856
Anal.
chem. 1804, 56. 858-859
1 !.)
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Flpure 3. Paper strip immediately after application of the detection procedure for 10 s to A 645 steel. The nickel band (a) appears pink In the wiainal and band (b). corresponding to iron. was dark brown. Cobm remain pwnmnenlly stable. The distance on the paper between the pointers from (a)to (b) corresponds approximately lo R, and ltm thickness of Me paper to R , as indicated in Figures 1 and 2.
tested hy this method, yielding a full color development in 10 s. However, alloys with lower contents of nickel were not analyzed by the present procedure. A paper strip obtained immediately after applying the detection procedure on A 645 steel is shown in Figure 3. These findings suggest that the method allows rapid and inexpensive large scale control tests of steel and other alloys. Since the method requires small areas of the sample, proximal mnes in the same sample and thin matinge muld be identified. Furthermore, samples located in arena of difficult access, e.g., pipes, covered with coatings as thick as 5-20 cm can he analyzed through a small hole. To sum up, this method allm the extraction, reaction, and separation of nickel and chromium in a few seconds from
different steel alloys and gold, copper, nickel, chromium, and iron from other alloys. At least for nickel, a comparable sensitivity to that of the standard methods was obtained. All these promsea ormr in the paper matrix of the probe. It must be mentioned that the probe itself is small, portable, versatile, and inexpensive. In addition, other elements may be detected by this procedure using a paper specifically sensitized. ACKNOWLEDGMENT We thank Luis F. Leloir, the other members of the Instituto de Investigaciones Bioquimicas "Fundacion Campomar" and Alberto Meschini from Universidad Nacional del Sur (Argentina) for helpful discussions and criticism. D.M.A. is a Fellow of the Consejo Nacional de Inveatigaciones Cientificas y Tecnicas (Argentina). H.C. and V.I.V. are Career Investigators a t the same institution. Registry No. Fe, 7439-89-6; Ni, 1440-02-0;Cr, 7440-47-3; Au, 7440-57-5; Cu, 7440-50-8; Chrome1 P, 12605-72-0;Constantan, 12605-79-7;Monel, 11105-19-4; Cunife, 39451-89-3; steel, 1259769-2; stainless steel, 12597-6&1; diacetyl dioxime, 95-45-4. LITERATURE C I T E D (1) RDurdve tu Ebardyik Spot Test A b y Cast. 811. 1945, 4 , 8. (2) Butler. J. A. V. '"ElacQical Rsnomena at Inlerlau~sin Chsm(rby. physics and B W . McMIllan Publishing CC.: New Y a k . 1951: p
123.
(3) Minlusd. C. L.: Meda. J.: Pas& R. LEMITAn. 1978. 2 . 15. (4) "Annual Book 01 ASTM Standards". R. 12; Chsmlcal Amtysb ot Metab; SampHng and ~nabsbof MI &ring Ores: ASTM: mladec ma. PA. 1981
REYXNEDfor review September 22,1983. Accepted December 9, 1983.
High-Pressure Acid Dissolution of Refractory Alumina for Trace Element Determination Henry A. Fonerl
. ..
Ceramics Deoartment. The Houldsworth School of ADDlied Science, The University of Leeds, Leeds LS2 GT, Great Britain High-purity refractory aluminas have important uses in the electronics and ceramics industries. Doped single-crystal aluminas such as ruby and sapphire are used in the construction of lasers. For electronics and electrical purposes sintered IreerystaUized" (i.e., polycrystalline)alumina is often used. Single-crystal alumina (mp 2050 "C) has been fused during fabrication while the other types have also been heated to very high temperatures. Above 1200 OC alumina converts into the a structural form. The physical, electrical, and manufacturing properties of alumina are all much affected hy the levels of impurities present, and hence the determination of trace elements in these materials is of great practical importance. The extreme hardness of the material (9 on the Mho d e ) makes it difficult to grind without introducing extraneous elements (2-3). It is impossible to dissolve a-alumina by a simple acid treatment and the necessity of using alkaline fluxes to dissolve the samples for analysis is a source of contamination. De arc spectrographic methods have also been used to analyze alu'Preaent addreea: The Geological Survey of Israel, 30 Malkhe Yisrael St., Jerusalem 95 501, Israel.
minas ( 3 , 4 ) , hut not very successfully (5). Acid solutions are convenient for analysis hy modern instrumental methods and. using high-purity acids, it is psaible to achieve very low blanks. A number of authors have attempted to dissolve a-alumina in acids a t normal pressures (6-9). This author has tried to dissolve finely ground fused alumina (-Zoo mesh) by these methods with very little success. By use of Mendlina's phosphoric/sulfuric acid mixture (3, the sample was dissolved with great difficulty (3 to 4 h) and with significant loss of material due to humping. Since, thermodynamically, a-alumina should be readily Boluble in acid, attempts have been made to achieve solution by increasing the reaction temperature (and hence pressure). Two principal approaches have been described in the literature: (a) the use of singlechamber sealed pressure vessels and (h) the use of double-chamber pressure vessels comprising a sealed tube containing the sample, with a compensating outer pressure to prevent the inner tube from bursting. Single-Chamber Pressure Vessels. The use of PTFElined bombs to decompose refractory materials and rocks is well-known (10-12) and this type of apparatus has also been used with alumina (13). A small (3.5 mL) platinum-lined 0 1984 Amdcan ChsmlcaI Socler,
ANALYTICAL CHEMISTRY. VOL. 56, NO. 4. APRIL 1984
Flgun 1.
057
Highpressure dlasolulbn vessel (bomb).
bomb (24, 15) has also been described. A number of workers have dissolved aluminas in sealed glass, or transparent vitreous silica (TVS)tubes (1fM9).This author has found i t imprmihle to disaolve fmely ground fused alumina (-200mesh) in mineral acids at 250 "C (48 h heating) using a PTFE-lined bomb. Attempts to dissolve the same alumina in TVS tubes using hydrochloric acid failed because the tubes burst at high temperatures. Double-Chamber P r e s s u r e Vessels. T h e problem of exploding tubes has been overcome hy using a douhle-chamber pressure vessel. Jannasch (20,22) used hydrochloric acid to dissolve refractory silicates at 400 "C in a n apparatus pressurized hy ether or benzene. Apart from the hazards of this apparatus, complete digsolution was not achieved because of its geometry (22). Wichers, Schlecht, and Gordon (23-25) described an improved form of J m a s c h ' s apparatus. They used sealed Pyrex glass tubes and equalized the pregsure by adding a weighed amount of solid carbon dioxide. The glass tubes used lead to signficant contamination. Bettison (3)has developed a similar apparatus with an improved sealing arrangement and with TVS tubes. Attempts to construct a double-chamber pressure vessel from stainless steel similar to the design of Gordon e t al. (25) failed due to (a) the difficulty of conslmcting a simple gastight screw-on seal on a large diameter pressure vessel and (h) the impossibility of packing enough solid carbon dioxide into the bomb to provide the necessary compensating pressure needed hy the large diameter (up to 14 mm overall) TVS tubes used. This paper describes a new high-pressure vessel (homh) using nitrogen gas as the pressurizing medium and with the possibility of measuring both the compensating pressure and the bomb temperature during heating. It is possihle to dipsolve quite large pieces (0.5 g) of unground (Le., uncontaminated) single-crystal a-alumina. EXPERIMENTAL SECTION Apparatus. High Pressure Vessel ond Furnace. The apparatus used consists of a high-pressure vessel (bomb) shown in Figures 1 and 2. The bomb (B) is connected to a pressure gage (P) and safety device (D), as shown in Figure 3. Arrangements are made for the entrance and release of pressurizing gas and the bomb itself is placed inside a specially designed electric tube furnace (E) fitted with insulating plugs (I). Detailed engineering design considerations are given in the supplementary material to this paper or may LE obtained from the author. Stainless steel was not used for the construction of the highpressure vessel because austenitic stainless steels have poor thermal conductivity and failure could occur due to thermal shock stress induced by rapid heating and moling. Additionally, stainleas steels are particularly liable to strewcorrosion cracking at about 300 "C in the presence of chloride (26),say from a burst tube. For these reasons a high tensile, creep resistant chromium-molybdenum steel ('Durehete 900", British Steel Corp., BSC Special Steels, Stockbridge, Sheffield 530 5JA, U.K.) was chosen as the body material. The high-pressure vessel is shown in detail in Figure 2. It is bored from 63.5 mm (2.5 in.) acrosa-flats hexagon steel bar. The plug and cap are made of 1- stainless steel,as are the six screws which jack the plug down onto the gasket. The jacking screws are at such an angle so that their line of action passes through
FWn 2. High-pressure dissolution vessel
U Flgure 3. High-pressure appralus (schematic): (E) highgressure vessel (bomb): (0)busting disk assembly: (E) elecflc furnace: (F) porous bronze flnm: ( I ) insulating plugs; (M) manifold block: (P) pressure gauge: (V,) lsohtlng valve: (V,) pressure rellel/lnlet valve.
the inner edge of the shoulder supporting the gasket. The plug is drilled to within 3.18 mm (0.125 in.) of its base with a 2.38 mm (0.094 in.) diameter hole to take a thermocouple The conical shouldersof the stainless steel plug have milled flatsto mate with the jacking screws. The seal on the pressure vessel shown in Figure 2 is much superior to a simple screw cap (25).in that (a) the total sealing tomue is soread out amone six bolts. and (b) there is no tendenev for ;he gasket tn rotate. 6 e apparat&su-fdly held a press& of 66 MPa (9600 Ibfin.2) st 400 "C. The aasket material chosen was 30 Kawe (0.30 mm, 0.012 in.) aluminum sheet. The use of a continuoussheet, instead of a ring gasket, enables the use of simple, flat sealing faces. These faces must be ground flat to achieve gas tightness. Before use, the sealing faces are lightly coated with a suspension of colloidal graphite in alcohol to prevent sticking, (Dag Dispersion 580, Acheson Colloids Co., Prince Rock, Plymouth, PL4 OSP, U.K., is suitable). The bomb is connected to the pipework hy a hemispherical screw union (Fpures2 and 3). All the other pipework and valves are stainless steel and are especially designed for high-pressure work up to 138 MPa (20000 l b / i ~ ~ . The ~ ) . seals used are metal to metal cones. The manifold block, M, is of mild steel. The pressure gauge, P, is specially designed for use with gases and is seated on a flat copper washer. The whole apparatus is protected by a bursting disk assembly (Elfab-HughesBursting Disc
858
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
Table I. Weight of Material Removed from Tubes Half Filled with Concentrated Hydrochloric Acid
tube material silica silica Pyrex glass a a
time at temp, max. time, "C h 350 350 300
13
46 24
material removed mg/100 cmz of surface 1.6 3.5 27
Taken from Wichers e t al. ( 2 4 ) .
Division, West Chirton Industrial Estate, North Shields, Tyne and Wear, NE29 8RQ, U.K.) set to burst a t 96 MPa (14000 lb/in.2). It was found necessary to protect valves VI and V, from abraaion by dust particles from both the pressurizing gas and the bomb itself. This was done by inserting conical sintered bronze filters (Sheepbridge Sintered Products, Ltd., Coxmoor Road, Sutton-in-Ashfield, Notts. NG17 5LA, U.K.), F1 and F,, in the pipelines as shown in Figure 3. In use, the bomb is placed inside a specially built electric tube furnace as shown schematically in Figure 3. The furnace is closed by two insulating brick plugs, I. The valve VI is then connected to the remaining pipework. The power input to the furnace is 2 kW. The furnace is electronically controlled and its temperature monitored by a thermocouple placed inside the heated chamber. The bomb temperature is monitored by a fine thermocouple placed in its thermowell (Figure 2). Between 250 "C and 400 "C, the bomb (plug) temperature is a constant 9 "C higher than the temperature meaaured inside the TVS tubes. The furnace is protected against accidental overheating by a thermal fuse working through a fail-safe relay. Low voltage and current run through the fuse-the former for the safety reasons and the latter in order to prevent self-heating effects. Water in the "dry" nitrogen used for pressure generation causes oorrodlion of the bomb body. This corrosion is prevented by placing a few grams of a molecular sieve (Linde 5A) inside the bomb. The sieve pellets are wrapped in silver mesh to keep them in place. Possible corrosion due to acid from a burst tube is minimized by adding a few grams of a marble chips to the bomb. The molecular sieve also reacts with acid. TVS tubes (Thermal Syndicate, Ltd., P.O. Box 6, Wallsend, Tynt and Wear NE8 606, U.K.) were selected for this work in preference to glass because of their superior tensile strength and purity. TVS tubes are available in very high grades of purity and undergo little attack from hydrochloric acid a t elevated temperatures-see Table I. Sample tubes were prepared by blowing a hemispherical end onto a piece of TVS tubing. A neck was then put into the tubing a t the required distance from the closed end and a length of at least 90 mm left above the neck, Tubes of 8 mm i.d. and 2 mm wall thickness (10 mL volume) were found to be the most convenient; they are easy to fill and seal and are readily emptied after use. Three such tubes fit into the bomb simultaneously. Before use the tubes are cleaned (and internal flaws removed) by filling them with 40% hydrofluoric acid for 15 min. They are then carefully washed with distilled water and oven dried. Blank values for sodium (the major contaminant) in the samples used were
