In the Laboratory
The Gravimetric Analysis of Nickel Using a Microwave Oven Nadia Carmosini, Sanaz Ghoreshy, and Marina C. Koether* Department of Chemistry, Queen’s University, Kingston, ON, Canada K7L 3N6 One experiment in our teaching laboratories is the gravimetric quantitative analysis of the percentage of nickel in steel. This classical method is very time consuming, which results in limited educational value for the students. A microwave method that modernizes the experiment and decreases the time consumption is proposed in this paper. Recently, Thompson and Ghadiali (1) utilized a household microwave oven to dry precipitates for three common gravimetric determinations: silver (as silver chloride), sulfate (as barium sulfate), and calcium (as calcium oxalate). The microwave oven not only produced results comparable to those of conventional drying methods, but also decreased the overall time required. Gulmini et al. (2) compared conventional and microwave heating procedures in the extraction of calcium, copper, iron, and manganese in a lagoon sediment and found that microwave heating produced results comparable to those of conventional heating, with a much shorter operating time. Since microwave digestion and drying have proven to be faster in the above applications, an investigation into its time efficiency, accuracy, and precision for the determination of Ni in a nickel ore was undertaken. Experimental Procedure (modified from ref 3)
Microwave Method The Ni ore (up to 6.3 g, Thornsmith Inc., 5.01% Ni w/ w) was dried in the microwave oven using cycles consisting of 3 min of radiation at medium power (Panasonic, model NN5572C—medium power—55% of 900 W) followed by 1 min of cooling. At most, seven cycles were required until a stable mass, indicating no further evaporation of moisture, occurred. (A “dummy” load of 200 mL of ice water, used to protect the magnetron from reflected microwaves, was placed in the microwave oven during all drying and digestion trials. The water in the dummy begins to simmer after 3 min.) The dried Ni ore was weighed by difference (0.6–0.8 g) into a cylindrical Teflon bomb made in-house. (The screwtop bombs consisted of a bottom and top portion with an exterior circular diameter of 8 cm and a combined height of 8.5 cm. Within the bottom half of the cylinder, an interior volume of 50 mL was bored out with a 4-cm diameter and a 4-cm depth.) Conc. HNO3 (5 mL) and conc. HCl (3 mL) were added to each bomb. The bombs were sealed with the screw-top lids and radiated (three at a time) for 1 min, water-bath cooled for 2 min, radiated for 2 min and water-bath cooled for 5 min. The entire heating and cooling process was repeated two times to ensure complete digestion. The final solution was light green—indicating that the solution had been boiling, which allowed for the release of the oxides. The solution was transferred to a beaker, and the volume made up to 50 mL with deionized distilled water (DDW) and vacuum-filtered with a Millipore filtering apparatus and filter paper (Whatman 42 ashless filter paper, 5.5 cm). The filter paper and residue were washed *Corresponding author. Current address: Department of Chemistry, Northeastern Illinois University, Chicago, IL 60625-4699.
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with hot NH4Cl solution (0.05 M). To the filtered solution, 75 mL of citric acid (0.2 M) was added and the volume was made up to 200 mL with DDW. The solution was made slightly basic (pH 8–9) with dropwise addition of aqueous NH4 OH and then acidified (pH ~2) with dropwise additions of HCl (4 M). The solution was brought up to a volume of 400 mL with DDW and 10 g of NH4Cl was added. The beakers were heated in the microwave (3 min, medium power) to 50–60 °C. The final step was addition of 20 mL of HDMG (1%) in ethanol and 30 mL of aqueous NH3 (3 M). The solutions were covered and left overnight to allow for complete precipitation. Medium-porosity Gooch crucibles (15-mm pores) were dried in the microwave using the heating regime of 3-min radiating periods at medium power with a 1-min cooling period in between. Typically, 10 periods were required. The Ni(DMG)2 precipitates were washed with hot DDW. The precipitates were dried in groups of three to six using the above heating regime. About 15 periods were required. SAFETY PRECAUTIONS: The normal safety procedures for an undergraduate analytical laboratory are generally sufficient for ensuring student safety in this experiment. Students should be required to wear rubber gloves to prevent injury due to acid spillage. If care is not taken, spillage may occur when opening the bombs after the digestion period. Students should also be reminded not to lean forward into the fume hood when opening the bombs, as a small amount of acidic fumes will escape. Utilizing the volumes of acids outlined in this paper, fumes were not found to pose a significant danger. However, instructors are cautioned that increasing the volumes of reagents without considering the sizes of the bombs could result in unsafe pressure buildup.
Table 1. Microwave Digestion and Drying Trial
Ni(DMG)2 wt found (g)
Ni(DMG)2 wt expected (g)
% Found 100.1
1
0.1576
0.1575
2
0.1609
0.1615
99.6
3
0.1787
0.1792
99.7
4
0.1712
0.1706
100.4
5
0.1538
0.1542
99.7
6
0.1508
0.1514
99.6
Table 2. Conventional Digestion and Drying Trial
Ni(DMG)2 wt found (g)
Ni(DMG)2 wt expected (g)
% Found 99.7
1
0.1908
0.1914
2
0.1686
0.1704
98.9
3
0.1655
0.1647
100.5
4
0.1700
0.1706
99.6
5
0.1494
0.1507
99.1
6
0.1662
0.1675
99.2
Journal of Chemical Education • Vol. 74 No. 8 August 1997
In the Laboratory Table 3. Comparison of Times Required for Microwave and Conventional Heating Methods Time Required for Step (min) Method
Ore Drying
Microwavea,b
28
Conventionalc
120 (o)
Crucibles
Digestion Heating
Cooling
Drying
Cooling
0
40 (3)
0
30 (3)
20–25
120 (o)
20–25
60 (h)
Precipitate Drying
Cooling
3 (3)
60 (6)
0
9 (h)
120 (o)
20–25
a
A 1-min cooling time is included in the drying cycles for the microwave oven. b The number in parentheses indicates the number of individual samples treated. c The conventional method includes both an oven (o) and a hot-plate (h).
Conventional Method The Ni ore (2.1 g) was dried for 2 h in a conventional oven at 110–120 °C. The dried Ni ore (0.6–0.8 g) was weighed by difference directly into a beaker and conc. HCl (10 mL) and conc. HNO3 (15 mL) were added. The beaker was then covered and set to simmer on the hot-plate for 20 min, after which the solution was boiled down to ~10 mL. This process was repeated and the volume was brought up to 50 mL with DDW. The remainder of the procedure was identical to the one above with a hot -plate taking the place of the microwave oven. Drying of the precipitate at 110–120 °C took 2 h in a conventional oven.
the Ni ore and crucibles, the conventional oven may be more time-effective than the microwave method, since other tasks could be performed during that long length of time. Typically, as more samples are treated, there is an increase in the time required for the microwave technique. On the other hand, due to the low cost of the household microwave ovens, more than one oven may be in use at one time. This is where the time advantage of the digestion and heating by the microwave may be more appropriately used, as the time is cut in half.
Results and Discussion
Accuracy and precision are not compromised by the use of the microwave oven and no chemical modification of the precipitate due to the microwave radiation occurred. As well, the microwave oven proved to be a very useful analytical tool for time efficiency, not only for the digestion and heating of the steel ore, but also for the drying of the ore, the crucibles, and the Ni(DMG)2 precipitate. However, constant attention, which is required with the microwave drying method, is not necessary for the conventional oven method. Since the two methods can be intermixed, in designing the experiment for undergraduate students, thought should be given to which part of the method should be performed with the microwave and which with the conventional oven in order to be more time effective.
Experiments performed with the microwave oven gave an average recovery of 99.9 ± 0.3%, whereas the conventional method gave a value of 99.5 ± 0.6%. Results show that the gravimetric analysis of Ni(DMG)2 by using a microwave oven (Table 1) can be performed without any loss in accuracy or precision when compared to the conventional oven method (Table 2). The Ni(DMG)2 samples, which were digested and dried in the microwave oven, showed no physical difference in color or consistency, indicating that there was no chemical modification of the precipitate due to the microwave radiation. Table 3 indicates that the microwave oven performs better in terms of time efficiency as well. Significant advantages occur with the cooling time. It can typically take up to 25 min for samples to cool when using a conventional oven (3), whereas it takes only 1 min for the microwave oven. However, it is not suggested that the entire experiment be performed with the microwave, as attendance between cycles is required. In some cases, such as drying of
Conclusion
Literature Cited 1. Thompson, R. Q.; Ghadiali, M. J. Chem. Educ. 1993, 70, 170–171. 2. Gulmini, M.; Ostacoli, G.; Zelano, V. Analyst 1994, 119, 2075– 2080. 3. Skoog, D. A.; West, D. M.; Holler. F. J. Fundamentals of Analytical Chemistry, 6th ed.; Saunders: New York, 1992.
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