Direct Reading Emission Spectrometric Determination of Trace Metals in Calcium Sulfate Minerals by Organic Enrichment Edgar F. Cruft and John Husler Department of Geology, University of New Mexico, Albuquerque, N. M . 87106
NATURALLY occurring gypsum, CaS04.2Hz0, and anhydrite, CaS04, contain minor and trace metals which reflect the environment-i.e., composition and character of the saline water-from which they formed. Hence, the analytical determination of these elements is important for the geochemist and chemical oceanographer as well as for the chemical and mineral processing engineer. While several elements--i.e., Sr, Mg, and t o a lesser extent Na and K-are present in concentrations from 10 ppm t o 1 or so in these minerals, the other metals, particularly those of the transition metal group are present in very low abundances. Determination of these elements directly by trace element instrumental methods-Le., atomic absorption and optical emission spectroscopy-is not feasible. Chemical preenrichment of some sort is necessary. In this study the trace metals were concentrated by an organic enrichment procedure and then analyzed by direct reading emission spectrometry using the solid precipitate phase. This organic enrichment technique is useful for other instrumental methods of analysis, and was used in this laboratory also for atomic absorption methods. Although the procedure has been used primarily for gypsum and anhydrite, it can be used for calcium sulfate hemihydrate (plaster of paris) and with little modification for other saline products. An early method involving the use of 8-hydroxyquinoline, thionalide, and tannic acid in combination to concentrate several metals was that of Mitchell and Scott ( I ) . They used a n acetic acid-ammonium acetate buffer at p H = 5.1 and added iron and aluminum as internal standard and collector. The elements Co, Ni, Mo, Pb, Zn, Ti, Ga, Al, Fe, Cd, Ag, and Au are quantitatively precipitated by 8-hydroxyquinoline and the addition of tannic acid makes the recovery of Cr, V, Be, Sn, and G e quantitative. Addition of thionalide allows the complete recovery of Sn and Pb. Calcium remains in solution. Various modifications of the procedure have been used. Heggen and Strock (2) used In instead of A1 as a n internal standard and carrier, although In is generally unsuitable as a n internal standard for metals with a wide range of volatilities. Chichilo, Specht, and Whittaker (3) precipitated at pH = 5.9 to effect the recovery of Mn. This p H was used in the present study and only AI was used as a collector. Internal standards used were Cd for the volatile elements and Lu for the involatiles, and these were fused in a sodium meta phosphate buffer and mixed with the solid enriched sample prior t o analysis. A major advantage of analyzing the solid phase is the extra sensitivity obtained from the dc arc methods compared to solution spark techniques. NaPO, is used extensively in our laboratories as a good general buffer t o pro-
(1) R. L. Mitchell and R. 0. Scott, Spectrochim. Acta, 3, 367
(1949). (2) G. E. Heggen and L. W. Strock, ANALCHEM,25, 859 (1953). (3) P. Chichilo, A. W. Specht, and C. W. Whittaker, J. Ass. Ofic. A g r . Chem., 315, 903 (1955).
Table I. Equipment and Instrument Parameters Spectrograph : CEC Maximum Versatility 3-m grating spectrometer with folded Roland Circle mounting. Reciprocal dispersion 4 A/mm, 21,000 lineiinch concave grating. Digital voltmeter readout. Anode: National Carbon Co. 0.180” Electrodes: diameter SPK high purity graphite rod, drilled to a depth of a/~6’’ and diameter of ‘18’’. Cathode: National Carbon Co. SPK high purity electrode, 0.180” diameter, sharpened to a point with a 15” included angle, 5 mm Arc gap: Entrance slit width: 25 microns Current and exposure: 20 dc amperes. Arced to completion in approximately 80 seconds, with no pre-arc. Atmosphere: 90% Argon-10% oxygen at a rate of 5 s.1.p.m. through a Stallwood jet.
Other Equipment pH Meter, Beckman Zeromatic or comparable instrument Filter paper, Whatman, No. 540 Crucibles, Glazed porcelain, 15-ml Wig-L-Bug, Crescent Manufacturing Co. Mixer-mill, Spex Ind. Inc.
vide a smooth burn, minimize matrix effects and to improve sensitivity for certain elements, notably the volatiles-i.e., Cu, Pb, and Zn. EXPERIMENTAL
Apparatus. A Consolidated Electrodynamics Corp. MVS instrument was used for the spectrochemical analysis with dc arc internal standard techniques and argon-oxygen atmospheres surrounding the arc, similar to those described by Cruft (4) and Cruft and Giles (5). The spectrochemical parameters are listed in Table I, and the spectral lines used are in Table 11. Reagents and Standards. Deionized, distilled water is used throughout. Hydrochloric acid is 10%. Acetic acid and ammonium acetate solutions are both 2 N. Ammonium hydroxide is concentrated, A.R. grade. Aluminum solution. Dissolve 5 grams Spex Industries or equivalent high purity aluminum metal in a minimum amount of HCI and dilute to one liter with H20. 8-Hydroxyquinoline, 5 %. Dissolve 6.50 grams reagent grade 8-hydroxyquinoline in 130 ml 2 N acetic acid. This is sufficient for 5 samples. Shake well in a polyethylene bottle and prepare as needed. Tannic acid, 10% in 2 N ammonium acetate. Place 2.50 grams reagent grade tannic acid in a polyethylene bottle. Add 25 ml 2N ammonium acetate and shake vigorously. Thionalide, 1% in acetic acid. Place 0.25 gram reagent grade thionalide (B-aminonaphthalide of thioglycollic acid)
(4) E. F. Cruft, lkon. Geof.,59,458 (1964). ( 5 ) E. F. Cruft and D. L. Giles, ibid., 62,406 (1967). VOL. 41, NO. 1, JANUARY 1969
175
Table 11. Spectral Lines Used for Direct Reading Spectrochemical Analysis Element Cda cu Pb
Zn LU5 La Ba
co Cr Y Fe
Mg Mn Ni Sr V a
Wavelength, A
Intensity
Slit width, f i
2288.02 3247.54 2833.07 2138.56 4518.57 4333.73 4554.03 3453.50 4254.35 3327.89 3719.94 2795.53 2576.10 3414.76 4607.33 3183.98
1500R 5000R 500R 800R 300 800 lOOOR 3000R 5000R 60 lOOOR 150 300R IOOOR lOOOR
75 50 50 150 75 50 75 75 100
150 75 50 50 75 50 50
500R
Internal standard.
in a polyethylene bottle. Add 25 ml A.R. grade acetic acid and shake vigorously. Buffer-internal standard mixture. Thoroughly mix 0.025 gram high purity CdO and 0.0175 gram high purity Lu203 with 10.16 grams NaNH4HP04. 4 H 2 0 and fuse in a silica dish for 30 minutes at 650 "C. Grind the fused product to 200 mesh in a agate mortar. Five-gram stock mixture of standard elements, 1.8% by weight. Accurately weigh out, into a 30-ml polystyrene vial containing three 3/d-diameter Plexiglas balls, the proper amounts of the high purity, crushed oxides. Make to exactly 5 grams by adding pure A1203 and shake the mixture for 10 minutes on a mixer-mill. Dilutions are made with A2103 to obtain 10-100 ppm element concentrations prior to mixing with the buffer and internal standard. Procedure. The sample is crushed in the agate mortar and pestle and sifted through a 100-mesh nylon screen. About 5 grams of weighed sample is dissolved in 300 ml of 10% HC1. Ten ml of 5 grams/l A1 solution and 25 ml of 5 % 8-hydroxyquinoline in 2N acetic acid are then added. Concentrated N H 4 0 H is added, by means of a buret, to the solution until a pH of 1.8 is attained. After buffering the solution with 60 ml 2N ammonium acetate, 4 ml of fresh 10% tannic acid in 2N ammonium acetate is added and stirred vigorously. Immediately after the addition of 4 ml fresh 1 % thionalide in glacial acetic acid, additional NH4OH is added to bring the pH up to 5.9. The organic reagents decompose in solution and hence cannot be stored. The solution is stirred vigorously, allowed to remain overnight at room temperature, and filtered through Whatman No. 540 filter paper. The precipitate is washed lightly with ice water and allowed to partially dry in a drying oven. The filter paper is then placed
Sample
cu=
Pb
1
