An Experiment in the Sampling of Solids for Chemical Analysis

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.