Chemical Analysis of Household Oxygen-Based Powdered Bleach: An Engaging Approach to Teaching Sampling of Heterogeneous Materials and Addressing Statistics

摘要

Crucial steps of a chemical analysis, such as sampling, determination, and statistical analysis of data, are addressed in a well-tested and stimulating laboratory activity that convincingly unveils the stratification of ingredients in a heterogeneous household product for laundry: a powdered oxygen bleach. Three different samples of bleach were taken from a single jar: one from the top, one from the bottom, and a more representative mixture collected by driving a core sampler (a graduated cylinder was found to be appropriate) throughout the full depth of the jar content. The resulting frequency distribution curves (histograms) of the H_(2)O_(2) titrations with MnO_(4)~(–) presented widely separated maxima for the three samples, while no statistically significant difference was found for the titration of CO_(3)~(2–) with HCl. These results intrigue the students, leading them to reflect until they understand that this is due to the presence of a single compound acting as a source of H_(2)O_(2) (sodium percarbonate), while two ingredients in the formulation of the stratified bleach powder provide CO_(3)~(2–) after dissolution. Large and light percarbonate grains are more frequent in the upper layer of the bleach jar, while smaller and denser Na_(2)CO_(3) particles are richer at the bottom.