In the Laboratory
Weigh This Way Judith M. Bonicamp Department of Chemistry, Middle Tennessee State University, Murfreesboro, TN 37132;
[email protected] For years, I taught quantitative analysis (Quant) students the following technique for weighing successive samples of the unknown from a weighing bottle on an electronic analytical balance. Students were to weigh the bottle plus contents and record the total weight. They were then to tamp the bottle above a suitable container, thus removing some of the unknown (sample 1), and then reweigh the bottle (1). They were now to calculate the weight of sample 1 by subtracting weight 2 from weight 1. For sample 2 and subsequent samples, students continued tamping, weighing, recording, and subtracting. Here is another way. Students first tare the weighing bottle and contents so that the scale reads zero. They next tamp out some unknown and reweigh the bottle. Now the scale reading is a weight in negative grams, corresponding to the weight lost from the bottle. Students record this as the (positive) weight of sample 1. Before removing the bottle and contents from the balance, students reset the scale to zero by taring. After the next sample is tamped out, the apparent weight of the bottle and remainder of the unknown are recorded as the weight of sample 2. The scale again gives a negative weight in grams, corresponding to the positive weight of sample 2. This process gives the same precision and accuracy as the more common technique (personal communication with Workman, D. [2000] and McLean, G. [2001]; MettlerToledo, Inc., Columbus, OH), requires no subtraction, and is faster. Weighing data that demonstrate this are in Table 1. The negative weight the student sees on the scale corresponds logically to the weight lost from the bottle, a pedagogically pleasing result. I have been unable to find this method described in the literature. So far no one in Quant has reported a negative percent analyte in a gravimetric determination using this method.
Table 1. Weights of Paper Clips Determined by Two Methods of Weighing by Difference A. Traditional Weighing by Difference Weight/g Trial
Aqua Clip
Remove Black Clip
Black Clip
Remove Blue Clip
Blue Clip
1
2.2758
1.9371
0.3387
1.561
0.3761
1.4207
0.1403
2
2.2759
1.937
0.3389
1.5609
0.3761
1.4206
0.1403
3
2.2758
1.9371
0.3387
1.5609
0.3762
1.4208
0.1401
4
2.2759
1.9371
0.3388
1.561
0.3761
1.4207
0.1403
5
2.2758
1.9371
0.3387
1.5611
0.376
1.4208
0.1403
6
2.2758
1.937
0.3388
1.5609
0.3761
1.4207
0.1402
7
2.2757
1.9371
0.3386
1.561
0.3761
1.4207
0.1403
8
2.2757
1.9371
0.3386
1.561
0.3761
1.4207
0.1403
9
2.2758
1.9371
0.3387
1.5609
0.3762
1.4207
0.1402
10
2.2758
1.9371
0.3387
1.5611
0.376
1.4207
0.1404
Av
—
—
0.33872
—
0.37610
—
0.14027
SD
—
—
9.19E᎑5
—
6.67E᎑5
—
8.23E᎑5
Remove Black Clip
Tare
Remove Blue Clip
B. Weighing by Taring between Steps Weight/g Trial
Boat and 3 Clips
Tare
Remove Aqua Clip
Tare
1
2.2757
0.0000
᎑0.3386
0.0000
᎑0.3761
0.0000
᎑0.1403
2
2.2758
0.0000
᎑0.3387
0.0000
᎑0.3761
0.0000
᎑0.1403
3
2.2759
0.0000
᎑0.3388
0.0000
᎑0.3761
0.0000
᎑0.1403
4
2.2761
0.0000
᎑0.3387
0.0000
᎑0.3761
0.0000
᎑0.1403
5
2.2758
0.0000
᎑0.3387
0.0000
᎑0.3762
0.0000
᎑0.1404
6
2.2758
0.0000
᎑0.3388
0.0000
᎑0.3761
0.0000
᎑0.1403
7
2.2758
0.0000
᎑0.3387
0.0000
᎑0.3761
0.0000
᎑0.1403
8
2.2759
0.0000
᎑0.3388
0.0000
᎑0.3761
0.0000
᎑0.1403
9
2.2758
0.0000
᎑0.3388
0.0000
᎑0.3761
0.0000
᎑0.1404
10
2.2758
0.0000
᎑0.3388
0.0000
᎑0.3761
0.0000
᎑0.1403
Av
—
—
᎑0.33874
—
᎑0.37611
—
᎑0.14032
SD
—
—
6.99E᎑5
—
3.16E᎑5
—
4.22E᎑5
NOTE: All weights were determined using the same Mettler AE100 four-place electronic balance.
Acknowledgment I thank Galen Longo for asking how to weigh this way during the fall 2000 Quantitative Analysis class.
476
Boat and Remove 3 Clips Aqua Clip
Literature Cited 1. Harris, D. C. Quantitative Chemical Analysis, 5th ed.; Freeman: New York, 1998; p 32.
Journal of Chemical Education • Vol. 79 No. 4 April 2002 • JChemEd.chem.wisc.edu
