In the Laboratory
An Exercise To Illustrate the Importance of Sample Preparation in Chemical Analysis
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Jeffrey G. Dunn, David N. Phillips, and Wilhelm van Bronswijk School of Applied Chemistry, Curtin University of Technology, P.O. Box U1987, Perth, Western Australia, Australia, 6001 The results of chemical analyses can only be as good as the sampling and sample preparation. Chemists need to recognize this problem and ensure that the analytical sample is representative and free from contamination. There is little point in using the latest analytical equipment and obtaining the best analytical precision, if the sample submitted for analysis is not representative in the first place. The basic rule for correct sampling is that all parts of the material being sampled must have an equal opportunity of being collected and becoming part of the final sample for analysis. If this rule is not respected, bias can easily be introduced. Unlike precision, which can be improved by replication, bias cannot be reduced once it is present. When sampling solids in industry, the most suitable sampling location is the discharge point of a process stream, where increments can be obtained by taking a complete cross-section of the stream with a sample cutter. Sampling devices that take only part of the stream may introduce serious bias. Usually the stream is divided into strata of equal time or mass, and increments are taken at the same point in each stratum (systematic sampling) or randomly within each stratum (stratified random sampling). Stratified random sampling should be used when periodic variations in quality are present. There are many texts that may be consulted for background reference when more sophisticated mathematical theories of sampling errors and variance need to be applied (1–5). At Curtin University we introduce chemistry students to the procedures of crushing, grinding, sieving and splitting a nickel ore for chemical analysis, together with an examination of the effects of particle size on the quantitative extraction of nickel by acid digestion. While this paper gives a brief description of the equipment and methods necessary for sample preparation, it concentrates more on the data obtained by our students and the conclusions that may be drawn from such data. This exercise may be used by class instructors when introducing students to the pitfalls in sampling and the power of statistics. Full details of the experimental procedure and raw data may be obtained by email from one of the authors (DNP:
[email protected]) or on JCE Online (http://jchemed.chem.wisc.edu).
Figure 1. The Tema Disc Mill head.
Figure 2. The sample splitter.
Equipment A jawcrusher is used to reduce material from a maximum particle size of about 5 cm to about 2 mm. It is customary to make several passes of material through the crusher, and the size of the exit material is governed by the number of blades set at the bottom of the sample holder. The lower the number of blades, the larger the exit material. A hammer may be effectively used as a substitute for a jawcrusher. A series of stacked sieves may then be used to fractionate the sample into various particle size ranges. The head of the Tema Disc Mill is shown in Figure 1. It is used to grind the various sized fractions to finer particle sizes. The material W Supplementary materials for this article appear on JCE Online at http://jchemed.chem.wisc.edu.
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Figure 3. A coned and quartered sample.
Journal of Chemical Education • Vol. 74 No. 10 October 1997
In the Laboratory
Figure 5. Percent nickel extracted vs. particle size.
Figure 4. Percent nickel content vs. particle size.
to be ground is placed between the rings in the grinding head. Grinding may take from a few seconds to one or two minutes, depending on the sample type and size. Reduction in the bulk sample may be achieved either by the use of a sample splitter or by coning and quartering. The sample splitter is shown in Figure 2. It consists of a number of chutes attached to either side of a center track. The sample is poured uniformly onto the track, and the sample is split into two portions. One half is rejected and the process is continued with the other half. In coning and quartering, the sample is poured onto a paper square in a cone shape. The cone is then flattened and quartered, and two opposite quarters are taken and retained. The other opposite two quarters are rejected. A coned and quartered sample is shown in Figure 3. Samples Two series of five samples are obtained in the experimental procedure as follows. The bulk sample is jawcrushed and sieved into the size fractions
