Page 242 - Modern Analytical Chemistry

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Chapter 7 Obtaining and Preparing Samples for Analysis 225

7K Suggested EXPERIMENTS

Experiments Bauer, C. F. “Sampling Error Lecture Demonstration,” J. collect a sufficient amount of data in a single laboratory
The following set of experiments introduce students to the important effect of sampling on the quality
of analytical results. Each experiment is annotated with a brief description of the principles that it
emphasizes.

Chem. Educ. 1985, 62, 253.
period. The overall variance of the analysis is partitioned into
components due to the method, to the preparation of
This short paper describes a demonstration suitable for
samples, and to sample collection. The validity of equation
use in the classroom. Two populations of corks are sampled
7.5 is also evaluated.
to determine the concentration of labeled corks. The exercise
demonstrates how increasing the number of particles
Kratochvil, B.; Reid, R. S.; Harris, W. E. “Sampling Error in a
sampled improves the standard deviation due to sampling.
Particulate Mixture,” J. Chem. Educ. 1980, 57, 518–520.
Clement, R. E. “Environmental Sampling for Trace Analysis,” In this experiment the overall variance for the analysis of
Anal. Chem. 1992, 64, 1076A–1081A. potassium hydrogen phthalate (KHP) in a mixture of KHP
Sampling of a large population (n = 900) of colored and sucrose is partitioned into that due to sampling and that
candies (M&M’s work well) is used to demonstrate the due to the analytical method (an acid–base titration). By
importance of sample size in determining the concentration having individuals analyze samples with different % w/w
of species at several different concentration levels. This KHP, the relationship between sampling error and
experiment is similar to the preceding one described by Bauer concentration of analyte can be explored.
but incorporates several analytes.
Lochmuler, C. “Atomic Spectroscopy—Determination of
Guy, R. D.; Ramaley, L.; Wentzell, P. D. “An Experiment in Calcium and Magnesium in Sand with a Statistical Treatment
the Sampling of Solids for Chemical Analysis,” J. Chem. Educ. of Measurements” published on the web at
1998, 75, 1028–1033. http://www.chem.duke.edu/~clochmul/exp4/exp4.html.
This experiment uses the molybdenum-blue method to This experiment introduces random sampling. The
determine the concentration of phosphate in a experiment’s overall variance is divided into that due to the
phosphate/sodium chloride mixture. Flow-injection analysis instrument, that due to sample preparation, and that due to
is used to increase the speed of analysis, allowing students to sampling.

The following experiments describe homemade sampling devices for collecting samples in the field.
Delumyea, R. D.; McCleary, D. L. “A Device to Collect This experiment describes the construction of an air
Sediment Cores,” J. Chem. Educ. 1993, 70, 172–173. sampler using an aquarium pump, a flow meter, a filter
Directions are provided for preparing and using a simple holder, and bottles that serve as traps for analytes.
coring device using PVC pipe. This experiment also details a Applications include the determinations of SO 2 , NO 2 ,
procedure for determining the weight percent of organic HCHO, and suspended particulate matter.
material in sediments as a function of depth.
Shooter, D. “Nitrogen Dioxide and Its Determination in the
Rockwell, D. M.; Hansen, T. “Sampling and Analyzing Air Atmosphere,” J. Chem. Educ. 1993, 70, A133–A140.
Pollution,” J. Chem. Educ. 1994, 71, 318–322. This paper describes the construction and use of a
Two simple air samplers are described as well as their use diffusion tube for sampling NO 2 from the atmosphere.
for determining particulates in air. Examples of its use include the determination of NO 2
concentrations at various heights above ground level in an
Saxena, S.; Upadhyay, R.; Upadhyay, P. “A Simple and Low- urban environment and through a tree’s leaf canopy.
Cost Air Sampler,” J. Chem. Educ. 1996, 73, 787–788.
—Continued