(2) Y. K. Chau, P. T. S. Wong, and H. Saitoh, J . Chromatogr. Sci., 14, 162 (1976). (3) S. Hancock and A. Slater, Ana/yst(London), 100, 422 (1975). (4) . , Kaarr Julshamn and Olaf R. Braekkan. At. Absorot. News/.. 14(3). 49 ( 1975). (5) "Analytical Methods Using The HGA Graphite Furnace", Perkin-Elmer Corp., Norwalk, Conn., 1974. (6) H. L. Kahn, At Absorpt. News/., 7 , 2 (1968). 1
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(7) Y. K. Chau, P. T. S. Wong, and P. L. Luxon, Nature(London), 253, 264 (1975). (8) J. E. Cremer, Occup. Health Rev., 17, 14 (1965).
,I
RECEIVED for review November 15,1976. Accepted January 28, 1977.
Determination of Traces of Arsenic in Siliceous Materials Cyrus Feldman Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
Finely ground rocks, slags, and fly-ash are completely converted to soluble form by HF-HN03 vapor phase attack at -70 OC. The treatment chamber is a comrnerclally available 1-L Teflon jar. Arsenic Is not volatlllzed by this procedure il In the As(V) form. Arsenlc contaminating the acld mlxture Is carrled over to the sample if the arsenic Is trivalent. Potassium permanganate, added to the acid mixture In advance, converts any As(II1) present to As(V), thus preventing contamination of the sample. Accurate results were obtained wlth NBS Coal Fly Ash (SRM 1633) and standard rocks.
Siliceous materials are usually dissolved for analysis with the aid of hydrofluoric acid, or are fused with alkali carbonates, borates, or fluoborates. These reagents are often contaminated with traces of foreign elements and, since the quantity of reagent used is much greater than the quantity of sample taken, the reagent blank may be unacceptably high for the determination of a given trace constituent. In 1957, N. N. Semenov (1) suggested the use of H F vapor, confined with the sample in a closed container, to circumvent the above difficulty in preparing quartz samples for the spectrographic determination of trace impurities. Morachevskii et al. (2) and Zilbershtein et al. ( 3 , 4 )subsequently used the vapors of a 1:l mixture of concentrated H F and HNOBin a similar way to eliminate the silicon from elemental silicon and/or high purity quartz. Piriutko (5)has shown that if this type of attack is used on elemental silicon, part of the nitrogen is reduced all the way to N(-111) and remains with the residue as (NH&3F6. Most of that part of the nitrogen which is reduced, however, is converted to NO and/or NOz. When it became necessary in this laboratory to determine low concentrations (10.05 ppm) of arsenic in coal slags, fly ash, and rocks, vapor phase HF-HN03 treatment was considered as a possible way to avoid contamination by reagents. The prospects seemed poor, however: the boiling point of AsFS is -53 "C, and that of AsF3,63 "C. Little encouragement was given by previous work in this field; according to Morachevskii et al. ( 2 ) ,76Astracer tests showed that if arsenic is added to H F or to 1:l H F + "03, the arsenic is volatilized completely when the liquid is evaporated to dryness a t 105-110 "C. Zilbershtein et al. ( 4 ) essentially confirmed this result, adding that all of the arsenic was also lost if added to elemental silicon which was then subjected to the vapor phase HF-HN03 treatment. Unfortunately, neither paper mentions the valence of the arsenic in the tracer or carrier used. It is well known, however, that traces of a given element may behave in one way in a dilute aqueous solution of carrier plus tracer, but in quite another way when the element exists
in a complex matrix. It therefore still appeared worthwhile to see whether conditions could be found under which the vapor phase HF-HN03 procedure could be used to make silicates soluble without either increasing or decreasing their arsenic content. Two types of experiment were necessary to test the applicability of the proposed procedure. First, it was necessary to demonstrate that a typical silicate sample of relatively high and accurately known As content does not lose arsenic when treated by this procedure. Such a sample was available in Standard Reference Material 1633 (Coal Fly Ash) (U.S. National Bureau of Standards, Washington, D.C., 20234). Several standard rock samples were also available from the U.S. Geological Survey and from the Zentrales Geologisches Institut, Berlin, DDR. The arsenic contents of these samples are known with greater accuracy in some cases than in others (6). Second, it would have to be shown that any arsenic which might be present originally in the H F and/or HNOB is not transferred to the sample during the vapor treatment. This could be determined by analyzing low-arsenic coal slags and U.S. Geological Survey standard rock samples (As 5 1 ppm in all cases) and comparing results obtained by the regular procedure with results obtained when substantial quantities of arsenic were added to the acid mixture before use. The reaction chamber was simple. Commercially available Teflon jars were employed to avoid use of the more expensive chambers machined from solid blocks of Teflon by Mitchell and Nash (7) and Wooley (8). Preliminary experiments indicated that -70 "C would be a suitable temperature for the present purposes. This temperature gave essentially complete conversion of the samples to fluorides and fluosilicates in a reasonable length of time, and was also considered low enough to minimize losses of arsenic and/or acids by volatilization.
EXPERIMENTAL Apparatus. A wide-mouth 32-02. cylindrical Teflon jar, with Teflon screw-on cap (Cole-Parmer Co., Chicago, Ill., Catalogue No. 6044-40) was modified slightly for use in the present procedure. The jar, with cap off, was laid on its side on a hotplate whose surface temperature was -350 O C . Pressure was exerted from above until the side wall of the jar softened to give an approximately flat area -1 in. X 61/2 in. Hotplate surface temperatures were measured with a bimetallic surface thermometer (Model 314C, Pacific Transducer Co., Los Angeles, Calif. 90064). All samples were held in 25-45 mL platinum dishes during exposure to the acid vapors. Heating was performed in an ordinary drying oven. See Figure 1. Reagents. All reagents (48% HF, 71% "Os, 72% HC104, and solid KMn04) were analytical reagent grade. The arsenic content of the HClO,, was found to be negligible compared with ANALYTICAL CHEMISTRY, VOL. 49,
NO. 8, MAY
1977
825
Table I. Effect of As(II1) and As(V) Contamination of Acid Mixture on As Concentrations Found for Silicates Made Soluble by Vapor Phase HF-"0, Attack Substances added to HF-HNO, mixture
No.
Line No.
Sample (particle size
