Objective
Reduce the time, complexity and expense of dissolving metal and mineral samples in solution in preparation for chemical analysis.
Approach
In earlier research, the Bureau of Mines developed a rapid and inexpensive method for the dissolution of mineral and metal samples in plastic pressure bottles heated in a boiling water bath. Using this technique, samples could be prepared for chemical analysis in less than one hour compared with several hours by traditional dissolution methods (see Technology News No. 85, December 1980 for further details). By heating in a microwave oven, sample dissolution time can be reduced to approximately 5 minutes.
An inexpensive microwave oven provides extremely precise and efficient heating of the sample and acid mixture. Increased temperature and pressure in the sealed bottles accelerate sample dissolution.
The Improvements
Samples are prepared as previously described by grinding them to minus 100- mesh and placing a small weighed portion of each into polycarbonate bottles. A mixture of hydrochloric, nitric, and hydrofluoric acids is added to each bottle. The bottle caps then are screwed on tightly to prevent contamination and to retain volatile gases which could be released during the dissolution process. To heat the samples, the bottles are placed in a microwave oven on “high” power for 2 minutes. Since uneven heating could occur in the oven due to the presence of hot spots, a polypropylene rack was designed that holds 12 sample bottles simultaneously and fits on a standard microwave carousel. Since the bottles are evenly spaced on the rack, which turns automatically on the carousel, the samples are heated more evenly.
After heating, the bottles are cooled in the oven for 1 to 2 minutes by a stream of CO2 gas rather than the water bath used in the earlier method. The CO2 not only cools the bottles more rapidly, but it is also safer to use since it removes any acid fumes from the oven.
A solution of boric acid is added to the cooled bottles which are then heated in the microwave oven for an additional 2 minutes. If after the second heating any undissolved residue remains in the bottles, they may be heated another 1 to 8 minutes for total dissolution. After the solutions are cooled, they are then ready for analysis.
Reliability of the Process
The microwave method was evaluated by using it to dissolve basic oxygen furnace slag and two Bureau of Standards reference materials. After dissolution, four sample solutions of each material were analyzed from separate bottles. Results of these analyses showed that the sample compositions were in excellent agreement with certified values. Relative standard deviations ranged from 0.0 to 3.92 percent with an average value of 1.09 percent. These values were obtained using standards uncorrected for matrix effects.
The microwave dissolution procedure described in this Technology News has been tested rigorously for safety. Tests have demonstrated that the bottle rack will contain any acid spills that might result from bottle failure, and any acid fumes are exhausted from the oven through a vent into a fume hood. The oven door effectively prevents any acid fumes from escaping into the workplace.