Aguzzi (4), Andermann (5), Larson, Winkler, and Guffy (a), Luke (7), Tingle and Matocha (8), and Strasheim and Brandt (9). The principal disadvantages of some of the reported flux fusion techniques are: the resulting glass is highly stressed and must either be annealed to remove stresses, or crushed and briquetted; each sample must be treated on an individual rather than a mass production basis; the use of ordinary graphite crucibles frequently leads to reduction of some elements to the metallic state. The procedures described here overcome these difficulties. EXPERIMENTAL
Development of Flux Compositions. Just as there is no “universal acid,” there is no one flux composition suitable for all sample materials. The aforementioned authors have developed many fluxes adequate for the particular sample compositions encountered in their studies. In the present investigation, prime consideration was given to simple fluxes containing no elements ordinarily determined by X-ray emission analysis, yet capable of dissolving a sufficient amount of nonmetallic samples to provide good yields of characteristic X-rays of the lighter elements (Z = 11 to 17) at the 1 wt % concentration level. Three lithium-borate-base fluxes have been developed which will dissolve a wide variety of sample materials and yield mechanically stable glass disks. Table I summarizes the composition of the fluxes, crucibles used to contain the melt, and typical sample materials suited to the particular flux. The flux components may be homogenized in large quantities in a roller mill for subsequent use. The fluxes should not be premelted, because the decomposition of lithium carbonate upon heating greatly aids in mixing the melt and sample during the fusion process. In fact, it is rarely necessary to swirl or stir the melt to achieve excellent homogeneity, even for fusion times as short as 5 minutes. Highly refractory samples such as ZrOz and HfOz may, however, require that the melt be swirled, particularly if the particle size of the sample is large. Flux A is used whenever the sample contains no reducible components or when reducible components are present in small concentrations and are unaffected by fusion in graphite. Flux B is employed when reduction problems are encountered. (1) F. Claisse, Norelco Reptr., 4, 3 (1957). (2) J. E. Townsend, Appl. Spectrosc., 17, 37 (1963). (3) H. J. Rose, Jr., I. Adler, and J. F. Flanagan, ibid., 17,81 (1963). (4) F. F. Rinaldi and P. E. Aguzzi, Spectrochim. Acta, 23B, 14 (1967). ( 5 ) G. Andermann, ANAL.GEM., 33, 1689 (1961). (6) J. 0. Larson, R. A. Winkler, and J. C. G d y , “Advances in XRay Analysis,” Vol. 10, Plenum Press, New York, N. Y.,1967. 35,56 (1963). (7) C. L. Luke, ANAL.CHEM., (8) W. H. Tingle and C. K. Matocha, ibid., 30, 494 (1958). (9) A. Strasheim and M. P. Brandt, Spectrochim. Acta, 23B, 183 (1967). 966
ANALYTICAL CHEMISTRY
A B C D Figure 1. Step in the flux-fusion process
(a) Sample and flux at beginningof fusion (b) Melted after 5 minutes at 1100 “C
Carbon flattening rod introduced and heating continued for 5 minutes (d) After cooling and removal of the flattening rod, the glass disk is freed from the crucible (c)
Besides being an excellent high-temperature oxidizing agent, the ceric oxide component is a strong X-ray absorber as favored by Rose et af. (3). Flux C works well for samples consisting mainly of alkaline-earth oxides that tend to produce glasses that crack easily. The SiOz, A1203, or GeOn component may be selected to avoid interferences in analysis of the sample. For particularly dficult sample compositions, the basic fluxes can usually be tailored by the addition of minor components. The review article by Hutchins and Harrington (IO) describes the role of many common oxides in glass-forming systems and can be used as an elementary guide to tailoring flux compositions. Carbon crucibles have been used throughout, because they are seldom wet by the melt and can be used repeatedly without fear of contamination. Platinum-gold alloy crucibles and palau crucibles, although not ordinarily wet by flux-fusion ~~
(10) J. R. Hutchins and R. V. Harrington, Encyc. Chem. Tech., Vol. 10,2 ed., 533 (1966), John Wiley & Sons, N. Y.
Table I. Fluxes for Fusion of Materials for X-Ray Emission Analysis Typical samples Flux Crucible Silicate glasses, silica- or Flux A Graphite alumina-bearing refrac90 wt Z LisB4G tories, geological sam10 wt Z LisCOt ples, cements. Vitreous carbon Samples containing components that wet graphite such as oxides of Pb, Bi, Sb, As, Ni, Cu, and others. Samples containing comFlux B Graphite ponents that are re90 wt Z LirB4Q duced in graphite cru8 wt % LizCor cibles using Flux A. 2wtZce0, Vitreous carbon Oxides of Pb, Bi, Sb, AS, Ni, Cu, Zn, and others that are still reduced in graphite crucibles. Oxides or carbonates of Flux C Graphite the alkaline earths, sam90 wt Z LirBO ples containing no glass8 wt Z Li, Cor forming components. 2 wt Z SiO:, Altoi, or GeOa
melts, are susceptible to attack by metals such as lead and arsenic that can drastically reduce the life of the crucible. Either graphite crucibles (#A2260 Fusion Crucible, Ultra Carbon Corp., Bay City, Mich.) or vitreous carbon crucibles (#170 Crucible, Beckwith Carbon Corp., 16140 Raymer St., Van Nuys, Calif.) are well suited for this flux-fusion technique. Vitreous carbon crucibles are employed when reduction of sample components is a problem or when melts tend to wet graphite crucibles. Although vitreous carbon crucibles are rather expensive, 20 to 25 melts can be made in one crucible before it is destroyed by oxidation. Mixtures consisting of 90 wt flux and 10 wt sample give good characteristic X-ray yields for the lighter elements. The flux/sample ratio can be changed within reasonable limits to suit particular needs. All samples should be ground to pass at least -100 mesh. Preparation of Flux-Fusion Glasses. After mixing the flux and sample, the mixture is transferred to the proper crucible and fused according to the schedule depicted in Figure 1. An initial 5-minute fusion at 1100 “C will completely dissolve all but the most refractory samples, in which case the initial fusion time may be increased or the melt swirled in the crucible to promote solution of the sample. A mediumsized muffle furnace is satisfactory for either single or multiple fusions. After the initial fusion, a short carbon rod is placed on top of the melt to flatten the surface and an additional 5-minute heating period is begun (no carbon rod is used with vitreous carbon crucibles.) The crucible, melt, and flattening rod are then removed from the furnace intact and are cooled for 10 or 15 minutes to room temperature. The glass disks can be removed easily by lifting off the flattening rod and tapping the inverted crucible against a flat surface. Machining about a 2” taper on graphite crucibles which originally have vertical walls greatly facilitates removal of the disks. Identification marks may be scribed on the bottom of the disks with a hand-held grinder or similar tool without fear of breakage.
Although the carbon flattening rod produces a fairly good sample surface, it is normally not adequate for accurate quantitative analysis. The quality of the surface finish required depends upon the longest characteristic X-ray wavelength used for analysis, longer wavelengths requiring a superior finish. The disks can be finished to 100, 200, 400, or 600 grit by wet grinding on silicon carbide paper or diamond-embedded grinding disks. The total grinding time for a 600-grit finish for one sample is around 3 minutes, depending upon the quality of the original surface.
RESULTS AND DISCUSSION The glass disks produced by this method may be presented directly to an X-ray spectrometer for analysis. They are stable indefinitely when stored in a desiccator to prevent the polished surface from picking up water. An electron microprobe study of a variety of flux-fusion glasses prepared from highly refractory samples (ZrOz, HfOz), easily reducible samples (PbO, CuO, NiO), and samples containing no glassforming components (limestone, dolomite) demonstrated no significant compositional variation across the glass disks. A similar study by Parker (11) reached the same conclusion, so grinding and briquetting the glass seems to be an unnecessary step. Because of the simplicity of this method and the fact that sample preparation times are much shorter than for the previously reported methods, batch processing of numerous samples is possible. Quantitative X-ray emission analysis of a variety of samples prepared by this technique showed excellent agreement with wet-chemical results.
RECEIVED for review January 21, 1969. Accepted March 21, 1969. (11) A. Parker, Anal. Chim. Acta., 40, 513 (1968).
Potentiometric Titrations of Sulfate Using an Ion-Selective Lead Ele’ctrode James W. Ross, Jr., and Martin S. Frant Orion Research, Inc., 1I Blackstone Street, Cambridge, Mass. 02139
IN SPITE of the large number of sulfate determinations being performed at the present time, there is still no simple, direct titration procedure available to the analytical chemist. Direct titrations using indicators have been reported ( I , 2), but these suffer from numerous interferences and are difficult to automate. Sulfate has been titrated potentiometrically with barium chloride as a reagent, and using a heterogeneous membrane electrode as an indicator (3). The extreme sensitivity of the electrode to chloride (and probably other anions) almost totally obscures the end point break, and renders the titration almost useless except in some special circumstances. Kolthoff and Pan (4) studied the titration of sulfate with lead (1) N. H. Furman, Ed., “Standard Methods of Chemical Analysis,” Vol. I, 6th ed., Van Nostrand, New York,1962, p 1011. (2) “1967 Book of ASTM Standards,” A.S.T.M., Philadelphia, Pa., 1967, p 57. (3) G. A. Rechnitz, Z. F. Lin, and S. B. Zamochnik, Anal. Letters, 1, 29 (1967). (4) I. M. Kolthoff and Y.D. Pan, J. Amer. Chem. Soc., 62, 3332 (1940).
nitrate, using an amperometric end point, and found the titration to be accurate and relatively free of interferences from most common ions. The necessity for purging the sample with nitrogen, however, limits the usefulness of the method in routine applications. We have found that the recent commercial introduction of a lead-selective electrode (5) allows the potentiometric determination of end points in the direct titration of sulfate with standard lead solutions. EXPERIMENTAL
Standard lead solutions were prepared from Pb(ClO& and distilled water, and were standardized against EDTA. Standard Na2S04solutions were prepared by weighing the dried reagent grade salt and dissolving in distilled water. Other chemicals and solvents used were reagent grade. ( 5 ) J. W. Ross, Jr., Symposium on Ion-Selective Electrodes, National Bureau of Standards, Gaithersburg, Md., Jan. 30, 1969; J. W. Ross, Jr., and M. S. Frant, Pittsburgh Symposium on An-
alytical Chemistry and Spectroscopy, Cleveland, Ohio, March 7, 1969. VOL. 41, NO. 7, JUNE 1969
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