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1998
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August
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In the Laboratory
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An Experiment in the Sampling of Solids for Chemical Analysis
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Robert D. Guy, Louis Ramaley, and Peter D. Wentzell
Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, NS, B3H 4J3 Canada
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August 1998
Vol. 75 No. 8
p. 1028
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| Abstract |
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An experiment is described that is suitable for demonstrating the principles of sampling uncertainty in the chemical analysis of particulate materials. Although uncertainty due to the sampling of heterogeneous materials is a pervasive problem in the analysis of "real world" samples, there are very few experiments designed to introduce this topic into the undergraduate laboratory. In the experiment described here, students analyze mixtures of KH2PO4 and NaCl (0.5 to 0.8% KH2PO4), with sample masses between 0.1 and 2.5 g, and observe the effect of sample size on analytical uncertainty. The variance due to sampling is isolated from other sources of variance and used to estimate Ingamells' sampling constant, Ks, for the mixture. The determination of phosphate is performed using the molybdenum-blue method implemented on a simple flow-injection apparatus, which permits a high sample throughput. The experiment is easily completed by two students in one laboratory period and consistently shows improved analytical precision with increasing sample size. Values for Ks typically range from 100 to 1000 g.
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| More Information |
 Citation |
Guy, Robert D.; Ramaley, Louis; Wentzell, Peter D. J. Chem. Educ. 1998 75 1028.
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Keywords |
Laboratory Instruction, Laboratory Equipment/Apparatus, Analytical Chemistry
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History |
Created:
Last Updated: |
June 22, 1999
June 24, 2005
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| Home > JCE Print > Journal of Chemical Education > Issues >
1998
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August
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1028
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