Anal. Chem. 1994,66, 2194-2196
Inorganic Microwave Digestions Incorporating Bases Bradley D. Zehr,' Judith P. VanKuren, and Hugh M. McMahon Osram Sylvania Inc., Chemical and MetaHurgical Pruducts, Hawes Street, TowanCra, Pennsylvania 18848
The use of bases in the microwave digestion of inorganics is discussed. Thisalternative to typical acid microwavedigestions has proven useful in specific cases. The specific examples include the digestion of materials which coatained barium aluminate (BaAI&), cobalt oxide (COO), molybdenumoxides ( M aand MOOS), thorium oxide and/or tungsten W4011,and woj). oxides (WOz,WZOS,
(m),
Microwave digestion technology has become an integral part of the analytical chemistry laboratory. Typically, microwavedigestionsuse acids e~clusively.*-~ Although many materials are digestible with acids in the microwave, some matrices are still encountered that call for alternative digestions. Normally, the digestion method used in this situation is a salt f ~ s i o n .This ~ work discusses an alternative, which includesthe use of bases in microwave digestions. Advantages of this type of nontypical microwave digestion over a salt fusion include decreased sample preparation time and lower risk of contamination. This work utilized pressure and temperature control microwave digestion systems (MDS 2000 and MDS 2100 by CEM Corp., Matthew, NC). RESULTS AND DISCUSSION In attempting to dissolve tungsten and various tungsten oxides, the use of bases in a microwave digestion system was explored. An initial observation included surface arcing within the digestion vessel during microwave operation, when using 10 M NaOH. The noted arcing was concluded to be most likely due to the formation of a layer of undissolved NaOH on the surface of the liquid. As a result of this observation, further work with 10 M NaOH was discontinued. However, this observation did bring up the question of solubility. Table 1 shows some properties of the common alkali bases. Our first successful basic digestion in the microwave was the dissolution of W03 with LiOH in an open vessel. For this technique, the microwave was used to increase the digestion speed. LiOH was chosen because of the need to determine Na and K in the digested material. The dissolution of other tungsten oxides with lower oxygen content (WOZ,WzO5, and W401I ) proved to be a greater challenge. With W02, W205, and W4O11, the addition of H202 to the LiOH was required in order to reach a complete dissolution. Utilizing the above observations, the dissolution of a tungsten scrap material was attempted. The tungsten scrap material contained a mixture of tungsten, tungsten carbide, and tungsten oxides. The combination of LiOH and H202 in (1) Zehr,
B. D.;Fcdorchak, M.A. Am. l a b . 1991, 4 0 4 5 .
(2) Zehr, B. D. Am. Lab. 1992,24. (3) Zchr, 8. D.;VanKurcn, J. P.Surf. Cwr. Technol. In press. (4) Zchr, B. D.;Zchr, T.J. Specrroscopy 1991, 6. 4447.
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AnaWcalChembtty, Vd. 66, No. 13, July 1, 1994
T a m 1. Pr0p.M.r of c0"on B
e
boiling saturation point solubility concentration MW ("C) (g/lOOO mL at 100 "C) (mol/L at 100 "C)
base
LiOH 23.95 KOH 56.11 NaOH 40.0
924 1320 1390
175 1780 3470
7.3 31.7 86.7
a From CRC Handbook of Chemistry and Physics, 5lst ed.; Robert C. Wcast, Ed.; CRC Press: Boca Raton, FL, 1970.
Tabla 2. Maowavo Base/Add VI Fudon-Add#loakn Comparkon Study (WCW oxkk Scrap for V)
Fusion (a) 0.5 g in Zr crucible (b) add 3 g of NazCO3 and 2 g of NazOz (c) mix and then fusc starting with low heat (d) allow fusion melt to cool (e) place crucible in Teflon beaker with H20 (f) slowly add 5 mL of HNOl into crucible (g) leach on hot plate (h) remove crucible when free of fusion melt (i) slowly add 5 mL of HF, 25 r L of HsFQ, and 5 mL of HCI to beaker (j)place on hot plate until dissolved (k) cool, add 1 mL of 191 g/L KCI, and take to 50 mL (1) analyze on flamc-AAS using V standards with W matrix Microwave (a) 0.5 g in single wall v-1 (b) add 5 mL of 10%(w/v) LiOH and 1 mL of H202 (c) microwave 50 psi for 15 min (d) cool and then add 5 mL of HNO, and 5 mL of H F (e) microwave 50 psi for 15 min (f) cool,add 1 mL of 191 g/L KCI, and take to 50 mL (8) analyze on flame-AAS using V standards with W matrix % Spike Recovery
sample A B sample A B C
D1 D2
D3
microwave
fusion
94 94
71 70
Sample Results (%) microwave
fusion
0.021
